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

Critical Masses

We now turn to how humanity rose to a position of dominance on the planet and to its chances for staying there. How, after billions of years of Earth's history, did we get to the place where we could consume our inheritance and destroy the global environment in the process? That story starts with our origins and reveals that the behavior of past civilizations has differed from ours not so much in kind as in scale. By necessity, it will also lead us to consider the demographic facts of life.

Human beings are a very recent form of life, with a history of only a few million years.1 During most of those years, the human forms that arose were fairly obscure omnivores (animals that eat both plants and animals) in warm parts of Africa and Asia, sharing habitats with their close primate relatives, monkeys and apes. Early members of our species evolved during the "ice age" era known as the Pleistocene, surviving intermittent glaciations in the northern continents. These primitive hunter-gatherers wandered and foraged in small groups, probably much as do chimpanzees today.


The total populations of the early human forms were quite small, probably not exceeding a few tens of thousands. Females probably bore several young during their lifetimes, but most likely at intervals of several years. Because the available foods were not easily digested by very young children, breastfeeding may have continued for three years or more, thus delaying the return of fertility after childbirth. Moreover, the wandering lifestyle and the need to carry very small children probably reinforced the long intervals between births. The birthrate in early hunter-gatherer groups was therefore lower than would be expected if births simply occurred as closely together as was biologically possible.

The birthrate is defined as the average number of children being born per 1,000 people in a population per year.2 In 1989, the human population of roughly 5.2 billion (5,200 million)3 produced about 144 million children, or an average of 28 children per 1,000 people.4

The counterpart of the birthrate is the death rate. In 1989, about 51 million people died in a population of some 5.2 billion, giving a death rate of 10 deaths per l,000.5

The growth rate of the human population (or that of any other animal) is simply the difference between the birthrate and the death rate.6 So in 1989 the growth rate of the human population was 28 births minus the 10 deaths to equal 18 per 1,000. That is, for every 1,000 human beings alive in the middle of that year, 28 more were born and 10 died during the year. Just to make things more complicated, while birth and death rates are conventionally given as rates per thousand, population growth rates are normally given per hundred -- that is, percent. Thus the growth rate of the human population in 1989 was about 1.8 percent.

"Natural increase" of populations of human beings (or other animals) ceases when the birthrate and the death rate are the same. Then, if there is no immigration or emigration, the population growth rate (births minus deaths) is equal to zero, and there is "zero population growth" (ZPG).7

The growth rate of early human beings must have been very, very close to zero over millions of years; otherwise, there would have been a prehistoric population explosion equivalent to today's. Even a growth rate as small as only 0.1 percent annually would have produced a population of over 6 billion from a starting population of, say, 100,000 individuals, in less that twelve thousand years. That is less than half of one percent of the stretch of time from Lucy to us.

This means that over most of our history death rates must have been much higher than they are today. And no doubt they were, due to accidents, exposure, and predators, as well as diseases. Then, as always, infants and small children must have been most vulnerable; yet even adults probably rarely survived to very old age. With a very small gap between average birth and death rates, growth of these early human populations was extremely slow in the best of times. And sometimes groups of people must have declined or even died out when food was scarce, diseases struck, or other human groups slaughtered them.

Over hundreds of thousands of years, the characteristics that distinguish human beings from other animals evolved: a larger brain, the ability to create and use tools, and, above all, the development of language and culture -- the capacity to transmit information from one generation to the next, nonge-netically. Cultural evolution, the process of change in that non-genetic information, indeed was the key to the phenomenal success of human beings. Adjustment to environmental changes or new environments no longer depended solely on the slow process of adaptation through natural selection of better-suited genetic types of individuals. People could simply change their behavior and tell others about the improvement in life brought by the change.

Cultural change in early human beings, while lightning-quick by the norm of biological evolution, was very slow by recent historical standards. Refinements in tool and weapon designs, hunting and gathering techniques, shelter-building, fire-preserving, etc., occurred rarely, but occur they did. Genetic evolution continued as well, and several human species appeared and thrived for a time. The brains of the earliest forms were roughly the same size as those of modern apes, and the use of tools was scarcely more advanced. Physical and cultural evolution seem to have advanced step by step and in tandem for a long time.

