Paul R. Ehrlich & Anne H. Ehrlich, The Population Explosion, 1990.

Global Ecosystem Health

Population size influences our health, and thus our life expectancy, in two different ways. One is indirect, through its impact on "ecosystem health" -- the integrity of Earth's life-support systems. The human population is supported by services received from Earth's natural ecosystems, which, among other things, control the mix of gases in the atmosphere, supply freshwater, control floods, supply food from the sea and products from forests, create soils, dispose of wastes, recycle essential nutrients, pollinate crops, and control the vast majority of pests that might attack them.1 If those ecosystems collapse, the human economy will collapse as well, and Homo sapiens will undergo an unprecedented population crash. The larger our population, the more ecosystem services it demands. So it's therefore ironic that one of the greatest threats to the health of natural ecosystems is itself the growing number of human beings.

The other way population size influences health is direct, affecting what is classically known as "public health" -- the [111] health of the community one lives in as controlled by sanitation, preventive medicine,, and social services.

Food is intimately involved in both kinds of health. Human attempts to produce more food more often than not reduce ecosystem health, which in turn undermines the ability of terrestrial and aquatic ecosystems to supply humanity with sustenance. Undernourished people are more susceptible to diseases and more likely to die from them. We have already discussed many of the local and regional impacts of people on ecosystems and the resultant loss of ecosystem services; in this chapter, we examine the health of ecosystems on a global scale. The population connection to global environmental problems is usually major and often obvious. In the next chapter, we look at public health, where the population connection is present, but often subordinate to other factors and sometimes difficult to demonstrate.


One of the most important ecosystem services, maintaining the appropriate mix of trace gases in the atmosphere, is also among those most tightly connected to population growth. The connection is two-way. The growing human population is a major factor in the disruption of this service, and that disruption may well have exceedingly dire consequences for humanity. These connections are most evident in the case of global warming.

Our planet is kept habitable by the presence in the atmosphere of tiny amounts of "greenhouse gases" that, in essence, trap heat close to the surface.2 The best known of these heat-holding gases are water vapor and carbon dioxide (CO2), but there are more than twenty others, including methane, nitrous oxide, and ozone. Were there too little of these, Earth would be a frozen sphere rather like Mars. With too much, Earth, like Venus, would be too hot to support life. In short, we benefit from just the right level of "greenhouse effect."

Since the start of the Industrial Revolution, however, humanity has been adding CO2 to the atmosphere, primarily by burning fossil fuels and secondarily by cutting down and [112] burning forests. Carbon dioxide is released when anything organic (containing carbon) is burned; and CO2 is removed from the atmosphere by plants in the process of photosynthesis on land and in the sea. It is also removed by a number of chemical and nonphotosynthetic biological processes, mainly in the oceans. Cutting and burning forests thus adds CO2 to the atmosphere, unless the forest is replanted and can reabsorb carbon from the air, sequestering it in leaves, branches, trunks, and roots. As Peter Raven, director of the Missouri Botanical Garden and home secretary of the U.S. National Academy of Sciences, recently pointed out, the fires in the Brazilian Amazon in 1987 (which covered 77,000 square miles) contributed about a fifth of all the CO2 that flowed into the atmosphere that year.3

All this CO2 flowing into the atmosphere, in combination with increasing releases of other greenhouse gases, is gradually warming the entire planet, turning up the heat on the atmospheric system. It now appears that the heating may be detectable in climatic records; one of the most compelling bits of evidence is that, globally, the six hottest years of this century were in the 1980s. Furthermore, scientists examining satellite data have recently concluded that the oceans have been heating up by nearly 0.2 degrees Fahrenheit per year,4 and at the same time evidence of an annual one-twelfth-inch rise in sea level as a result of warming has been reported by other scientists.5

Whether or not the warming has yet had a detectable influence on the weather is hotly debated. Detecting a subtle warming trend is one thing; identifying its effects on weather is another. For example, the 1988 drought was precisely the sort of event that computer models predict will become more frequent as the warming continues. So were the near-record-size hurricanes in the Gulf of Mexico the same year and the West Indies and South Carolina in 1989. In October 1989, the Philippines were struck by three powerful typhoons. Hurricanes can be thought of as devices for transferring enormous amounts of heat and moisture from equatorial regions toward the poles.

Still, observation of events of a sort predicted to become more frequent with global warming does not demonstrate that [113] they were caused by a warming. We may never know whether the drought or the hurricanes were a result of the buildup of greenhouse gases or merely events that are part of normal climatic variability. Unless the climate slips over an unpredicted threshold, it will be extremely difficult to identify the start of climatic change due to the warming. If that change has started, it could be a decade or so before scientists have the data to be certain they have detected it. Several years of cool, wet summers and bumper grain crops in the United States could easily occur, even if the computer models of the impacts of global warming are correct. Such a stretch of favorable weather would very likely cause a relaxation of concern about the possibility of catastrophic change in climate. But the long-term environmental trends matter more than the transient events to which we more often pay attention.6

The role of population size and growth in generating the excess greenhouse gases can be seen with a few simple calculations. It is widely recognized that industrialized countries, with less than a quarter of the world's population, are responsible for roughly three quarters of the CO2 released by burning fossil fuels -- in automobiles, power plants, and other industrial apparatus used mainly by the rich. Coal is the worst offender among fossil fuels in terms of carbon dioxide per unit of energy generated; natural gas releases only a little more than half as much as coal, and petroleum is in between.

Suppose the United States decided to take the dramatic step of cutting its contribution to the CO2 component of the global warming by terminating all burning of coal. That would necessitate substantial readjustments in our economy, since coal now supplies almost a quarter of U.S. annual energy consumption. Replacing coal with energy sources that don't release CO2 (conservation, wind power, solar-voltaic panels, passive solar, hydroelectric, geothermal, nuclear fission) would require considerable effort and would carry other environmental costs.7

Suppose also that China's population remained at 1.1 billion -- a very optimistic assumption, since demographers project it to rise at least to 1.4 or 1.5 billion, and some Chinese experts claim it has already exceeded 1.2 billion.8 Suppose [114] further that China scaled back its development plans so that it only doubled its per-capita consumption of commercial energy (it presently plans to more than double its use of coal by 2000).9 That would raise Chinese per-capita energy use to some 14 percent of the U.S. level, about on a par with Algeria. Assume further that China produced that energy by using its vast stocks of coal. This modest development advance by China, certainly a legitimate goal by any standard, would more than offset the reduction of CO2 emissions achieved by America's abandonment of coal.

Even without considering the growth of populations of either rich or poor countries, the huge populations we already have can magnify small and reasonable per-person changes into gigantic impacts. Small per-person changes can have very large effects when multiplied by enormous numbers of persons!10 The P factor in the I = PAT equation is critical here.

Accounting for projected population growth makes the situation look even bleaker. What if, in the course of development, India's per-capita energy consumption rose only to about the level of China's today -- about 7 percent of U.S. per-capita consumption? That, combined with India's projected population growth, by the end of the next century would inject as much additional CO2 into the atmosphere as would result from doubling China's per-capita energy use with no increase in China's population. That calculation is based on the projection we made earlier that India's population will reach two billion near the end of the next century, assuming success in family planning.11 The dilemmas of both China and India underline the latent problems the world must face because of previous uncurbed population growth and demographic momentum.

