What adaptations does the human body have to handle frequent and sustained changes in orientation?

What adaptations does the human body have to handle frequent and sustained changes in orientation?

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Most animals are horizontal with the ground. When they lie down, they get closer to the ground, but their orientation does not change.

Humans, on the other hand, stand up straight and tall, perpendicular with the ground. And when we lie down, we make a complete 90° turn. So we spent 2/3 of our day at one orientation, and 1/3 of our day at the exactly orthogonal orientation.

It's difficult to think of many other animals that change their orientation so completely on a regular basis, or spend much time in different orientations. (Sure, prairie dogs, bears, lemurs, and apes stand upright now and then, and I've seen pictures of whales sleeping vertically, but it's never for long, sustained periods. Giraffes seem so tall and vertical, but it's all neck and legs - the organs of their body are always horizontal with the ground, and this orientation is maintained when they lie down. Birds, maybe?)

For humans, regular changes in orientation must surely cause biological stresses, and therefore there must be some biological adaptations to those stresses.

Even non-biologists are aware of plenty of orientation-related malfunctions:
- middle-aged people start to get acid reflux and indigestion when they lie horizontally
- elderly people with certain heart conditions and arrhythmias cannot lie horizontally
- some people are affected by changes in blood pressure when they lie down or stand up
- some people have dizziness or balance issues when lying down or standing up
- knees and feet start to have problems from a lifetime of bearing all the weight

What specific adaptations does the human body have to handle these daily changes in orientation?

What adaptations does the human body have to handle frequent and sustained changes in orientation? - Biology

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See the video clips associated with this section.


The user must keep in mind that much is still unknown about the over-all, long term effects of various space environments on performance capabilities. The data included here were derived from past experience with high-performance aircraft, and the relatively limited experience, particularly with respect to long orbital stays, with past space programs. A lot of the information in this chapter has been derived from one-g data to which trend information from the sources cited above was applied. Although less than perfect or complete data were compiled for this chapter, it is the best information in this field known to exist at this time.

This chapter is based on the premise that designers and mission planners will do a better job if they are familiar with the capabilities of the people for whom they are designing. When people go into space their performance capabilities may change in important ways. The purpose of this chapter is to document these changes.

The voluminous data that exists on human performance capabilities under 1-G (Earth) conditions are not included here. This material is covered in other sources (see refs. 4, 19, 143, and especially 336).


4.2.1 Introduction

This section discusses aspects of visual performance that are, or are likely to be, modified by space travel. For more general information on vision, consult the references provided in Paragraph 4.1.

4.2.2 Vision Design Considerations

Space-related factors that may affect visual perception as listed below.

a. Acceleration - The effects of acceleration on vision depend on the direction of the force vector.

(Refer to Paragraph 5.3, Acceleration, for additional information on the affects of acceleration.)

1. +Gz acceleration (eyeballs down) results in dimming of vision, followed by tunnel vision loss of sight which begins on the periphery and gradually narrows down until only macular (central) vision remains. This is followed by total blackout and then loss of consciousness.

2. +Gx acceleration (eyeballs in) results in loss of peripheral vision. This typically occurs at slightly over 4-G (based on a rate of onset of 1-G per second). Complete loss of vision varies between individuals, and with physical conditioning, training, and experience.

3. -Gz acceleration (eyeballs up) results in diminished vision, red-out (red vision), an increase in the time for the eyes to accommodate, and a blurring or doubling of vision.

4. When exposed to -Gx acceleration (eyeballs out), crewmembers will experience visual symptoms associated with -Gz acceleration (see 3" above).

5. Visual reaction time may be defined as the interval between the onset of a stimulus and the initiation of the crewmember's response. This interval is, in general, lengthened by increased G level.

6. Visual tracking is moderately degraded by increased G level.

b. Vibration - If vibration is sufficiently severe, visual performance will be degraded. The severity depends on the frequency and amplitude of the vibration along with the resonance frequency of the body part involved. Unfortunately, the times when vibration is most likely to be encountered (e.g., liftoff and landing) also tend to be times when vision is important. Displays that must be read during projected periods of high vibration should be designed accordingly. Design techniques to be considered should include display characters which are sufficiently large to be perceived even when blurred and sufficient illumination to avoid scotopic vision which results in a lower Critical Flicker Fusion Frequency.

(Refer to Paragraph, Human Response to Vibration, and reference 19 for additional information.)

c. Light in Space - Differences in light transmission and reflectance in space result in some significant differences in available perceptual cues in the extravehicular environment as compared to earth atmosphere.

1. Light Scatter - Atmospheric light scatter does not exist in space due to the lack of particulate and gaseous material. Thus, aerial perspective cues are absent. Figure-ground contrast is increased and shadows appear darker and more clearly defined. Loss of these cues along with other environmental consequences discussed below can degrade perception of object shape, distance, location and relative motion.

2. Luminance Range (Contrast) - The extravehicular environment is marked by a wider range of light intensities than normally encountered on Earth. Shifting gaze from a brighter to a substantially dimmer scene will require time for the eyes to adapt to the lower light level. For example, problems arise on EVA missions when crewmembers go from working in sunlight to working in shadows.

In Figure 4.2.2-1, adaptation time requirements are shown for shifting gaze from a brighter to a dimmer environment. For comparison, Figure 4.2.2-2 indicates luminance values for some typical visual stimuli.

Figure 4.2.2-1 Dark Adaptation Thresholds

Reference: 338, p. 187 NASA-STD-3000

Figure 4.2.2-2 Luminance Values for Typical Visual Stimuli

Example Scale of Luminance (millilamberts) Effect
Sun's Surface at noon 10 9 Damaging
10 8
10 7 Phototopic
Tungsten Filament 10 6
10 5
White paper in sunlight 10 4
10 3
10 2
Comfortable reading 10
1 Mixed
10 -1 Scotopic
White paper in moonlight 10 -2
10 -3
White paper in starlight 10 -4
10 -5
Absolute threshold 10 -6

Reference: 337, p. 26 NASA-STD-3000 194

d. Absence of Other Earth Cues:

1. Absence of a Fixed Vertical Orientation - Recognition of familiar objects, faces, and areas (e.g., workstation) is poor when viewed from an orientation significantly different from the established vertical. The viewer must be oriented within approximately 45 degrees of this vertical to perceive the surroundings in a relatively normal fashion. This fact argues for the establishment of a local vertical for each living and working area within a space module.

2. Absence of Fixed Horizon and its accompanying foreground and background cues can be expected to degrade extravehicular perception of object shape, distance, location and relative motion.

3. Absence of a fixed, overhead sun position and its effects on shadow cues is expected to have similar effects as those in 2 (above).

e. Light Flashes - The perception of light flashes has been reported by many crewmembers during periods of darkness at specific orbital locations. The cause is thought to be cosmic rays and/or heavy-particle radiation traversing the head or eyes and triggering a neural response that results in these perceptions.

f. Potential deficits - While visual perception in space is normal in many respects, there are reports of various changes in vision (some of them contradictory) that point out the complex consequences of the above factors. These include Soviet reports of a shift in perceived colors and a reduction in contrast sensitivity, along with a seemingly contradictory report indicating improved visual acuity for distant objects. Some U.S. astronauts have indicated a reduction in near acuity with no apparent change in far acuity, while some crewmembers who wear reading glasses on Earth found they were more dependent on them while in space. Clearly, more research is needed before we can say more about these effects.


4.3.1 Introduction

There is no evidence that human auditory functioning changes in space. However, there are several factors (e.g., the effects of noise) that should be considered in designing the space habitat.

(Requirements pertaining to acceptable noise levels are described in Paragraph 5.4.3, Acoustic Design Requirements.)

4.3.2 Auditory System Design Considerations Auditory Response

Figure shows human auditory responses as a function of frequency.

Figure Human Auditory Response as a Function of Frequency

Table supplementing the graph above
Human Auditory Response Decibels (db)
Threshold of pain 140
Subway - local station with express passing 120
Average factory, large store, or noisy office 80
Average residence 40
Low whisper 1.5 m (5 ft) 20

Reference: 335 NASA-STD-3000 195

c. Communication - Even low levels of noise can interfere with communication.

d. Task Complexity - Noise can adversely affect performance, with the effects being greater for more complex asks.

e. Intermittent Noise - Intermittent noise has more adverse effects than steady-state noise.

f. Adverse Effects - General effects of a noisy environment include fatigue, distractibility, sleep disturbance, irritation, and aggressive behavior.

g. Psychological Factors - The level of annoyance that noise produces depends on a number of factors. Sensitivity varies greatly among individuals.

1. People are generally less sensitive to noise related to their well-being.

2. People are more sensitive to unpredictable noise.

3. People are more sensitive to noise they feel is unnecessary.

4. People who are most sensitive to noise become increasingly disturbed as the noise persists, whereas the annoyance level of less sensitive individuals remains constant over time.

5. The perceived abrasiveness of certain sounds is subjective and varies considerably among individuals (e.g., consider the potential conflict between opera and rock music lovers).

h. Cabin Pressure - Reduced cabin pressure causes a reduction in sound transmission. This means that crewmembers have to talk louder to be heard which can potentially lead to hoarseness on the part of some crewmembers. The problem becomes more noticeable as the distance between individuals increases.

(Refer to Paragraph 5.4, Acoustics, for additional information.) Noise Design Considerations

Noise can have many adverse effects on humans and must be considered when designing the human habitat. Considerations include:

a. Extreme Noise - Extreme noise can cause pain and temporary or permanent hearing loss. The adverse effects of pure tones occur at a level about 10 dB lower than for broad band noise.

b. Extended Exposure - Exposure to loud noise for extended periods of time can cause permanent hearing loss. The degree of exposure that will result in damage depends on intensity and individual susceptibility.


4.4.1 Introduction

Changes in our senses of smell and taste might occur in space. These changes are described below.

4.4.2 Olfaction and Taste Design Considerations Olfaction

Aspects of olfaction (smell) that could influence design are presented below.

a. Decreased Sensitivity - There are frequently reported problems with nasal congestion while living in the microgravity environment.

b. Adverse Effects - Unpleasant odors have been associated with a number of medical symptoms including nausea, sinus congestion, headaches, and coughing. Such odors also contribute to general annoyance.

c. Microgravity Odors - Because particulate matter does not settle out in a weightless environment, odor problems in a space habitat may be more severe than under similar Earth conditions. Circulation and filtering will influence the extent of the problem.

d. Visual Cues and Odors - Responses to odors can be accentuated by the presence of visual cues. This increased responsiveness applies to pleasant and unpleasant odors and is something that a designer could potentially put to good use. Taste

Generally there is a decrement in the sense of taste in microgravity. This is probably caused by the upward shift of body fluids and accompanying nasal congestion. Reports indicate that food judged to be adequately seasoned prior to flight tasted bland in space. Given the important role that food is likely to play in maintaining morale on extended space missions, attention should be paid to this problem.


4.5.1 Introduction

Microgravity results in two categories of vestibular side effects: spatial disorientation and space adaptation syndrome (space sickness), both of which can impair crewmember performance.

4.5.2 Vestibular System Design Considerations Spatial Disorientation

Spatial disorientation is experienced by some crewmembers and should be considered in the design of hardware and the planning of missions.

a. Spatial Disorientation - Responses include postural and movement illusions and vertigo. For example, stationary crewmembers may feel that they are tumbling or spinning. These illusions occur with the eyes open or closed.

b. Frequency of Occurrence - The percentage of crewmembers who experience spatial disorientation varies from mission to mission, but averages approximately 50%. The conditions that determine the likelihood and intensity of this disorientation are not well understood.

c. Duration - Some crewmembers may experience spatial disorientation for the first 2 to 4 days of a mission.

d. Activity Schedule - While spatial disorientation need not cause any serious problems, it is advisable not to schedule activities that depend heavily on spatial orientation early in a mission. Space Adaptation Syndrome

Aspects of space adaptation syndrome (SAS) relevant to the design of space modules and mission planning are presented below.

a. Symptoms - SAS symptoms range from stomach awareness and nausea to repeated vomiting. Symptoms also include pallor and sweating.

b. Incidence and Duration - It appears that approximately 50% of the crewmembers are affected by SAS. Symptoms last for the first 2 to 4 days of flight.

c. Performance Decrements - A highly motivated crewmember may be able to maintain a high level of performance despite the presence of mild SAS. However, if motion sickness is severe, some crewmembers will be unable to work until the symptoms lessen.

d. Cause - The leading theory as to the cause of SAS is the sensory conflict theory. This theory states that space sickness occurs when patterns of sensory input to the brain from different senses (vestibular, other proprioceptive input, vision) are markedly rearranged, at variance with each other, or differ substantially from expectations.

e. Volume Effects - The severity of SAS tends to increase as the motion which induces sensory conflict and sickness (particularly head movements in the pitch and roll modes) increases. It follows then that as the volume in which a crewmember is working becomes larger, the chances for this sickness inducing motion increases.

f. Space and Motion Sickness - It is assumed that the mechanism of SAS and 1-G motion sickness are similar, but are similar, but it is not possible to predict an individual's susceptibility to space sickness from their susceptibility to Earth motion sickness.

g. Space Sickness Countermeasures.

1. Drugs - Anti-motion sickness pharmaceuticals (usually Scopedex) have reduced the severity of SAS symptoms for some crewmembers, but have appeared to be ineffective for others. It is likely that they would be more universally effective if they were administered prophylactically, either by injection or orally. The drug should be taken before symptoms develop and absorption from the gut is severely hampered due to the cessation of propulsive motions of the stomach., If a swallowed drug becomes trapped in the stomach, little absorption will take place.

2. Head movements - In some cases restricting head movements has been found effective in reducing the incidence of, and ameliorating the symptoms of, space motion sickness.


4.6.1 Introduction

Kinesthesia is the sense mediated by end organs located in muscles, tendons, and joints, and stimulated by body movements and tensions. Present knowledge of kinesthetic changes occurring when one enters microgravity is limited to estimation of mass and limb position sense.

4.6.2 Kinesthetic Design Considerations

One experiment has indicated that some kinesthetic sensitivity degradation occurs for a few crewmembers. The indications of this experiment are provided below.

a. Mass Versus Weight - In a weightless environment, increments in mass must be at least twice as large as weight increments in a 1-G environment before they can be discriminated (see Figure 4.6.2-1).

b. Barely Noticeable Differences - For two masses to be perceived as different under microgravity conditions, they must differ by at least 10% (see Figure 4.6.2-1).

c. Mass and Acceleration - Differential sensitivity for mass under microgravity conditions can be improved by increasing the acceleration force imposed on the object.

d. Mass Estimation - Absolute judgments of mass tend to be lower under microgravity than under 1-G.

Figure 4.6.2-1 Mean Difference Thresholds (DL) and Associated Standard Deviations (SD) Plotted for Each Standard Under Both Weight and Mass Conditions.

Reference: 78, p. 1-90 NASA-STD-3000 196


4.7.1 Introduction

There appears to be some slowing of reaction times in space, although little precise data are available.

The subject of this Section is Response Time. This time period consists of two phases: 1) Reaction Time which is the time between the presentation of a stimulus to a subject and the beginning of the response to that stimulus, and 2) the time during which the actual response to the stimulus is accomplished. It is believed that this section is actually referring to Response Time and the titles and references should be changed accordingly. However, Reaction Time should not be slowed in micro-gravity as it has more to do with motivation and the effects of microgravity on the subject's physiological and emotional states. A good definition of the difference between Response Time and Reaction Time would help in the solution of this dilemma.

4.7.2 Reaction Time Design Considerations

Information on reaction time that should be considered by designers is provided below.

a. Object Mass - The time required to move an object in microgravity increases as the mass of the object increases.

b. Control Operation - In microgravity, the speed of operating switches (pushbuttons, toggles, rotary switches) is significantly lower than in the 1-G condition.

(Refer to Reference 171 for more information on visual reaction times and Reference 347, for 1-G muscular-reaction time information.)

4.8 MOTOR SKILLS (Coordination)

4.8.1 Introduction

There is a minor impairment of motor skills upon first entering microgravity. This decrement is reduced or eliminated after a short period of adaptation.

4.8.2 Motor Skills (Coordination) Design Considerations

Aspects of human motor skills in space that should be considered by individuals designing for space are provided below.

a. Adaptation Period - Motor skills are somewhat affected when crewmembers are first exposed to microgravity, although these effects tend to diminish or disappear with adaptation. During the period that the crew is adapting to microgravity, fine motor movements are more adversely affected than either medium or gross motor movements. Designers should minimize requirements for crewmembers to exercise fine motor control early in the mission. Switches should be easy to manipulate and care should be taken to preclude accidental activation.

During periods that motor coordination is adapted for the micro-g environment, short returns to an altered g-state (as in reentry, maneuvers, landings, etc.) may result in dyskinesia and dysmetria. This can cause undershooting when reaching for switches for buttons or applying force to control sticks, pedals, knobs, handles, etc.

b. Postural Changes - A change in body posture in microgravity results in a change in the relative position of body parts and can cause decrements in coordination until adaptation occurs. Changes in body posture result from the crewmembers assuming the increase in height due primarily to spinal column expansion.

Refer to Section 3, Anthropometric and Mobility, for additional information on microgravity posture.

c. Body Part Weight - When moving in microgravity, the muscular system does not have to compensate for the weight of body parts. This changes the muscular forces required for coordinated movement and requires the system to readapt.

d. Large Mass Handling - When properly planned, no difficulty has been encountered by crewmembers in moving large masses in a microgravity environment.



Physical work can be divided into two parts: power and endurance (anaerobic and aerobic performance).

The next section addresses the first of these (power), and how it can influence the design of facilities and equipment to achieve optimal crewmember performance. (Endurance is addressed in Paragraph 4.10.2a).

4.9.2 Strength Design Considerations

Aspects of human strength that should be understood and considered in designing for the space environment are presented below.

a. Strength - Strength is the ability to generate muscular tension and to apply it to an external object through the skeletal lever system. Sheer muscle mass (thus, body size) is a significant factor, with cross-sectional area of the muscle fibers being a major determinant of the maximum force that can be generated. Maximum muscular force (strength) can be exerted for only a few seconds.

b. Muscular Endurance - Muscular endurance is the duration a submaximal force may be held in a fixed position (Isometric), or the number of times a movement requiring a submaximal force may be repeated (Isotonic). The duration that a fixed percentage of maximum can be held is reasonably constant across individuals.

c. Counterforces - Microgravity does not have certain counterforces that allow people to effectively perform physical work in 1-G. Traction which depends on body weight is absent, as are forces that result from using body weight for counterbalance.

d. Working While Restrained - Crewmembers' work capabilities while restrained can approach the efficiency experienced on Earth-based tasks, but only where workstation design (including fixed and loose equipment) and task procedures are optimized for the microgravity environment.

e. Working Without Restraints - Without proper restraints, a crewmember's work capabilities will generally be reduced and the time to complete tasks increased.

f. Improved Performance - There are situations where a crewmember can achieve improved strength performance in microgravity. These situations occur when the crewmember uses the greater maneuverability of microgravity to achieve a more efficient body position to be able to push off solid surfaces.

g. Deconditioning - Experience in space indicates that both the strength and aerobic power of load bearing muscles in crewmembers decreases during missions exposing them to microgravity. Exercise programs have been used to counter these deficits but to date have been only partially effective.

(Refer to Paragraph 7.2.3, Reduced Gravity Countermeasures, for information on maintaining strength in space.)

h. Kinematics - The linear motion of free-floating crewmembers can be described by relatively simple equations. The time crewmembers can exert force is governed by the distance they can push before losing physical contact. The force exerted during this time will typically vary as in Figure 4.9.2-1.

The important aspects of this curve are the impulse (Fdt, or the area under the curve), which will determine departure velocity and the peak force, which will determine peak acceleration. In the simplest case, for a subject of mass m, an impulse I with a peak force F acting through the subject's center of mass will result in a velocity

v = I /m where v is in ft/x, I is in lbfs, and m is in slugs or v is in m/x, I is in Ns, and m is in kg and a peak acceleration

a=F/m where a is in ft/s2, F is in lbf, and m is in slugs or a is in m/s2, F is in N, and m is in kg.

In reality, of course, an impulse will rarely go exactly through the center of mass to produce pure linear motion. For any offset of the force from the center of mass, a percentage of the impulse will go toward producing angular (tumbling) motion, with a corresponding decrease in linear velocity. This percentage depends on the offset distance and the subject's moment of inertia. (Moment of inertia varies considerably with body position, and so is difficult to analyze parametrically, but there will be some tumbling in practically all cases.)

Figure 4.9.2-1 Representation of Force Generated by Free-Floating Crewmember Pushing Off

Figure 4.9.2-2 shows the time that a particular force can be exerted as a function of the magnitude of the force exerted, the mass of the individual, and the distance pushed. The velocity that the crewmember will have as they lose contact with the surface is also given.

Figure 4.9.2-2 Force Application and Push-Off Velocity

Time of Force Application and Push-Off Velocity
(95th percentile American male -- 99.3 kg (219 lb))
Force N(lb) Time in sec. for 0.3 m (1 ft) push-off Push-off velocity m/sec (ft/sec) Time in sec. for 0.6 m (2 ft) push-off Push-off velocity m/sec (ft/sec)
4.45 (1) 3.66 0.16 (0.52) 5.18 0.23 (0.75)
22.25 (5) 1.64 0.37 (1.21) 2.31 0.52 (1.71)
44.50 (10) 1.16 0.52 (1.71) 1.64 0.73 (2.40)
89.00 (20) 0.82 0.73 (2.40) 1.16 1.04 (3.41)

Time of Force Application and Push-Off Velocity
(72.6 kg (160 lb individual))
Force N(lb) Time in sec. for 0.3 m (1 ft) push-off Push-off velocity m/sec (ft/sec) Time in sec. for 0.6 m (2 ft) push-off Push-off velocity m/sec (ft/sec)
4.45 (1) 3.12 0.19 (0.63) 4.42 0.27 (0.89)
22.25 (5) 1.40 0.42 (1.41) 1.98 0.61 (2.00)
44.50 (10) 0.99 0.61 (2.00) 1.40 0.86 (2.82)
89.00 (20) 0.70 0.86 (2.82) 0.99 1.21 (3.97)

Time of Force Application and Push-Off Velocity
(5th percentile Japanese Female -- 40.3 kg (89 lb))
Force N(lb) Time in sec. for 0.3 m (1 ft) push-off Push-off velocity m/sec (ft/sec) Time in sec. for 0.6 m (2 ft) push-off Push-off velocity m/sec (ft/sec)
4.45 (1) 2.33 0.26 (0.85) 3.30 0.36 (1.18)
22.25 (5) 1.04 0.57 (1.87) 1.47 0.81 (2.66)
44.50 (10) 0.74 0.82 (2.69) 1.04 1.15 (3.77)
89.00 (20) 0.52 1.15 (3.77) 0.74 1.63 (5.35)

Note: Please be aware that all of the above data was gathered under 1-g conditions.

Reference: 335 NASA-STD-3000 197

4.9.3 Strength Design Requirements

Strength data that shall be used to guide design work are provided below. The weakest crew member in the specified design population shall be accommodated.

(Refer to Reference 16 for additional data on 1-G strength.)

1. Grip strength, as a function of the size of the gripped object, is provided for men in Figure 4.9.3-1.

2. Maximum grip strength for men (5th, 50th, and 95th percentile) is given in Figure 4.9.3-2.

3. Grip strength for females is shown in Figure 4.9.3-3.

b. Arm, Hand, and Thumb/Finger Strength - Figure 4.9.3-4 presents arm, hand and thumb/finger strength for fifth percentile males. These figures must be corrected for females (see Figure 4.9.3-5).

c. Male/Female Muscular Strength - Figure 4.9.3-5 provides a comparison of male and female muscular strength for different muscle groups. These data allow a more accurate extrapolation from male to female strength data than is provided by the old method of assuming females have two thirds the strength of men.

(Refer to Reference 16 for more detailed male/female comparison data.)

d. Static Push Force - Maximal static push forces for adult males are shown in Figure 4.9.3-6. While these data were collected in a 1-G situation, the fact that they do not depend on friction resulting from body weight makes them applicable to microgravity. Corrections will have to be made for females (see Figure 4.9.3-5).

e. Leg Strength - Leg strength for the 5th percentile male as a function of various thigh and knee angles is reported in Figure 4.9.3-7. Estimates of female leg strength can be made from these data using the correction factors provided in Figure 4.9.3-5.

f. Torque Strength - Maximum hand torque data are provided in Figure 4.9.3-8.

Figure 4.9.3-1 Male Grip Strength as a Function of the Separation Between Grip Elements

Note: 44 subjects, all pilots or aviation cadets

Reference: 1, p. 2.5-19 NASA-STD-3000 200

Figure 4.9.3-2 Grip Strength for Males

Population Percentiles, N (lb)
U.S. Air Force personnel, air crewmen 5th 50th or mean 95th Population S.D.
Right hand 467 (105) 596 (134) 729 (164) 80.1 (18.0)
Left hand 427 (96) 552 (124) 685 (154) 71.2 (16.0)

Reference: 1, p. 2.5-18 NASA-STD-3000 201

Figure 4.9.3-3 Grip Strength for Females

Population Percentiles, N (lb)
5th 50th or mean 95th Population S.D.
U.S. Navy personnel
Mean of both hands
258 (58) 325 (73) 387 (87) 39.1 (8.8)
U.S. Industrial workers:
Preferred hand
254 (57) 329 (74) 405 (91) 45.8 (10.3)

Reference: 1, p. 2.5 - 18 NASA-STD-3000 202

Figure 4.9.3-4 Arm, Hand, and Thumb/Finger Strength (5 th Percentile Male Data)

(1) (2) (3) (4) (5) (6) (7)
Degree of elbow flexion (rad) Pull Push Up Down In Out
L** R** L R L R L R L R L R
π 222 231 187 222 40 62 53 75 58 89 36 62
5/6 π 187 249 133 187 57 80 80 89 67 89 36 67
2/3 π 151 137 116 160 76 107 93 116 89 98 45 67
1/2 π 142 165 98 160 76 89 93 116 71 80 45 71
1/3 π 116 107 96 151 67 89 80 89 76 89 53 76
Hand and thumb-finger strength (N)
(3) (9)
Hand grip Thumb-finger grip Thumb-finger grip
Momentary hold 250 260 60 60
Sustained hold 145 155 35 35

* Elbow angle shown in radians
** L=Left R=Right

Reference: 2, p. 113 NASA-STD-3000 203

Figure 4.9.3-5 Comparison of Female vs. Male Muscular Strength

Female strength as a percentage of male strength for different conditions. The vertical line within each shaded bar indicates the mean percentage difference. The end points of the shaded bars indicate the range.

Reference: 16, p. VII-50 NASA-STD-3000 204

Figure 4.9.3-6 Maximal Static Push Forces

(1) Height of the center of the force plate - 200 mm (8 in.) high by 254 mm (10 in.) long - upon which force is applied. (2) Horizontal distance between the vertical surface of the force plate and the opposing vertical surface (wall or footrest, respectively) against which the subject brace themselves.

*Thumb-tip reach - distance from backrest to tip of subjects thumb as thumb and fingertips are passed together.