The resources used by the earliest human beings were primarily food, water, and resting places. Their food probably consisted niainly of fruits, nuts, and vegetables, with insects, eggs, and occasional small animals providing a protein supplement. Meat most likely was not a major element in human diets until hunting skills had developed sufficiently to produce regular kills of large game animals. Besides consuming food and water, earW people may have used tree branches or leaves to construct crude shelters, skins for clothing and other uses, and bones, sticks, and stones for tools and weapons. The discovery of how to control fire, of course, must have led to an appreciable use of wjood for fuel.

Except for the stones, the resources used by the early human groups therefore were consumed no faster than natural processes produced them. The raw material for stone artifacts, however, wits abundant. Thus, few in number and relying all but entirely on naturally replenished resources, the first human creatures "trod lightly on this Earth." They were not capable of doing more.


Perhaps 300,000 years ago, Homo sapiens appeared, eventually supplanting the other human forms. During the most recent ice age, these modern human beings spread to occupy most of the planet's continental areas, and their population expanded accordingly. With superior hunting skills and much more sophisticated tools and weapons than those of earlier groups, the expanding human population began to exert a noticeable impact on the rest of the planet's flora and fauna. It has been suggested that the extensive savannas of Africa are a result of repeated forest-burning by human groups to facilitate hunting; if so, it was a significant alteration in biotic communities over a wide area.

The impact of hunting seems to have been even greater on animal populations, however. Increasingly skillful human hunters are thought by many biologists to have caused, or at least abetted, the extinction of many species of large herbivorous (plant-eating) mammals, including wooly rhinoceroses and mammoths, giant sloths, and one kind of bison. The disappearance of these animals occurred gradually in Eurasia and might have been caused by the changing climate as glaciers retreated. But similar large mammals in the New World vanished relatively suddenly and suspiciously soon after human beings with fully developed hunting abilities invaded North America about twelve thousand years ago.8 It is likely that the substantial widespread climate changes taking place during that period, as the glaciers retreated, played a role in the extinctions of these large herbivores -- as well as of some of their other predators such as saber-toothed cats -- but the additional hunting pressure from human groups seems to have dealt the final blow.9

The demise of the giant herbivores is the first instance we know about in which human populations exploited a resource with such ferocity that it was extinguished, although at a lower level of exploitation it would have survived. The loss of that important resource may have led to a renewed dependence by human groups on plant foods for sustenance and consequently spurred the invention of agriculture as the ice age waned about ten thousand years ago. At that time, the population had reached perhaps 5 million people.10 Agriculture, of course, opened the door to exploitation of renewable resources on a scale and in ways never seen before.


The deliberate cultivation of plants for food is thought to have been first practiced in Asia Minor and the Near East. The earliest farming communities grew wheat, barley, and rye -- all grasses native to the region -- as well as pulses (various kinds of beans and peas) and fruits. A second center of agricultural development, based on rice, is believed to have originated at about the same time in Southeast Asia. A third (and possibly fourth) independent invention of agriculture, based on potatoes and maize, occurred in the New Worl several thousand years later.

Although the archaeological record shows farming villages in the Near East appearing rather suddenly, the development of agriculture was very likely a gradual process, arising from the increasingly intimate knowledge by the food gatherers of the ecology of their preferred food plants. Deliberate planting of seeds and casual "weeding" of competing plants in favorable places presumably led, through repeated beneficial results, to human groups carrying out these activities in a more and more systematic fashion. And over many centuries, by choosing and planting seeds primarily from individual crop plants with desired traits, early farmers slowly transformed plant species from their wild forms to much more productive ones -- to the domesticated crops familiar today.

Exactly when people began to control herds of herbivorous animals in order to maintain access to steady supplies of meat, and perhaps to facilitate the animals' reproduction, isn't known. But nomadic herding cultures in some semiarid regions unsuitable for cultivation may date as far back as the earliest farming. Like the development of agriculture, that of herding may have been a gradual process, as groups of people following migrating herds step by step asserted control over the animals, while protecting them against other predators.

However the processes may have occurred, farming and herding represented a radical new departure in the ability of human beings to manipulate their environments and the renewable resources on which they depended. The planting of crops meant, first, a replacement of the natural flora. By extension, it also spelled the displacement of much of the animal life that had depended on the flora, although some other animal species -- pests -- were favored by the bountiful new plantings.