Poor nations are now relatively minor contributors to the CO2 load generated by burning fossil fuels, but a significant realization of their legitimate aspirations to develop, multiplied by their population growth, will change that very quickly. While the United States might manage its energy use in a way that would compensate for the per-capita increases just discussed for either China or India, it certainly would be hard pressed to compensate for both without dramatic changes in [115] lifestyle. Western Europe and Japan use their energy much more efficiently than we do and so have considerably less potential for energy conservation. Likelier sources of compensation through conservation are the Soviet Union and Eastern Europe, whose per-capita level of fossil-fuel use is high, but whose energy technology is very inefficient. The USSR uses roughly two thirds as much energy as the United States does with a modestly larger population, and has a standard of living less than half of ours. Poland's energy use is equivalent to Sweden's and higher than that of Switzerland or France; but since it's much less efficient, the standard of living is much lower.

It is clear that significant changes in energy patterns will be required in all rich countries in order to slow the injection of CO2 into the atmosphere if poor nations are to enjoy even moderate levels of development. But the changes are technically feasible, and fast becoming more so. The political determination to make the changes has yet to become widespread, however.

Other contributions of population growth to the CO2 problem are also very substantial, but more difficult to measure. Plants take up carbon dioxide in the process of photosynthesis; when they die and decay or are burned, they release it again. When trees are cut, carbon stored over decades or centuries is released. In the tropics, much clearing of forests is for agriculture (often unsustainable) to meet the food needs of increasing populations. The amount of carbon dioxide being added to the atmosphere in this way is uncertain -- on the order of a fifth to a half as much as that contributed by fossil fuels.12

One obvious long-term measure that would help mitigate the greenhouse problem, therefore, is to regenerate forests.13 For this purpose, trees whose wood would be preserved even after cutting rather than burned should be favored as much as possible -- ideally, high-quality hardwoods in the tropics. These woods are preferred for construction and furniture.

But, unfortunately, it is precisely in the tropics that expanding human populations are contributing most heavily to the destruction of forests. The relationship is complex. As described earlier, industrialization of agriculture in southern [116] Brazil has created an army of landless people whom the government funnels toward the "frontier" of the Amazon. The government also has tried to encourage migration into the Amazon from the desperately poor, famine-afflicted northeast.14 Once in Amazonia, the migrants clear and burn the forests in an attempt, usually unsuccessful,15 to make a living by farming.

Brazil's high rate of population growth is an important factor in the destruction of its rainforest riches, stimulating the government's promotion of migration to the Amazon. But projects sponsored by various international "aid" agencies, which have facilitated the destruction by financing roads and dams to support settlement, have been at least as important.

On the Indian subcontinent, the destruction of forests is even more directly tied to overpopulation, as timber-cutters respond to the needs of cities for firewood and lumber. In the foothills of the Himalayas, nocturnal tree-cutters, called "owl men," sneak out to harvest the last of a dwindling lumber supply.16 A countervailing force in India and Nepal is the Chipko movement -- "tree huggers." These are men and women who understand the critical role trees play in the local economy and attempt to save trees by education and, if necessary, blocking axmen with their bodies.17

Similarly, in China, the original destruction of the forest cover also had close connections with that nation's enormous population growth.18 And, in spite of much propaganda about reforestation, the loss of China's forests continues. One of China's leading environmentalists, He Bochuan, estimates that since the Communists took over in 1949, forest cover has declined from 12 percent to less than 11 percent, despite massive attempts at reforestation, and that it will fall to 8.3 percent by 2000.19 In a quarter of a century, forest fires have consumed the equivalent of a third of all the surviving saplings from China's reforestation programs. Exact figures are difficult to determine, since it seems that about half of the trees planted in official statistics are in fact imaginary, and only 40 percent of those actually planted survive. The Chinese forestry situation is obviously rather grim. The People's Daily reported that annual consumption of wood for building, paper, and fuel was [117] 50 percent higher than regrowth, and that if this overdraft on the forests continued, "state timber enterprises would have nothing to log by the end of the century."20

China is only one developing nation facing this problem. A major source of the assault on forests and woodlots, especially in the arid and semiarid subtropics, is the dependence of more than 2 billion people on firewood for fuel.21 As Peter Raven put it, "The basic reason this [assault] is happening is population growth, brought into intense focus in the warmer parts of the world by extreme poverty."22

Overconsumption and overpopulation in rich nations are also responsible for deforestation in the tropics. Demand for cheap beef in fast-food outlets has created the "hamburger connection." In much of Central America and Amazonia, forests have been cut down to provide temporary pasture for cattle raising -- at least 10,000 square miles annually.23 For a few years, those pastures can produce cattle destined to be devoured by citizens of developed nations. Then the pastures are abandoned as wasteland, and other pieces of forest are cut down to replace them. The immediate economic yield from destroying the forests is greater than it would be from using them in any sustainable way, and immediate yield is the main goal of the present economic system.24

Similar stories could be told about Japanese woodchipping of forests in Papua New Guinea, Thailand, Malaysia, Colombia, and Cameroon and elsewhere,25 grinding them up to make cardboard for packing around new electronic equipment, or about the destruction caused by demand in rich nations for tropical hardwoods. In each case, the consumption and technological parts of the I = PAT equation are important, but so is the population factor. Everything else being equal, if there were only half as many people in the rich world, they would be responsible for only half as much tropical deforestation.

Some rich countries are destroying their own forests as well. In Australia, some of the most interesting of all tropical forests, those in the politically backward northern state of Queensland,26 are being destroyed simply because the timber industry wants to prove it can do it -- regardless of what the rest of the world thinks.27 In British Columbia, the Ministry of [118] Forestry is cooperating in the unsustainable destruction of that province's precious virgin forests. The rape of U.S. forests, especially old-growth forests, in Alaska and the Pacific Northwest (largely at taxpayers' expense) is a continuing disgrace -- one only partially hidden from the public by the practice of leaving a narrow band of trees lining main highways. The forests of those areas are being destroyed by large corporations which care nothing about either the environment or long-term employment for local people in the timber industry.28

Again, this destruction, all of which contributes to global warming, can be partly assigned to faulty harvesting and inadequate reforestation, as well as to overconsumption of paper and other forest products. But population plays its inevitable multiplicative role: large (and still growing) numbers of Americans, Canadians, Europeans, and Japanese want homes, furniture, paper, and other products in which wood is used.29

In the next few decades, methane could almost equal CO2 in importance as a trace gas in causing greenhouse warming. A molecule of methane traps roughly twenty-five times as much of the sun's heat as a molecule of carbon dioxide, and the concentrations are rising in the atmosphere twice as fast. The population connection with methane emissions is very clear, because the principal known sources of methane include rice paddies,30 the flatus of cattle,31 and soils of forests and fields that are cleared and burned by farmers. Another major source of methane appears to be the putrefying contents of garbage dumps, and it has been suggested that sun-baked asphalt is yet another.32 All of these sources are intimately tied to the size of the burgeoning human population, so substantial reductions in methane emissions will not be easily achieved without substantial success at population control.

If the climatologists are correct, and the vast weight of the evidence indicates they are, global warming means much more than simply a few degrees Fahrenheit increase in average temperature between now and the middle of the next century.33 The need for more air-conditioning to deal with more and hotter heat waves, some rise in sea level, and more frequent and destructive hurricanes could be the least of our problems. The worst consequence of global warming is likely to be alterations [119] of climatic patterns caused by the rising temperatures, changes that will occur at a rate unprecedented in history.