**Span - the maximal distance between a persons fingertips as he extends his arms and hands to each side. (3) 1-g data

Reference: 1, pp. 2.5-5, 2.5-6 NASA-STD-3000 205

Figure 4.9.3-7 Leg Strength at Various Knee and Thigh Angles (5th Percentile Male Data)

Reference: 1, p. 115 NASA-STD-3000 206

Figure 4.9.3-8 Torque Strength

Maximum Torque Strengths
Maximum Torque Type Unpressurized suit, bare handed
Nm (lb-in)
Nm (lb-in)
Maximum Torque Supination
13.73 (121.5) 3.41 (30.1)
Maximum Torque Pronation
17.39 (153.9) 5.08 (45.0)

Reference: 1, pp. 2.5 - 20 NASA-STD-3000 207



This section covers workload considerations including aerobic power, aerobic endurance, and aerobic efficiency, as well as design factors such as optimum workload, task selection, and task complexity.

4.10.2 Workload Design Considerations

Workload related factors that should be considered when designing for optimum crewmember performance are presented below.

a. Endurance (Aerobic Power) - Two complex factors determine the limits of an individual's capacity to produce work and generate the requisite power. One of these is the capacity to sustain output over a period of time (this is a function of aerobic power). The second is strength (discussed in Paragraph 4.9).

1. Aerobic power - Aerobic power is the total power that an individual generates. It is related to usable power output by an efficiency factor (see 5" below). Aerobic power is expressed as volume of oxygen used per unit time. It is also commonly expressed in food calories oxidized per unit time, when referring to workload for a given task.

2. Resting metabolic rate - At rest (zero external workload), the ratio of oxygen consumed to body mass has been found to be quite consistent across individuals [3.5 mL/kg/min (0.1 in3/lb/min)] and is called the resting metabolic rate or 1 MET.

3. Maximum aerobic power - An individual's maximum aerobic power can range from two times the resting rate for an invalid to 23 times for a champion marathon. The average person will have a maximum aerobic power of 8 to 12 times resting metabolic rate. As with rest, the energy demands for a given workload are reasonably consistent across individuals. Thus, their ability to perform becomes a function of the ratio of their capacity to the demand.

4. Aerobic endurance - Aerobic endurance is a function of the individual's maximum aerobic power, and determines how long an individual can perform tasks of moderate to heavy intensity. Maximum effort can be maintained for only a few minutes, while up to 40% of maximum can be maintained over an 8-hr work shift with typical rest breaks (see Figure 4.10.2-1). Most people would judge work requiring 40% of their maximum aerobic capacity as moderate to heavy, but tolerable for 8 hours. Tasks that may be performed by any of a number of crewmembers should keep metabolic energy requirements 10 to 20% lower than that which would be considered tolerable by the least fit of the users.

Figure 4.10.2-1 Aerobic endurance: Duration and Workload

1. Vo2 = aerobic power (consumed volume of O2 per unit time)
Exemplary fitness levels:
28 mL/kg/min (0.78 in 3 /lb/min) would be considered "fair" for the general female population and is below the average of the U.S. female astronauts selected to date.
42 mL/kg/min (1.16 in 3 /lb/min) would be considered "average" for males and approximates the average for the U.S. male astronauts selected to date.
56 mL/kg/min (1.55 in 3 /lb/min) would be considered "high" for the males and is well above average for the U.S. male astronauts selected to date.

2. Nominal durations that individuals can maintain aerobic power levels as percent of their maximums. Durations greater than one hour normally require 10 minutes rest per hour, greater than 4 hours, a "lunch (rest) break" of approximately one hour.

3. Upper values assume a person of 54 kb (120 lb) and lower values one of 74 kb (163 lb).

4. Rate of caloric expenditure (kcal/hr) that can be maintained as tolerable for the corresponding duration. (NOTE: EVA activities have averaged about 230 kcal/hr).

Reference: 351, NASA-STD-3000 208

5. Aerobic efficiency - In a shirtsleeve environment on Earth, human efficiency ranges from approximately 35% to below 10%, depending on specific movement patterns. In cycling, for example, the human has an efficiency of about 21%. Thus, the useful power output for an individual expending 500 kcal/hr cycling would be 122 W rather than the 581 W that would result from 100% efficiency. Most of the wasted energy results in metabolic heat that must be dissipated by the person.

b. Optimum Workloads - It is important to try to maintain work loads that are close to optimum for each individual. This is especially true on longer duration flights. Optimum work loads mean not only to avoid overloading the individual but also not to underload them. Both of these conditions have been shown to lead to decreased performance.

c. Biomedical Changes - Biomedical changes, such as diminished musculoskeletal strength and reduced cardiac activity, can adversely affect work capacity. In-flight decrements in exercise capacity approaching 10% have been observed in some astronauts. These effects are likely to be more severe on longer missions and should be controlled to the extent possible by in-flight countermeasures such as exercise and diet.

d. Workload Prediction - It should be noted that a preponderance of evidence from previous flight experience implies several mechanisms which contribute to the difficulty of predicting workloads and task times during missions. These mechanisms include:

1. Effects of Space Adaptation Syndrome. These tend to increase task times due to the tendency for affected crewmembers to limit head motions. The effects are particularly evident during activation phases involving unstowage and frequent movements within the spacecraft, and are less evident with fully adapted crewmembers after the first few days in orbit.

2. Effects of Inappropriate Workstation Design - As noted in paragraph 4.9.2.d, workstation design can either support or confound task performance microgravity, with task difficulties ranging from slightly easier to significantly more difficult than the same task performed in one-g, depending on the success of the workstation design.

3. Proficiency Loss - Depending on the criticality of a task and its occurrence within the mission timeline, the length of time since a particular task was last performed in a training exercise may be significantly greater than the time between training sessions leading up to launch.

4. Adaptation to Microgravity Operations - This is a steep but significant learning curve associated with living and working in microgravity which often results in greatly decreased task times for second and subsequent performances of similar tasks as compared with the initial performance.

These mechanisms act independently and together to increase task times, particularly during early portions of a mission. Designers and mission planners should anticipate these changes and should allow for task time increments of from 25% to 100% compared with one-g experience.

e. Task Complexity and Fatigue - Simple tasks can be performed effectively at much higher levels of fatigue than more complex tasks. Thus, in designing the daily schedules, it would be beneficial to place the complex tasks during periods of least fatigue.

4.11 Effects of Deconditioning

4.11.1 Introduction

4.11.2 Effects of Deconditioning Design Considerations

4.11.3 Effects of Deconditioning Design Requirements

Figure 4.11.3-1 presents design requirements and constraints for accommodating deconditioned crewmembers. In establishing these requirements, different levels of conservatism were applied to normal, and to backup/contingency activities. Activities normally required for safe return must assure success for highly deconditioned crews. Activities associated with off-nominal, low probability situations are based on more optimistic estimates of crew capability. In applying these requirements, the following must be observed:

a. Crew activities and implementation methods listed are not presented as requirements, but as a catalog of candidates for which the crew may be used if the associated requirements and constraints are met. If activities or implementation methods not listed herein are intended, they must be submitted to the emergency vehicle Project Office for approval and subsequent incorporation into this document.

b. For design purposes, deconditioning effects are assumed significant only during reentry and subsequent mission phases. For operations prior to entry interface (0.2g), other sections of this document are to be applied without derating for deconditioning.

c. All crewmembers will remain in their couches or seats appropriately restrained, throughout reentry and landing. After landing, the crew will not be required to leave their couches or seats or release their restraints until the vehicle is upright. For nominal mission, post landing activities must not require the crew to stand without assistance by ground personnel.

d. The crew shall not be required to perform any tasks during transient environments associated with parachute opening or disreefing, landing retrorocket firing, or landing impact.

e. Not accommodated as used in Figure 4.11.3-1 specifies that the crew shall not be required to perform the activity. This does not necessarily imply that the crew is not able to perform the activity.

f. Post Landing items 10 thru 14 are considered off-nominal/non-routine activities.

Figure 4.11.3-1 Capabilities of a Deconditioned Crew Re-entry Through Final Descent

Potential Crew Activity/Implementation Design Requirements/Constraints
-2 < Gx < 2 2 < Gx < 4
1. Monitor displays:
- Alpha-numeric
- Graphical
- Analog
- Discrete
a. Displays must be within eye and head movement limits of Fig. with lateral head movement of ±30 degrees and viewing distance limits of para.

b. Must not require lifting the head.

b. Must not require lifting the head.

b. Hardcopy must be within visibility limits of 2.a. and reach envelope of para.

b. Hardcopy must be within visibility limits of 2.a. and reach envelope of para.

a. Controls must be within visibility limits of 1.a or meet the blind operation actuation requirements of para. and within reach envelope of para.

b. Keystroke requirements should be minimized.

a. Controls must be within visibility limits of 1.a or meet the blind operation actuation requirements of para. and within reach envelope of para.

b. Keystroke requirements should be minimized.

b. Specific applications must be approved.

a. Specific applications must be approved.

b. Specific applications must be approved.

5. Communicate with Mission Control & SAR:
a. Vox
b. PTT

b. Aural alarms must meet the requirements specified in para. 9.4.4.

c. Specific applications must be approved.

d. Specific applications must be approved.

b. Aural alarms must meet the requirements specified in para. 9.4.4.

b. The medical support equipment must be visible within the field of view specified in para.

Figure 4.11.3-1 Capabilities of a Deconditioned Crew Post-Landing

Potential Crew Activity/Implementation Design Requirements/Constraints
1-G Upright 1-G Inverted
1. Monitor displays:
- Alpha-numeric
- Graphical
- Analog
- Discrete
a. Displays must be within eye and head movement limits of para. and viewing distance limits of para. Rapid head movement should not be required a. Displays must be within eye and head movement limits of para., and viewing distance limits of para. Rapid head movement should not be required.
2. Read checklist data:
a. Computer screen
b. Hard copy
a. See 1.a.

b. Hardcopy must be within visibility limits of 2.a. and reach envelope of para.

b. Hardcopy must be within visibility limits of 2.a. and reach envelope of para.

b. Specific applications must be approved.

b. Specific applications must be approved.

b. Crew must be restrained in couch or seat.

b. Aural alarms must meet the requirements specified in para. 9.4.4. Crew must be able to discern cues from couch or seat.

c. Specific applications must be approved.

d. Specific applications must be approved.

b. Aural alarms must meet the requirements specified in para. 9.4.4. Crew must be able to discern cues from couch or seat.

c. Specific applications must be approved.

d. Specific applications must be approved. Crew must be able to discern cues from couch or seat.

b. Medical support equipment must be visible within the field of view specified in para.

b. Attendant may exit couch or seat to provide assistance. The crewmember must not be required to stand.

b. Attendant may exit couch or seat to provide assistance. The crewmember must not be required to stand.

b. Unrestrained mass should be less than 12 lbs.

c. Control actuation must meet the requirements specified in para. 9.3.3.

d. Crew strength capabilities should be as specified in para. 4.9.3.

e. Contingency operations which require the crew member to stand must be approved.

b. Unrestrained mass should be less than:
- dynamic (water) envir. - 12 lbs.
- static (land) envir. - 20 lbs

c. Restrained loads should not exceed capabilities specified in para. 4.9.3.

d. Contingency operations which require the crew member to stand must be approved.

c. Restrained loads should not exceed crew strength capabilities specified in para. 4.9.3.

b. Crew is assumed to have the physical capability and strength of a normally conditioned crew as stated in para. 4.9.3..

b. Mass of single package of survival equipment should not exceed 50 lbs.

Preventing Back Injuries in Health Care Settings

Healthcare workers often experience musculoskeletal disorders (MSDs) at a rate exceeding that of workers in construction, mining, and manufacturing. 1 These injuries are due in large part to repeated manual patient handling activities, often involving heavy manual lifting associated with transferring, and repositioning patients and working in extremely awkward postures. The problem of lifting patients is compounded by the increasing weight of patients to be lifted due to the obesity epidemic in the United States and the rapidly increasing number of older people who require assistance with the activities of daily living. 2,3

Costs associated with overexertion injuries in the healthcare industry were estimated to be $1.7 billion in 2015.* 4a, 4b Additionally, nursing aides and orderlies suffer the highest prevalence (18.8%) and report the most annual cases (269,000) of work-related back pain among female workers in the United States. 5 In 2000, 10,983 registered nurses (RNs) suffered lost-time work injuries due to lifting patients. Twelve percent of nurses report that they left the nursing profession because of back pain. 6

As our nursing workforce ages (average age 46.8 years) and we face a critical nursing shortage in this country (an expected 20% shortage by 2015 and 30% by 2020), preserving the health of our nursing staff and reducing back injuries in healthcare personnel is critical. The National Institute for Occupational Safety and Health (NIOSH) has a comprehensive research program aimed at preventing work-related MSDs with major efforts to reduce lifting injuries in healthcare settings. NIOSH’s research with diverse partners has already made great strides in developing and implementing practical intervention strategies, with further progress expected.

The first research effort was a comprehensive lab and field study to identify safer ways to lift and move nursing home residents by removing the excessive forces and extreme postures that can occur when manually lifting residents. Historically, the caregiver has used his or her own strength to provide manual assistance to the resident. NIOSH conducted a large field study to determine if an intervention consisting of mechanical equipment to lift physically dependent residents, training on the proper use of the lifts, a safe lifting policy, and a preexisting medical management program would reduce the rate and the associated costs of the resident handling injuries for the nursing personnel in a real world setting. 7

During the 6-year period, from January 1995 through December 2000, 1,728 nursing personnel were followed before and after implementation of the intervention. After the intervention, there was a significant reduction in injuries involving resident handling, workers’ compensation costs, and lost work day injuries. When injury rates associated with patient handling were examined, workers’ compensation claims rates per 100 nursing staff were reduced by 61% Occupational Safety and Health Administration (OSHA) recordable injury rates decreased by 46% and first reports of employee injury rates were reduced by 35%. The initial investment of $158,556 for lifting equipment and worker training was recovered in less than 3 years on the basis of post-intervention savings of $55,000 annually in workers’ compensation costs and potentially more quickly if indirect costs (lost wages, cost of hiring and retraining workers, etc.) are considered. This is significant given that cost is an often cited barrier to purchasing lifting equipment. Another advantage of lifting equipment is the reduction in the rate of assaults on caregivers during resident transfers—down 72%, 50%, and 30% on the basis of workers’ compensation, OSHA recordable incidents, and the first reports of injury data, respectively.

More information on this study can be found in the NIOSH publication Safe Lifting and Movement of Nursing Home Residents. Based on the successes achieved in the long-term care industry, NIOSH is undertaking a six-year longitudinal research study to evaluate the effectiveness of a “best practices” safe patient handling program at two large acute-care hospitals in the United States.

Another major study demonstrating success in reducing back injuries to health care workers was funded by NIOSH through a cooperative agreement. The study examined the long-term effectiveness of a safe lifting program with the primary objective to reduce injuries to healthcare workers resulting from manual lifting and transferring of patients. 8 The safe lifting programs, which used employee management advisory teams (participatory-team approach), were implemented in seven nursing homes and one hospital. The eight facilities varied in the available number of beds and number of nursing personnel. In this study, manual lifting and transferring of patients was replaced with modern, battery operated, portable hoists, and other patient-transfer assistive devices. Ergonomics committees with nearly equal representation from management and employees selected the equipment and implemented the safe lifting programs.

Injury statistics were collected post-intervention for 51 months and were compared with 37 months of pre-intervention data. The results were compelling. The number of injuries from patient transfers decreased by 62% (range = 3979%), lost work days by 86% (range = 5099%), restricted workdays by 64% (96% decrease to 17% increase), and workers’ compensation costs by 84% (range = 5399%). Overall, the eight facilities experienced decreases of 32% in all injuries, 62% in all lost work days, 6% in all restricted work days, and 55% in total workers’ compensation costs. The program produced many intangible benefits including improvements in patient comfort and safety during transfers and patient care. The nursing personnel perceived that their backs were less sore and that they were less tired at the end of their shifts. More pregnant and older workers were able to perform their regular duties and stay on the job for a longer period.

Despite the obvious advantages to using lifting equipment, schools of nursing continue to teach, and nurses’ licensure exams 9 continue to include, outdated and unsafe manual patient handling techniques. This is due in large part to outdated books and curricula which promote unsafe patient handling practices. To address this, a team of experts from NIOSH, the American Nurses Association, and the Veterans Health Administration developed and evaluated an evidence-based training program on safe patient handling for educators at schools of nursing. The study found that when using the curriculum, nurse educator and student knowledge improved significantly as did the intention to use mechanical lifting devices in the near future. 10,11 The curriculum module, which won the 2008 National Occupational Research Agenda (NORA) Partnership Award, is ready for broad-scale dissemination across nursing schools to reduce the risk of MSDs among nurses.

Looking ahead: Beginning in 2009, NIOSH will conduct a project aimed at improving safety while lifting and moving bariatric patients. In healthcare settings, the term “bariatric” is used to refer to patients whose weights exceed the safety capacity of standard patient lifting equipment (300 lbs), or who otherwise have limitations in health, mobility, or environmental access due to their weight/size. 12 Compared to the non-obese population, obese individuals require more frequent and extensive healthcare due to obesity-related health problems, and healthcare personnel are encountering hospitalized and critical-care bariatric patients on an increasingly frequent basis. 13,14,15 In the extreme, such patients can weigh over 1,200 pounds. The upcoming NIOSH project will evaluate bariatric patient handling practices at multiple hospitals, including intervention programs and health/safety outcomes, in order to identify and promote evidence-based best practices.

We all have a vested interest in taking care of those who help take care of us and our families when we need medical attention. It is likely that the implementation of the research presented here will significantly reduce injuries and illnesses for healthcare workers and increase the quality of patient care. In turn, reducing MSDs among nurses may help address the critical issues of nurse recruitment and retention.

As we contemplate further research, we would like to hear about your experiences with lifting equipment and practices in medical settings. Additionally, your thoughts about retooling student nursing curriculum as well as your opinions on state laws regulating safe patient handling and movement would be appreciated.

—Jennifer Bell, PhD Jim Collins, PhD, MSME Traci L. Galinsky, PhD Thomas R. Waters, PhD, CPE

Dr. Bell is a research epidemiologist in the Analysis and Field Evaluations Branch in the NIOSH Division of Safety Research.

Dr. Collins (Captain, U.S. Public Health Service) is the Associate Director for Science for the NIOSH Division of Safety Research.

Dr. Galinsky (Captain, U.S. Public Health Service) is a research psychologist in the NIOSH Division of Applied Research and Technology.

Dr. Waters is a research safety engineer in the Division of Applied Research and Technology.

* The cost figure was revised 9/22/2017 to reflect most current data available.



Cette revue vise à définir la notion de fatigue neuromusculaire et à présenter les connaissances actuelles relatives aux facteurs centraux et périphériques à l’origine de ce phénomène au niveau du muscle sain. Cette revue aborde également la littérature traitant des mécanismes mobilisés afin de s’adapter à la fatigue.


Cent quatre-vingt-deux articles indexés dans PubMed (1954–2010) ont été considérés.


La fatigue neuromusculaire a des origines centrales et périphériques. La fatigue centrale, prépondérante pour les exercices de faible intensité et de longue durée, impliquerait une baisse de la commande centrale (cortex moteur, motoneurone) influencée par l’activité des neurotransmetteurs cérébraux et les afférences musculaires. La fatigue périphérique, associée à une altération des mécanismes allant de l’excitation à la contraction musculaire, serait induite par une perturbation des mouvements d’ions calcium, une accumulation de phosphate, et/ou une baisse des réserves en adénosine triphosphate. Pour pallier cette diminution de la force produite, l’organisme développe plusieurs mécanismes adaptatifs impliquant notamment l’activité des unités motrices.


L’apparition de la fatigue est accompagnée d’une altération des mécanismes impliqués dans la production de force. L’interaction entre ces mécanismes centraux et périphériques engendre alors une cascade d’événements participant à la baisse de la force produite.

Climate change and human health: questions and answers

Weather is the continuously changing condition of the atmosphere, usually considered on a time scale that extends from minutes to weeks. Climate is the average state of the lower atmosphere, and the associated characteristics of the underlying land or water, in a particular region, usually spanning at least several years. Climate variability is the variation around the average climate, including seasonal variations and large-scale regional cycles in atmospheric and ocean circulations such as the El Niño/ Southern Oscillation (ENSO) or the North Atlantic Oscillation.

Climate change occurs over decades or longer time-scales. Until now, changes in the global climate have occurred naturally, across centuries or millennia, because of continental drift, various astronomical cycles, variations in solar energy output and volcanic activity. Over the past few decades it has become increasingly apparent that human actions are changing atmospheric composition, thereby causing global climate change (1).

The Climate System

Earth’s climate is determined by complex interactions between the Sun, oceans, atmosphere, cryosphere, land surface and biosphere. The Sun is the principal driving force for weather and climate. The uneven heating of Earth’s surface (being greater nearer the equator) causes great convection flows in both the atmosphere and oceans, and is thus a major cause of winds and ocean currents.

Five concentric layers of atmosphere surround this planet. The lowest layer (troposphere) extends from ground level to around 10-12 km altitude on average. The weather that affects Earth’s surface develops within the troposphere. The next major layer (stratosphere) extends to about 50 km above the surface. The ozone within the stratosphere absorbs most of the sun’s higher-energy ultraviolet rays. Above the stratosphere are three more layers: mesosphere, thermosphere and exosphere.

Overall, these five layers of the atmosphere approximately halve the amount of incoming solar radiation that reaches Earth’s surface. In particular, certain "greenhouse" gases, present at trace concentrations in the troposphere (and including water vapour, carbon dioxide, nitrous oxide, methane, halocarbons, and ozone), absorb about 17% of the solar energy passing through it. Of the solar energy that reaches Earth’s surface, much is absorbed and reradiated as long-wave (infrared) radiation. Some of this outgoing infrared radiation is absorbed by greenhouse gases in the lower atmosphere, which causes further warming of Earth’s surface. This raises Earth’s temperature by 33ºC to its present surface average of 15ºC. This supplementary warming process is called "the greenhouse effect" (Figure 2.1).

Greenhouse Gases

Human-induced increases in the atmospheric concentration of GHGs are amplifying the greenhouse effect. In recent times, the great increase in fossil fuel burning, agricultural activity and several other economic activities has greatly augmented greenhouse gas emissions. The atmosphere concentration of carbon dioxide has increased by one-third since the inception of the industrial revolution (Figure 2.2).

Table 2.1 provides examples of several greenhouse gases and summarizes their 1790 and 1998 concentrations, their rate of change over the period 1990 to 1999 and their atmospheric lifetime. The atmospheric lifetime is highly relevant to policy makers because the emission of gases with long lifetimes entails a quasi-irreversible commitment to sustained climate change over decades or centuries.

Studying the Health Impacts of Climate

Studying the impact of weather events and climate variability on human health requires appropriate specification of the meteorological "exposure". Weather and climate can each be summarized over various spatial and temporal scales. The appropriate scale of analysis, and the choice of any lag period between exposure and effect, will depend on the anticipated nature of the relationship. Much of the research requires long-term data sets with information about weather/climate and health outcome on the same spatial and temporal scales. For example, it has proven difficult to assess how climate variability and change has influenced the recent spread of malaria in African highlands because the appropriate health, weather and other relevant data (e.g. land use change) have not been collected in the same locations and on the same scales.

In all such research, there is a need to accommodate the several types of uncertainty that are inherent in these studies. Predictions about how complex systems such as regional climate systems and climate-dependent ecosystems will respond when pushed beyond critical limits are necessarily uncertain. Likewise, there are uncertainties about the future characteristics, behaviours and coping capacity of human populations.


  • Albritton DL, Meiro-Filho LG. Technical Summary. In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY.
  • US Environmental Protection Agency. Greehouse effects schematic (2001).
  • Watson RT and the Core Writing Team. Climate Change 2001: Synthesis Report. Summary for Policymakers. A Report of the Intergovernmental Panel on Climate Change. IPCC Secretariat, c/o World Meteorological Organization, Geneva, Switzerland (2001).

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What is the International Consensus on the Science of Climate and Health?

The IPCC Third Assessment Report

Through recent research, our understanding of climate-health relationships has increased rapidly, largely due to the stimulus of the IPCC and other policy-related reviews at regional and national levels.

In the early 1990s there was little awareness of the health risks posed by global climate change. This reflected a general lack of understanding of how the disruption of biophysical and ecological systems might affect the longer-term wellbeing and health of populations. There was little awareness among natural scientists that changes in their particular objects of study – climatic conditions, biodiversity stocks, ecosystem productivity, and so on – were of potential importance to human health. Indeed, this was well reflected in the meagre reference to health risks in the first major report of the UN’s Intergovernmental Panel on Climate Change (IPCC), published in 1991.

Subsequently, the situation has changed. The IPCC Second Assessment Report (1996) devoted a full chapter to the potential risks to health. The Third Assessment Report (2001) did likewise, this time including discussion of some early evidence of actual health impacts, along with assessing potential future health effects. That report also highlighted the anticipated health impacts by major geographic region.

The IPCC was established by WMO and UNEP in 1988. The IPCC’s role is to assess the world’s published scientific literature on: (i) how human-induced changes to the lower atmosphere, via the emission of greenhouse gases, have influenced and are likely to influence world climatic patterns (ii) how this does, and in future would, affect various systems and processes important to human societies and (iii) the range of economic and social response options available to policy-makers to avert climate change and to lessen its impacts.

The IPCC’s work has been done by many hundreds of scientists, worldwide. On a five-yearly basis, national governments propose scientists with expertise in the many topic areas included within this comprehensive review task. Topic review teams are then chosen to ensure proper geographic and disciplinary representation. Excluding the small number of scientists working at IPCC secretariat level, all this work of reviewing, discussing and writing is contributed voluntarily.

The IPCC’s draft assessments are subject to a series of internal and external peer-review processes. The final wording of IPCC report summaries are subject, via formal international conferences, to detailed and systematic scrutiny by governments.

The IPCC’s assessment of health impacts

In its Third Assessment Report the IPCC concluded that: “Overall, climate change is projected to increase threats to human health, particularly in lower income populations, predominantly within tropical/subtropical countries.”

That summary went on to state:“Climate change can affect human health directly (e.g., impacts of thermal stress, death/injury in floods and storms) and indirectly through changes in the ranges of disease vectors (e.g., mosquitoes), water-borne pathogens, water quality, air quality, and food availability and quality. The actual health impacts will be strongly influenced by local environmental conditions and socio-economic circumstances, and by the range of social, institutional, technological, and behavioural adaptations taken to reduce the full range of threats to health.” (1).

Broadly, a change in climatic conditions can have three kinds of health impacts:

  • Those that are relatively direct, usually caused by weather extremes.
  • The health consequences of various processes of environmental change and ecological disruption that occur in response to climate change.
  • The diverse health consequences – traumatic, infectious, nutritional, psychological and other – that occur in demoralized and displaced populations in the wake of climate-induced economic dislocation, environmental decline, and conflict situations.

These several pathways are illustrated in Figure 3.1.

Our understanding of the impacts of climate change and variability on human health has increased considerably in recent years. However, several basic issues complicate this task:

  • Climatic influences on health are often modulated by interactions with other ecological processes, social conditions, and adaptive policies. In seeking explanations, a balance must be sought between complexity and simplicity.
  • There are many sources of scientific and contextual uncertainty. The IPCC has therefore sought to formalise the assessment of level of confidence attaching to each health impact statement.
  • Climate change is one of several concurrent global environmental changes that simultaneously affect human health – often interactively (3). A good example is the transmission of vector-borne infectious diseases, which is jointly affected by climatic conditions, population movement, forest clearance and land-use patterns, biodiversity losses (e.g., natural predators of mosquitoes), freshwater surface configurations, and human population density (4).