While the causes of global warming can be traced through various paths to the activities of a growing human population, the climatic effects of global warming on producing food for that population cannot be predicted with certainty. Computer models, however, suggest that climatic change from greenhouse warming will be rapid -- possibly ten to sixty times faster than the average natural rates of change since the last ice age.34 Those models also suggest that one of the more likely results will be a decrease in water availability in the world's principal grain belts.35 This pace and kind of change will inevitably cause large-scale disruptions in world agriculture. As climate belts shift rapidly, major adjustments in irrigation and drainage systems will be required at a cost that could be as high as $200 billion worldwide.36 Farmers in many areas will have to switch to drought-resistant crops, and will be forced to accept the lower yields that such crops produce. Drought-reduced harvests, like those of 1988, can be expected to occur with increasing frequency and severity.

A northward migration of climatic belts favorable to grain production might at first glance appear beneficial to agriculture in regions like Canada and much of the Soviet Union, where low temperatures and growing-season frosts are limiting factors. But if grain production is shifted to those areas, their often thin, infertile soils will limit yields.

Because CO2 is an essential raw material for photosynthesis, it has been speculated that an increase in CO2 concentration would enhance productivity. In our view, it is doubtful that this will yield a net benefit in the face of so many other limitations. Higher temperatures and increased CO2 may unfavorably change relationships between crops and their pollinators, competitors, or pests.37 For instance, crop plants might grow larger, but supply proportionately less nutritive value per unit of produce, as the ratio of carbon to nitrogen in the tissues increases. Moreover, insect pests may well eat more of the [120] crop to compensate.38 No one has any idea what shifts might occur in which pest attacks what crop. At the very least, depending on beneficial effects on crops from higher concentrations of CO2 to compensate for the climatic impacts of warming would be an extremely dangerous gamble.

Finally, the conservatism of governments will result in considerable delay and exacerbate the problems involved in making adjustments. For instance, the United States Congress had only begun to discuss taking steps to prepare for or to delay the possible effects of global warming a year after the magnitude of the 1988 harvest disaster was apparent. The administrative branch had not taken any initiatives by mid-1989. Even though it was not certain that the drought was caused by the warming, the hot, dry summer did elicit more than enough scientific testimony to make a prudent government take out some "insurance."39

Food security will be influenced by global warming in ways other than through change of climate. The rise in sea level will cause losses in food production through flooding of coastal agricultural land and damage to fisheries by inundating coastal wetlands that support them.40 Low-lying, fertile, and heavily populated deltas such as those of the Nile and the Brahmaputra/Ganges (Bangladesh) will be submerged first. Developed countries, though more capable of resisting the rising seas, will not be immune. Holland may have to flood some of its reclaimed agricultural land with Rhine River water to prevent saltwater intrusion into groundwater supplies.41 Florida and its citrus industry may eventually disappear.


To examine the possible effects of climate change on food production, our group at Stanford constructed a simple global model that simulated population growth, annual agricultural output, annual food consumption, and the effects of unfavorable weather patterns such as those that occurred in 1988.42 The model determines the amount of food available for consumption (production plus carryover stocks) in each year over [121] a twenty-year period. For all runs of the model, it was assumed that average increases in grain production in years with favorable weather would keep up with population growth (1.7 percent annually). In those years, a surplus of 50 million metric tons of grain was stored. The model then was used to examine the effects of different frequencies and severities of widespread unfavorable weather patterns.

Under the most "optimistic" scenario, unfavorable climatic events occurred on average once every five years and caused a 5 percent reduction in global grain harvest, roughly the size of the drought-caused drop in 1988. Under the most "pessimistic" scenario, the average time between unfavorable climatic events was set at 3.3 years, and each event was assumed to cause a 10 percent drop in grain production below the trend.

In order to simulate the feedback between availability of food and population size, it was assumed that a food deficit of one ton of grain resulted in two deaths. Roughly three people are supported by each ton of grain produced now, but about one third of all grain is fed to animals; so theoretically some of the shortfall could be made up by feeding people the grain directly. Even so, actual death rates from starvation might be raised further than the model indicates. Undernutrition occurs mainly among the poorest people, say the bottom quarter or fifth of the population. This group bears the brunt of any deficits, while the rest usually maintain adequate diets, albeit at higher prices. Because of the disproportionate burden on the poor, diseases and hunger may take a heavier toll on them than our all-or-nothing simplification suggests.43

Results of the model suggest that the optimistic scenario (a 5 percent reduction in grain harvest on average twice per decade) would not lead to complete depletion of world grain stocks, although world food security (adequate carryover supplies to compensate for unexpected crop failures) would be threatened. These reductions would have little effect on overall population growth. Under the pessimistic scenario (10 percent reductions on average three times per decade), however, severe deficits in grain stocks occur about twice per decade, each causing the deaths of between 50 and 400 million people. [122]

Weather patterns that might cause such drops include, for instance, repeats of the 1988 North American/Chinese/Soviet drought, equally or more severe, and totally different weather patterns involving other areas. In short, the model ignored the question of the pattern of crop failures that would lead to large declines in grain production. It also did not consider compensatory actions such as bringing set-aside land in the United States back into production, conversion from animal feed to food crops, or the general intensification of agricultural activity that would result from increased demand for food, except to the degree they are included in our "constant average increase" assumption. The model may also have been pessimistic in not incorporating increases in production that might result from technical innovations stimulated by famines.

On the other hand, some of the assumptions about carrying capacity were optimistic. It did not, for example, incorporate additional drops in harvest due to social breakdown related to famines, the spread of disease through malnourished populations, or inappropriate aid programs that damage the agricultural sectors of recipient nations. Indeed, the basic assumption of food production keeping pace with population growth in the absence of unfavorable climatic events should be considered very optimistic, since this is no longer the case in Africa or Latin America, and it is almost twice the rate of production increase projected as the maximum by Lester Brown.

Such a model, of course, is simply an aid to thinking about the possible consequences if short-term climatic change were to cause drops in grain production roughly comparable to those known to have occurred before, and considering the rest of the system to be essentially "surprise free." The results were not predictions, simply indications of the nature of problems that may occur if the global warming leads to more frequent and more severe climatic events that are deleterious to agriculture. They show that, if global warming progresses as many climatic models suggest it might, there is a risk of serious famines, each of which could kill more people than any war in human history. The results also indicate that climate change at the very least would reduce the margin of safety in the global food system. [123]

The population-food system has no "fail-safe" backup mechanisms designed into it, even if climates should remain favorable to food production. The world depends upon the statistical "cushion" that adverse weather and unusual pest outbreaks do not occur everywhere at once.44 To the degree that global food production becomes more concentrated (as in North America), humanity becomes more vulnerable.


Whereas the greenhouse warming represents an impending catastrophe, serious damage from acid rain45 is already upon us. Across North America and Europe, loss of life in lakes, streams, and forests -- and of the services of those ecosystems -- originates in the sulfur and nitrogen oxides that emanate from smokestacks and vehicle exhaust pipes.46 Too many automobiles, too many industrial products, and too much energy use per person inflate the consumption factor of the Impact = Population X Affluence x Technology (I = PAT) equation. Failure to invest in smaller, more energy-efficient cars and a refusal to pay for adequate pollution controls in factory and power-plant stacks make the ecologically damaging technology (T) factor substantial as well. But the P factor is important here, too. If there were only half as many Americans driving cars, using manufactured devices, and consuming electric power, acid-rain problems would be comparatively negligible, even if levels of per-capita consumption and pollution control were identical.47

The problem is becoming truly global. Vast areas of the world have precipitation substantially more acidic than preindustrial natural levels, including nonindustrial regions. Very acid precipitation has been recorded in remote, nonindustrial parts of China, due to coal burning for heating, cooking, and water purification.48 Recent reports of acid precipitation have come from tropical Africa, produced there by agricultural burning to clear shrubland and encourage the growth of grass.49 One does not have to await more detailed studies of the impact of increased acidity on living systems to be very apprehensive about this. Biologists know from first principles [124] that rapidly changing the acidity of an ecosystem is a good way to disrupt its functioning.50

In the African case, the population element of the I = PAT equation obviously dominates. As atmospheric scientist Paul Crutzen pointed out, there will be increased air pollution from the tropics as the population grows and more forests and savannas are turned into fields and grasslands that are burned more frequently.51 It is not yet known how sensitive the African rain forest will prove to be to acid deposition, which in this case is combined with an assault from toxic ozone, also generated by the fires.

The prospects of nearly worldwide damage to vulnerable forests and tree crops in the future is not a cheering one in a world in which both forest and agricultural ecosystems are likely to be badly stressed from climate change -- and many forests will be under a multitude of other population-related assaults. Furthermore, forest damage may itself contribute to climate change by putting still more CO2 into the atmosphere, reducing the amount of moisture recycled through vegetation, and changing the reflectivity of the land surface.52


The global environmental problem that has the loosest population connection is depletion of the ozone layer in the upper atmosphere which shields people, other animals, and plants from dangerous ultraviolet-B (UV-B) radiation. Without the ozone layer, life on land would be transformed into something like life under an ultraviolet sterilizer on an old-time roadhouse toilet seat -- it would essentially be impossible. Depletion of the ozone layer threatens the flow of services from terrestrial ecosystems by damaging or destroying the ecosystems themselves.

For every 5 percent decrease in the ozone layer, there is a 10 percent increase in the UV-B reaching Earth's surface. That increment of radiation would produce roughly twenty thousand additional skin cancers per year in the United States, of which about one thousand would be fatal. So far the ozone layer over the United States has thinned 2 to 3 percent.

Everyone by now is aware of the danger of increased skin [125] cancers from exposure to UV-B, and most know that it increases the chances of cataracts. But the public is less aware that the impacts of increased UV-B are far broader than these risks suggest; we hear about them because scientists know more about them. But UV-B is widely injurious to virtually all forms of life. It damages DNA (the genetic material), impairs the immune systems of human beings, and inhibits photosynthesis. Algae, the base of marine food chains, are extremely sensitive to UV-B, and their populations in surface waters (which are penetrated by UV-B) could be reduced, with deleterious effects on fisheries dependent on them. More UV-B exposure also could make human populations more susceptible to disease and have damaging effects on fisheries, natural ecosystems, and crops. Broad-leaved crops such as soybeans appear especially susceptible. Exactly what the total biological impacts will be at various levels of exposure are unknown; all we can say is that as the ozone shield thins, life on Earth's land surfaces will become more difficult, and more skin cancers could be the least of our problems.

The threat to the ozone layer comes largely from the synthetic compounds known as chlorofluorocarbons (CFCs), which are used as refrigerants, foaming agents in plastics, and aerosol propellants -- and are potent greenhouse gases to boot.53 CFCs in aerosol spray cans have been banned for most uses in the United States since 1977, but are still widely used elsewhere.

Substitutes can be found for CFCs in all these uses; some may cost more to produce or will require changes in refrigerator design that will make them more expensive. Some substitutes will cause refrigerators to be less efficient.

The CFC threat to the ozone layer can be abated by operating only on the affluence and technology factors of the I = PAT equation. But even here the job would be eased if there were fewer people -- especially in poor countries such as China. That nation has the goal of providing refrigeration for its entire population. China had planned to use CFCs rather than more expensive substitutes, since it must strive to minimize the costs of development, which are already enormous because of its huge population.

Nonetheless, the ozone situation is a bellwether, because [126] eliminating the threat to the ozone shield is simple compared to the efforts that will be required to slow the global warming, abate acid precipitation, arrest the general toxification of the planet, or save a substantial portion of biodiversity. A first international protocol on reducing CFC production was reached in Montreal in 1987. Then it was found that ozone depletion was occurring faster than thought earlier. Negotiations for a necessary strengthening of that protocol with the goal of ending all production and use by 2000 have proceeded rapidly. An agreement to that end was signed in Helsinki in May 1989 by eighty nations, including the U.S.A., China, and India. The nations also agreed to establish a fund to help poor nations to develop alternates.54 We hope that complete elimination of these compounds will occur, and that the rich will honor their commitments to help the poor with the inevitable costs. Most of all, we hope that the ozone protocols will serve as a model for dealing with other global environmental problems.


One of the most widespread population-related environmental problems is the ecological degradation of Earth's land surface in a process called "desertification." Desertification is caused by destruction of vegetation by woodcutting, burning, and overgrazing, by erosion by water and wind as a result of poor land-management, by salinization and waterlogging of irrigated fields, and by soil compaction (by cattle hoofs, tractors, drying, and the impact of raindrops on denuded soil surfaces).55 Its terminal stage is easily recognizable -- a barren wasteland, virtually devoid of vegetation, familiar to those who have seen TV stories of famine in the Sahel. A functional ecosystem is degraded to the point where it can provide few, if any, services to humanity.

But in its earlier stages, desertification can go unrecognized by most people. For instance, overgrazing has ruined much of the grasslands of the western United States. Nonetheless the average citizen of, say, Albuquerque, New Mexico, does not realize that he or she lives in an area desertified by [127] human action -- that the upper Rio Grande Valley was once a rich grassland.56

The United Nations has estimated that globally about 13 million square miles (almost four times the area of the fifty United States) of arid and semiarid land have lost about a quarter of their potential productivity due to desertification classified as "moderate." Almost 6 million square miles have lost over half of their potential productivity, and are severely desertified. Over 80,000 square miles have been reduced to zero economic productivity annually.57 The areas most affected include the margins of the Sahara, eastern and southern Africa, much of south-central Asia, Australia, the western United States, and southern South America. Desertification even threatens relatively humid tropical areas where deforestation can change local climates and turn an area previously rich with life into a wasteland.58 Approximately 230 million people, mostly in poor nations, are said to be directly and deleteriously affected by desertification.59

Unfortunately, these numbers are but rough estimates, and present a picture of "deserts on the march" that greatly understates the complexity of the situation. For instance, the image of the Sahara moving inexorably southward may well be inaccurate. Satellite studies in the 1980s show "a generally southward retreating vegetation front in the Sahel in 1982 to 1984 and a generally northward advancing vegetation front in 1985, 1986, and 1987."60 It is probably more accurate to view desertification as a process of repeated pulses of land deterioration "from centres of excessive population pressure"61 than as a process originating from edges of established deserts.