The IPCC concluded, with high confidence, that climate change would cause increased heat-related mortality and morbidity, decreased cold-related mortality in temperate countries, greater frequency of infectious disease epidemics following floods and storms, and substantial health effects following population displacement from sea level rise and increased storm activity.

For each potential impact of climate change, certain groups will be particularly vulnerable to disease and injury. The vulnerability of a population depends on factors such as population density, level of economic development, food availability, income level and distribution, local environmental conditions, pre-existing health status, and the quality and availability of public health care (5). For instance, those most at risk of being harmed by thermal extremes include socially isolated city dwellers, the elderly and the poor. Populations living at the present margins of malaria and dengue, without effective primary health care, will be the most susceptible if these diseases expand their geographic range in a warmer world.

The IPCC report also underscores that our understanding of the links between climate, climate change and human health has increased considerably over the last ten years. However, there are still many gaps in knowledge about likely future patterns of exposure to climatic-environmental changes, and about the vulnerability and adaptability of physical, ecological and social systems to such climate change.


  • IPCC. Synthesis Report, Third Assessment Report. Cambridge University Press, 2001.
  • Patz, J.A. et al. The potential health impacts of climate variability and change for the United States: executive summary of the report of the health sector of the U.S. National Assessment. Environmental Health Perspectives,108(4): 367-76 (2000).
  • Watson, R.T. et al. (eds.) The Regional Impacts of Climate Change. An assessment of vulnerability: A Special Report of IPCC Working Group II . pp 517 Cambridge, U.K: Cambridge University Press (1998).
  • Gubler, D.J. Dengue and dengue haemorrhagic fever. Clinical Microbiology Review 11: 480-96 (1998).
  • Woodward AJ, et al. Protecting human health in a changing world: the role of social and economic development Bulletin of the World Health Organization. 78: 1148-1155 (2000).

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What are the major challenges for scientists studying climate change and health linkages?

Research on climate change and health spans basic studies of causal relationships, risk assessment, evaluation of population vulnerability and adaptive capacity, and the evaluation of intervention policies (Figure 4.1).

The challenges in identifying, quantifying and predicting the health impacts of climate change entail issues of scale, “exposure” specification, and the elaboration of often complex and indirect causal pathways (1). First, the geographic scale of climate-related health impacts and the typically wide timespans are unfamiliar to most researchers. Epidemiologists usually study problems that are geographically localised, have relatively rapid onset, and directly affect health. The individual is usually the natural unit of observation.

Second, the “exposure” variable – comprising weather, climate variability and climate trends – poses difficulties. There is no obvious "unexposed" group to act as baseline for comparison. Indeed, because there is little difference in weather/climate exposures between individuals in the same geographic locale, comparing sets of persons with different “exposures” is usually precluded. Rather, whole communities or populations must be compared – and, in so doing, attention must be paid to intercommunity differences in vulnerability. For example, the excess death rate during the severe 1995 Chicago heatwave varied greatly between neighbourhoods because of differences in factors such as housing quality and community cohesion.

Third, some health impacts occur via indirect and complex pathways. For example, the effects of temperature extremes on health are direct. In contrast, complex changes in ecosystem composition and functioning help mediate the impact of climatic change on transmission of vector-borne infectious diseases and on agricultural productivity.

A final challenge is the need to estimate health risks in relation to future climatic-environmental scenarios. Unlike most recognized environmental health hazards, much of the anticipated risk from global climate change lies years to decades into the future.

Research strategies and tasks

While much health-impacts research focuses on future risk, empirical studies referring to the recent past and present are important. Standard observational epidemiological methods can illuminate the health consequences of local climatic trends in past decades – if the relevant data-sets exist. Such information enhances our capacity subsequently to estimate future impacts. Meanwhile, we should also seek evidence of the early health effects of climate change, since change has been underway for several decades.

The health impacts of future climate change, including changes in climatic variability, can be estimated in two main ways. First, we can extrapolate from analogue studies that treat recent climatic variability as a foretaste of climate change. Second, we can use predictive computer models based on existing knowledge about relationships between climatic conditions and health outcomes. Such models cannot predict exactly what will happen, but they indicate what would occur if certain future climatic (and other specified) conditions were fulfilled.

The five main tasks for researchers are:

1. Establishing baseline relationships between weather and health

There are many unresolved questions about the sensitivity of particular health outcomes to weather, climate variability, and climate-induced environmental changes. For example, the major pathogens that cause acute gastroenteritis multiply faster in warmer conditions. Do higher ambient temperatures cause more illness? Apparently so – as is evident from the monthly salmonella infection count in New Zealand in relation to average monthly temperature (Figure 4.2).

2. Seeking evidence of early effects of climate change

There have been many, coherent, observations on physical and ecological changes attributable to recent global warming – but few indications yet of human health effects. Amongst these are changing patterns of infectious disease (such as tick-borne encephalitis (2) and cholera (3)). Health researchers must allow for the fact that humans have many coping strategies, ranging from planting shade trees, to changing work-hours, to installing air-conditioning.

The challenge is to pick the settings, populations and health outcomes with the best chance of: (i) detecting changes, and (ii) attributing some portion of these to climate change. Impacts are likely to be clearest where the exposure-outcome gradient is steepest, the local population’s adaptive capacity is weakest, and when there are few competing explanations for observed relationships.

3. Scenario-based predictive models

Unlike most other environmental exposures, we know that the world’s climate will continue to change for at least several decades. Climatologists now can satisfactorily model the climatic consequences of future scenarios of greenhouse gas emissions. By linking these climate scenarios with health impact models, we can estimate the likely impacts on health.

Some health impacts are readily quantified (deaths due to storms and floods for instance) others are more difficult to quantify (e.g., the health consequences of food insecurity). We need models with sufficient representation of the multi-faceted future world to provide useful, or credible, estimates of future health risks. Where possible, we should use a high level of “integration” to achieve realistic modelled forecasts of impact in a world that will have undergone various other demographic, economic, technological and social changes.

4. Evaluating adaptation options

Adaptation means taking steps to reduce the potential adverse impact of environmental change (see chapter 11).

5. Estimating the co-incidental benefits and costs of mitigation and adaptation

Steps to reduce GHG emissions (mitigation) or to lessen health impacts (adaptation) may have other coincidental health effects. For example, promotion of public transport relative to private vehicles may not only reduce CO2 emissions, but also improve public health in the near-term by reducing air pollution and road traffic injuries and increasing physical activity. Information about these "ancillary" costs and benefits is important for policy-makers. Note, however, for impacts that are either deferred in time or that extend into the distant future, the costing is not straightforward.

General issues concerning uncertainty

Researchers should describe, communicate and explain all relevant uncertainties. This gives the decision-maker important insight into the conditions needed for a particular outcome to occur. Since environmental risk perception varies with culture, values and social status, “stakeholders” should assist both in shaping the assessment questions and in interpreting the risk.


  • Walther, G. et al. Ecological responses to recent climate change. Nature 416: 389-395 (2002).
  • Lindgren, E. & Gustafson, R. Tick-borne encephalitis in Sweden and climate change. Lancet 358(9275): 16-87 (2001).
  • Pascual M et al., Cholera dynamics and El Niño Southern Oscillation. Science 289: 1766-69 (2000).

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What are the major health impacts of climate extremes?

Climatic factors are an important determinant of various vector-borne diseases, many enteric illnesses and certain water-related diseases. Relationships between year-to-year variations in climate and infectious diseases are most evident where climate variations are marked, and in vulnerable populations. The El Niño phenomenon provides an analogue for understanding the future impacts of global climate change on infectious diseases.

Extreme climate events are expected to become more frequent with climate change. These disruptive events have their greatest impact in poor countries. The two categories of climatic extremes are:

  • Simple extremes of climatic statistical ranges, such as very low or very high temperatures
  • Complex events: droughts, floods, or hurricanes

The Pacific-based El Niño- Southern Oscillation (ENSO), an approximately semi-decadal cycle, influences much of the world’s regional weather patterns. Climate change is likely to increase the frequency and/or amplitude of El Niño (1). It illustrates well how climatic extremes can affect human health.

Climate, weather, El Niño and infectious diseases

Both temperature and surface water have important influences on the insect vectors of vector-borne infectious disease. Of particular importance are vector mosquito species, which spread malaria and viral diseases such as dengue and yellow fever. Mosquitoes need access to stagnant water in order to breed, and the adults need humid conditions for viability. Warmer temperatures enhance vector breeding and reduce the pathogen’s maturation period within the vector organism. However, very hot and dry conditions can reduce mosquito survival.

Malaria, today, is mostly confined to tropical and subtropical regions. The disease’s sensitivity to climate is illustrated by desert and highland fringe areas where higher temperatures and/or rainfall associated with El Niño may increase transmission of malaria (2). In areas of unstable malaria in developing countries, populations lack protective immunity and are prone to epidemics when weather conditions facilitate transmission.

Dengue is the most important arboviral disease of humans, occurring in tropical and subtropical regions, particularly in urban settings. ENSO affects dengue occurrence by causing changes in household water storage practices and in surface water pooling. Between 1970 and 1995, the annual number of dengue epidemics in the South Pacific was positively correlated with La Niña conditions (i.e., warmer and wetter) (3).

Rodents, which proliferate in temperate regions following mild wet winters, act as reservoirs for various diseases. Certain rodent-borne diseases are associated with flooding, including leptospirosis, tularaemia and viral haemorrhagic diseases. Other diseases associated with rodents and ticks, and which show associations with climatic variability, include Lyme disease, tick borne encephalitis, and hantavirus pulmonary syndrome.

Many diarrhoeal diseases vary seasonally, suggesting sensitivity to climate. In the tropics diarrhoeal diseases typically peak during the rainy season. Both floods and droughts increase the risk of diarrhoeal diseases. Major causes of diarrhoea linked to heavy rainfall and contaminated water supplies are: cholera, cryptosporidium, E.coli infection, giardia, shigella, typhoid, and viruses such as hepatitis A.

Temperature extremes: heatwaves and cold spells

Extremes of temperature can kill. In many temperate countries, death rates during the winter season are 10-25% higher than those in the summer. In July 1995, a heatwave in Chicago, US, caused 514 heatrelated deaths (12 per 100,000 population) and 3300 excess emergency admissions.

Most of the excess deaths during times of thermal extreme are in persons with preexisting disease, especially cardiovascular and respiratory disease. The very old, the very young and the frail are most susceptible. In terms of the amount of life lost, the mortality impact of an acute event such as a heatwave is uncertain because an unknown proportion of deaths are in susceptible persons who would have died in the very near future.

Global climate change will be accompanied by an increased frequency and intensity of heatwaves, as well as warmer summers and milder winters. Predictive modelling studies, using climate scenarios, have estimated future temperature-related mortality. For example, the annual excess summer-time mortality attributable to climate change, by 2050, is estimated to increase several-fold, to between 500-1000 for New York and 100-250 for Detroit, assuming population acclimatisation (physiological, infrastructural and behavioural) (4). Without acclimatisation the impacts would be higher.

The extent of winter-associated mortality directly attributable to stressful weather is less easy to determine. In temperate countries undergoing climate change, a reduction in winter deaths may outnumber the increase in summer deaths. Without better data, the net impact on annual mortality is difficult to estimate. Further, it will vary between populations.

Natural disasters

The effects of weather disasters (droughts, floods, storms and bushfires) on health are difficult to quantify, because secondary and delayed consequences are poorly reported. El Niño events influence the annual toll of persons affected by natural disasters (5). Globally, disasters triggered by droughts occur especially during the year after the onset of El Niño.

Globally, natural disaster impacts have been increasing. An analysis by the reinsurance company Munich Re found a tripling in the number of natural catastrophes in the last ten years, compared to the 1960s. This reflects global trends in population vulnerability more than an increased frequency of extreme climatic events. Developing countries are poorly equipped to deal with weather extremes, even as the population concentration increases in high-risk areas like coastal zones and cities. Hence, the number of people killed, injured or made homeless by natural disasters has been increasing rapidly.

Table 5.1. shows the numbers of events, deaths and people affected by extreme climatic and weather events in the past two decades, by geographic region.


The increasing trend in natural disasters is partly due to better reporting, partly due to increasing population vulnerability, and may include a contribution from ongoing global climate change. Especially in poor countries, the impacts of major vector-borne diseases and disasters can limit or even reverse improvements in social development. Even under favourable conditions recovery from major disasters can take decades.

Short-range climatic forecasts may help reduce health impacts. But early warning systems must also incorporate monitoring and surveillance, linked to adequate response capacities. Focusing attention on current extreme events may also help countries to develop better means of dealing with the longer-term impacts of global climate change, although this capacity may itself decline because of cumulative climate change. For example, increased food imports might prevent hunger and disease during occasional drought, but poor, food-insecure, countries may be unable to afford such measures indefinitely in response to gradual year-by-year drying.


  • IPCC. Climate Change 2001, vol 1. Cambridge University Press (2001).
  • Bouma MJ, van der Kaay HJ. Epidemic Malaria in India's Thar Desert. Lancet 373: 132-133 (1995).
  • Hales S, et al. Dengue Fever Epidemics in the South Pacific Region: Driven by El Nino Southern Oscillation? Lancet 348: 1664- 1665 (1996).
  • Kalkstein, L.S. & Greene, J.S. An Evaluation of Climate/Mortality Relationships in Large US Cities and the Possible Impacts of Climate Change. Env.Hlth.Pers. 105(1): 84-93 (1997).
  • Bouma MJ, et al. Global Assessment of El Nino's Disaster Burden. Lancet 350: 1435- 1438 (1997).

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How does climate change affect infectious disease patterns?

Today, worldwide, there is an apparent increase in many infectious diseases, including some newly-circulating ones (HIV/AIDS, hantavirus, hepatitis C, SARS, etc.). This reflects the combined impacts of rapid demographic, environmental, social, technological and other changes in our ways-of-living. Climate change will also affect infectious disease occurrence (1).

Humans have known that climatic conditions affect epidemic diseases from long before the role of infectious agents was discovered, late in the nineteenth century. Roman aristocrats retreated to hill resorts each summer to avoid malaria. South Asians learnt early that, in high summer, strongly curried foods were less likely to cause diarrhoea.

Infectious agents vary greatly in size, type and mode of transmission. There are viruses, bacteria, protozoa and multicellular parasites. Those microbes that cause “anthroponoses” have adapted, via evolution, to the human species as their primary, usually exclusive, host. In contrast, non-human species are the natural reservoir for those infectious agents that cause “zoonoses” (Fig 6.1). There are directly transmitted anthroponoses (such as TB, HIV/AIDS, and measles) and zoonoses (e.g., rabies). There are also indirectly-transmitted, vector-borne, anthroponoses (e.g., malaria, dengue fever, yellow fever) and zoonoses (e.g. bubonic plague and Lyme disease).

Vector-borne and water-borne diseases

Important determinants of vectorborne disease transmission include: (i) vector survival and reproduction, (ii) the vector’s biting rate, and (iii) the pathogen’s incubation rate within the vector organism. Vectors, pathogens and hosts each survive and reproduce within a range of optimal climatic conditions: temperature and precipitation are the most important, while sea level elevation, wind, and daylight duration are also important.

Human exposure to waterborne infections occurs by contact with contaminated drinking water, recreational water, or food. This may result from human actions, such as improper disposal of sewage wastes, or be due to weather events. Rainfall can influence the transport and dissemination of infectious agents, while temperature affects their growth and survival.

Observed and predicted climate/infectious disease links

There are three categories of research into the linkages between climatic conditions and infectious disease transmission. The first examines evidence from the recent past of associations between climate variability and infectious disease occurrence. The second looks at early indicators of already-emerging infectious disease impacts of long-term climate change. The third uses the above evidence to create predictive models to estimate the future burden of infectious disease under projected climate change scenarios.

Historical Evidence

There is much evidence of associations between climatic conditions and infectious diseases. Malaria is of great public health concern, and seems likely to be the vector-borne disease most sensitive to long-term climate change. Malaria varies seasonally in highly endemic areas. The link between malaria and extreme climatic events has long been studied in India, for example. Early last century, the river-irrigated Punjab region experienced periodic malaria epidemics. Excessive monsoon rainfall and high humidity was identified early on as a major influence, enhancing mosquito breeding and survival. Recent analyses have shown that the malaria epidemic risk increases around five-fold in the year after an El Niño event (2).

Early impacts of climate change

These include several infectious diseases, health impacts of temperature extremes and impacts of extreme climatic and weather events (described in chapter 5).

Predictive Modeling

The main types of models used to forecast future climatic influences on infectious diseases include statistical, process-based, and landscape-based models (3). These three types of model address somewhat different questions.

Statistical models require, first, the derivation of a statistical (empirical) relationship between the current geographic distribution of the disease and the current location-specific climatic conditions. This describes the climatic influence on the actual distribution of the disease, given prevailing levels of human intervention (disease control, environmental management, etc.). By then applying this statistical equation to future climate scenarios, the actual distribution of the disease in future is estimated, assuming unchanged levels of human intervention within any particular climatic zone. These models have been applied to climate change impacts on malaria, dengue fever and, within the USA, encephalitis. For malaria some models have shown net increases in malaria over the coming halfcentury, and others little change.

Process-based (mathematical) models use equations that express the scientifically documented relationship between climatic variables and biological parameters – e.g., vector breeding, survival, and biting rates, and parasite incubation rates. In their simplest form, such models express, via a set of equations, how a given configuration of climate variables would affect vector and parasite biology and, therefore, disease transmission. Such models address the question: “If climatic conditions alone change, how would this change the potential transmission of the disease?” Using more complex “horizontal integration”, the conditioning effects of human interventions and social contexts can also be incorporated.

This modelling method has been used particularly for malaria and dengue fever (4). The malaria modelling shows that small temperature increases can greatly affect transmission potential. Globally, temperature increases of 2-3ºC would increase the number of people who, in climatic terms, are at risk of malaria by around 3- 5%, i.e. several hundred million. Further, the seasonal duration of malaria would increase in many currently endemic areas.

Since climate also acts by influencing habitats, landscape-based modeling is also useful. This entails combining the climate-based models described above with the rapidly-developing use of spatial analytical methods, to study the effects of both climatic and other environmental factors (e.g. different vegetation types – often measured, in the model development stage, by ground-based or remote sensors). This type of modelling has been applied to estimate how future climate-induced changes in ground cover and surface water in Africa would affect mosquitoes and tsetse flies and, hence, malaria and African sleeping sickness.


Changes in infectious disease transmission patterns are a likely major consequence of climate change. We need to learn more about the underlying complex causal relationships, and apply this information to the prediction of future impacts, using more complete, better validated, integrated, models.


  • Patz, J.A., et al., Effects of environmental change on emerging parasitic diseases. Int J Parasitol, 30(12-13): p. 1395-405 (2000).
  • Bouma, M. and H. van der Kaay, The El Niño Southern Oscillation and the historic malaria epidemics on the Indian subcontinent and Sri Lanka: an early warning system for future epidemics? Tropical Medicine and International Health, 1(1): p. 86-96. (1996).
  • Martens WJM, Rotmans J, Rothman DS In: Martens WJM, McMichael AJ (eds). Environmental Change, Climate and Health: Issues and Research Methods. Cambridge: Cambridge University Press, 2002, pp. 197- 225.
  • Hales, S., et al., Potential effect of population and climate changes on global distribution of dengue fever: an empirical model. Lancet, 360: p. 830-834 (2002).
  • Wilson, M.L., Ecology and infectious disease, in Ecosystem Change and Public Health: A Global Perspective, J.L. Aron and J.A. Patz, Editors. 2001, Johns Hopkins University Press: Baltimore. p. 283-324.

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How much excess disease can climate change cause?

To inform policies, an estimation of the approximate magnitude of the health impacts of climate change is needed. This will indicate which particular impacts are likely to be greatest and in which regions, and how much of the climate-attributable disease burden could be avoided by emissions reduction. It will also guide health-protective strategies.

The global burden of disease attributable to climate change has recently been estimated as part of a comprehensive World Health Organization project (1). This project sought to quantify disease burdens attributable to 26 environmental, occupational, behavioural and life-style risk factors in 2000, and at selected future times up to 2030.

Disease burdens and summary measures of population health

The disease burden comprises the total amount of disease or premature death within the population. To compare burden-fractions attributable to several different risk factors requires, first, knowledge of the severity/disability and duration of the health deficit, and, second, the use of standard units of health deficit. The widely-used Disability-Adjusted Life Year (DALY (2)) is the sum of:

  • years of life lost due to premature death (YLL)
  • years of life lived with disability (YLD).

YLL takes into account the age at death. YLD takes into account disease duration, age at onset, and a disability weight reflecting the severity of disease.

To compare the attributable burdens for disparate risk factors we need to know: (i) the baseline burden of disease, absent the particular risk factor, (ii) the estimated increase in risk of disease/death per unit increase in risk factor exposure (the “relative risk”), and (iii) the current or estimated future population distribution of exposure. The avoidable burden is estimated by comparing projected burdens under alternative exposure scenarios.

Disease burdens have been estimated for five geographical regions (Figure 7.1). The attributable disease burden has been estimated for the year 2000. For the years 2010, 2020 and 2030, the climate-related relative risks of each health outcome under each climate change scenario, relative to the situation if climate change did not occur, were estimated (3). The baseline scenario is 1990 (the last year of the period 1961 to 1990 – the reference period used by the World Meteorological Organization and IPCC).

Figure 7.1 Estimated impacts of climate change in 2000 by region

The future exposure scenarios assume the following projected GHG emission levels:

  • Unmitigated emission trends (approximating the IPCC "IS92a" scenario)
  • Emissions reduction, achieving stabilization at 750 ppm CO2- equivalent by 2210 (s750)
  • More rapid emissions reduction, stabilizing at 550 ppm CO2- equivalent by 2170 (s550).

Health outcomes assessed

Only some of the health outcomes associated with climate change are addressed here (Table 7.1). These were selected on the basis of: (a) sensitivity to climate variation, (b) predicted future importance, and (c) availability/feasibility of quantitative global models.

Additional likely health impacts that are currently not quantifiable include those due to:

  • changes in air pollution and aeroallergen levels
  • altered transmission of other infectious diseases
  • effects on food production via climatic influences on plant pests and diseases
  • drought and famine
  • population displacement due to natural disasters, crop failure, water shortages
  • destruction of health infrastructure in natural disasters
  • conflict over natural resources
  • direct impacts of heat and cold (morbidity).

All independently-published models linking climate change to quantitative, global, estimates of health impacts (or health-affecting impacts – e.g. food yields) were reviewed. Where global models do not exist, local or regional projections were extrapolated. Models were selected according to their assessed validity. Linear interpolation was used to estimate relative risks for inter-scenario years.

Summary of results

Climate change will affect the pattern of deaths from exposure to high or low temperatures. However, the effect on actual disease burden cannot be quantified, as we do not know to what extent deaths during thermal extremes are in sick/frail persons who would have died soon anyway.

In 2030 the estimated risk of diarrhoea will be up to 10% higher in some regions than if no climate change occurred. Since few studies have characterized this particular exposure-response relationship, these estimates are uncertain.

Estimated effects on malnutrition vary markedly among regions. By 2030, the relative risks for unmitigated emissions, relative to no climate change, vary from a significant increase in the South- East Asia region to a small decrease in the Western Pacific. Overall, although the estimates of changes in risk are somewhat unstable because of regional variation in rainfall, they refer to a major existing disease burden entailing large numbers of people.

The estimated proportional changes in the numbers of people killed or injured in coastal floods are large, although they refer to low absolute burdens. Impacts of inland floods are predicted to increase by a similar proportion, and would generally cause a greater acute rise in disease burden. While these proportional increases are similar in developed and developing regions, the baseline rates are much higher in developing countries.

Changes in various vector-borne infectious diseases are predicted. This is particularly so for malaria in regions bordering current endemic zones. Smaller changes would occur in currently endemic areas. Most temperate regions would remain unsuitable for transmission, because either they remain climatically unsuitable (e.g., most of Europe) or socioeconomic conditions are likely to remain unsuitable for reinvasion (e.g., southern United States). Uncertainties relate to how reliable is extrapolation between regions, and to whether potential transmission will become actual transmission.

Application of these models to current disease burdens suggests that, if our understanding of broad relationships between climate and disease is realistic, then climate change may already be affecting human health.

The total current estimated burden is small relative to other major risk factors measured under the same framework. However, in contrast to many other risk factors, climate change and its associated risks are increasing rather than decreasing over time.


  • WHO. The World Health Report 2002. Geneva: WHO, 2002.
  • Murray, C.J.L. Quantifying the Burden of Disease - the Technical Basis for Disability- Adjusted Life Years. Bulletin of the World Health Organization. 72(3): 429-445 (1994).
  • McMichael, A.J. et al. Climate Change. In: Comparative quantification of Health Risks. Geneva: World Health Organization, 2003. (in press).

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Can stratospheric ozone depletion and ultraviolet radiation affect health?

Strictly, stratospheric ozone depletion is not part of “global climate change”, which occurs in the troposphere. There are, however, several recently described interactions between ozone depletion and greenhouse gas-induced warming.

Scientists 100 years ago would have been incredulous at the idea that, by the late twentieth century, humankind would be affecting the stratosphere. Yet, remarkably, human-induced depletion of stratospheric ozone has recently begun – after 8,000 generations of Homo sapiens.

Stratospheric ozone absorbs much of the incoming solar ultraviolet radiation (UVR), especially the biologically more damaging, shorter-wavelength, UVR. We now know that various industrial halogenated chemicals such as the chlorofluorocarbons (CFCs – used in refrigeration, insulation and spray-can propellants) and methyl bromide, while inert at ambient Earth-surface temperatures, react with ozone in the extremely cold polar stratosphere. This destruction of ozone occurs especially in late winter and early spring.

During the 1980s and 1990s at northern mid-latitudes (such as Europe), the average year-round ozone concentration declined by around 4% per decade: over the southern regions of Australia, New Zealand, Argentina and South Africa, the figure approximated 6-7%. Estimating the resultant changes in actual ground-level ultraviolet radiation remains technically complex. However, exposures at northern mid-latitudes, for example, are likely to peak around 2020, with an estimated 10% increase in effective ultraviolet radiation relative to 1980s levels (1).

In the mid-1980s, governments recognised the emerging hazard from ozone depletion. The Montreal Protocol of 1987 was adopted, widely ratified, and the phasing out of major ozone-destroying gases began. The protocol was tightened in the 1990s. Scientists anticipate a slow but near-complete recovery of stratospheric ozone by the middle of the twenty-first century.

Main types of health impacts

The range of certain or possible health impacts of stratospheric ozone depletion are listed in Table 8.1, with a summary evaluation of the evidence implicating UVR in their causation.

Many epidemiological studies have implicated solar radiation as a cause of skin cancer (melanoma and other types) in fair-skinned humans (2). Recent assessments by the United Nations Environment Program project increases in skin cancer incidence and sunburn severity due to stratospheric ozone depletion (1) for at least the first half of the twenty-first century (and subject to changes in individual behaviours).

The groups most vulnerable to skin cancer are white Caucasians, especially those of Celtic descent living in areas of high ambient UVR. Further, culturally-based behavioural changes have led to much higher UV exposure, through sun-bathing and skin-tanning. The marked increase in skin cancers in western populations over recent decades reflects, predominantly, the combination of background, post-migration, geographical vulnerability and modern behaviours.