While understanding the precise pattern of land deterioration is necessary to reversing that deterioration, we must not let disagreements over that pattern or over estimates of the amount of desertification obscure the basics of the situation. It is a huge global problem; too many people is one of its major causes; and population growth interacts with bad land-use policies and changing socioeconomic conditions to produce it.

It is no accident that the most serious desertification is found in areas where burgeoning human populations are contributing to rapidly changing land-use patterns. For instance, [128] the 1950s and 1960s were a period of unusually favorable rainfall in the Sahel. As a result, cash-crop agriculture expanded along with the human population. Specifically, the population of Niger increased from 2.5 to 3.8 million from 1954 to 1968, and peanut farming expanded from just over 500 square miles to some 1700 square miles. Nomadic herders of the Sahel, who previously grazed animals on land that had disappeared under cash crops, were displaced to the north. They stocked new lands (which tribal traditions taught were undependable for the long term), and their herds increased during the moist phase. Then, as tradition predicted, the climate turned dry again. The vegetation was completely removed by cattle, camels, and goats, and millions of animals died. An unknown number of people, probably around 100,000, perished in the resulting famine.62

Overpopulation of grazing animals and the people who depend on them in the Sahel often resulted from the drilling of tube wells. The wells allowed herds to build up beyond the long-term carrying capacity of an area. Cattle must trek daily to water in order to survive, and their movements destroy vegetation and compact the soil. They concentrate around the well sites, eating and trampling vegetation, and degrading the soil of an ever-increasing area. Even the droppings of cattle add to the process. "Cowpats" dry rapidly in the sun, heating up and killing the bacteria and fungi that otherwise would speed their decomposition. The dried cowpats form a "fecal pavement" that discourages the sprouting of fresh grass.63 The drilling of even more wells is often seen as the solution to this problem, but clearly it is much more likely simply to exacerbate it.64

The Sahel tragedy is just an extreme example of a general trend on Earth's grasslands. As human populations expand, so do those of the livestock that supply food, draft power, and dung used as fertilizer or (in extremis) fuel. Not only in the developing world but in much of the intermountain United States,65 herds now exceed the carrying capacity of the land. The animals eat the grass faster than it can grow. In each of nine nations in southern Africa, cattle exceed the carrying capacity of the range by 50 to 100 percent.66 In desertified Indian states such as Karnataka and Rajasthan, the range can [129] carry only 50 to 80 percent of the cattle herds now on them. Many of the animals are emaciated, and droughts kill hundreds of thousands.67

In China, between 1949, when the Communist government took over, and 2000, it is estimated that the total area of desert will have doubled. At the moment, about one sixth of the nation is desert. In Inner Mongolia, some 33,000 square miles are threatened with desertification, and in northern China 15,000 square miles of farmland and 20,000 square miles of grasslands are also threatened. Between 1983 and 2000, an increase of almost 30,000 square miles of desert is expected in northern semiarid and arid areas.68


If one wants to see blatant evidence of the squandering of the human inheritance in the United States, perhaps the best way to do so is to travel to one of our most grossly overpopulated areas: south Florida. South Florida is an ideal real-life laboratory in which to observe the impacts of overpopulation on the quality of life in general and on ecosystem services in particular. If Florida were an independent nation, it would be one of the fastest growing in the world. Its population growth rate is about that of Bangladesh, 2.8 percent per year, which if continued unchanged would increase its 1987 population of 12 million to 17.5 million by 2010. A major difference, however, is that Florida's growth is not the result of a high birthrate but of immigration, about a quarter of which is of elderly people choosing to retire in a benign climate.

The signs of explosive growth are everywhere evident in south Florida. Lake Okeechobee is heavily polluted, and groundwater tables are dropping. Suburban developments are marching steadily into the once-wildlife-rich Everglades,69 their invasion made possible by the draining of the marshes and the termination of the ecosystem services they once provided. Over this scene looms the highest point in south Florida, majestic "Mount Garbage," the Miami sanitary landfill. Mount Garbage doesn't get the job done, though. Informal dumps line [130] the area's back roads. Plastic garbage bags and derelict refrigerators decorate those roads and complement the plastic-bottle-festooned shorelines of the Florida Keys, accented by old fishing nets and floats, six-pack wrappers, and other colorful debris made from processed petroleum. Mixed in with this are the globs, cakes, and stains of unprocessed petroleum, washed from the holds of passing tankers. All this and the generally sleazy development are the most obvious symptoms of a population already too large, growing too fast, and overconsuming nonrenewable resources.

Those superficial symptoms tend to distract attention from much more fundamental problems, most of which are connected to the freshwater flows through the marshes of the southern peninsula. Before European settlement, water drained southward along a gentle gradient from the northern part of the state into Lake Okeechobee. From there it flowed as a sheet, many miles wide and a few inches deep, to Florida Bay, the body of water between the Keys and the mainland. That sheet flow is the famous "river of grass" -- the central part of the Everglades. The rich estuarine waters of Florida Bay and the vast Everglades marshes supported a diverse flora and fauna, including deer, cougars, and millions of nesting wading birds. The influx of people and their agriculture and industry has put heavy demands upon this water supply, in terms of both its use and the course of its flow. The result has been a disaster for the area's wildlife. Only remnants of the previous bird populations -- about 10 percent -- now occupy the Everglades. As they so often do, the bird populations signal difficulties within the ecosystem.

The most poignant indicators of distress are the "panhandling" great white herons.70 Many of these 3-feet-tall, long-legged birds now patrol regular territories on docks and in the backyards of local human residences, begging for fish. The birds nest on mangrove islets in Florida Bay, where they are secure from raccoons and other mammalian nest-robbers. Once the herons supported themselves and raised their young on the fish of Florida Bay. But, as Audubon Society biologist George Powell and his colleagues have shown, the herons can no longer find enough of their natural food to raise enough [131] young to maintain their populations. Only the panhandlers are sufficiently successful reproductively to fledge enough young to replace themselves.71 They are dependent on the kindly retirees of the Keys.

The human-population-related changes in the Everglades wetlands apparently have reduced the productivity of Florida Bay, something that bodes ill for the commercial and sport fisheries that are critical to the economy of the Florida Keys. In addition, subtle changes in coastal waters are suspected of causing a decline in the coral reefs, a major tourist attraction in the area. And, unless effective steps are taken to restore the wetlands ecosystem, Everglades National Park, a major tourist attraction important to the local economy, may close its doors in a decade or so.

Florida not only has serious difficulties created by human intervention in its freshwater flows, but also is the state most at risk from a sea-level rise resulting from global warming. The state is low and flat; the bottom of Lake Okeechobee is at sea level. The 2- or 3-foot rise in sea level that may occur in the next half century would flood a substantial portion of the state. A big chunk of the southwestern Everglades will disappear as salt water intrudes at rates that neither mangroves nor marsh grasses can adjust to. More serious, the porous limestone shelf on which most of the state rests will permit salt to penetrate aquifers far inland as sea level rises. It has been estimated that for every foot of rise in sea level, there will be about a 40-foot reduction in the depth of fresh water in Florida aquifers.72 Indeed, the threat of salinization of aquifers is already present because of human manipulation of surface water flows even without rising seas.73

Most frightening of all, however, is the prospect of a higher sea level combined with the predicted increase in both the frequency and intensity of hurricanes. Storm surges will be carried far inland. It is likely, if the greenhouse warming continues as projected, that many acres of Florida now seemingly remote from the sea will, in the next century, find themselves swept by a fathom or two of fast-moving salt water.