Scientists expect the combined effect of recent stratospheric ozone depletion and its continuation over the next 1-2 decades to be (via the cumulation of additional UVB exposure), an increase in skin cancer incidence in fair-skinned populations living at mid to high latitudes (3). The modelling of future ozone levels and UVR exposures study has estimated that, in consequence, a ‘European’ population living at around 45 degrees North will experience, by 2050, an approximate 5% excess of total skin cancer incidence (assuming, conservatively, no change in age distribution). The equivalent estimation for the US population is for a 10% increase in skin cancer incidence by around 2050.

Laboratory studies demonstrate that exposure to UVR, in particular to UVB, in various mammalian species induces lens opacification. The epidemiological evidence for a role of UVR in human lens opacities is mixed. Cataracts are more common in some (but not all) countries with high UVR levels.

In humans and experimental animals, UVR exposure, including within the ambient environmental range, causes both localised and whole-body immunosuppression (4). UVR-induced immunosuppression could influence patterns of infectious disease. It may also influence the occurrence and progression of various autoimmune diseases and less certainly, vaccin efficacy (5).

Finally, there is a wider, ecological, dimension to consider. Ultraviolet radiation impairs the molecular chemistry of photosynthesis both on land (terrestrial plants) and at sea (phytoplankton). This could affect world food production, at least marginally, and thus contribute to nutritional and health problems in food-insecure populations. However, as yet there is little information about this less direct impact pathway.


Encouraging total sun avoidance (with the related notion of solar radiation as a “toxic” exposure) is a simplistic response to the hazards of increased ground-level UVR exposure due to stratospheric ozone depletion, and should be avoided. Any public health messages concerned with personal UVR exposure should consider the benefits as well as the adverse effects. Nevertheless, we must be alert to the potential increase in some particular risks to health posed by stratospheric ozone depletion.


  • Environmental effects of ozone depletion: 1998 assessment. Nairobi, Kenya, United Nations Environment Program, 1998. Also: Kelfkens, G. et al. Ozone layer-climate change interactions. Influence on UV levels and UV related effects. Dutch National Research Programme on Global Air Pollution and Climate Change. Report no.: 410 200 112.
  • IARC. Solar and Ultraviolet Radiation. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Vol 55. Lyon, France, International Agency for Research on Cancer, 1992.
  • Madronich S, de Gruijl FR. Skin cancer and UV radiation. Nature, 366 (6450): 23 (1993).
  • Ponsonby A-L, McMichael AJ, van der Mei I. Ultraviolet radiation and autoimmune disease: insights from epidemiological research. Toxicology 181-182: 71-78 (2002).
  • Temorshuizen F, et al. Influence of season on antibody response to high dose recombinant Hepatitis B vaccine: effect of exposure to solar UVR? Hepatology, 32 (4): 1657 (2000).
  • Slaper al. Estimates of ozone depletion and skin cancer incidence to examine the Vienna Convention achievements. Nature 384 (6606): 256-8 (1996).

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What methods do countries use to estimate how climate may affect health conditions?

National assessments of health impacts of climate change

Estimates, even if approximate, of the potential health impacts of climate change are an essential input to policy discussion on reducing greenhouse gas emissions and on social adaptation to climate change. Societies must respond despite the unavoidable uncertainties. Indeed, national governments have a responsibility, under the UN’s Framework Convention on Climate Change (1992), to carry out formal assessments of the risk to their population’s health posed by global climate change.

Health impact assessment (HIA) has been defined as “a combination of procedures, methods and tools by which a policy, project or hazard may be judged as to its potential effects on the health of a population, and the distribution of those effects within the population” (1).

Despite recent advances in health impact assessment methods, its integration into mainstream policy-making has yet to be satisfactorily achieved. Besides, impact assessments typically refer to health impacts over the next 10 to 20 years (e.g. due to current smoking rates, obesity levels, or population ageing), rather than the 50 to 100 year time-scale appropriate to climate change projections. So there is need for scenario-based impact assessments that incorporate, and communicate, a higher level of uncertainty. The steps in climate change impact and adaptation assessment are shown in figure 9.1.

Several types of national health impact assessments have been undertaken. A basic assessment identifies the types, but not much about the magnitudes, of potential impacts. In contrast, comprehensive well-funded and well-supported assessments are undertaken. For example, in the United States assessment, published in 2000, population health was one of the five target sectors included in the 16 detailed regional assessments and in the overall assessment. The US assessment involved stakeholder participation and extensive consultation and peer review (3). Further Comparative details of two national assessments are shown in the box.

Comparing Assessments: UK and Fijii

The UK assessment concentrated on producing quantitative results for the following health outcomes (4), for three time periods and for four climate scenarios:

  • Heat-related and cold-related deaths and hospital admissions
  • Cases of food poisoning
  • Changes in distribution of Plasmodium falciparum malaria (global) and tick-borne encephalitis (Europe), and in seasonal transmission of P. vivax malaria (UK)
  • Cases of skin cancer due to stratospheric ozone depletion.

The large uncertainty surrounding these estimates was acknowledged. The main conclusions of the report were the impact of increases in river and coastal flooding, and severe winter gales. This report also clearly addressed the balance between the potential benefits and adverse impacts of climate change: the potential decline in winter deaths due to milder winters is much larger than the potential increase in heat-related deaths. Climate change is also anticipated to lessen air pollution-related illnesses and deaths, except for those associated with tropospheric ozone, which will form more readily at higher temperatures.

The Fijian assessment addresses health impact in the context of current health services. Fiji’s main concerns were dengue fever (recent epidemic in 1998), diarrhoeal disease and nutrition-related illness. The islands are malaria free and an anopheline mosquito vector population has not been established despite a suitable climate. Hence, the risk of introduction and establishment of malaria and other mosquito-borne diseases due to climate change was considered to be very low. Filariasis, an important vector-borne disease on the islands, is likely to be increased by warmer temperatures. The distribution of the vector (Aedes polynesiensis) may also be affected by sea level rise, since it breeds in brackish water. A dengue fever transmission model was incorporated into a climate impacts model developed for the Pacific Islands (PACCLIM). The modelling indicates that climate change may extend the transmission season and geographic distribution in Fiji.

Diarrhoeal disease may increase in Fiji because of increased temperature and altered patterns of rainfall. However, no evidence was presented on the association between flooding or heavy rainfall and cases of diarrhoea. The 1997/1998 drought (associated with El Nino) had widespread health impact, including diarrhoeal disease, malnutrition and micronutrient deficiency in children and infants (5).

Comprehensive multi-sectoral assessments have been conducted by the USA, Canada, the UK and Portugal. Assessments in developing countries have been undertaken only under the auspices of donor-funded capacity-building initiatives. (Other sub-national or local assessments of potential health impacts may have been undertaken for climate change, but, if so, such studies are in the “grey” literature, not widely available.) The outcomes listed refer to the likely health impacts reported on for that particular country. The level of uncertainty accompanying these estimates is usually not described. Vector-borne diseases, particularly malaria, have been widely addressed. Other potentially greater impacts, such as from weather disasters, have been less well addressed.

Out of these experiences, several conclusions can be drawn:

  • Assessments should be driven by region and country priorities in order to determine which health impacts are considered. No single set of guidelines covers all health and institutional situations.
  • HIA is a policy tool, therefore the actual process of conducting assessments, particularly the involvement of stakeholders, is very important.
  • Assessments should set an agenda for future research. Nearly all the assessments done to date have identified research gaps, and they often specify detailed research questions.
  • Assessment should be linked to follow-up activities such as monitoring and updated reports.

The development of formal guidelines for the national assessment of health impacts will improve methods used, will achieve some standardization, and will facilitate the development of relevant indicators. Health Canada has prepared an initial framework (6), proposing that there are three distinct phases to the assessment task:

  • Scoping: to identify the climate change problem (concerns of vulnerable groups) and its context, describe the current situation (health burdens and risks) and identify key partners and issues for the assessment.
  • Assessment: estimations of future impacts and adaptive capacity, and evaluation of adaptation plans, policies and programmes.
  • Risk management: actions to minimize the impacts on health, including follow-up assessments.

This type of health impact assessment, in relation to large-scale climatic-environmental changes, requires guidelines that accord with the mainstream HIA framework of WHO and other international agencies. Achieving this would help to move the climate change policy discussion beyond the environmental impact domain and into the social and public health impacts arenas. Currently, in most countries, sector differentiation and the associated policy environment neither facilitates nor fosters intersectoral collaboration. Within the health sector, resources are allocated primarily in relation to dealing with existing problems, taking some account of the relative burden of disease.

A major shortcoming of many climate change health impact assessments has been the superficial treatment of the population’s adaptive capacities and policy options. Strategies to enhance population adaptation should promote measures that are not only appropriate for current conditions, but which also build the capacity to identify and respond to unexpected future stresses/hazards. The restoration and improvement of general public health infrastructure will reduce population vulnerability to the health impacts of climate change. In the longer-term, and more fundamentally, improvements in the social and material conditions of life and the reduction of inequalities within and between populations are required for sustained reduction in vulnerability to global environmental change.


  • WHO Health impact assessment as a tool for intersectoral health policy. WHO European Centre for Environment and Health/European Centre for Health Policy, 1999.
  • Parry, M.L. & Carter, T. Climate impact and adaptation assessment. London, UK, EarthScan, 1998.
  • Patz, al. The potential health impacts of climate variability and change for the United States: executive summary of the report of the health sector of the US National Assessment. Environ Health Perspect 108: 367-376 (2000).
  • Dept of Health (UK) Health Effects of Climate Change in the UK. London: DoH 2002.
  • OCHA. UNDAC Mission Report Fiji Drought. UN Office for Co-ordination of Humanitarian Affairs,1998.
  • Health Canada. National Health Impact and Adaptation Assessment Framework and Tools. Ottawa: Climate Change and Health Office, Health Canada, 2002.

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How can countries monitor their health effects to see if climate change is affecting them?

Both the detection and measurement of health effects of climate change are necessary as evidence underpinning national and international policies relating to measures to protect public health. Those measures include mitigation of greenhouse gas emissions.

Good evidence requires good data. The climate varies naturally as well as in response to human influences, and, in turn, climate is only one of many determinants of population health. Therefore, assessing the health impacts of climate change poses challenges. Further, the process of climate change is detectable only over decades, and the resultant health impacts will be similarly slow to emerge.

Monitoring is “the performance and analysis of routine measurements aimed at detecting changes in the environment or health of populations” (1). In many public health investigations, it is possible to measure changes in a defined health impact and to attribute this trend to changes in a directly-acting risk factor. However, the monitoring of the impacts of climate change on health is more complex. There are three main issues:

(i) Distinguishing apparent from real “climate change”

Climate is always fluctuating naturally, and many indices of health show seasonal and interannual fluctuation. The demonstration of such a relationship provides no direct evidence that climate change per se has occurred — rather, it merely confirms that these diseases have a seasonal or climatic dependence.

An excess of heat-related deaths in a particularly hot summer, or even a succession of hot summers, indicates the potential for climate change to increase mortality, but it does not prove that mortality has increased as a result of climate change. That would require evidence of a change in the 'baseline' climate conditions – i.e. that the sequence of hot summers was exceptional, and due to climate change rather than random variation.

(ii) Attribution

Since climate is one of many influences on health, the attribution of an observed change in population health to an associated change in climate is not straightforward. The influence of concurrent changes in other environmental, social or behavioural factors must be first allowed for.

(iii) Effect modification

Over time, as the climate changes, other changes may also occur that alter the population’s vulnerability to meteorological influences. For example, vulnerability to extreme weather events, including floods and storms, will depend on where and how residential housing is built, what flood protection measures are introduced, and how land-use is changed. Effective monitoring must include parallel measurements of population and environmental data, to allow study of potential modifying influences.

General Principles

The principal criteria for selecting diseases and settings for monitoring should include the following:

  • Evidence of climate sensitivity - to be demonstrated through either observed health effects of temporal or geographical climate variation, or evidence of climate effects on components of the disease transmission process in the field or laboratory.
  • Significant public health burden - monitoring should be preferentially targeted towards significant threats to public health. These may be diseases with a high current prevalence and/or severity, or considered likely to become prevalent under conditions of climate change.
  • Practicality – logistical considerations are important given that monitoring requires dependable and consistent longterm recording of health-related indices and other environmental parameters. Monitoring sites should be chosen where change is most likely to occur, but where appropriate capacity for reliable measurement exists.

Data Requirements and Sources

The data needed for monitoring climate effects on health comprise: (i) climatic variables (ii) population health markers and (iii) other nonclimatic explanatory factors (Table 10.1).

Table 10.1. Data required to monitor climate impacts on health

Thermal extremesExtreme weather events (floods, high winds, droughts)Food- & waterborne diseaseVector-borne disease
Principal health outcomesDaily mortality hospital admissions clinic/emergency room attendanceAttributed deaths hospital admissions infectious disease surveillance data mental health nutritional statusRelevant infectious disease deaths & morbidityVector populations disease notifications temporal and geographical distributions
Which populations/ locations to monitorUrban populations, especially in developing countriesAll regionsAll regionsMargins of geographical distribution (e.g: changes with latitude, altitude) and temporality in endemic areas
Sources and methods for acquiring health dataNational and sub-national death registries (e.g. city specific data)Use of sub-national death registries local public health recordsDeath registries national & subnational surveillance notificationsLocal field surveys routine surveillance data (variable availability)
Meteorological dataDaily temperatures (min/max or mean) & humidityMeteorological event data: extent, timing & severityWeekly/daily temperature rainfall for water-borneWeekly/daily temperature, humidity and rainfall
Other variablesConfounders: influenza & other respiratory infections air pollution
Modifiers: housing conditions (e.g. household/workplace air conditioning), availability of water supplies
Disruption/contamination of food & water supplies disruption of transportation. Population displacement
The above parameters will have an indirect impact on health
Long term trends dominated by host-agent interactions (e.g. S enteritidis in poultry) whose effects are difficult to quantify. Indicators may be based on examination of seasonal patterns.Land use surface configurations of freshwater

The choice of non-climatic variables will depend on the specific disease, but the principal categories of confounding or modifying factors include:

  • age structure of population
  • underlying rates of disease, especially cardiovascular and respiratory disease and diarrhoeal illness
  • level of socio-economic development
  • environmental conditions, e.g. land-use, air quality, housing conditions
  • quality of health-care
  • specific control measures, e.g. vector control programmes.

Specific Categories of Health Impacts: Data Needs, Opportunities

To monitor the health effects of thermal extremes, reliable long time-series of temperature and mortality/morbidity data are available in many countries. An important focus of research data should be the assessment of how the temperature-mortality/morbidity relationship is modified by individual, social and environmental factors. Existing databases (e.g. EMDAT) for extreme weather events may be a key resource. To maximize their usefulness, complete and consistent reporting of extreme weather events across a wide geographical area, along with standard definitions of events and methods of attribution, is needed.

Current monitoring data can provide only a broad quantification of the relationship between climate and most vector-borne disease. Assessment of the climate contribution to long-term trends requires linked data on factors such as land-use, host abundance and intervention measures. Clearer understanding of relationships should result from high-quality serial data on vectors at a modest number of sites within or at the margins of endemic areas. Data from sites along specified transects could indicate changing vector distributions (including altitude). Geographical comparisons based on remote sensing data may give additional insights into disease trends.


With all forms of monitoring, interpretation of evidence will be strengthened by procedures for standardization, training and quality assurance/quality control. Long time-series of health changes in populations in relation to steep (i.e. sensitive) climate-disease relationships will be the most informative. Such monitoring will become more effective through international collaboration and integration with existing surveillance networks.


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What can be done to protect populations from climate impacts? What is Adaptation?

Adaptation and adaptive capacity, to lessen health impacts

Even if greenhouse gas emissions are reduced in the near future, Earth’s climate will continue to change. Hence, adaptation strategies must be considered to reduce disease burdens, injuries, disabilities and deaths.

The IPCC has defined the following two closely-related terms (1):

Adaptation:Adjustment in natural or human systems in response to actual or expected climatic stimuli or their effects, which moderates harm or exploits beneficial opportunities.

Adaptive capacity:The ability of a system to adjust to climate change (including climate variability and extremes) to moderate potential damages, to take advantage of opportunities, or to cope with consequences.

The extent to which human health is affected depends on: (i) the exposures of populations to climate change and its environmental consequences, (ii) the sensitivity of the population to the exposure, and (iii) the ability of affected systems and populations to adapt (Figure 11.1). We therefore need to understand how decisions are made about adaptation, including the roles of individuals, communities, nations, institutions and private sector.

Adaptation and prevention

Many adaptive measures have benefits beyond those associated with climate change. The rebuilding and maintaining of public health infrastructure is often viewed as the “most important, cost-effective and urgently needed” adaptation strategy (1). This includes public health training, more effective surveillance and emergency response systems, and sustainable prevention and control programs.

Extreme weather events can have vastly different impacts because of differences in the target population’s coping capacity. For example, cyclones in Bangladesh in 1970 and 1991 are estimated to have caused 300,000 and 139,000 deaths respectively (2). In contrast, Hurricane Andrew struck the United States in 1992, causing 55 deaths (although also causing around $30 billion in damages (3)). Climate-related adaptation strategies must therefore be considered in relation to broader characteristics – such as population growth, poverty, sanitation, health care, nutrition, and environmental degradation – that influence a population’s vulnerability and capacity to adapt.

Adaptations which enhance a population’s coping ability may protect against current climatic variability as well as against future climatic changes. Such “no-regrets” adaptations may be especially important for less developed countries with little current coping capacity.

Adaptive capacity

Adaptive capacity refers to both actual and potential features. Thus, it encompasses both current coping ability and the strategies that expand future coping ability. For example, access to clean water is part of the current coping capacity for developed countries – but represents potential adaptive capacity in many less developed countries.

Highly-managed systems, such as agriculture and water resources in developed countries, are thought to be more adaptable than less-managed or natural ecosystems. Unfortunately, some components of public health systems are often relaxed when a particular health threat recedes. For example, the threat of infectious diseases appeared to be retreating thirty years ago because of advances in antibiotic drugs, vaccines and pesticides. Today, however, there is a general resurgence of infectious diseases – and relevant public health measures need to be reinvigorated.

The main determinants of a community’s adaptive capacity are: economic wealth, technology, information and skills, infrastructure, institutions, and equity. Adaptive capacity is also a function of current population health status and pre-existing disease burdens.

Economic resources

Wealthy nations are better able to adapt because they have the economic resources to invest, and to offset the costs of adaptation. In general, poverty enhances vulnerability – and we live in a world in which approximately one-fifth of the world’s population lives on less than US$1 per day.


Access to technology in key sectors and settings (e.g., agriculture, water resources, health-care, urban design) is an important determinant of adaptive capacity. Many health-protecting adaptive strategies involve technology – some of which is well established, some new and still being disseminated, and some still being developed to enhance coping with a changing climate.

The health risks from proposed technological adaptations should be assessed in advance. For example, increased air conditioning would protect against heat stress, but could increase emissions of greenhouse gases and other air pollutants. Poorly designed coastal "defences" may increase vulnerability to tidal surges if they engender false security and promote low-lying coastal settlements.

Information and skills

In general, countries with more “human capital” or knowledge have greater adaptive capacity (1). Illiteracy increases a population’s vulnerability to many problems (4). Health systems are labor-intensive and require qualified and experienced staff, including those trained in the operation, quality control, and maintenance of public health infrastructure (5).


Infrastructure specifically designed to reduce vulnerability to climate variability (e.g., flood control structures, air conditioning, and building insulation) and general public health infrastructure (e.g., sanitation facilities, wastewater treatment systems, laboratory buildings) enhance adaptive capacity. However, infrastructure (especially if immovable) can be adversely affected by climate, especially extreme events such as floods and hurricanes.


Countries with weak institutional arrangements have less adaptive capacity than countries with wellestablished institutions (1). For example, institutional and managerial deficiencies contribute to Bangladesh’s vulnerability to climate change.

Collaboration between public and private sectors can enhance adaptive capacity. For example, the Medicines for Malaria Venture – a joint public-private initiative to develop new antimalarial drugs – is developing new products for use in developing countries.


Adaptive capacity is likely to be greater when access to resources within a community, nation, or the world is equitably distributed (6). Under-resourced and marginal populations lack adaptive resources. While universal access to quality services is fundamental to public health, many still lack access to health care. Overall, the developing world, with 10 per cent of the world’s health resources, carries 90 per cent of the disease burden (5).

Health Status and Pre-existing Disease Burdens

Population well-being is an important ingredient and determinant of adaptive capacity. Great progress has been achieved in public health, yet 170 million children in poor countries are underweight, of whom over three million die each year. Many countries face the double burden of increases of non-communicable diseases, but with continued prevailing infectious diseases.


Adaptive strategies intended to protect public health will be needed whether or not actions are taken to mitigate climate change. Building capacity is an essential preparatory step. Adapting to climate change will require more than financial resources, technology, and public health infrastructure. Education, awareness-raising and the creation of legal frameworks, institutions and an environment that enables people to take well-informed, long-term, sustainable decisions are all needed.


  • IPCC, 2001. Climate Change 2001: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK.
  • NOAA. NOAA releases century’s top weather, water, and climate events. 1999. .htm.
  • US Centers for Disease Control (CDC). Rapid health needs assessment following Hurricane Andrew - Florida and Louisiana, 1992. Morbidity and Mortality Weekly Report, 41 (37): 685 (1992).
  • UNDP. 2000 Human Development Report 2000: Human rights and human development. United Nations Development Program. Oxford University Press, New York, NY, USA.
  • WHO. 2000. World Health Report 2000: Health systems: Improving Performance. World Health Organization, Geneva, Switzerland.
  • Rayner, S. & Malone, E.L. Climate change, poverty and intragenerational equity: the national level. In: Climate change and its linkages with development, equity and sustainability. Proceedings of the IPCC Expert Meeting held in Colombo, Sri Lanka, 27-29 April, 1999. Munasinghe, M. & Swart, R. eds. Colombo, Sri Lanka, LIFE Bilthoven, The Netherlands, RIVM and Washington D.C., USA, World Bank, pp. 215-242, 1999.

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How to develop policy responses to climate change?

Policy choices are guided by several principles. These include considerations of equity, efficiency and political feasibility. The usual public health ethics considerations may also apply: respect for autonomy, nonmaleficence (not doing bad), and justice and beneficence (doing good).

To make informed decisions about climate change, policy-makers will need timely and useful information about the possible consequences of climate change, people’s perceptions of those consequences, available adaptation options, and the benefits of slowing the rate of climate change (1). The challenge for researchers is to provide this information.

Once policy-makers have received input from the impact assessment community, they must integrate this information into a broader policy portfolio. Response options include actions to mitigate greenhouse gas emissions to slow the rate of climate change measures to adapt to a changing climate in order to increase society’s resilience to the changes that are coming activities to increase the public’s awareness of the climate change issue investments in monitoring and surveillance systems and investments in research to reduce key policy-relevant uncertainties.

Climate change, however, should not be considered in isolation from other global environmental stresses. Further, policy-makers usually deal with multiple social objectives (e.g., poverty elimination, promotion of economic growth, protection of cultural resources), while competing stakeholder desires compound the allocation of scarce resources. Climate change should therefore be viewed as part of the larger challenge of sustainable development.

Using the information provided by the research community, risk managers must make decisions despite the existence of scientific uncertainties. Policy-focused assessments analyze the best available scientific and socioeconomic information to answer questions being asked by risk managers. They characterize and, if possible, quantify scientific uncertainties to the extent possible, and explain the potential implications of the uncertainties for the outcomes of concern to the decision makers. Ultimately, it is up to society to decide whether a perceived risk warrants action. But the scientific uncertainty, by itself, does not excuse delay or inaction.

Decision-making criteria

Many different criteria exist for making decisions about climate change policy. Two approaches to decision making that are often discussed are the “precautionary principle” and “benefit-cost” analysis.

The precautionary principle is a risk management principle applied when a potentially serious risk exists, but significant scientific uncertainty also exists (2). The precautionary principle allows some risks to be deemed unacceptable not because they have a high probability of occurring, but because the consequences if they occur may be severe or irreversible. This principle was featured in the 1992 Rio Declaration on Environment and Development as Principle 15, stating: “Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation.”

Another widely used approach is the “benefit-cost” criterion, weighting the expected benefits and costs of a proposed action. Questions arise about how benefits and costs should be measured, and how they should be compared among different societies. The benefit-cost criterion emphasizes the efficient use of scarce resources – but does not deal with equity. Nor does it deal well with consequences that are displaced into the future, and therefore, by economic convention, often discounted. Climate change has the potential for catastrophic outcomes in the distant future, the “present value” of which would be small if discounted. Despite these concerns, benefit-cost analysis should not be dismissed. This would only deprive decision makers of one set of insightful information.

Response options

The mitigation of greenhouse gases provides a mechanism for slowing, and perhaps eventually halting, the buildup of greenhouse gases in the atmosphere. A slowing of the rate of warming could yield important benefits in the form of reduced impacts to human health and other systems however, the inertia in the climate system means that there will be a significant temporal lag between emission reduction and slowing in the rate of warming.

Adaptation (discussed in chapter 11) is another important response option. Such actions enhance the resilience of vulnerable systems, thereby reducing potential damages from climate change and climate variability.

Communication of information about climate change, its potential health impacts, and response strategies, is itself a public policy response to climate change. So, too, are the development and implementation of monitoring and surveillance systems, and investments in research. Monitoring and surveillance systems are integral and essential to providing the information needed to support decisions by public health officials.

Building the bridge from science to policy: policy-focused assessment

Policy-focused assessment is a process that can help resource managers and other decision makers meet the challenge of assembling an effective policy portfolio. It is a process by which the best-available scientific information can be translated into terms that are meaningful to policy makers.

A policy-focused assessment is more than just a synthesis of scientific information or an evaluation of the state of science. Rather, it involves the analysis of information from multiple disciplines – including the social and economic sciences – to answer the specific questions being asked by stakeholders. And it includes an analysis of adaptation options to improve society’s ability to respond effectively to risks and opportunities as they emerge. Formulating good policy requires understanding the variability in vulnerability across population sub-groups, and the reasons for that variability.

In the assessment of adaptation options, a number of factors related to the design and implementation of strategies need to be considered. These include the fact that (1) the appropriateness and effectiveness of adaptation options will vary by region and across demographic groups (2) adaptation comes at a cost (3) some strategies exist that would reduce risks posed by climate change, whether or not the effects of climate change are realized (4) the systemic nature of climate impacts complicates the development of adaptation policy and (5) maladaptation can result in negative effects that are as serious as the climate-induced effects being avoided.

Complicating the assessment process is the fact that there are significant scientific and socioeconomic uncertainties related to climate change and its potential consequences for human health. Uncertainties exist about the potential magnitude, timing and effects of climate change the sensitivity of particular health outcomes to current climatic conditions (i.e., to weather, climate, and climate-induced changes in ecosystems) the future health status of potentially affected populations (in the absence of climate change) the effectiveness of different courses of action to adequately address the potential impacts and the shape of future society (e.g., changes in socioeconomic and technological factors).

A challenge for assessors is to characterize the uncertainties and explain their implications for the questions of concern to the decision makers and stakeholders. If uncertainty is not directly addressed as part of the analysis, a health impacts assessment can produce misleading results and possibly contribute to ill-informed decisions.

Public awareness: communicating assessment results

Stakeholders should be engaged throughout an assessment process. A communication strategy must ensure access to information, presentation of information in a usable form, and guidance on how to use the information. Risk communication is a complex, multidisciplinary, and evolving process. Often information has to be tailored to the specific needs of risk managers in specific geographic areas and demographic groups. This requires close interaction between information providers and those who need the information to make decisions.


Some have argued that the existence of scientific uncertainties precludes policy makers from taking action today in anticipation of climate change. This is not true. In fact, policy makers, resource managers, and other stakeholders, despite the existence of uncertainties, make decisions every day. The outcomes of these decisions may be affected by climate change. Or the decisions may foreclose future opportunities to adapt to climate change. Hence, the decision makers would benefit from information about the likely impacts of climate change. An informed decision is always better than an uninformed decision.

Care must be taken to respect the boundary between assessment and policy formation. The goal of policy-focused assessment is to inform decision-makers, not to make specific policy recommendations.


  • Scheraga, Joel D., and Anne E. Grambsch, “Risks, opportunities, and adaptation to climate change,” Climate Research, Vol. 10, 1998, 85-95.
  • Tamburlini, G., and K. L. Ebi, “Searching for evidence, dealing with uncertainties, and promoting participatory risk-management,” in Children’s health and environment: A review of evidence, Tamburlini G., O.S. von Ehrenstein, R. Bertollini, editors. A Joint Report from the European Environment Agency and the WHO Regional Office for Europe, EEA, Copenhagen, 2002, 199-206.

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What types of climate related activities does WHO support for action?

Conclusions and recommendations for action

Sustainability is essentially about maintaining Earth’s ecological and other biophysical life-support systems. If these systems decline, human population wellbeing and health will be jeopardised. Technology can buy time, but nature’s bottom-line accounting cannot be evaded. We must live within Earth’s limits. The state of human population health is thus a central consideration in the transition towards sustainability (1).

Climate change, like other human-induced large-scale environmental changes, poses risks to ecosystems, their life-support functions and, therefore, human health (Figure 13.1) (2,3). WHO, WMO and UNEP collaborate on issues related to climate change and health, addressing capacity building, information exchange and research promotion.


Climate-related exposures

The IPCC’s Third Assessment Report projected that, as we continue to change atmospheric composition, global average surface temperature will rise by 1.4 to 5.8ºC in this century, along with changes in precipitation and other climatic variables. Research needs include developing innovative approaches to analysing weather and climate in relation to human health setting up long-term data sets to answer key questions and improving understanding of how to incorporate outputs from Global Climate Models into human health studies.

Reaching consensus on the science

The science of climate change has achieved increasing consensus among scientists. There is increasing evidence that human health will be affected in many and diverse ways. Knowledge is still limited in many areas, for example on the contribution of short-term climate variability to disease incidence on development of early warning systems for predicting disease outbreaks and extreme weather events and on understanding how recurring extreme events may weaken adaptive capacity.

Challenges for scientists

Climate change poses some special challenges, including the complexity of causal process, the unavoidable uncertainties, and temporal displacement of anticipated impacts into the future. Some key research topics to address include identifying where first effects of climate change on human health will be apparent improving estimates of climate change impacts and better expressing the uncertainties associated with studies of climate change and health.

Extreme climate events

The IPCC’s Third Assessment Report projected changes in extreme climate events that include more hot days and heat waves more intense precipitation events increased risk of drought increase in winds and tropical cyclones (over some areas) intensified droughts and floods with El Niño events and increased variability in the Asian summer monsoon. Research gaps to be addressed include further modelling of relationships between extreme events and health impacts improved understanding of factors affecting vulnerability to climate extremes and assessment of the effectiveness of adaptation in different settings.

Infectious diseases

Infectious diseases, especially those transmitted via insect vectors or water, are sensitive to climatic conditions. Disease incidence data is needed to provide a baseline for epidemiological studies. The lack of precise knowledge of current disease incidence rates makes it difficult to comment about whether incidence is changing as a result of climatic conditions. Research teams should be international and interdisciplinary, including epidemiologists, climatologists and ecologists to assimilate the diversity of information from these respective fields.

The burden of disease

The stock of empirical evidence relating climatic trends to altered health outcomes remains sparse. This impedes estimating the range, timing and magnitude of likely future health impacts of global environmental changes. Even so, an initial attempt has been made, within the framework of the WHO Global Burden of Disease 2000 project. Analyzing only the better studied health outcomes, the climate change that occurred since the climate baseline period 1961-1990 was estimated to have caused 150,000 deaths and 5.5 million DALYS in the year 2000 (5).

Stratospheric ozone depletion, climate change and health

Stratospheric ozone depletion is essentially a different process from climate change. However, greenhouse-warming is affected by many of the chemical and physical processes involved in the depletion of stratospheric ozone (6). Also, because of changes in climate (in addition to public information and education campaigns), patterns of individual and community sun exposure behaviour will change – duly affecting received doses of ultraviolet radiation.

National assessments

Several developed and developing countries have undertaken national assessments of the potential health impacts of climate change, including reference to vulnerable areas and populations. There is a need to standardize the health impact assessment procedures, and tools and methods are being developed. More accurate climate information at the local level, particularly on climate variability and extremes, is needed.

Monitoring climate change impacts on human health

Climate change is likely to affect diseases that are also influenced by other factors. Monitoring to assess climate-change impacts on health therefore requires data-gathering coupled with analytical methods able to quantify the climate-attributable portion of such diseases. Monitoring and surveillance systems in many countries currently cannot provide useful data on climate-sensitive diseases. Less developed countries should strengthen existing systems in order to meet current needs.

Adapting to climate change

Since climate change is already underway, we need adaptation policies to complement mitigation policies. Efficient implementation of adaptation strategies can significantly reduce adverse health impacts of climate change. Human populations vary in their susceptibility, depending on factors such as population density, economic development, local environmental conditions, pre-existing health status and health-care availability. Adaptation measures usually will have near-term as well as future benefits, by reducing the impacts of current climate variability. Adaptation measures can be integrated with other health strategies.

Responses: From science to policy

The magnitude and character of global climate change necessitates a community-wide understanding and response, guided by policies informed by good scientific advice. A successful policy-focused assessment of the potential health impacts of climate change should include: i) a multidisciplinary assessment team ii) responses to questions asked by all stakeholders iii) evaluation of risk management adaptation options iv) identification and prioritisation of key research gaps v) characterization of uncertainties and their implications for decision-making and vi) tools that support decision-making processes.


International agreements on global environmental issues such as climate change should consider the principles of sustainable development proposed in Agenda 21 and the UNFCCC. These include the “precautionary principle”, the principle of “costs and responsibility” (the cost of pollution or environmental damage should be borne by those responsible), and “equity” – both within and between countries and over time (between generations).

Adherence to these principles would help prevent future global environmental threats and reduce existing ones. With climate change already underway, there is need to assess vulnerabilities and identify intervention/adaptation options (7). Early planning for health can reduce future adverse health impacts. The optimal solution, however, lies with governments, society and individuals – and requires changes in behaviour, technologies and practices to enable a transition to sustainability.


McMichael AJ et al. The Sustainability Transition: A new challenge (Editorial). Bull WHO, 78: 1067 (2000).

R. Watson, et al. Protecting Our Planet Securing Our Future: Linkages Among Global Environmental Issues and Human Needs. UNEP, NASA, World Bank, 1998.

McMichael, A.J. Population, environment, disease, and survival: past patterns, uncertain futures. Lancet, 359: 1145-48 (2002).

Patz, J.A. et al. The potential health impacts of climate variability and change for the United States: executive summary of the report of the health sector of the U.S. National Assessment. Environ Health Perspect,108(4): 367-76 (2000).

World Health Organization. World Health Report, 2002.

WMO/UNEP. Scientific Assessment of Ozone Depletion, 2002.

IPCC. Climate Change 2001, Impacts, adaptation and vulnerability. Published for the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press, 2001.

Help Is Available for Stress

Stress is a part of life. What matters most is how you handle it. The best thing you can do to prevent stress overload and the health consequences that come with it is to know your stress symptoms.

If you or a loved one is feeling overwhelmed by stress, talk to your doctor. Many symptoms of stress can also be signs of other health problems. Your doctor can evaluate your symptoms and rule out other conditions. If stress is to blame, your doctor can recommend a therapist or counselor to help you better handle your stress.

Global Environmental Change: Understanding the Human Dimensions (1992)

All the human causes of global environmental change happen through a subset of proximate causes, which directly alter aspects of the environment in ways that have global effects. We begin this chapter by outlining and illustrating an approach to accounting for the major proximate causes of global change, and then proceed to the more difficult issue of explaining them. Three case studies illustrate the various ways human actions can contribute to global change and provide concrete background for the more theoretical discussion that follows. We have identified specific research needs throughout that discussion. We conclude by stating some principles that follow from current knowledge and some implications for research.


The important proximate human causes of global change are those with enough impact to significantly alter properties of the global environment of potential concern to humanity. The global environmental properties now of greatest concern include the radiative balance of the earth, the number of living species, and the influx of ultraviolet (UV-B) radiation to the earth's surface (see also National Research Council, 1990b). In the future, however, the properties of concern to humanity are likely to change&mdashultra-violet radiation, after all, has been of global concern only since the 1960s. Consequently, researchers need a general system for

moving from a concern with important changes in the environment to the identification of the human activities that most seriously affect those changes. This section describes an accounting system that can help to perform the task and illustrates it with a rough and partial accounting of the human causes of global climate change.


A useful accounting system for the human causes of global change has a tree structure in which properties of the global environment are linked to the major human activities that alter them, and in which the activities are divided in turn into their constituent parts or influences. Such an accounting system is helpful for social science because, by beginning with variables known to be important to global environmental change, it anchors the study of human activities to the natural environment and imposes a criterion of impact on the consideration of research directions (see also Clark, 1988). This is important because it can direct the attention of social scientists to the study of the activities with strong impacts on global change.

Because the connections between global environmental change and the concepts of social science are rarely obvious, social scientists who begin with important concepts in their fields have often directed their attention to low-impact human activities (see Stern and Oskamp, 1987, for elaboration). An analysis anchored in the critical physical or biological phenomena can identify research traditions whose relevance to the study of environmental change might otherwise be overlooked. For example, an examination of the actors and decisions with the greatest impact on energy use, air pollution, and solid waste generation showed that, by an impact criterion, studies of the determinants of daily behavior had much less potential to yield useful knowledge than studies of household and corporate investment decisions or of organizational routines in the context of energy use and waste management (Stem and Gardner, 1981a,b). Theories and methods existed for each subject matter in relevant disciplines such as psychology and sociology, but much of the research attention had been misdirected.

The idea of tree-structured accounting can be illustrated by the following sketch of a tree describing the causes of global climate change.

The chief environmental property of concern is the level of greenhouse gases in the atmosphere. The major anthropogenic

greenhouse gases, defined in terms of overall impact (amount in the atmosphere times impact per molecule integrated over time), are carbon dioxide (CO2), chlorofluorocarbons (CFCs), methane (CH4), and nitrous oxide (N2O). If the trunk of the tree represents the greenhouse gas-producing effect of all human activities, the limbs can represent the contributing greenhouse gases. Table 3-1 presents the limbs during two different time periods and a projection for a future period.

Both natural processes and human activities result in emissions of greenhouse gases. For instance, carbon dioxide is emitted by respiration of animals and plants, burning of biomass, burning of fossil fuels, and so forth. If each limb of the tree represents human contributions to global emissions of a greenhouse gas, the branches off the limbs can represent the major anthropogenic sources of a gas, that is, the major categories of human activity that release it. These are proximate human causes of climate change, and their impact is equal to their contribution of each greenhouse gas times the gas's radiative effect, integrated over time. For the same emissions, the representation of impact will vary with the date to which the impact is projected. Tables 3-2 and 3-3 allocate emissions of the most important greenhouse gases during the late 1980s to human activities.

Major human proximate causes, such as fossil fuel burning, are conducted by many actors and for many purposes: electricity generation, motorized transport, space conditioning, industrial process heat, and so forth. A tree branch, such as one representing fossil fuel burning, can be divided into twigs that represent these different actors or purposes, each of which acts as a subsidiary proximate cause, producing a proportion of the total emissions. It is possible to make such a division in numerous ways. Fossil fuel burning can be subdivided according to parts of the world (countries, developed and less-developed world regions, etc.), sectors of an economy (transportation, industrial, etc.), purposes (locomotion, space heating, etc.), types of actor (households, firms, governments), types of decisions determining the activity (design, purchase, utilization of equipment), or in other ways. Different methods may prove useful for different purposes. Table 3-4 illustrates one way to allocate the carbon dioxide emitted from fossil fuel consumption to the major purposes (end uses) of those fuels.

The tree structure can be elaborated further by dividing the subsidiary proximate causes defined at the previous level into their components. Such analysis is important for high-impact activities.

TABLE 3-1 Estimated Human Contributions Per Decade to Global Warming of Major Greenhouse Gases During Three Time Periods, in Watts per square meter (percentage in parentheses)

These estimates are of "radiative forcing" by greenhouse gases, that is, the change they produce in the earth's radiative balance that in turn changes global temperature and climate. Radiative forcing is calculated from current gas concentrations in the atmosphere, which include gases remaining in the atmosphere from all emissions since the beginning of the industrial era, set here at 1765. It is not identical to the "global warming potential" of gases emitted by human activity, a property that integrates the effects of gas emissions over future time. Global warming potential is affected by the different atmospheric lifetimes of greenhouse gases before breakdown, so that the relative importance of gases for global warming depends on the future date to which effects are estimated. In addition, chemical reactions in the atmosphere convert some radiationally inactive compounds into greenhouse gases over time. The estimation of the global warming potential of currently emitted gases is quite uncertain due to incomplete knowledge of the relevant atmospheric chemistry. An early estimate of the 100-year global warming potential of gas emissions in 1990 allocates it as follows: CO2, 61% CH4, 15% CFCs, 12% N2O, 4% other gases (NOx, nonmethane hydrocarbons, carbon monoxide), 8% (Shine et al., 1990). Although these estimates differ from the radiative forcing estimates in the table, the differences are not great in terms of the relative importance of the gases for the global warming phenomenon. Our analysis uses the estimates of radiative forcing because they are far less uncertain.

a Source: Shine et al. (1990:Table 2.6).

b Source: Shine et al. (1990:Table 2.7), assuming a "business-as-usual" scenario with a coal-intensive energy supply, continued deforestation and associated emissions, and partial control of CO and CFC emissions.

Uncertainties for the future projections are very large. Total effects of greenhouse gases projected for 2025-2050 varied by a factor of 5 from the "accelerated policies" scenario, which projected the lowest level of emissions, to the "business-as-usual" scenario, which projected the highest.

c Stratospheric water vapor is believed to increase as an indirect effect of CH4 emissions.

TABLE 3-2 Global Emissions of CO2, CH4, and N2O From Human Activities in the Late 1980s

CO2 emissions (Mt carbon per year)

CH4 emissions (Mt CH4 per year)

Function of acreage and cropping intensity

N2O emissions (Mt N2O per year) a

Increased cultivation of land

Fuel wood and industrial biomass

Note: Mt = million metric tons

a Estimates of N2O emissions are highly uncertain. For example, Watson et al. (1990) give a range of 0.01-2.2 for fertilization. In addition, N2O releases from unknown sources are probably larger than all anthropogenic releases. It is not clear how much of the unaccounted releases is anthropogenic.

Sources: For CO2 and CH4, Watson et al. (1990) for N2O, National Academy of Sciences (1991a).

For instance, automobile fuel consumption can be analyzed as the product of number of automobiles, average fuel efficiency of automobiles, and miles driven per automobile the determinants of each of these factors can be studied separately. Researchers might then investigate the social factors that affect change in the number of automobiles and their typical life span, such as household income, household size, number employed per household, and availability of public transportation. More detailed analysis can be carried out until it no longer would provide information of high enough impact to meet some preset criterion. Again, there are many ways to ana-

lyze an activity such as automobile fuel consumption, and the most useful approach is not obvious a priori.

The task of making such accounts, even for a single tree, is enormous. The work can be eased by using the impact criterion: analysts might reasonably choose to move from trunk to limb to branch to twig only until the contribution falls below a preset level of impact for the time period of concern. Data collection and substantive analysis of the thinnest twiglets can be deferred. Table 3-5 presents a composite of the accounts of individual green

TABLE 3-3 Anthropogenic Sources of Atmospherically Important Halocarbons in the Late 1980s

Aerosols, refrigeration, foams

Aerosols, refrigeration, foams

Cleaning electronic components

Note: Production estimates are from Watson et al. (1990), except for CH3CCl3, which comes from World Meteorological Organization (1985). Projections of future production are very sensitive to changes in economic growth, and relatively quick substitution is possible when alternative chemicals become available. CFC 22 production doubled between 1977 and 1984 (e.g., fast-food packaging), as did CFC 113 production (electronics industry).

a Numbers represent the integrated effects over 100 years of release of one unit mass of the compound, relative to CO2. Integration over other time horizons would change the relative potentials because of differing atmospheric residence times. Source: Shine et al. (1990:Table 2.8).

b Percentage of 100-year effects of all 1990 halocarbon emissions. Source: Shine et al. (1990:Table 2-9).

c Projected atmospheric effects depend not only on total production but also on the balance between end uses. When CFC 11 and CFC 12 production shifted from aerosols to other applications after 1976, the result was a longer lag time from production to entry into the atmosphere.

TABLE 3-4 Disaggregation of Carbon Dioxide Emissions by Economic Sector and End Use (percentages, United States, 1987)

Steam power, motors, appliances

Personal transportation (automobiles, light trucks)

Freight transport (heavy truck, rail, ship, other)

Heating for industrial processes

Note: U.S. data are unrepresenative of world energy use in various ways. However, the United States is responsible for approximately 20 percent of global CO2 emissions.

a 2 percent in the single category of heating, ventilating, air conditioning, and lighting was allocated one percent each to heating and lighting.

Source: U.S. Office of Technology Assessment, 1991.

TABLE 3-5 Estimated Composite Relative Contributions of Human Activities to Greenhouse Warming

Gases (Relative Contribution in percent)

Source: Compiled from Tables 3-1, 3-2, and 3-3. For interpretation of the data, see the note at Table 3-1.

sonal transportation) than for explaining the choice or operation of water heating systems for buildings. For a policy-oriented analysis based on such an approach, see National Academy of Sciences, 1991b.

Accountings such as the one represented in Figure 3-1 can help guide the research agenda for the human causes of global change. They are critically dependent, however, on analyses from the natural sciences to sketch the trunk and major limbs, that is, to identify the most important environmental effects of human action and the technologies that produce those effects. Natural science can help social science by providing an improved picture of the trunk and limbs, and particularly by improving estimates of the uncertainties of their sizes. The uncertainties of some components are quite large (see, for instance, Table 3-2. estimating the relative contributions of different human activities to methane releases), and attention should be paid to whether, in the full account, these uncertainties compound or cancel each other. Research that estimates the relative impacts of proximate human causes of global change on particular environmental changes of concern, specifying the uncertainty of the estimates, is essential for understanding the human dimensions of global change.

As tree diagrams move from the trunk out toward the branches and twigs, analysis depends more on social science. For each important environmental change, there are several possible accounting trees, each consistent with the data but highlighting different aspects of the human contribution. Social science knowledge is needed to choose accounting procedures to suit specific analytic purposes. Whatever accounting system is used, social scientists conducting research on the human causes of global change should focus their attention on factors that are significant contributors to an important global environmental change.


Because many different tree diagrams may be consistent with the same data, tree diagrams must be treated as having only heuristic, not explanatory, value. They are useful but not definitive accounts. A more serious limitation of tree-structured accounts is that they do not by themselves illuminate the driving forces behind the proximal causes of global change. Social forces that have only indirect effects on the global environment, and that may therefore be omitted from tree accounts, can have at least as

much impact as the direct effects. Consider, for instance, the rate of female labor force participation, which affects energy use in many different ways. With an increase in the proportion of women in the labor force, there tend to be more automobiles and miles driven per household, increased travel by plane, and, because of the associated decrease in household size, increased per capita demand for residential space conditioning and household appliances (see Schipper et al., 1989). Because these factors appear in different branches of Figure 3-1, the figure is not useful for representing the effect of female labor force participation on energy demand. The broader social process&mdashthe changing role of women in many societies&mdashhas even wider effects on energy use, but is still harder to capture in the figure. Despite these limitations, the accounting tree is useful as a preliminary check on the likely impact of a major social variable. When such a variable has a high impact, it is worth considering for inclusion in models of the relevant proximal causes of global change.

Tree-structured accounting is also limited in that it can evaluate human activities against only some criteria of importance (such as high and widespread impact), but not others (such as irreversibility). Consideration of criteria of importance other than current impact may require detailed empirical analyses of factors that look small in an accounting of current human causes of environmental change. An example, elaborated in the next section, concerns future CO2 emissions from China. If per capita income grows rapidly there, Chinese emissions may increase enough to become tremendously important on a world scale. To make projections, it would be very useful to have detailed studies of the effects on emissions of increased income in other countries that have undergone recent spurts of economic growth, such as Taiwan and South Korea, even though these countries have no major impact on the global carbon dioxide balance.


As we have shown, all human activity potentially contributes, directly or indirectly, to the proximate causes of global change. This section presents three rather detailed cases of human action with high impact on important global environmental changes to explore what lies behind the proximate causes. Taken together, the cases illustrate human causes that operate through both industrial and land-use activities and in both developing and devel-

oped countries. They illustrate how multiple driving forces interact to determine the proximate human causes of global change and why systematic social analysis is necessary for understanding how human actions cause it. In the section that follows, we discuss the interrelationships among the driving forces at a more theoretical level.


In 1985, the head of the British Antarctic Survey, Joseph Farman, reported that his team had discovered a heretofore unobserved atmospheric phenomenon: a sudden springtime thinning of the ozone layer over Antarctica, allowing ultraviolet radiation to reach the ground much more intensely than was ordinarily the case (Farman et al., 1985). Subsequent scientific investigations soon led to what is now the most widely accepted explanation of what was happening. Chlorine compounds derived mostly from chlorinated fluorocarbon gases (CFCs), mass-produced by industrial societies for a variety of purposes, reacted in the stratospheric clouds over Antarctica during the cold, dark, winter months to produce forms of chlorine that rapidly deplete stratospheric ozone when the first rays of the Antarctic spring sunlight arrive (Solomon, 1990). Massive destruction of ozone followed very quickly, until natural circulation patterns replenished the supply and closed what came to be known as ''the ozone hole.'' Human activities in distant areas of the planet had brought a sudden and potentially devastating change to the Antarctic and its ecosystems, a change that did not bode well for the ozone layer in other parts of the planet (Stolarski, 1988).

To understand this event and the political controversies that followed in its wake, one has to reach back through almost a century's worth of history, long before CFCs existed. Until almost the end of the nineteenth century, refrigeration was a limited technology, based almost entirely on natural sources of supply. Urban Americans who could afford to drink chilled beverages relied on metropolitan ice markets, which cut ice from local ponds in the winter and stored it in warehouses for use during the warm months of the year. Breweries and restaurants were the heaviest users of this stored winter ice, which was sometimes shipped hundreds of miles to provide refrigeration. Boston ice merchants, for instance, were regularly delivering ice to consumers in Charleston, South Carolina, and even the Caribbean by the fourth decade of the nineteenth century (Hall, 1888 Cummings, 1949 Lawrence, 1965).

Given the expense and difficulty of obtaining this stored winter ice, food preservation was accomplished largely with chemical additives, the most common being ordinary table salt: sodium chloride. In the United States, pork was the most popular form of preserved meat because of the ease with which its decay could be arrested by salt. Beef was much less popular in preserved form, so those who ate it preferred to purchase it freshly slaughtered from local butchers. Then, in the 1870s, meatpackers began experimenting with ice-refrigerated railroad cars that could deliver dressed beef, slaughtered and chilled in Chicago, to consumers hundreds of miles away. Dressed beef, which was cheaper than fresh beef for a variety of reasons, soon took the country by storm, driving many wholesale butchers out of business and giving the Chicago packing companies immense economic power. The packers initially relied on complicated ice storage and delivery networks, cutting and storing millions of tons of winter ice along the railroad routes that delivered beef from Chicago to urban customers throughout the East. Their investment in ice storage technology contributed to dramatic shifts in the American food supply and was soon affecting foods other than meat. Fruits and vegetables from California and Florida and dairy products from metropolitan hinterlands throughout the East, were among the most important to benefit from the new ice delivery system (Cronon, 1991 Yeager, 1981 Kujovich, 1970 Giedion, 1948 Clemen, 1923 Swift and Van Vlissingen, 1927 Neyhart, 1952 Unfer, 1951 Fowler, 1952).

But natural ice was unreliable: two warm winters in 1888-1889 and 1889-1890 brought partial failures of the ice crop that encouraged the packers to turn to a more reliable form of refrigeration. Although the principle of mechanical refrigeration, in which compressed gas was made to expand rapidly and so lower temperatures, had been known since the middle of the eighteenth century, its first application on a large commercial scale was not found until the second half of the nineteenth century (Anderson, 1953). Urban brewers, especially in the warm climates of the South, were the first to make wide use of it. As the meatpackers sought to solve their problems with erratic winter ice supply, they too adopted mechanical refrigeration on a large scale after 1890. By the first quarter of the twentieth century, the delivery of perishable foods throughout the United States&mdashand international food shipments as well&mdashhad come to depend on mechanical refrigeration. By drastically lowering the rate at which food decayed and hence making perishable crops available to consum-

ers through much of the year, refrigeration changed the whole nature of the American diet.

The most widespread early refrigeration technology depended on compressed ammonia gas, which easily produced desired drops in temperature for effective food storage. But ammonia (like other refrigerant gases such as sulfur dioxide and methyl chloride) had serious problems. For maximum efficiency, it had to attain high pressures before being released, which increased the likelihood that the compression equipment might fail. Accidental explosions were frequent, and the toxic nature of the gas caused a number of fatalities. Toxicity and the need for large expensive compressors kept mechanical refrigeration from making headway with retail customers, who represented an immense potential demand. That is why Thomas Midgely Jr.'s 1931 invention of Freon 12 represented a revolution for the refrigeration industry. Midgely, working at the request of the General Motors Frigidaire division, developed the new chlorinated fluorocarbon as the perfect alternative to all other refrigerant gases then on the market.

Nonflammable, nonexplosive, noncorrosive, and nontoxic, the various forms of Freon gas seemed the perfect technical solution to a host of environmental and safety problems. They also required less pressure to produce the desired cooling effect, so compressors could be smaller and less expensive. Freon soon came to dominate the market for refrigeration and opened up new retail markets because of its diminished capital requirements. Previously, consumers had bought their refrigerated food at the store just before eating it, since efficient and reliable household refrigeration was not generally available. Now American households could own their own refrigerators, making it possible for the food industry to shift much of its marketing apparatus toward selling chilled food in retail-sized packages. Frozen foods burst onto the American marketplace in the 1950s, as did fresh vegetables, dairy products, and other foods that are today accepted as ordinary parts of the national diet. Although European countries were slower to adopt these technologies, they too eventually followed suit.