Florida, even more than California, appears to be at the "edge of history." Growth is rampant, and human and natural [132] values are being lost in a development mania in many areas. Environmental groups are working hard to prevent development in critical parts of the Everglades ecosystem and pushing attempts to restore something resembling the original hydrologic regime. But, as in much of the rest of the world, their chances of success depend on curing both local and global problems.

Population growth and "development" in Florida must be halted, and the lifestyles and attitudes of Floridians changed. But that won't save the state if the world doesn't solve its global problems. Ozone depletion could make sunbathing lethal and help kill the state's agriculture (both directly and by contributing to climate change). Climate change due to greenhouse warming also could greatly exacerbate Florida's freshwater-supply problems. If that warming continues unabated, most of the state may disappear beneath the sea in a few centuries or less.


Overpopulation in rich nations obviously represents a much greater threat to the health of Earth's ecosystems than does population growth in poor nations. The rich contribute disproportionately to the problem of global warming, being responsible today for about 80 percent of the injection of carbon dioxide into the atmosphere from burning fossil fuels, and sharing responsibility for tropical deforestation, which also adds to the CO2 load. The developed nations probably also contribute more than their share of methane emissions, the second-most-important greenhouse gas. Similarly, most of the responsibility for ozone depletion, acid precipitation, and oceanic pollution can be laid at the doorstep of the rich. So can the local and regional environmental consequences of much of the cash-crop agriculture, tropical deforestation, and mining operations carried out worldwide.

Unfortunately, nations do not even attempt to keep statistics on average per-capita environmental impact of their citizens -- which, of course, js simply the combined A and T factors of the I = PAT equation. So, in order to make [133] reasonable comparisons, we must use a surrogate statistic for A x T: per-capita use of commercial energy. Much environmental damage is done in the mobilization of energy, and even more is done by its use. Energy use is central to many things we consider affluence (A), and lack of energy efficiency in the devices that provide affluence is a major cause of environmental damage (T).

Hundreds of thousands of fishes, sea birds, and mammals killed at Prince William Sound in Alaska, the death of lakes in the Northeast from acid precipitation, originating largely in midwestern power plants, and a contribution to global warning and acidification of ecosystems (CO2 and nitrogen oxides from vehicles and hundreds of power plants) all follow from mobilization of energy to power American society.

Energy is also used to pave over natural ecosystems to create airports and parking lots; energy is required to produce the plastic and paper and aluminum cans that clog our landfiUs and decorate our highways and seashores; energy powers the boats that slaughter the manatees in Florida lakes; energy was used to produce the pesticides and to mobilize the selenium from soils that kills birds in California's Kesterson National Wildlife Refuge; energy cools the offices of Arizona developers as they plan the further unsustainable suburbanization of the American southwestern deserts; energy warms the offices of oil-company officials in Anchorage as they plan the exploitation of the Alaskan National Wildlife Refuge.

Energy is being used to pump the Ogallala aquifer dry, and energy lets us fly in jet airplanes 30,000 feet above the circular irrigation patterns created by the pumping -- energy that did environmental damage when oil was pumped out of the ground and now is causing environmental damage as jet exhausts are spewed into the atmosphere. And, of course, energy damages when it is used to mine ores, win metals from those ores, and use those metals and other energy-intensive materials to manufacture automobiles, airplanes, TVs, refrigerators, and all the other paraphernalia of our civilization.

Poor people don't use much energy, so they don't contribute much to the damage caused by mobilizing it. The average Bangladeshi is not surrounded by plastic gadgets, the average [134] Colombian does not fly in jet airplanes, the average Kenyan farmer does not have a tractor or a pickup, and the average Chinese does not have air-conditioning or central heating in his apartment. In 1980, of some 400 million motor vehicles in the world, 150 million were in the United States, 36 million in Japan, 24 million in West Germany, 1.7 million each in India and China, and 181,000 in Nigeria.74

So statistics on per-capita commercial energy use are a reasonable index of AT -- of the responsibility for damage to the environment and consumption of resources of an average citizen of a nation. According to that index of AT, a baby born in the United States represents twice the destructive impact on Earth's ecosystems and the services they provide as one born in Sweden, 3 times one born in Italy, 13 times one born in Brazil, 35 times one in India, 140 times one in Bangladesh or Kenya, and 280 times one in Chad, Rwanda, Haiti, or Nepal.75

These statistics should lay to rest the myth that population problems arise primarily from rapid growth in poor nations -- although their impact is nontrivial and increasing very rapidly. They remind us that pppulation shrinkage is essential among the rich, since each birth forgone relieves on average much more of the pressure on Earth's resources and environment than a birth forgone in a poor nation.


In summary, overpopulation is rapidly degrading Earth's ecosystems in both rich and poor nations. The future of humanity probably depends much more heavily on the health of global ecosystems than on public health in the classic sense. Civilization can't persist without ecosystem services, and these are threatened in innumerable ways by the expanding scale of human activities. Curing cancer, for example, would increase the life expectancy of Americans by only a year or so; a collapse of ecosystem services will lower life expectancy by decades.

Perhaps the best way to end this discussion of the human environment is with a quote from The Population Bomb: [135]

How well are we treating these symptoms of the Earth's disease of overpopulation? Are we getting ahead of the filth, corruption, and noise? Are we guarding the natural cycles on which our lives depend? Are we protecting ourselves from subtle and chronic poisoning? The answer is obvious -- the palliatives are too few and too weak. The patient continues to get sicker.76


1. For more discussion of ecosystem services, see P. Ehrlich and A. Ehrlich, Extinction: The Causes and Consequences of the Disappearance of Species (Random House, New York, 1981).

2. For a detailed and authoritative treatment of this topic, see S. H. Schneider's Global Warming (Sierra Club Books, San Francisco, 1989). Also excellent, but more technical, is D. E. Abrahamson, ed., The Challenge of Global Warming (Island Press, Washington, D.C., 1989).

3. Calypso Log, June 1989, p. 7.

4. Nature, April 20, 1989.

5. T. Maugh II, "Ocean Data Shows Global Warming May Have Begun, Los Angeles Times, April 20, 1989.

6. For a detailed discussion of the evolutionary reasons why we have so much trouble paying attention to long-term trends, and what should be done about it, see R. Ornstein and P. Ehrlich, New World/New Mind (Doubleday, New York, 1989).

7. Conservation is by far the environmentally most benign and readily available alternative. With appropriate incentives we believe the transition could be made in about 15 years.

8. He Bochuan, a professor of philosophy at Zhongshan Univ., Guangdon, claims that to evade the one-child policy, many families do not register their children, so that the standard 1.1 billion figure is an undercount. Reported in K. Forestier, "The Degreening of China," New Scientist, July 1, 1989, p. 53.