No less importantly, the nontoxicity of Freon made it possible for refrigeration technology to be applied to the ambient cooling of buildings, so that air conditioning came to be an ever more important market for the gas. Air conditioning had been used in specialized industrial applications ever since Willis H. Carrier's use of the technique for a climate-controlled lithography plant in 1902. The introduction of Freon meant that air conditioning suddenly became much cheaper and safer in a way that allowed it to

be applied to office buildings and finally to residences as well. Air conditioning played a key role in the years following World War II in promoting urban growth in the region known as the Sun Belt, as well as in tropical areas around the globe. From Florida to Texas to southern California, the massive influx of new residents depended in no small measure on the ability of buildings to protect their occupants from summer heat. Air conditioning became a fact of life in such places, so much so that it is hard to imagine urban life in the Sun Belt without it. Its significance can be captured by two phenomena of striking environmental significance: the shift in the seasonal consumption of electricity from peak load during the winter months (when energy consumption for lighting and space heating had always traditionally been at its highest) to peak load during the summer and the steeply upward slope in the production and consumption of chlorinated fluorocarbons. The upward trend in CFC production was also aided by the development of still other uses for CFCs: as nontoxic propellants in aerosol sprays and later, in the 1960s and 1970s, as solvents in the manufacture of integrated circuits.

CFCs are very stable gases: that is in fact one of the properties that made them seem so benign when measured by their toxicity and immediate environmental effects. But the very stability that made CFCs so attractive for so many applications proved finally to be their greatest hazard. Once released into the environment&mdashand the proliferation of refrigerators, freezers, and air conditioners meant that Freon escaped at an ever increasing rate&mdashCFCs began to permeate the atmosphere, eventually reaching its upper regions. There they encountered the ozone layer, the thin belt of unstable tripartite oxygen molecules that filters out much of the sun's ultraviolet radiation and protects living organisms on the surface of the planet from the effects of that radiation. In the presence of sunlight, CFC molecules became chemical agents capable of destroying many times their number of ozone molecules. This effect was first hypothesized in 1974 by the chemists Mario Molina and Sherwood Rowland of the University of California at Irvine, writing in the wake of the controversy over supersonic transport aircraft and with recent knowledge, developed through new detection technology, that CFCs were present in the atmosphere (Molina and Rowland, 1974). Their hypothesis was controversial but convincing enough to produce action by the United States and eight other countries to ban the use of CFCs in aerosol sprays in the late 1970s (unquestionably the most marginal of their uses). Significantly, the suggestion that CFCs might possibly be damaging

to the ozone layer did not have much effect on uses that were much more central to the industrial economy: food refrigeration, ambient air conditioning, and electronic manufacturing solvents. (The knowledge that CFCs account for a significant proportion of the human contribution to the greenhouse effect&mdashabout 25 percent by the mid-1980s&mdashalso did not have much effect.)

Not much effect, that is, until 1985 and the discovery of the ozone hole over Antarctica. Within two years' time, the scientific community agreed that CFCs were the most likely culprit officials at DuPont, which produced 25 percent of the world's CFCs, declared the company's intent to phase out CFC production over the next decade and a half and an international protocol was signed at Montreal, in which signatory countries declared their intention to cut CFC production and consumption in half by the end of the century (Benedick, 1989a, b U.S. Office of Technology Assessment, 1988 Haas, 1989).

The lessons of this story about CFCs and the ozone hole are several. On the positive side, the rapid response of the scientific, industrial, and policy-making communities to the discovery of the ozone hole over Antarctica is reassuring proof that international agreements in response to global change are in fact possible. That the Montreal Protocol and the later, even stronger London amendments to it could be signed even in the absence of environmentally benign alternatives to the CFCs suggests people's perception of how serious and urgent the problem had become, but also their faith&mdashencouraged by DuPont's actions&mdashthat alternatives would in fact be available by the time the agreement's deadline fell due. Indeed, the Montreal Protocol is a paradigmatic case of a quick technical fix, in which people respond to the environmental problems of a particular substance by finding (or hoping to find) a technology that can be used for exactly the same purposes without requiring any fundamental change in human economies or societies.

And that suggests some of the less reassuring lessons of this story. Refrigeration and air conditioning have today become so embedded in the American way of life, and in the ways of life of many people the world over, that it is hard to imagine modern food supplies and urban life styles without them. The very form of the post-World War II city, with its tall office buildings, fixed windows, and energy-intensive controlled climate systems, presumes a significant commitment to refrigeration and cooling. Almost no one has responded to the ozone hole by suggesting a retreat from these fundamental technologies of modern life: al-

most everyone assumes that existing technologies can be sustained more or less unaltered by introducing some other gas as an alternative to Thomas Midgley's 1931 invention. A quick technical fix may well be all that is needed, in which case the refrigeration-intensive (and energy-intensive and greenhouse gas-intensive) food and architectural systems of the twentieth century First World will continue to proliferate around the planet, with countries of the tropics presumably adopting them with even greater reason and greater intensity than those living in temperate regions.

Of course, such a chain of events might well accelerate global climate change. The invention of CFCs started a process that led to building practices and patterns of human settlement with two unexpected and long-term effects on the global environment: a built-in demand for CFCs and a built-in demand for energy, not only for space cooling but also for transportation to and between the new dispersed, warm-climate population centers. A quick fix for the effects of CFCs on the ozone layer might encourage the spread of the American pattern of energy-intensive settlement. A possible result is more rapid growth of greenhouse gas emissions than would otherwise be the case.

The encouraging policy success at Montreal in 1987 was dramatic, but may have depended on special circumstances: there were only about two dozen CFC producers worldwide, and reductions threatened few of the existing infrastructures that had developed over the previous century and a half. For that reason, the signing of the Montreal Protocol is a risky predictor of how other international negotiations may turn out when the response to global change seems to require greater alterations in historical practice, when there are many millions of responsible actors, or when the costs and benefits of change are less evenly distributed around the planet.

There is one final lesson of the CFC story that is most ironic of all. We would do well to remember that chlorinated fluorocarbons were themselves a response to serious environmental problems. They reduced the occupational hazard of compressor explosions, they all But ended toxic pollution (and deaths) from refrigerant gases, and they dramatically increased the variety and safety of the human food supply. For 50 years, they seemed a perfect example of a benign technical solution to environmental and engineering problems, with no negative side effects of any kind. We now understand that the very quality that made them seem so safe&mdashtheir stability&mdashmeans that they will continue to destroy ozone molecules far into the future even if we were to end their production and use at this instant.

The history of CFCs demonstrates, above all else, that human activities can have quite unexpected long-term effects on the environment. CFCs, initially developed to support a limited set of end uses in the refrigeration industry, have changed not only that industry but also significant aspects of human civilization. As a result, they have made major contributions both to stratospheric ozone depletion and to global climate change. Moreover, because CFCs have contributed to social changes that are built into national building stocks, transportation systems, and even political structures (congressional representation from the Sun Belt, for example), the indirect effects of CFCs on climate change may be very difficult to reverse, even if substitutes are found that do not harm the ozone layer. Dependence on refrigeration has created social pressures to resolve the ozone problem by technical means, a strategy that could have paradoxical results: the solution to the ozone problem could accelerate social processes that cause climate change. The CFC story demonstrates the tremendous difficulty of understanding the environmental effects of technological change. It suggests that connections need to be traced through a greater variety of technological and social systems and over longer periods of time than usually covered in social scientific studies. We return to these difficult long-term scientific challenges in Chapter 5. The CFC story also drives home the point that we cannot anticipate all the environmental or social effects of our own activities, suggesting that the best policies are those designed with considerable robustness to unintended consequences.


Fossil fuel consumption accounts for over half the human contribution to the greenhouse effect, chiefly through the emission of carbon dioxide. Although the People's Republic of China is only the world's third-largest producer of carbon dioxide (after the United States and the Soviet Union), it is increasing its rate of production faster than any other country (750 million metric tons more in 1988 than in 1980&mdashNational Academy of Sciences, 1991a). Three-quarters of the Chinese emissions come from burning coal. The rapid increase in Chinese coal consumption&mdashfrom 62 million tons (Mt) in 1952 to 812 Mt in 1985&mdashcan be traced to industrialization, electrification, and population growth (Xi et al., 1989). The trend seems likely to continue over the next several decades because China is in an energy-dependent phase of development and has few alternatives to coal. China has the world's third largest

coal reserves, after the Soviet Union and the United States, but is very limited in reserves of other fossil fuels (Xi et al., 1989) and lacks the capital for major investments in nuclear power or development of its large, but inconveniently located, hydroelectric potential.

Causes of Present Coal Burning

A simple way to analyze energy use in China is to use the accounting equation:

where E is energy consumption, P is population, and GNP is gross national product. Thus, energy use is the product of population, per capita economic output, and energy intensity&mdashthat is, energy use per unit of output. Chinese energy use in 1987 was 435 percent of what it was in 1965, while population was 147 percent, GNP per capita 305 percent, and GNP 97 percent of 1965 levels:

(data from World Bank, 1989:Tables 1 and 5). This analysis suggests that roughly two-thirds of the rapid increase in Chinese energy use was a result of economic development, and the rest was due to population increase. But a closer look at the relationship of energy use and GNP gives a different picture&mdashone that puts much more emphasis on technology and its social control.

China's energy use is a story not only of economic development, but also of persistently intensive energy use. China's economy is far more energy-intensive than that of most other countries or, put another way, China gets much less economic output from each unit of energy. Tables 3-6 and 3-7 show that China's economy may be the most energy-intensive in the world. In terms of CO2 emissions per unit of economic output, China is by far the world leader (National Academy of Sciences, 1991a).

The few available analyses of energy use in China suggest that its energy intensity has two main sources: industrialization and inefficiency. Industry is more energy-intensive than other productive sectors, and China devotes a greater proportion of its recorded energy consumption to industry and is more dependent on coal in that sector, than most other countries (see Table 3-8). This pattern may be traceable to a Stalinist development policy that

favors heavy industry on ideological grounds. The government, which determines production by directive rather than allowing it to respond to demand, is said to continue to command steel production, despite huge surpluses (Smil, 1988, and personal communication).

TABLE 3-6 Energy Intensities in Selected Countries and Groups of Countries, 1987

40 other low-income countries

53 middle-income countries

a Kilograms of oil equivalent per U.S. dollar of GNP.

b U.S. dollars of GNP per barrel of oil equivalent (1 barrel = 137.2 kg).

Source: Calculated from data in World Bank (1989).

TABLE 3-7 The Most Energy-intensive Economies in the World, 1987

Yemen, People's Democ. Republic

Note: Complete data not available for Afghanistan, Albania, Angola, Bhutan, Bulgaria, Burkina Faso, Burma, Chad, Cuba, Czechoslovakia, German Democratic Republic, Guinea, Iran, Iraq, Ivory Coast, Kampuchea, Korea (Democratic People's Republic), Mongolia, Namibia, Romania, U.S.S.R., Vietnam, and countries with less than 1 million population.

a Kilograms of oil equivalent per U.S. dollar of GNP.

b U.S. dollars of GNP per barrel of oil equivalent (1 barrel = 137.2 kg).

Source: Calculated from data in World Bank (1989).

The main reason for energy intensity, however, appears to be an inefficiency that has several contributing causes:

Inefficient End Uses Coal burning in China is typically done in small, old units owned by households or small enterprises&mdashcharacteristics that spell inefficiency. In the United States, 85 percent of coal is burned to generate electric power, at an average efficiency of 36 percent. By contrast, 22 percent of Chinese coal is converted to electric power, with an overall efficiency of only 29-31 percent (Kinzelbach, 1989 Xi et al., 1989). The bulk of Chinese coal is burned at still lower efficiencies, in industry (46 percent of 1985 coal use) and for commercial and residential heating (26 percent). Residential coal stoves often have only 10-18 percent efficiency (Xi et al., 1989). Adoption of more efficient furnaces and replacement of coal-fired space heating with combined heat and power installations proceed slowly for lack of capital.

Price Structure Policy sets coal prices for the state-owned mines artificially low, below the cost of production. Although the industry operates at a loss (Xi et al., 1989), the government is said to be reluctant to raise prices for fear of inflation and urban unrest. Many analysts see price as the key source of continuing

TABLE 3-8 Percentage of Commercial Energy Used for Industrial Purposes and Percentage of Industrial Energy Supplied by Coal in Selected Countries and Groups of Countries

Direct Coal Use (% of Industrial Energy)

a Bulgaria, Czechoslovakia, German Democratic Republic, Hungary, Poland, and Romania.

Source: Calculated from World Resources Institute and International Institute for Environment and Development (1988:Table 7.4).

inefficiency, in that efforts to improve efficiency in either mining or consumption look uneconomic with current prices.

The Command Economy The practice of government-dictated production, combined with the price structure, allows highly inefficient enterprises to continue operating despite financial losses. Enterprises that could compete by using energy more efficiently do not have incentives to do so. Moreover, the system of production quotas encourages the shipment of uncleaned, unsorted coal with an energy value of 30 percent less than actual tonnage (Smil, 1988). Such coal fulfills quotas easily, inflating production statistics by over 100 Mt per year, but it strains the Chinese railroads, 40 percent of whose cars are devoted to moving coal wastes fuel in transport and results in substantial emissions of unburned particulates when the coal is used.

Table 3-7 offers a rough guide to the amount of inefficiency a command economy can produce. Although data are available only for a few such economies, among these are four of the five least energy-productive economies in the world. The other large-population, low-income countries of the world, India, Indonesia, Nigeria, Bangladesh, and Pakistan, get 2.5 to 6 times as much production as China out of each unit of energy they use (data from World Bank, 1989). Although China cannot be expected to increase its energy productivity 2.5 times to India's level&mdashthe ample availability of low-cost coal in China gives it less incentive to economize on energy&mdashit seems to have room for huge improvements in efficiency.

Determinants of Future Coal Burning

The future of global climate change depends very much on how energy-intensive future Chinese development will be. Between 1965 and 1987, Chinese coal use&mdashand CO2 emissions&mdashincreased at the same rate as total economic output. If both continue to increase at the recent historic rate of 4 percent per year, the Chinese contribution to global CO2 emissions will quadruple in less than 40 years and surpass that of the United States, presuming that the latter also follows recent trends. However, if future economic growth can be less energy-dependent than past growth has been, the picture would be quite different (data from World Bank, 1989 Fulkerson et al., 1989).

What determines whether economic growth will or will not increase CO2 emissions? Historical data show that successful

economic development in Western countries has been marked by a period of rapid industrialization and consumption highly dependent on increased use of energy from fossil fuels, followed by a period in which economic growth becomes less energy intensive. Economic growth can proceed with decreasing energy intensity because of shifts of production from industrial to service sectors and adoption of more energy-efficient processes and technologies (World Resources Institute and International Institute for Environment and Development, 1988:114) energy use per capita, however, has continued to increase in these countries (World Bank, 1989:173).

The future of China's energy use can be analyzed in the terms of the accounting equation: population growth, economic development, and changes in energy intensity or productivity. A fourth factor&mdashshifts from fossil fuels to other energy sources&mdashis unlikely to have much influence in China for several decades unless there is a major international effort to promote such shifts.

Population growth, barring wars, epidemics, and the like, is easier than the other variables to forecast, because it is driven mainly by the current age distribution and slowly changing fertility trends. Beyond a few decades, though, uncertainties increase: desired family size may change, as may population policy, which has recently been holding family sizes below the levels parents seem to prefer. Forecasts for the year 2025 give a range of 1.4 to 1.6 billion for China's population (29 to 51 percent above the 1985 level) (United Nations, 1989).

Economic growth and energy intensity are closely interrelated and very difficult to forecast. The Chinese national growth plan calls for quadrupling GNP from 1980 to 2000, but for coal use to increase only 2.3 times (Smil, 1988 Xi et al., 1989). These forecasts call for the elasticity of energy/GNP to decrease from the 0.97 of the 1965-1987 period to about 0.6, a change that would save more fossil fuel in 2000 than China used in 1985, if the quadrupling of GNP is achieved.

There are tremendous uncertainties in predicting whether these goals can be met, even though Chinese energy use is certainly inefficient enough to allow this much technological improvement. Some observers believe the goals can be met only after continued economic liberalization, including price reform and market incentives, and political reforms that would overhaul wasteful management practices and attract needed foreign technology, expertise, and capital (e.g., Smil, 1988). The probability of such policy changes is notoriously uncertain, as the political events of 1989 in China

and Eastern Europe attest. And given the current level of knowledge about the functioning of command economies, even if policy changes were known in advance, the success of their implementation, and therefore their precise effects on energy productivity, would be hard to predict.

No one knows how the Chinese will use the fruits of future economic development. If they make major investments in energy productivity&mdashfor instance, modernizing the coal industry, using electricity to replace inefficient coal burning, and developing the service sector of the economy&mdashmuch can be done to mitigate CO2 emissions. But other directions of investment, focusing on new manufacturing and expanded energy services such as refrigeration and personal transportation, would be much more energy intensive. If China makes a major shift toward market incentives, the decentralization of choice will promote efficiency in production, but it might also encourage energy-intensive consumption, as individuals gain disposable income. The net effect on energy intensity is still unknown.

Another important unknown is whether government policies will emphasize energy efficiency and the global environment. China already has policies to reduce coal use, but not in order to improve energy efficiency. The priorities are urban air pollution, freeing rail cars for noncoal cargo, and reduction of sulfur oxide emissions. These priorities encourage some energy-productive investments, such as combined heat and power plants that capture waste heat to warm buildings. But other important energy-productive investments do not fit these priorities. The future thrust of Chinese environmental policy depends, of course, on politics. Current environmental policies have been set from the top down, influenced by the exposure of traveling Chinese officials to the environmental concerns of foreign scientists, international organizations, and investors (Ross, 1987). If China turns inward to resist democratization, global concerns about energy efficiency may not influence Chinese policy for a long time. If environmental politics in China decentralizes and democratizes, an opening will appear for local environmental movements, which have been prevented from forming horizontal linkages in the past (Ross, 1987). Freedom for Chinese environmentalists, however, might lead to pressure for local changes, rather than for policies that improve energy efficiency nationally.

In sum, the Chinese contribution to global climate change depends on the interactions of technology with social factors, including population growth, economic development, policy, and

ideology. Scientists know much about the technical changes that could mitigate China's greenhouse gas emissions, but they have relatively little quantitative understanding of the social factors that make possible, and interact with, technological change. Enough is known to identify some of the critical determinants of Chinese energy intensity, but not to quantify their effects or specify their interactions. That will require further research. For example, critical changes in policy, such as increased emphasis on market incentives and decentralized decision making, might greatly improve energy productivity. Studies of transitions to increased market control in other command economies might provide valuable knowledge for projecting the likely effects of such policy changes on energy efficiency in China. The future of Chinese energy demand also depends on changes in the structure of the Chinese economy and of consumer demand. Careful comparative studies of the social determinants of energy intensity and changes in energy intensity at the level of nation-states are critical for understanding and projecting China's future contribution to the greenhouse effect.


Clearing of tropical forests is generally considered to be the most important single cause of recent losses in the earth's biological diversity. It also accounts for about 15 percent of the effect of human greenhouse gas emissions. Clearing has been very extensive in recent years, and the disturbances are not readily reversible, as deforestation by indigenous slash-and-burn techniques had previously been (Conklin, 1954 Nye and Greenland, 1966 Sanchez et al., 1982). The damage is now so extensive and severe as to preclude regeneration to original cover without special measures that are only now being developed (Uhl et al., 1989).

The most widely used definition of biological diversity includes three levels: genetic, species, and ecosystem diversity (Norse et al., 1986 U.S. Office of Technology Assessment, 1987). Deforestation reduces diversity at all three levels. Genetic diversity, or the diversity of genes within a species, provides the raw material for evolution, as it allows some individuals of a species to survive environmental changes that prevent other individuals from living or reproducing. Species diversity, which refers to the many million species now estimated to exist on earth, is richest in the tropical forests, particularly in the Amazon Basin (Erwin, 1988). Many of the Amazonian species are closely tied to particular forest ecosystems and tree species, so that they are very narrowly

distributed and especially vulnerable to extinction by regional or even local forest clearing. Ecosystem diversity, that is, the existence of distinctive communities of species in different physical situations based on factors such as soil types or height above the river channel (Prance, 1979), is also great in the Amazon Basin, even between physical situations that look identical to the untrained eye.

Amazonian deforestation threatens these forms of biological diversity in many ways. Elimination of forests destroys the habitat of many species that are closely tied to particular trees or ecosystem types. Species whose habitats are not totally destroyed may become extinct when an insufficient number are left in the remaining habitat, or remaining patches do not contain the resources they need (such as nest sites or food from a particular tree necessary to sustain the species). Species may be eliminated because of ecosystem simplification, as when removal of a single species eliminates the many species dependent for their existence on the local population of that species, 1 or when cutting eliminates the cool, moist, windless microclimates of the forest interior that many species require. Diversity is vulnerable to drying of the regional climate, because evapotranspiration from the forest generates about half the rainfall in the Amazon Basin (Salati and Vose, 1984). 2 Deforestation can damage biological diversity by contributing to both global and regional climate change, especially if the result is a drier climate in the Amazon Basin. Road building, in addition to destroying the forest, increases access to it and facilitates further deforestation. Deforestation favors species that occur only in highly disturbed areas, such as weeds, mosquitoes, and cattle, and that spread disease, compete with native organisms, and change the soil structure (Denevan, 1981). Finally, much deforestation is a by-product of industries such as mining, which not only destroys the forest at the industrial site, but may also use large numbers of trees for fuel.

Deforestation reduces biological diversity in several ways. In general, the species hardest hit are likely to be the ones with large area requirements, narrow ranges, or value to humans for food, medicine, or timber, yet the entire taxonomic spectrum may suffer major losses. 3 Some threatened species may be important to the region's economy and culture, some are used far beyond the Amazon Basin, and some have potential value to humans that is not yet known. The threatened ecosystems provide regionally important services, such as creating soils, moderating temperatures, reducing soil erosion, cleaning the air and water, and preventing flood and drought (Smith, 1982). The net effect on hu-

mans is impossible to estimate in advance but, whatever its size, it is likely to be irreversible.

Causes of Deforestation

Amazonian forest land is cleared for many purposes. Logging is a major industry, with four of the six states in the Brazilian Amazon Basin depending on wood products for more than 25 percent of their industrial output (Browder, 1988). Other industries destroy forest both as an integral part of the manufacturing process and as a by-product. For example, 610,000 hectares (ha) of forest per year are used to produce charcoal for iron smelting in the Gran Carajas region of Brazil (Treece, 1989). Damming rivers to generate electricity for aluminum refining and for urban power inundates huge areas because of the low relief of the land. But the largest single source of Amazonian deforestation and the focus of this discussion is cattle raising, which now covers an estimated 72 percent of the cleared area (Browder, 1988).

The transformation of forest into inferior, rapidly degrading pasture was not inevitable. It was strongly influenced by national policies and supported by international development agencies, which encouraged migration and land clearing through land-titling arrangements, provided a publicly financed infrastructure of roads, and established credit and tax incentives to benefit ranching. Given these institutional conditions and the presence of abundant, accessible, and relatively cheap land in the Amazon, individual actors made rational economic choices that furthered their own best interests and helped create a system with its own economic and social momentum that continues deforestation even after state incentives have been removed.

Road Building With support from the World Bank, the Interamerican Development Bank, and other international lending institutions, the Brazilian government improved and paved major north-south (Belem-Brasilia) and east-west (Cuiaba-Porto Velho) highways, hoping to tap the wealth of the Amazon, make minerals and timber accessible, and promote agricultural enterprises (Fearnside, 1989). If, as the planners intended, settlers had migrated from the poor, drought-stricken northeast to settle along the trans-Amazon highway, they might have developed the area intensively, with permanent, smallholder farming and agroforestry, and limited deforestation. But mass migration did not occur in the northeast, and much of the area was abandoned to pasture

(Browder, 1988 Moran, 1976, 1990). The opening of new lands and the relative absence of people favored extensive development, such as ranching, over intensive development.

Land Tenure Rights For centuries, it has been the legal practice to grant rights of possession to whoever deforests a piece of Brazilian land. Rights of ownership soon follow (Fearnside, 1989). Squatters on public land can gain the rights to 100 ha by living on it and using it, but 100 ha is not sufficient for ranching. Ranchers often buy up the lots of failed farmers, and in 1974 it became possible for a company to acquire a tract of up to 66,000 ha (Smith, 1982). Large individual and corporate ranchers can build their own access roads and lay claim to extensive plots far from major highways. By the time roads are constructed, most state land in the Amazon is already claimed (Binswanger, 1989). Brazilian land laws encourage both extensive holdings and extensive use. For instance, the 1988 constitution provides that land ''in effective use,'' that is, cleared, cannot be expropriated for the purpose of agrarian reform (Hecht, 1989b).

Speculation Land holding has been a useful hedge against Brazil's galloping inflation and an excellent speculative investment. Mahar reports that a farm laborer can "net the equivalent of $9,000 from clearing 14 ha of forest, planting pasture, and a few crops for a few years, and selling the 'improvements' to a new settler" (1988:38). This is four times what the laborer could hope to earn from ordinary farm work. The largest speculative gains accrue to large investors with good connections in government and the courts because the value of land is greatly influenced by "institutional factors such as validity of title, [and] access to credits" (Hecht, 1989b:229).

Financial Incentives from Government To encourage development in the Amazon, the Brazilian government made rural credit available to those with a land title or a certificate of occupancy at low, indeed at negative, interest rates. The credits were so attractive that money flowed from the nonagricultural sector into extensive ranching (Binswanger, 1989). Small farms were not taxed on land, large ones could reduce their already low taxes by converting forest to pasture or crops (Binswanger, 1989), and corporations could deduct up to 75 percent of the cost of approved development projects in the Amazon from their federal tax liability (Browder, 1988). Corporations could also write off losses on Area-

zon projects against taxable income earned elsewhere (Browder, 1988). These incentives favored extensive enterprises and encourage livestock production even when returns from beef alone did not pay the cost of production. Fiscal incentives for livestock raising have largely dried up since 1985, but the cattle population has continued to grow at an annual rate of 8 percent (Schneider, 1990), suggesting that profit can now be made without subsidies, partly from the appreciation of land values (Binswanger, 1989).

Livestock and Crop Economics The strategy that is generally most immediately profitable when land is plentiful and labor scarce is one of extensive and often transitory use. An example is shifting cultivation, the predominant indigenous strategy of land use. Fire removes cut brush and trees, and there is no need to turn the soil, weed frequently, irrigate, drain, or terrace. Beef production demands even less work per unit output and, with the help of modern technology and fossil-fuel energy for clearing forests, can be much more extensive than shifting cultivation. Fattening cattle on grass requires little labor or expenditure on fencing and corrals, and no weeding. Ranchers can take advantage of the highly productive first years after forest clearance to overstock the range and increase short-term profit. Cattle projects supported by the Brazilian development agency, SUDAM, operated with as few as one employee per 400 ha (Denevan, 1981). Such ranches, established with government subsidies, are now able to survive without them by marketing more timber from the land, selling beef to recent migrants to the new urban centers in the region, walking their cattle to market, and using new and better-adapted grass species and selectively bred cattle (Schneider, 1990).