9. China in 1987 burned some 650 millions tons of coal, accounting for about three quarters of its total energy consumption (about 850 million tons of coal-equivalent). The official government plan is for an annual consumption of between 1.4 and 1.5 billion tons of coal-equivalent by 2000, with coal accounting for an even higher proportion of energy use than it does today. In 1985, U.S. fossil-fuel consumption injected some 1.2 billion tons of carbon into the atmosphere (CO2 emissions are usually measured in tons of carbon); China's injected slightly over 500 million. Under the scenario presented here, China would very nearly have caught up with the U.S. by the turn of the century, provided U.S. emissions did not increase. The basic source for this information is Stephen Meyer, ed., Proceedings of the Chinese-American Symposium on Energy Markets and the Future of Energy Demand, Nanjing, China, June 22-24, 1988, published by Lawrence Berkeley Laboratory (available from NTIS, Springfield, VA 22161). Interestingly, in the same symposium the 3.2 million cars in China in 1985 were projected to increase to 13 million by 2000. Professor Lu Yingzhong of the Institute of Nuclear Energy and Technology, Beijing, looks further ahead in his apparently unpublished 1989 paper, "Some Comments on CO2 Issues in PRC." He gives a "low-nuclear-power" projection of coal use in 2025 as 2.6 billion tons; a "high-nuclear-power" projection of 1.75 billion tons. It is therefore pretty clear that China plans to emit more CO2 than the U.S. could offset by 2025 at the latest. China, by the way, had coal reserves in 1985 of 780 billion tons, about a third of the world total. Global warming will clearly restrain China's coal use long before supply becomes a significant constraint.

10. Material on population, coal, and CO2 is from P. R. Ehrlich and A. H. Ehrlich, "How the Rich Can Save the Poor and Themselves: Lessons from the Global Warming," in press in proceedings of conference on Global Warming and Climate Change: Perspectives from Developing Countries, held in New Delhi, India, Feb. 21-23, 1989, sponsored by the Tata Energy Research Institute, New Delhi.

11. The projection, of course, also optimistically presumes that India will somehow avoid massive rises in its death rates from starvation, disease, or social breakdown -- something that seems highly unlikely to us.

12. Roughly 1-3 billion tons of carbon are added annually by deforestation as compared to about 5.5 billion tons from burning of fossil fuels. There is considerable controversy surrounding the estimates of the contributions of deforestation.

13. S. Postel, "A Green Fix to the Global Warm-up," World Watch, September/October 1988, pp. 29-36.

14. Norman Myers, The Primary Source: Tropical Forests and Our Future (Norton, New York, 1984); J. O. Browder, "Public Policy and Deforestation in the Brazilian Amazon," in R. Repetto and M. Gillis, eds., Public Policies and the Misuse of Forest Resources (Cambridge Univ. Press, Cambridge, 1988), pp. 247-97.

15. Because of poor and erosion-prone soils, crop pests, malaria and other tropical diseases, insufficient farm credit, and lack of sound agricultural advice and support.

16. Associated Press, Oct. 27, 1987.

17. R. Hutchinson, "A Tree-Hugger Stirs Villagers in India to Save Their Forests," Smithsonian, February 1988.

18. Li Jinchang, Kong Fanwen, He Naihui, and L. Ross, "Price and Policy: The Keys to Revamping China's Forestry Resources," in R. Repetto and M. Gillis, eds., Public Policies and the Misuse of Forest Resources, p. 211.

19. Reported in Forestier, "The Degreening of China." The information that follows on fires is attributed to him and other forestry experts.

20. Quoted in Forestier, "The Degreening of China."

21. S. Postel, "Global View of a Tropical Disaster," American Forests, November/December 1988. According to FAO estimates, in 1980 almost 1.2 billion people in poor countries were cutting fuelwood faster than it grew; by 2000 half the people in developing countries will lack a sustainable supply of fuelwood. In many areas women and children spend a large portion of their time traveling in search of wood, which keeps them from other productive activities. It also affects diets, since quick-cooking cereals and tubers are substituted for more nutritious, slower-cooking foods such as beans.

22. Calypso Log, June 1989, p. 8.

23. See N. Myers, The Primary Source: Tropical Forests and Our Future (Norton, New York, 1984), chap. 7; also N. Myers, ed., Gaia: An Atlas of Planet Management (Doubleday, New York, 1984).

24. This holds true only as long as more forests exist to move to as each previous tract gives out. A new study indicates that even this destruction procedure is less remunerative than sustainable use even in the relatively short term (W. Booth, "Study Offers Hope for Rain Forests," Washington Post, June 29, 1989; C. Peters, A. Gentry, and R. Mendelsohn, "Valuation of an Amazonian Rainforest," Nature, vol. 339, pp. 655-56, June 29, 1989), although some of the estimates in the study were probably too optimistic.

25. N. Myers, Primary Source, pp. 104-5.

26. The state has long been run by a reactionary rural minority given control by gerrymandered electoral districts.

27. The industry claims the forests will regenerate, apparently not realizing that many of the plants require the microclimatic conditions of the forests to grow, and that the animals can't go into suspended animation to wait for the forests to return.

28. The economic reasons for "taking the capital" rather than harvesting on a sustainable-yield basis are discussed on pp. 164-65. In at least one case, the corporation is under pressure to pay off the junk bonds used in the takeover of the lumber company.

29. For an informative overview of forest policies globally, see R. Repetto and M. Gillis, eds., Public Policies and the Misuse of Forest Resources (Cambridge Univ. Press, Cambridge, 1988).

30. Natural bogs and marshes and termite flatus are also major sources.

31. It has been estimated that cow farts contribute annually almost 100 million tons of methane to the atmosphere (F. Pearce, "Methane: The Hidden Greenhouse Gas," New Scientist, May 6, 1989). A cow produces over 700 times as much methane as a human being, so the most obvious direct connection to human population growth is not critical. (See P. J. Crutzen, I. Anselmann, and W. Seiler, "Methane Production by Domestic Animals and Humans," Tellus, vol. 388, pp. 271-80, 1986.)

32. For a recent review of the role of methane in global warming see Pearce, "Methane: The Hidden Greenhouse Gas."

33. There are so many uncertainties in the levels of greenhouse gas emissions and the speed of the climatic system's response to it, that average warming by 2050 could be anything from as much as 9° F. to as little as 0.65° F., with about a 10 percent chance of the change being outside of those boundaries. The chances are around 50-50 that the pace of change will be some 10 to 60 times faster than long-term average natural rates of change. See S. H. Schneider, Global Warming.

34. S. H. Schneider, "The Greenhouse Effect: Scientific Basis and Policy Implications," testimony before Subcommittee on Water and Power Resources, Committee on Interior and Insular Affairs, U.S. House of Representatives, Sept. 27, 1988.

35. S. H. Schneider, Global Warming, and personal communication.

36. S. Postel, "Stabilizing Chemical Cycles," in L. Brown et al., State of the World 1987 (Norton, New York, 1987).

37. About all that can be said of the studies done so far (many of them nicely summarized in Schneider, Global Warming) is that they indicate the situation will be very complex.

38. See, for example, D. Lincoln, D. Couvet, and N. Sionet, "Response of an Insect Herbivore to Host Plants Grown in Carbon Dioxide Enriched Atmospheres," Oecologia (Berlin), vol. 69, pp. 556-60 (1986); D. Lincoln and D. Couvet, "The Effect of Carbon Supply on Allocation to Allelochemicals and Caterpillar Consumption of Peppermint," ibid., vol. 78, pp. 112-14 (1989); E. Fajer, M. Bowers, and F. Bazzaz, "The Effects of Enriched Carbon Dioxide Atmospheres on Plant-Insect Herbivore Interactions, Science, vol. 243, pp. 1198-1200 (1989).