Ideology, Politics, and Economics of Development Throughout much of the 1960s and 1970s, the Brazilian government with support from international financial institutions pursued a strategy of large-scale, capital-intensive development projects. These often meant monocropping, relatively low labor inputs, mechanization, and the maximization of short-term financial returns. The strategy, elaborated in textbooks on development (e.g., Rostow, 1960), was based on shared premises about the essential goodness of economic growth and made deforestation for lumbering and extensive cattle raising the most profitable activity on the Brazilian frontier (Partridge, 1984). The international debt incurred in part to promote such development increased demands for rapid returns, high profits, and the production of exports to pay the interest

charges (Hildyard, 1990). Recently, disappointing economic returns, declining international aid, and an awareness of rapid ecological deterioration are becoming associated with changing priorities, and analysts in the World Bank and elsewhere are becoming critical of the old development philosophy (Binswanger, 1989 Mahar, 1988 Schneider, 1990).

The Role of Population Growth It is easy to see Brazil's average population growth of 2.8 percent in the 1970s as the source of land hunger and migration, raising the Amazon population by 6.3 percent annually (Browder, 1988). However, the period witnessed stronger movements of population from the already settled hinterland to cities, combined with considerable natural increase in urban areas. The decline in rural population density is reflected in the phrase, "Quando chega o boi, o homen sai," (When the cattle arrive, the men leave) (Browder, 1988:254). The extensive clearing of forest on the frontier reflects population pressure and food needs outside the local region, combined with a lack of population pressure locally (Denevan, 1981).

Alternative Futures for Amazonia

The Amazonian case illustrates the difference between intensive and extensive patterns of land use in tropical forests. Table 3-9 provides a summary representation of the extremes of these patterns, presented as ideal types (actual land use almost always has features of both types). The Amazonian forest has long been inhabited by peoples that used a mixture of these strategies to support their economies. Indigenous groups combined relatively extensive strategies, such as temporary or shifting cultivation followed by natural forest regeneration and hunting and gathering of dispersed game, fish, and wild food plants, with more intensive farming of alluvial riverine and other soils of high, renewable fertility. More recently, both native American (Posey, 1989 Prance, 1989) and immigrant populations such as the rubber tappers have maintained the forest by a mixed-management strategy that mimics rather than replaces the biologically diverse natural environment (Browder, 1989).

The modern forms of land use most implicated in deforestation&mdashcattle ranching, crop agriculture, and logging and other industrial uses&mdashare extensive and rapidly expansive, market and capital dependent, specialized in one or a few commodities, and mechanized or labor saving. Some observers point to modern strat-

TABLE 3-9 Land Use Type

Low density, growing migratory

High density, permanent settlement

Low average yields, high variability, low crop diversity

High average yields, low variability, high diversity (cereals, tubers, vegetables, trees, livestock)

Large, tendency to increase land area

Small, balanced fragmentation anti accretion

Low total inputs, seasonally variable, unskilled, high returns per hour, often hired

High total inputs, steady inputs throughout year, skilled, low returns per hour, often household

Mechanical, energy imported, nonrenewable, capital intensive

Simple, often manual, energy local, renewable, labor intensive

High, output sold, inputs largely purchased, national and international commodity markets

Subsistence combined with cash production, not totally dependent on market prices, some purchased inputs

Private, land values speculative but initially low, legal access politically determined

Private and common property rights, land values high, inheritance important, legal protections

High, growing polarization, landlord elite and landless wage laborers

Moderate, stratified, smallholders significant, social mobility

Response to Environmental Change

Vulnerability, boom/bust cycles

Resilience, buffering, flexibility

Degradation, decline in biodiversity, nutrient loss

Sustainability, fertility renewed, conservation

egies of mixed development as an alternative way of using the forest for human needs. They claim that intensive, stable agricultural land use with a mixture of crops and livestock can be combined with labor-intensive efforts to maintain soil quality by careful, thorough tillage, agroforestry, manuring, terracing, irrigation, and drainage. Thus they can provide high, reliable, sustainable production from smallholdings with high inputs of household labor and little capital or fossil fuel energy. These systems may also help preserve mature forest ecosystems from destruction by reducing development pressure on them (Anderson, 1990).

The potential for a future of less-extensive forest use in the Amazon Basin relates in part to land distribution. Inequality of land holdings in Brazil has increased greatly over the last few decades, with 70 percent of Brazilian farmers now landless and 81 percent of the farmland held by just 4.5 percent of the population (Hildyard, 1990). This pattern of increasing inequality also holds in the Amazon, making access to resources more difficult for subsistence farmers and hunters and gatherers and threatening indigenous land tenure systems based on communal rights (Chernela, n.d. Poole, 1989). Larger landholdings bring more extensive use. In Pará state, for instance, small farms cultivate an average of 50 percent of their area, while farms of over 1,000 ha cultivate only 26 percent (Hecht, 1981). More intensive cultivation means that less forest must be displaced to meet human needs. Moreover, stable smallholders have an incentive to economize on land and keep it productive, so that land degradation can be slower with more intensive use. Thus, the current pattern of extensive development, by displacing indigenous peoples and small-scale extractors, has removed a brake on deforestation and threatens a store of valuable knowledge about the intensive management of forest species for human consumption.

There are barriers to a transition to a mixed-development strategy in the Amazon. One is the social change resulting from the current extensive strategy. Another is the politics of change. With rural poverty increasing and a political choice between dividing up large landholdings and encouraging the landless to colonize unclaimed or "unused" frontier lands, migration and resettlement policies are much the more palatable alternative (Macdonald, 1981). And finally, there are intrinsic social limits. Although portions of the local environment could support intensive land use like that of the wet rice/garden systems of south China and Java, the necessary local density of population with plentiful labor and nearby markets are not present (Moran 1987:75). Extensive, extractive

land use with deforestation is likely to remain the most economically feasible and politically viable development strategy in the Amazon region because vast areas of cheap land are accessible and markets are distant.

In sum, the causes of Amazon deforestation lie partly in the same frontier conditions that have led to extensive land use in nineteenth century North America and elsewhere. In addition, development policy around the world has supported capital-intensive development of export monocultures. The unique institutional and political history of Brazil has helped determine the particular development pattern there, a pattern significantly different from that of tropical forest development in Zaire or Indonesia (Allen and Barnes, 1985 Brookfield et al., 1991 Lal et al., 1986). A key to the future of the forests lies in policy changes that could limit deforestation and extensive land use while increasing food production from existing agricultural areas. However, the social and economic changes brought about by Amazonian development have created barriers to making and implementing such policies.


The examples above illustrate how the proximate causes of global environmental change result from a complex of social, political, economic, technological, and cultural variables, sometimes referred to as driving forces. They also show that studies of driving forces and their relationships have been and can be done (National Research Council, 1990b Turner, 1989). However, little of this research has been conducted on a global scale, for at least three important reasons: demand for such studies is a very recent phenomenon relevant data at the global level are scarce and social driving forces may vary greatly with time and place. Consequently, much additional work is needed to support valid global generalizations.

We distinguish five types of social variables known to affect the environmental systems implicated in global change: (1) population change, (2) economic growth, (3) technological change, (4) political-economic institutions, and (5) attitudes and beliefs.

Vocal arguments have been made for each of these as the exclusive, or the primary, human influence on global environmental change. In each instance, supportive evidence exists below the global level. Evidence at the global level, however, is generally insufficient either to demonstrate or dismiss claims that a par-

ticular variable causes global environmental change or is more important than some other variable.

We briefly outline the evidence supporting and qualifying claims that each class of variable is an independent influence on global environmental change. Our citations are not meant to be exhaustive, but rather to refer the reader to typical sources and critiques of claims about the importance of particular variables. For many of the authors cited, links between key explanatory variables and global environmental change are only implicit in such instances, we draw out the implications for global environmental change. We also outline some of the key unanswered but researchable questions regarding these driving forces.


Of all the possible driving forces of environmental change, none has such a rich history in Western thinking as population growth. Starting with Malthus, scholars have attempted to understand the effects of population growth on resource use, social and economic welfare, and most recently the environment. Few debates in the social sciences have been so heated or protracted as that around the impacts of population growth. Clearly, each person in a population makes some demand on the environment and the social system for the essentials of life&mdashfood, water, clothing, shelter, and so on. If all else is equal, the greater the number of people, the greater the demands placed on the environment for the provision of resources and the absorption of waste and pollutants. Stated thus, the matter is a truism. The source of controversy centers around more complex questions. Does all else remain equal in the face of population growth? Do simple increases in numbers account for most of the increase in environmental degradation in the modern world? Can population growth occur without major environmental damage? If not, is population growth a root cause of the degradation that follows, or merely an effect of more deeply underlying causes, such as changes in technology and social organization?

Ehrlich and others (Ehrlich, 1968 Ehrlich and Holdren, 1971, 1988 Ehrlich et al., 1977 Ehrlich and Ehrlich, 1990) hold that population growth is the primary force precipitating environmental degradation. They argue that the doubling of the world's population in about one generation accounts for a greater proportion of the stress placed on the global environment than has increased per capita consumption or inefficiencies in the production-

consumption process. They do not hold that other factors are unimportant in placing stress on the earth's resources and on the biosphere, only that population growth must be considered primary, because if all other factors could be made environmentally neutral, population growth of this magnitude would still spur resource stress and environmental degradation. Indeed, it is argued that once population has reached a level in excess of the earth's long-term capacity to sustain it, even stability and zero growth at that level will lead to future environmental degradation (Ehrlich and Ehrlich, 1990).

The critiques of this position are many. One strand of criticism argues that technological and socioeconomic factors are primary (e.g., Coale, 1970 Commoner, 1972 Harvey, 1974 Ridker, 1972a Schnaiberg, 1980). Another criticism comes from those who argue that population, though it may be a driving force of change, is not necessarily a driving force of degradation (Boserup, 1981 Simon, 1981 Simon and Kahn, 1984). Rather, they view population growth as a driving force of improvement, which increases the capacity of society to transform the environment for the better, or as a reflection of society's success in improving the environment so as to support greater numbers. These critics offer evidence from long sweeps of history, such as the relationships between major sociotechnical changes in society and global increases in population (Deevey, 1960 Boserup, 1965). Others have suggested that these population increases are also associated with increasing global environmental change (Whitmore et al., 1991).

Since World War II, concern with rapid population growth has motivated the U.S. government, private foundations, and multilateral aid agencies to fund a substantial body of research on the causes of population growth. In addition to supporting individual studies, these bodies have devoted substantial resources to institutional development by subsidizing education, professional journals, and centers of excellence. The result has been impressive in building demography as a respected, interdisciplinary field within the social sciences, and in gaining knowledge of the causes of population growth. As we note in Chapter 7, this experience provides a useful model for advancing interdisciplinary social science research on global change.

Research on the causes of population growth provides some useful insight into the causes of global change and strategies to deal with them. For example, current fertility and mortality patterns suggest that world population will continue to increase well into the next century. But if fertility declines as fast throughout

the developing world as it has in a few developing countries, this growth might be reduced by almost 2 billion people by the time that the population of the developing world would otherwise have reached 8 billion (World Bank, 1984). This research helps clarify how much growth is more or less inevitable because of the momentum built into the age structure of the world population.

Compared with research on the causes of population growth, very little research has been devoted to understanding its consequences for environmental quality. This is ironic, because it is concern with the consequences that motivates much support for research on the causes of growth. There is some research on the effects of population growth on economic growth and social welfare, though the topic is still subject to some controversy (much of this literature is summarized in National Research Council, 1986). Only a handful of empirical studies have examined the effects of population growth on the environment, and many of these are quite dated [e.g., Ridker, 1972b Fisher and Potter, 1971). As a result, it is difficult to assess just how important population may be as a driving force. For example, in 1986 a National Research Council study committee composed of economists and demographers concluded that slower population growth might assist less-developed countries in developing policies and institutions to protect the environment, but could find little empirical work on the link between population growth and environmental degradation (National Research Council, 1986).

Research Needs

We believe an extensive research program is needed to explicate the environmental consequences of population growth and provide a sounder basis for deciding what actions may be appropriate in response. Such research should begin by acknowledging that the environmental consequences of population growth depend on other variables. For instance, a population increase of people with the standard of living and technological base of average North Americans in 1987 would use 35 times as much energy as an increase of the same number of people living at India's standards&mdashand their respective effects on the global climate would be in roughly the same proportion. The critical questions for research, then, are about the conditions determining the environmental effect of a projected population increase at a particular place and time. What are the multipliers that represent the environmental impacts of a new person in a particular year and coun-

try? To what extent are multipliers such as annual income or annual distance traveled constant for a country, and to what extent are they contingent on other factors that may change over time, such as the manufacturing intensiveness or energy supply mix of the country's economy or the country's policies on income distribution or energy development?


Global economic growth, defined as increases in the measured production of the world's goods and services, is likely to continue at a rapid rate well into the future. The human impulse to want more of the material things of life appears to be deep-seated, and the areas of the world in which people are most lacking in material goods are those with the greatest&mdashand most rapidly increasing&mdashpopulation. Assuming United Nations and World Bank projections for world population to double to about 10 billion in about 50 years, with 90 percent or more of that growth occurring in the developing countries of Africa, Asia, and Latin America, and assuming that per capita income grows 2.5 percent and 1.5 percent annually in the developing and developed countries, respectively (a low projection, in both cases, by standards of the last several decades), global economic output would quadruple between 1990 and 2040.

Under these conditions the relative gap between per capita income in developing and developed countries would narrow, but the absolute gap would increase substantially. To the extent that per capita income aspirations in the developing countries are driven by comparison of their incomes with those in developed countries, aspirations for additional income growth in the developing countries may be even stronger in 50 years than they are now.

Increased income or economic activity as measured by such indicators as gross national product is not, of course, equivalent to increased well-being. There is considerable debate in the economic literature on how to measure welfare, focused on such questions as how to count things people value that are not traded in markets and whether expenditures for pollution control should be considered an addition or a subtraction from net welfare (e.g., Daly, 1986 Repetto et al., 1989). Although these questions are very important for analyzing human-environment interactions, most current analyses of the effects of economic growth and environmental quality are based on conventional definitions of economic activity.

Economic activity has long been a major source of environmental change and, for the first time in human history, economic activity is so extensive that it produces environmental change at the global level. The key issues concern the extent to which current and future economic activity will shape the proximate causes of global change.

The production and consumption of goods and services is bound by a fundamental natural law&mdashthe conservation of matter. Whatever goes into production and consumption must come out, either as useful goods and services or as residual waste materials. Since the conversion of inputs to useful outputs is never entire, it is fair to say economic activity inevitably stresses the environment by generating residual wastes.

Wastes must be disposed of somewhere in the environment. Economists note that disposal presents no important social problem if it is managed to reflect its true social costs and to be equitable in the sense that the costs are borne by those who generate the residuals. However, true social costs can be very difficult to determine, especially when wastes alter biogeochemical processes that are poorly understood. And when the wastes are released to the atmosphere, rivers, and oceans, it is difficult to ensure that those who generate the waste pay the costs. The problem of defining social cost and the separation of those who generate the costs of waste disposal from those who bear them are the keys to the waste-induced environmental problem (Kneese and Bower, 1979).

Economic growth also depletes the stock of nonrenewable natural resources such as coal, oil, natural gas, and metallic minerals and, in some cases, the stock of renewable resources as well, as when the rate of soil erosion exceeds the rate of restoration of soil and nutrients. Environmental degradation follows when extraction disturbs land or biota and when resource use generates wastes. Economic growth may also destroy aspects of the natural landscape, for example, pristine wilderness areas or vast geological features such as the Grand Canyon. Continued use of depletable resources will create economic pressure to develop renewable energy resources, expanded recycling, and substitute materials (see, e.g., Barnett and Morse, 1963 Smith, 1979 Simon and Kahn, 1984), but a quadrupling of the global economy over 50 years would result in continued resource depletion.

Depletion of nonrenewable resources need not threaten long-run economic growth if management of the resources takes adequate account of their future value and the likelihood of finding substitutes. This condition may be easier to meet than the condi-

tions for managing wastes because it is much easier to establish clear, enforceable property rights in nonrenewable resources and because such property rights permit creation of markets that provide price signals of changing resource scarcity and incentives to take future as well as current resource values into account. Property rights are relatively easy to establish because, unlike in the atmosphere and the oceans, nonrenewable resources are localized, spatially well defined, and fixed in place.

But markets in nonrenewable resources are no panacea for the environmental effects of minerals extraction or fossil energy use. Current markets have no sure way to anticipate, and therefore reflect, the value future generations will put on the depleted resources. This is the issue of intergenerational equity in resource management, and there are strong arguments that markets cannot deal adequately with the issue (Sen, 1982 Weiss, 1988 MacLean, 1990). The values future generations will hold can only be guessed at, drawing on human experience so far. Given this uncertainty, most analysts advocate more cautious resource management than what current market signals indicate.

So economic growth necessarily stresses the environment directly by increasing quantities of wastes and indirectly by depleting resources. However, the relationship between economic growth and environmental stress is not fixed. The key analytic questions concern the conditions under which a given amount of present or future economic growth produces larger or smaller impacts on the environment.

Several conditions apply. It matters which pattern of goods and services is produced. An economy heavily weighted toward services appears to generate fewer wastes and less resource depletion per unit of output than one weighted toward manufactured goods. Experience so far indicates that consumption patterns shift toward services as per capita income rises, suggesting that the process of growth itself may induce less than proportional increases in environmental stress. It seems that past some point, consumers use their economic resources to purchase well-being that is decreasingly dependent on material goods (see Inglehart, 1990). If the historic pattern holds, future economic growth in the low-income developing countries will be materials and energy intensive for quite some time before a transition to a service economy sets in. But this projection is uncertain because of incomplete knowledge about the causes of that transition and the ways it might be altered by deliberate action.

Other shifts in economies can also change the relationship between economic growth and environmental quality. Per capita

use of many materials has been declining in North America and western Europe for some time (Herman et al., 1989). Waste management based on recycling, redesign of production processes, and the treatment of the wastes of one process as raw materials for another can reduce the environmental impact of economic activity (e.g., Ayres, 1978 National Research Council, 1985 Haefele et al., 1986 U.S. Office of Technology Assessment, 1986 Friedlander, 1989). And an observed trend in the United States, in which the main source of pollution has shifted from production activities to consumption activities, has effects on the overall economy-environment relationship that are not yet clear (Ayres and Rod, 1986 Ayres, 1978).

The environmental effect of economic growth may also depend on forms of political organization. The comparison of emissions of CO2 and pollutants in Eastern and Western Europe suggests that democratic countries may be able to deal more effectively with the effects of wastes than nondemocratic countries. When people who feel the effects, or become concerned about the effects on others, have ready access to political power, their concerns may possibly have more influence on policy. If this hypothesis is correct, then political trends toward democracy, such as in Eastern Europe, will tend to reduce the amount of degradation resulting from economic growth there.

National policies also help determine the environmental costs of economic growth. In many developing countries, policies have favored extensive use of ''unused'' resources and "underpopulated" land to increase national power and improve the welfare of their citizens. Countries such as the United States, Canada, Argentina, and Australia had such policies during rapid development phases, and other countries have followed the example. This model of development through frontier occupation and rapid creation of wealth required cheap food and raw materials from rural areas, an infrastructure of roads and transport to open up these areas, and huge infusions of capital for enterprises and settlement. An alternative development model generates increased production per unit of land by agricultural intensification rather than by extensive land uses such as shifting agriculture or ranching (Boserup, 1990 Turner et al., n.d.). Development of this kind can be carried out in a sustainable manner (Conway and Barbier, 1990 Sublet and Uhl, 1990).

Research Needs

The effects of economic development on the proximate causes of global change appear to be contingent, among other things, on

the structure of consumer demand, the population and resource base for agricultural development, forms of national political organization, and development policies. However, the nature of these contingent relationships, particularly the relationships between policy and the other variables, is not understood in detail. Research is critically needed on the ways consumer demand changes as income increases, the effects of national policies on patterns of production and consumer demand, the effects of agricultural intensity on economic growth and the environment, and the causes of shifts from more to less energy-and materials-intensive economies. These questions call for research both within and across the boundaries of disciplines and academic specialties.


Technological change affects the global environment in three ways. First, it leads to new ways to discover and exploit natural resources. Second, it changes the efficiency of production and consumption processes, altering the volume of resources required per unit of output produced, the effluents and wastes produced, and the relative costs and hence the supply of different goods and services. Third, different kinds of technology produce different environmental impacts from the same process (e.g., fossil-fuel and nuclear energy production have different effluents). Some technologies have surprising and serious secondary impacts, as the history of refrigeration illustrates (see also Brooks, 1986).

In one view, technological development tends to hasten resource depletion and increase pollutant emissions. In this view, technology as currently developed is a Faustian bargain, trading current gain against future survival (e.g., Commoner, 1970, 1972, 1977). Modern technology is seen as a much more significant contributor to environmental degradation than either population or economic growth. One reason is that modern technological innovation progresses much faster than knowledge about its damaging effects, both because the effects are intrinsically difficult to understand and because the powerful economic interests that benefit from new technologies influence research agendas to favor knowledge about the benefits over knowledge about the costs (Schnaiberg, 1980).

Three arguments are advanced to oppose or qualify the Faustian theme. In the first, technology's contribution to environmental change is deemed relatively unimportant (Ehrlich and Holdren, 1972). In the second, technological innovation and adoption are

seen as induced by other forces, particularly demand from population (Boserup, 1981) or market forces (Ruttan, 1971) and therefore not a driving force. The third argument is that technological change is a net benefit to the environment because it can ameliorate environmental damage through more efficient resource use and the lessening of waste emissions (e.g., Simon and Kahn, 1984 also Ausubel et al., 1989 Gray, 1989 Ruttan, 1971).

These contradictory arguments, all plausible, can be weighed only by research that is specific (e.g., which technology, in which society, at what time) and that takes into account the other major social forces that cause or are affected by technological progress. For instance, technological progress is affected by the relative prices of energy, materials, and labor, with inventors and entrepreneurs having a built-in incentive to develop technologies that economize on the more expensive factors of production. As a result, technological development starting in countries with low-cost energy will be more energy intensive than technologies developed in countries in which energy is expensive and therefore more likely to have negative environmental effects. The effects of technology on the environments of poor countries may reflect the fact that much of the technological innovation adopted in poor countries originated in rich countries, which face different economic and environmental problems. National economic policies, as well as environmental and energy policies, can favor particular kinds of technological innovation and thus hasten or forestall environmental degradation. In the United States, debates about apportioning government energy research funds between nuclear, fossil, conservation, and renewable energy development have always been, in part, debates about the effect of these technologies on the environment. And the environmental effects of technology look quite different depending on the time scale being considered or the state of environmental knowledge when the analysis is done. For example, the environmental effects of refrigeration technology look much different now than they would have looked in an analysis done in the 1950s.

Research Needs

As with other human influences on the global environment, the effects of technology are likely to be contingent on the other driving forces. Consequently, research on the effects of technology on global change will need to consider the social context. Several critical topics for research are obvious: one involves com-

parisons of the environmental impacts of different technologies for energy production and consumption, food production, and other human activities that can have major impacts on the global environment, a topic that has received some attention in the past (e.g., Inhaber, 1978 Holdren et al., 1979, on energy production). Such studies should be specific at first, focusing on the alternatives available in a particular place and time, and should examine the technologies as they are implemented in actual social systems rather than under idealized conditions. Another involves diffusion of production technologies across national boundaries, particularly from more-developed to less-developed countries: How do the environmental impacts differ between the innovating countries and the adopting countries, and how do the differences depend on the social organizations using the technologies (e.g., Covello and Frey, 1989)? A third concerns the effect of government policies on the development, adoption, and use of technologies with different kinds of environmental effects: What policy choices influence technology and its use in environmentally destructive or beneficial ways, and how do the effects of policy depend on the political, economic, and social context where they are adopted (e.g., Zinberg, 1983 Clarke, 1988 Jasper, 1990)?


It seems reasonable that the social institutions that control the exchange of goods and services and that structure the decisions of large human groups should have a strong influence on the effects of human activity on the global environment. These institutions include economic and governmental institutions at all levels of aggregation.

A key institution is the market. Neoclassical economic theory argues that free markets efficiently allocate goods and services to the most valued ends. Thus, environmental problems can be analyzed in terms of market failures, that is, conditions that prevent markets from operating freely. Several types of market failure are relevant to environmental problems. First, the costs of the transactions necessary to resolve environmental problems in an optimal fashion may be prohibitively high because of the costs of collecting information, for example on the net present value to all affected of the future effects of resource use (e.g., Coase, 1960 Baumol and Oates, 1988). Second is the problem of "externalities." Individuals not involved in buying or selling a good or service may nevertheless be affected by the transaction, for ex-

ample, if it alters the earth's ozone layer. But because they do not know what the effect will be, they may not engage in transactions to maximize their preferences. Third, government action may supersede the market (e.g., Burton, 1978 Coase, 1960), leading to inefficiencies, for instance, excessive and uneconomic cutting in U.S. national forests, or profligate use of coal in China due to artificially low prices and a production quota system that gives no premium for quality. Fourth, a lack of clearly defined private property rights may leave no one with the incentive to pay to prevent degradation. This situation can arise because of traditional social arrangements that allow free access to all (Hardin, 1968) or because of the indivisible, common-pool nature of resources such as open-access marine fisheries (Gordon, 1954) and the world atmosphere.

The analysis that traces environmental degradation to the absence of free markets is criticized on several grounds. First, even smoothly working markets are likely to produce undesirable outcomes. Questions have been raised regarding the theoretical assumption that a dollar has the same value regardless of a party's wealth and the morality of treating polluters and pollution recipients as symmetric and reciprocal sources of harm to one another (Kelman, 1987 Mishan, 1971). Second, the tendency of markets to place a higher value on possible impacts in the near future than on those in the distant future conflicts with the goal of long-term sustainability and reduces the rights of future generations effectively to zero (Weiss, 1988 Pearce and Turner, 1990). Third, goods that have no price, whose production is highly uncertain, or that are valued by nonparticipants in markets, for instance, the survival of nonhuman species, tend to be systematically undervalued in markets (e.g., Krutilla and Fisher, 1975). Fourth, the theory of market failures does not compare the environmental effects of different kinds of imperfect markets. Knowledge does not support the easy inference that the more a market resembles theoretical perfection, the more of the benefits of free markets it provides (Lipsey and Lancaster, 1956 Dasgupta and Heal, 1979). This is a serious limitation because, for environmental resources such as the stratospheric ozone layer, the only markets are imperfect.

Some analysts trace the roots of environmental problems to the system of free-enterprise competition that underlies markets (e.g., Schnaiberg, 1980). They argue that the capitalist, cash-based market system rewards those who exploit the environment for maximum short-term gain, an incentive structure fundamentally at

odds with conservation and long-term sustainability and, moreover, that the capitalist class exacerbates the process through its strong influence on public policy. The argument is sometimes illustrated with the case of development in the Amazon.

The critique of capitalism can be criticized for relying on a global, highly generalizing contrast between capitalist market economies and precapitalist, subsistence, socially undifferentiated groups that presumably maintain a delicate balance with the natural environment. It does not account for the fact that noncapitalist societies without private property may perpetuate large-scale environmental abuses, as in the case of the drying of the Aral Sea for irrigation purposes in the Soviet Union (Medvedev, 1990) or the reliance on inefficient coal burning technology in China. It does not account for labor resistance to environmental protection when it seems to threaten loss of jobs, such as opposition to restrictions on mining and burning Appalachian coal. And it does not acknowledge the existence within fully integrated market economies of stable, intensively producing family farmers and smallholder land-use regimes that modify but do not permanently degrade their habitat.