39. We do not count the (tiny and overdue) raising of the automobile fuel-efficiency standards in 1989 as "significant" -- although it was at least a move in the right direction.

40. New wetlands will doubtless eventually replace the old ones in some areas, but the rate of their formation is likely to lag far behind the rate of inundation. Furthermore, many new shoreline areas will be already occupied by cities, highways, farms, and so on, and will not be readily available for conversion to coastal marshes.

41. Schneider testimony, "The Greenhouse Effect." See also Schneider's Global Warming, chap. 6.

42. See P. Ehrlich, G. Daily, A. Ehrlich, P. Matson, and P. Vitousek, Global Change and Carrying Capacity: Implications for Life on Earth, Paper 0022, Morrison Institute for Population and Resource Studies, Stanford University, 1989; in press, proceedings of National Academy of Sciences/Smithsonian Institution Forum on Global Change, Washington, D.C., May 3, 1989. The details of the model will be given in G. Daily and P. Ehrlich, "An Exploratory Model of the Impact of Rapid Climate Change on the World Food Situation," in preparation.

43. A food model developed at the International Institute for Applied Systems Analysis (Options, 1987, no. 1-2) concludes that lower food prices do not eliminate hunger, but "high-price scenarios yield increased numbers of starving, despite the consequent higher long-term food production in developing countries."

44. At the moment, we also have the theoretical option of consuming less livestock (especially those that are grain-fed) and using land now devoted to growing feed to producing food for human beings. But this "fail-safe" mechanism is certainly not "designed" for that role, and it is highly unlikely to function (we suspect the rich would continue to eat meat while the poor died in large numbers).

45. Technically, "acid deposition," since acid reaches the ground through rain, fog, snow, and dry deposition.

46. For an excellent overview, see J. Harte, "Acid Rain," in P. Ehrlich and J. Holdren, eds., The Cassandra Conference (Texas A & M Press, College Station, 1988), pp. 125-46.

47. The production of significant acid precipitation is a complex response to the injection of sulfur and nitrogen oxides into the atmosphere. For instance, when local pollution problems resulted in the building of much taller power-plant stacks, local air-pollution problems were rapidly converted to regional acid-precipitation problems. There also may be significant nonlinearities in the atmospheric chemistry.

48. For an extreme example, see J. Harte, "An Investigation of Acid Precipitation in Qinghai Province, China." Atmospheric Environment, vol. 17, pp. 403-8 (1983). In northern China, airborne dust tends to neutralize acid rain, but in southern China rainfall with an average pH less than 4.5 falls in 13 cities (reported in J. Silvertown, "A Silent Spring in China," New Scientist, July 1, 1989, p. 57). In Guandong, Guangxi, and Hubei provinces (all southern) the pH of the rain is between 4 and 4.2 (Forestier, "The Degreening of China"). Rainfall with a pH of about 4.5 caused great biological difficulties in Adirondack lakes. Its impacts on Chinese natural aquatic communities and aquaculture facilities will depend on the buffering capacity of the soil.

49. M. Simons, "High Ozone and Acid-Rain Levels Found Over African Rain Forests, New York Times, June 19, 1989.

50. The severity of damage to biological systems from acid deposition, and the speed with which that damage appears, are very largely a function of the buffering capacity of the soil. Ecosystems with very alkaline soils may show very little damage over very long time periods.

51. Quoted in Simons, "High Ozone and Acid Rain."

52. How the climate system works was described early in chapter 5; more information is in the Appendix. See also S. H. Schneider and R. Londer, The Coevolution of Climate and Life (Sierra Club Books, San Francisco, 1984).

53. CFCs, if uncontrolled, could account for as much as a quarter ot warming in the next century (Schneider, Global Warming).

54. Development Forum, May-June 1989. ,

55. H. E. Dregne, Desertification of Arid Lands (Harwood, New York, 1983), and "Combating Desertification: Evaluation of Progress," Environmental Conservation, vol. 11, pp. 115-21 (1984).

56. D. Ferguson and N. Ferguson, Sacred Cows at the Public Trough (Maverick Publications, Bend, Ore., 1983).

57. M. A. F. Kassas, "Ecology and Management of Desertification," in H. J. De Blij, ed., Earth '88: Changing Geographic Perspectives (National Geographic Society, Washington, D.C., 1988), p. 198; UNEP, General Assessment of Progress in the Implementation of the Plan of Action to Combat Desertification, Report of the Executive Director, 1984, Nairobi, UNEP/GC.12/9.

58. Dregne, op. cit.

59. Independent Commission on International Humanitarian Issues, The Encroaching Desert: The Consequences of Human Failure (Zed Books, London, 1986).

60. R. Nelson, quoted in B. Forse, "The Myth of the Marching Desert," New Scientist, Feb. 4, 1989, p. 32.

61. Ibid.

62. N. Myers, Gaia: An Atlas of Planet Management; P. R. Ehrlich, A. H. Ehrlich, and J. P. Holdren, Ecoscience: Population, Resources, Environment (Freeman, San Francisco, 1977), p. 628.

63. P. R. Ehrlich, The Machinery of Nature (Simon and Schuster, New York, 1986); David Hopcraft, personal communication.

64. R. Baker, "Famine: The Cost of Development," Ecologist, vol. 4, pp. 170-75 (June 1974).

65. Ferguson and Ferguson, Sacred Cows at the Public Trough. Overstocking in the U.S. West has no significant human-population component, since the relatively tiny amount of beef produced does not go to hungry people or even allow pressures to be reduced on tropical forests being cleared for pasture. It is mostly a story of greed, stupidity, and the ignorance and incompetence of people ranging from senators to bureaucrats, as this fine book shows.

66. Southern African Development Coordination Conference, SADCC Agriculture: Toward 2000 (FAO, Rome, Italy, 1984).

67. L. R. Brown, and C. Flavin, "The Earth's Vital Signs," in Brown et al., State of the World 1988 (Norton, New York, 1988), p. 9.

68. Forestier, "The Degreening of China."

69. The Everglades have been under threat for a long time; see J. Harte and R. Socolow, "The Everglades: Wilderness Versus Rampant Land Development in South Florida," in J. Harte and R. Socolow, eds., Patient Earth (Holt, Rinehart and Winston, New York, 1971), pp. 181-202.

70. A distinctive, pure white south-Florida population of the common great blue heron.

71. G. V. N. Powell, A. H. Powell, and N. K. Paul, "Brother, Can You Spare a Fish?," Natural History, February 1988, pp. 34-38.

72. John Harte, personal communication. Fresh water, of course, floats on salt water, so the saltwater intrusion amounts to making the under-ground reservoir shallower.

73. Harte and Socolow, "The Everglades."

74. U.S. Bureau of the Census, Statistical Abstract of the United States: 1982-83 (103d ed.; Washington, D.C., 1982). The Chinese number is said to have increased to 3.2 million by 1985.

75. These estimates are based on 1986 statistics from WRI and IIED, World Resources, 1988-89 (Basic Books, New York, 1988). Note that the assumptions in the statement include that energy-use differentials will remain the same as the babies grow up, and that technological changes will be parallel in all nations. Statistics are also very rough estimates, especially in the poorer nations, and a disproportionately larger fraction of damage from energy use is likely to come from noncommercial energy use (such as agricultural burning and the gathering of fuelwood by individual families). None of this changes the validity of the basic point.

76. p. 130.