Some analysts trace environmental deterioration, particularly in developing countries, to an international division between rich Western industrial and poor Third World raw material-producing nations that fosters political-economic dependence. Unequal terms of trade drain capital from peripheral or satellite regions to core areas. Underdevelopment and poverty are "developed" and perpetuated by market mechanisms (Wallerstein, 1976 Frank, 1967). This analysis emphasizes the effects of foreign investment, loans, the operations of large corporations, and quantifiable movements of capital, labor, imports, and exports on particular changes in the environment. Again, the Amazon case is sometimes offered as an example.

This dependency model highlights the important role of foreign capital and extractive industries, but because it pits a monolithic global capitalism against a similarly undifferentiated and largely passive Third World, it cannot account for the historical specificity of particular cases or the variability in internal dynamics as systems adapt (Wolf, 1982). Dependency theorists often overlook the role and complicity of national elites (Hecht and Cockburn, 1989). The model has been criticized as imprecise in that the notion of unequal terms of trade is inadequately defined. And contrary to the simple view of dependency, pressures from international lending institutions are now beginning to influence Area-

zonian land use in a positive way (Schmink and Wood, 1987:50 but see Price, 1989). Some Latin American countries, such as Costa Rica, have taken leadership in setting aside tracts of tropical forests as parks and conservation areas, despite high debt levels and dependence on exports to the United States [Gamez and Ugalde, 1988]. A range of other factors in addition to dependency must be considered to account for the variety of resource use patterns in the Third World.

The state is a major institution affecting global environmental change because state actions modify economic institutions and affect a wide range of human actions, including those with global environmental impacts. As already noted, democratic states may be more responsive to popular pressures to take action on environmental problems than nondemocratic states. It may be more difficult in the latter for nonelite groups to get environmental issues on national policy agendas and then to influence the legislative process through the expression of public opinion. Another critical dimension may be the degree of centralization of the political system. One perspective argues that systems in which decisions are decentralized, primarily through markets, are apt to respond more readily to resource constraints. However, under certain circumstances, a more centralized, state-controlled form of decision making might be better able to take a long-term and broader perspective.

Specific public policies can also have significant environmental consequences, both intentionally and inadvertently. Many governments have pursued policies aimed at maximum exploitation of natural resources in pursuit of economic growth that give environmental concerns a low priority. However, many governments, primarily in the West, have also enacted policies to ameliorate the effects of industrial growth on the environment. State action can also have large unintended effects on the environment. For example, emissions of greenhouse gases and air pollutants in the United States have been greatly affected by the many policy choices of the U.S. government that have encouraged the use of the automobile as a form of personal transportation. Similarly, policies pursued by such federal agencies as the Army Corps of Engineers, the Department of the Interior, and the Atomic Energy Commission have affected environmental quality, even though&mdashor perhaps because&mdashenvironmental quality was not an issue in their policy deliberations. Knowledge about why different governments develop different environmental policies is discussed in more detail in Chapter 4.

Research Needs

Clearly, political-economic institutions can affect the global environment along many causal pathways. We have identified some of the important areas in which more knowledge could add greatly to understanding of the causes of global change. One is the comparative study of the effects of different imperfect-market methods of environmental management&mdashincluding the various pricing systems and regulatory approaches in operation around the world, market-like approaches not in use but potentially usable, and various mixtures&mdashto determine their effects on global environmental variables as a function of where and when they are used, and on which human activities. Theoretical work classifying and analyzing the varieties of market imperfection could also make great contributions to understanding if directed toward the kinds of market imperfection characteristic of global environmental resources. A second important research area concerns the comparison of national policies in terms of their origins and their environmental effects. A third concerns the commonly alleged short-sightedness of corporate decisions about the environment. Under what conditions do capitalist actors adopt practices of natural resource use or waste management that preserve environmental values? What national policies affect the likelihood that they will adopt such practices? A fourth concerns the variation in development policies adopted by countries that are similar to each other in terms of level of development and dependency. To what extent is such variation dependent on the political structure of the state, national political culture, level of centralization of decision-making power, and other variables at the national level?


Widely shared cultural beliefs and attitudes can also function as root causes of global environmental change. Many analysts focus on broad systems of beliefs, attitudes, and values related to the valuation of material goods. An early argument in this vein attributed the modern environmental crisis to the separation of spirit and nature in the Judeo-Christian tradition (White, 1967) another traces the rise of capitalism with its materialist values and social and economic structures back to Protestant theology (Weber, 1958). The Frankfurt school of critical theory accorded a similar role to the spread of purely instrumental rationality (Hab-

ermas, 1970 Offe, 1985). Bias toward growth and a hubristic disregard for physical limits, others have argued, are today the principal driving forces (e.g., Boulding, 1971, 1974 Daly, 1977). Some point to "humanistic" values, derived from the Enlightenment, that put human wants ahead of nature and presume that human activity (especially technology) can solve all problems that may arise (Ehrenfeld, 1978). Some assert that increased environmental pressures are associated with materialistic values of modern society (e.g., Brown, 1981), implying that materialism is amplified in the social atmosphere of the Western world. Sack (1990) argues that environmental degradation is intimately tied to social forms and mechanisms that have divorced the consumer from awareness of the realities of production, hence leading to irresponsible behavior that exacerbates global change. And some analysts have traced environmental problems to a set of values, rooted in patriarchal social systems, that identify woman and nature and define civilization and progress in terms of the domination of man over both (e.g., Merchant, 1980 Shiva, 1989).

Some researchers argue that a secular change in basic values is occurring in many modern societies. Inglehart (1990) presents survey data to suggest that across advanced industrial societies, a value transition from materialist to postmaterialist values is occurring that has significant implications for the ability of societies to respond to global change with mitigation strategies that involve changes in life-style (see also Rohrschneider, 1990). Along a similar line, Dunlap and Catton have argued that a "dominant social paradigm" that sets human beings apart from nature encourages environmentally destructive behavior but that a "new environmental paradigm" that considers humanity as part of a delicate balance of nature is emerging (Dunlap and Van Liere, 1978, 1984 Catton, 1980 Catton and Dunlap, 1980). Other writers claim that a change in environmental ethics is necessary to prevent global environmental disaster (e.g., Stone, 1987 Sagoff, 1988).

Short-sighted and self-interested ways of thinking can also act as underlying causes of environmental degradation. The inexorable destruction of an exhaustible resource that is openly available to all, what Hardin (1968) called the "tragedy of the commons," is, at a psychological level, a logical outcome of this sort of thinking. Individuals seeking their short-term self-interest exploit or degrade open-access resources much faster than they would if they acted in the longer-term or collective interest (Dawes, 1980 Edney, 1980 Fox, 1985).

Direct challenges to these analyses are few, in part because they are compatible with analyses that emphasize the role of other driving forces. Cultural values, short-sightedness, and self-interestedness can both cause and respond to other major social forces, such as political-economic institutions and technological change. For example, global expansion of capitalism is seen by some as inextricably linked to a transformation of attitudes toward material production (Cronon, 1983 Merchant, 1991 Worster, 1988). Economists treat market behavior as an expression of preferences, which are ultimately attitudes, so the treatment of the environment is an indirect result of attitudes, even in economic analysis. Where controversy tends to arise is over the relative primacy and hierarchical ordering of attitudes and beliefs relative to other causal factors, especially the degree to which beliefs and attitudes can be given causal force in their own right or are products of more fundamental forces. The empirical associations underlying some claims have also been called into question (e.g., Tuan, 1968, on White, 1967). On the side of human response, however, at least some sense of the autonomy of attitudes and beliefs is implicit in every analysis that offers explicit recommendations for action.

Research Needs

As with the other driving forces, the most interesting questions for research concern the ways in which the central variables&mdashhere, cultural and psychological ones&mdashinteract with other driving forces to produce the proximate causes of global change. Observational and experimental studies of these relationships have been done, although almost always with relatively small numbers of individuals in culturally and temporally restricted settings (see, e.g., Stern and Oskamp, 1987, for a review). They indicate that attitudes and beliefs sometimes have significant influence on resource-using behavior at the individual level, even when social-structural and economic variables are held constant, and that attitudinal, economic, and other variables sometimes have interactive effects as well. But these studies do not explain the sources of variation in individual attitudes. It seems likely that attitudes and beliefs have significant independent effects on the global environment mainly over the long-term&mdashon the time scale of human generations or longer&mdashand that within single lifetimes, attitudes function as intervening variables between aspects of an individual's past experience and that individual's resource use.

Testing this hypothesis would require research conducted over longer time scales than is common in psychological research.


This section distills some general conclusions or principles from the chapter and outlines their implications for setting research priorities.


Research on the human causes of global environmental change should be directed at important proximate sources. It is critical to develop reasonably accurate assessments of the relative impact of different classes of human activity as proximate causes of global change. This chapter offers such an analysis&mdashwhat we call a tree-structured account&mdashfor the human contribution to the earth's accumulation of greenhouse gases. Similar accounts should be made for the human contributions to other problems of global change. The task is relatively simple in the sense that the initial accounts need not have great precision. For social scientific work to begin, it will be sufficient to know whether a particular human activity contributes on the order of 20 percent, 2 percent, or 0.2 percent of humanity's total contribution to a global change. Such knowledge will allow social scientists to set worthwhile research priorities until more precision is available.

Current impact is not the only criterion of importance. Estimating the relative contributions of different future human activities to global changes is a more difficult, but equally important, part of assessing the importance of proximal causes. The difficulty lies in predicting future human activities, particularly the invention and adoption of new technologies. Initially, projections of the future accounts based on simple models will suffice to guide the research plan for human dimensions. However, researchers should be aware of their limitations and should occasionally test their analyses against a variety of scenarios of future human contributions to global change. Although it is more difficult to quantify other aspects of importance, these can provide strong justifications for research. For example, human actions that may be proximate causes of irreversible environmental change must be considered important beyond the magnitude of the change they may cause.

Researchers should be able to demonstrate the significance of

their chosen subjects not only in terms of the theoretical and empirical issues in their fields, but also in terms of importance to global environmental change. All the research needs identified in this chapter presume that the importance criterion is applied to particular efforts to meet the needs.


Understanding human causes of environmental change will require developing new interdisciplinary teams and will take lead time to build the necessary understanding. Listed below are some central considerations for guiding research.

The driving forces of global change need to be conceptualized more clearly. Different kinds of technological change and of economic growth clearly have different implications for the global environment, but much still needs to be learned about which aspects of change in these and other variables drive environmental change. A better typology of development paths is needed, so that researchers can identify the ways different styles of development affect the environment and the conditions under which a country or region takes one path or another. The same is true for research on the ways nation-states organize the management of natural resources.

Driving forces generally act in combination with each other. As the case studies demonstrate, the driving forces of global change are highly interactive. Brazilian deforestation is due to the combined effects of economic incentives, land tenure institutions, and government policy Chinese coal use depends on the combined effects of economic development, the country's technological state, its political structure, and its economic policies the development of CFCs was a function of population migration, economic incentives, and new technology. An additive model of these relationships is not viable, so the study of single causal factors in isolation is misleading.

The various driving forces should be studied in combination, using multivariate research approaches. These include quantitative multivariate studies that treat particular proximate causes (e.g., emissions of carbon dioxide and other greenhouse gases) as a joint function of population, economic activity, technological change, and political structures and policies. Such studies may be conducted using both national-level data on demographic and economic variables and indicators of policy and social-structural vari-

ables, some of which might have to be constructed for the purpose. Detailed case studies using qualitative methods are also important, as the case summaries in this chapter illustrate. Qualitative methods can offer a depth of understanding not available from quantitative analyses, which by their nature are limited to those variables already quantified. Moreover, each method acts as a check on conclusions drawn from the other.

Driving forces can cause each other. For example, new technologies can promote economic growth, which in turn allows for further technological development materialistic ideologies contributed to the rise of capitalism, which promotes materialistic ideas. More complex mutual causal links also exist among several driving forces. Such relationships are difficult to disentangle and further complicate analyses of the human causes of global change. To understand the nature of these interactive relationships, it is important to compare different places and to follow the relationships over time.

The forces that cause environmental change can also be affected by it. Population growth is a good example of feedbacks between human actions and the global environment. Population growth increases the demand for food, which creates pressure to make agriculture both more intensive and extensive. These changes eventually bring diminishing returns, reducing food production per capita and creating downward pressure on population. The diminishing returns can be postponed by improved technology, but technology also interacts with the environment. Humans can increase food production by using tractor power, chemical fertilizers, pesticides, and herbicides, but these technologies rely on fossil energy and therefore eventually reach limits imposed by scarcity, price, or environmental consequences.

Relationships among the driving forces depend on place, time, and level of analysis. It is easy to illustrate the principle. For places: economic growth has been more dependent on fossil-fuel energy in China than in other countries, even other developing countries the causes of deforestation in Brazil are distinct from its causes in other countries. For times: fossil-fuel energy use increased almost in lockstep with economic activity in industrialized countries for many years since the 1970s, the correlation has been nearly zero (see Chapter 4). Also, the long-term effects on the global environment of a technology such as refrigeration with CFCs have been much different from the effects over a shorter time span&mdashnot only because of increasing use of the technology, but also because of the secondary effects of migrations made

possible by the technology. For levels of analysis at the local level, the inefficiency of Chinese energy use can be understood in terms of outmoded technology and lack of funds for replacing it at the national level, low prices for coal and the system of production quotas appear as critical factors at the world level, the entire system of command economies is implicated. All the relationships are equally real and important, yet answers derived at each level are incomplete.


1. The highest priority for research is to build understanding of the processes connecting human activity and environmental change. Better studies focused on the driving forces and their connections to the proximate causes are necessary for effective integrative modeling of the human causes of global change. Quantitative models will be of limited predictive value, especially for the decades-to-centuries time frame, without better knowledge of the processes.

More is generally known about the causes of population growth, economic development, technological change, government policies, and attitudes and culture&mdashthe driving forces of global change&mdashthan about their interrelationships and environmental effects. This is so because study of the driving forces is supported by organized subdisciplines or interdisciplinary fields in social science, such as population studies, development studies, and policy analysis, whereas an interdisciplinary environmental social science&mdasha field that examines the environmental effects of the driving forces&mdashis not yet organized. There is a critical need for support of the research that would constitute that field. Research on the processes by which human actions cause environmental change should begin from the basic principle that the relationships are contingent: the effect of such variables as population on environmental quality depends on other human variables that change over time and place. This fact has three major implications for research strategy: understanding the human causes is an intrinsically interdisciplinary project the important human causes of global change are not all global and comparative studies to specify the contingencies are critically needed (see #2 and #3 below). Research at the global level is important but far from sufficient for understanding the human causes of global change.

2. Over the near term, research on the human causes of environmental change should emphasize comparative studies of glob-

al scale. We can distinguish three types of global-scale analysis: aggregate, systemic, and comparative. Aggregate analysis at the global level examines human-environmental relationships on the basis of measures of the entire planet. Such analysis uses a small number of time-series data points and considers the entire planet the unit of analysis. For example, total atmospheric carbon dioxide can be correlated with global fossil fuel combustion over a period of time.

Systemic analysis of human-environmental relationships emphasizes facets of human activity that operate as a global system (i.e., a perturbation anywhere in the system has consequences throughout). For example, the world oil market is a global system in that changes in oil production anywhere reverberate through the system and may have global environmental impacts, for example, by changing the rate of consumption of oil or other fuels. Analyses of such relationships may use globally aggregate data or local and regional data linked to the phenomena of interest.

Comparative analysis at the global scale can take various forms. It might employ a large number of local or regional data points, worldwide in coverage. For example, the relationships of population, economic development, and government policies to deforestation may be studied by comparing data with the nation-state as the unit of analysis (e.g., Rudel, 1989). This approach is limited by the availability and comparability of relevant data (see Chapter 6). In contrast, case-based comparative studies can be selected so that a sample of units represents the range of socioeconomic and environmental contexts of the world. The case-comparison approach allows for more contextual detail at the expense of complete coverage. For example, a set of cases could be used to explore the various pathways that lead to conversion of wetlands to other uses.

Aggregate studies at the global level have limited value because the small number of data points make it impossible to identify the contingent relationships that shape the proximate human causes of global change. Systemic approaches have greater value in principle, but few human activities have the kind of systemic character that makes general circulation models of atmospheric processes valuable. Even the world oil market, one of the most globally systemic of environmentally relevant human systems, is affected by national policies such as trade restrictions and tax policies that interfere with world flows. Perhaps the most valuable research over the near-term will come from comparative studies that involve either a large number of representative data points or a smaller number of selected regional case studies from around

the world. The social sciences have a long tradition of comparative research and can usefully apply the conceptual and methodological tools they have developed to the problem of global environmental change.

3. Human dimensions research should prominently include comparisons of human systems that vary in their environmental impact. Comparisons between countries or localities or of the same place at different time periods can show why some social systems produce as much human welfare as others with less adverse impact on the global environment. A number of important issues lend themselves to comparative and longitudinal approaches, including:

the causes of deforestation (studies can compare deforestation rates in countries that vary in their land tenure systems, development policies, and governmental structures)

the effects of imperfect markets on release of greenhouse gases and air pollutants (studies can compare the emissions of countries or industries with different regulatory or pricing regimes)

the sustainability of different agricultural management systems (studies might compare nearby localities in the same country)

the effects of different industrial development paths on fossil fuel demand (studies might compare time-series data for different countries)

the determinants of adoption of environmentally benign technologies or practices (for example, studies might compare industries or firms that do and do not recycle waste products)

the relationship of attitudes about environmental quality and materialism to environmental policies in different countries.

Such studies can ''unpack'' broad concepts, such as technological change, economic growth, and population growth, that are frequently offered as explanations of how human activities cause global change. Comparative studies offer the best way to get inside the broad concepts and identify more specifically the features of growth and change in human activity that drive environmental change.

4. Researchers should study the causes of major environmental changes both globally and at lower geographic levels. Global aggregate analysis may show a very different picture from analyses at lower levels of aggregation. It is important to have both pictures because aggregate data can obscure the variety of causal processes that can produce the same outcome. For example, the global relationship between economic growth and greenhouse gas

emissions may change considerably if centrally planned economies become extinct. To estimate the size of any such effect, it is necessary to have studies at the national level. In addition, policy responses, particularly mitigation responses, require understanding of the activities that drive global change at the level at which the responses will be made. Depending on the topic, it may be important to conduct studies at the level of the nation-state, the community, the industry, the firm, or the individual. For studies below the global level, priorities should be set on the basis of the potential to gain understanding of the global picture or to make significant responses to global change. Thus, a high-priority study might be one that focuses on a country or activity that by itself contributes significantly to global change or one that is expected to generalize to a sufficient number of individuals, firms, or communities to matter on a global level or one that illuminates variables that explain important differences between actors at the chosen level of analysis. At each level of analysis, projects that meet such criteria are worthy of support, independently of what is known at the global level.

5. Important questions should be studied at different time scales. The full effects of technological and social innovations&mdashboth on society and on the natural environment&mdashare often unrecognized for decades or centuries. The CFC case shows how the effects of human activities can look very different depending on the time scale used for analysis: a technology developed to refrigerate food had much wider global implications several decades later, after it was applied to refrigerating buildings. Such cases need to be collected so they can be studied systematically and testable hypotheses derived about what kinds of innovations are likely to acquire the social momentum that produces long-lasting and increasing effects on the global environment, such as has resulted from CFC technology or from the Brazilian development strategy used in the Amazon Basin. Theory is particularly weak for this purpose. Historians can offer convincing accounts of the current effects of changes of the distant past, but social scientists have little ability to project the effects of current changes in human systems equally into the future.

6. Research should build understanding of the links between levels of analysis and between time scales. For example, social movements mediate between individual attitudes and national policies the interactions of individuals and firms can result in the creation of national and global markets and national policies can stand or fall depending on whether thousands of firms or millions of individuals willingly comply. Because of these linkages, hu-

man action at one level of aggregation may depend on events at another level. Theory about these relationships is relatively weak, but the problem is of active interest to social scientists in several disciplines. If excellent data sets are compiled, the problem of connecting levels of analysis may attract leading disciplinary researchers to the topic of global change to build theory that would aid in understanding it while advancing their own fields.

Linking time scales is also critical to the global change agenda. The question is this: Which social changes, occurring on the time scale of months to years, are likely to persist or be amplified over time, to the extent that they will be significant to the global environment on a scale of decades to centuries? Obversely, which short-term changes are likely to disappear over time? Physical scientists know which halocarbons are long-lived catalysts for the destruction of stratospheric ozone and which ones are quickly destroyed social scientists do not yet know much about which social changes catalyze other changes or about which ones are relatively irreversible. Historical cases, such as the CFC case, suggest some interesting hypotheses over the near-term, efforts to catalogue and compare such hypotheses would be a useful first step toward a theory of the long-term effects of social change. The general problem has received very little attention from social scientists. Improved understanding of the human analogues of long-lived catalysts may contribute to increased interest in long-term phenomena in social science.


Some species, such as rosewood, are selectively eliminated from the forest for economic reasons. It is reasonable to expect that in an ecosystem characterized by many smaller species, such as insects dependent on a single species for food, that the selective cutting of one tree species will cause multiple extinctions.

The mechanism is rather complex. Evapotranspiration in the Amazon forest appears to cause a regional climatic increase in precipitation. In such a regime, large-scale clearing, which reduces evapotranspiration per land area even if trees are replaced by other vegetation, will decrease rainfall downwind. Because species diversity in Amazonia is directly related to levels of rainfall, lower rainfall in any region can be expected to reduce the number of species in that region.

Species with large area requirements are disproportionately affected when forest clearing is fragmented, as it typically is in Amazonia. Under those conditions, an individual or functional group of individuals with a large area requirement is less likely to find adequate forest resources

Broad Misconceptions by Student Writers

The goal of writing is to put an idea or information on paper and communicate it clearly to others. The most important aspect of a scientific paper, arguably, is content. For science students, learning what to communicate is as important as how to write it. If a writer has nothing new or interesting to communicate, then how well she writes is not relevant. But poor writing also can undermine, or mask, a writer’s ability to communicate his good thoughts. Presenting ideas and information in a manner that conforms to the expectations of the audience is essential to communicating complex and new ideas and information clearly (10). This skill is enhanced by practice, emulating word choice, style, and organizational principles from examples within the literature, and soliciting detailed and consistent feedback from peers or mentors. As a general start, a writer should avoid the following two common mistakes for any assignment.

1. Do not write using the “spoken voice.”

One of the biggest misconceptions we have observed is the student assumption that, “if I can speak, then I can write.” It is difficult for students to turn off their “spoken voice” and turn on their “writing voice.” We suspect that this difficulty is the consequence of a lack of routine and sustained formal writing opportunities compared with occasions for informal writing during college or elsewhere. The informal writing styles acceptable for email, texting, social media posts, blogs, or diaries likely encourage the use of spoken voice in students’ formal work. Students’ written works for the sciences are submitted as hybrids of spoken and written voice, which must be rectified to be acceptable for a science audience and highlights the importance of proofreading.

2. Do not turn in the first complete draft.

Writing guides recommend free-writes for first drafts to focus on content without the constraint of style or formality (6, 30). Although it is not always the case, undergraduates often delay paper writing until the final day or hours before the due date. Ultimately, the first draft, which often reads as a free-write, is submitted with errors and spoken language throughout the document. Reading and evaluating a poorly written paper is frustrating for an instructor because it can be difficult to determine whether the bad writing is covering up good ideas or content. If bad writing is the source of the problem, then the student can remedy this by proofreading the paper or enlisting a peer to give comments. Experienced writers understand that producing a badly written first draft is an accepted part of the writing process. This understanding of the process makes it easier to put ideas on paper without constraints, subject to corrections in subsequent versions. Unfortunately, students often feel that it is more expedient to combine these steps in fact, the added pressure “to get it right” in the first draft may ultimately prolong the entire writing process.

Aging changes in the heart and blood vessels

Some changes in the heart and blood vessels normally occur with age. However, many other changes that are common with aging are due to or worsened by modifiable factors. If not treated, these can lead to heart disease.

The heart has two sides. The right side pumps blood to the lungs to receive oxygen and get rid of carbon dioxide. The left side pumps oxygen-rich blood to the body.

Blood flows out of the heart, first through the aorta, then through arteries, which branch out and get smaller and smaller as they go into the tissues. In the tissues, they become tiny capillaries.

Capillaries are where the blood gives up oxygen and nutrients to the tissues, and receives carbon dioxide and wastes back from the tissues. Then, the vessels begin to collect together into larger and larger veins, which return blood to the heart.

  • The heart has a natural pacemaker system that controls the heartbeat. Some of the pathways of this system may develop fibrous tissue and fat deposits. The natural pacemaker (the sinoatrial or SA node) loses some of its cells. These changes may result in a slightly slower heart rate.
  • A slight increase in the size of the heart, especially the left ventricle occurs in some people. The heart wall thickens, so the amount of blood that the chamber can hold may actually decrease despite the increased overall heart size. The heart may fill more slowly.
  • Heart changes often cause the electrocardiogram (ECG) of a normal, healthy older person to be slightly different than the ECG of a healthy younger adult. Abnormal rhythms (arrhythmias), such as atrial fibrillation, are more common in older people. They may be caused by several types of heart disease.
  • Normal changes in the heart include deposits of the "aging pigment," lipofuscin. The heart muscle cells degenerate slightly. The valves inside the heart, which control the direction of blood flow, thicken and become stiffer. A heart murmur caused by valve stiffness is fairly common in older people.
  • Receptors called baroreceptors monitor the blood pressure and make changes to help maintain a fairly constant blood pressure when a person changes positions or is doing other activities. The baroreceptors become less sensitive with aging. This may explain why many older people have orthostatic hypotension, a condition in which the blood pressure falls when a person goes from lying or sitting to standing. This causes dizziness because there is less blood flow to the brain.
  • The capillary walls thicken slightly. This may cause a slightly slower rate of exchange of nutrients and wastes.
  • The main artery from the heart (aorta) becomes thicker, stiffer, and less flexible. This is probably related to changes in the connective tissue of the blood vessel wall. This makes the blood pressure higher and makes the heart work harder, which may lead to thickening of the heart muscle (hypertrophy). The other arteries also thicken and stiffen. In general, most older people have a moderate increase in blood pressure.
  • The blood itself changes slightly with age. Normal aging causes a reduction in total body water. As part of this, there is less fluid in the bloodstream, so blood volume decreases.
  • The speed with which red blood cells are produced in response to stress or illness is reduced. This creates a slower response to blood loss and anemia.
  • Most of the white blood cells stay at the same levels, although certain white blood cells important to immunity (neutrophils) decrease in their number and ability to fight off bacteria. This reduces the ability to resist infection.

Normally, the heart continues to pump enough blood to supply all parts of the body. However, an older heart may not be able to pump blood as well when you make it work harder.

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Keywords: walking, body center of mass, pathological gaits, system approach, gait rehabilitation

Citation: Tesio L and Rota V (2019) The Motion of Body Center of Mass During Walking: A Review Oriented to Clinical Applications. Front. Neurol. 10:999. doi: 10.3389/fneur.2019.00999

Received: 27 March 2019 Accepted: 02 September 2019
Published: 20 September 2019.

Eric Yiou, Université Paris-Sud, France

Manh-Cuong Do, Université Paris-Sud, France
Romain Tisserand, University of British Columbia, Canada

Copyright © 2019 Tesio and Rota. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.


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