The advantages of the new way of life to the first farmers were quite clear. Food supplies were more dependable and much more abundant; many times more people could be supported by the agricultural production of a given area of land than through hunting and gathering. A settled way of life, as necessitated by farming, was a more secure existence than constant wandering in search of food.

Whether death rates declined because of the greater security is a matter of debate, because the denser populations may have been more susceptible to communicable diseases. But the settled life and the more easily digested foods produced by agriculture may have enabled mothers to wean their children earlier and thus to bear infants more often. In any case, some combination of these factors -- an increased, more dependable food supply, greater security, and more frequent childbearing -- led to a gradual expansion of the human population.

After a time, farming became so efficient that one farmer could produce enough food for several people, a circumstance that permitted some people to specialize in other occupations and eventually made possible the development of towns, cities, and governments. Mining and metalworking were among the new occupations; these marked the first important use of non-renewable resources -- the first inroads on our capital inheritance. The use of metals undoubtedly contributed to more efficient farming, as well as providing weapons for protection against other, hostile human groups. Because metals are unevenly distributed in Earth's crust, their use also presumably stimulated trade over long distances and, quite likely, the spread of ideas and technologies among different groups.

Not long after cultivation of crops began, people learned to channel supplies of surface water in areas where rainfall was sparse or unreliable. Some very early farming cultures developed complex irrigation systems to divert water from streams and rivers to the fields -- another instance of human manipulation of a renewable resource. The ancient Sumerian culture of the Tigris and Euphrates valleys (in what is now Iraq), some five thousand years ago, was based on irrigation. One cause of the demise of that early civilization is believed to have been the result of centuries of irrigation: an inexorable accumulation of salts in the soil and siltation of the extensive irrigation systems.11

The increasingly efficient use of both renewable and non-renewable resources to support people and the expansion of farming to ever larger areas fueled an increase of the human population from 5 million before the invention of agriculture around 8000 B.C. to 200-300 million some two thousand years ago. That population growth, while imperceptibly slow by today's standards, represented an unprecedented outbreak of a single animal species, namely us. Moreover, it was accompanied by substantial changes in the biotic communities over much of the planet's land surface, especially in southern Europe and parts of Asia. Agriculture increasingly replaced natural ecosystems in favorable areas, and forests were cut down for fuel, for construction materials, and to clear more land for farming.

The Mediterranean basin, once heavily forested and well watered, then converted to agriculture, supported the ancient Egyptian, Phoenician, Greek, and Roman civilizations, among others. But, over the centuries, deforestation, overcultivation, and overgrazing by domestic animals led to a gradual depletion of soils and possibly contributed to a gradually drier climate. The fading of those once-brilliant civilizations may have been due in part to such environmental damage and depletion of the renewable resource base, although sources are not adequate to make judgments on the degree of responsibility. The exception was surely Egypt, whose civilization outlasted many others because the fertility of its soil was continually restored by the annual floods of the Nile.

Historians trying to explain why past civilizations rose, flourished for a time, then usually declined or fell prey to some conquering outside force have customarily looked for causes in social, economic, or political factors. Rarely have they considered population pressures, and their contributions to environmental deterioration and depletion of resources, as underlying causes of a civilization's downfall. Yet numerous contemporary accounts documented problems with soil erosion, recurring floods and droughts associated with deforestation, and so forth. The Greek philosophers described such processes and warned of the consequences of continued deforestation and of overgrazing, especially by goats. The warnings went unheeded; Greece today is nearly a desert, its soils thin and poor, the vast majority of its original forests long vanished.12

Similar trends were described by Roman writers, who also mentioned serious air and water pollution in Rome that may have caused significant but subtle public-health problems such as lead poisoning. The fall of Rome may have had less to do with the growing power of the barbarians who overcame it than with the declining health and vigor of the Romans themselves. The quality and abundance of food supplies may have declined as well, and it seems possible that the region's population, after a considerable expansion during Rome's heyday, may have dwindled with the decline and collapse of the empire and the onset of the Dark Ages.

Such phenomena were not confined to the Mediterranean basin. Similar damage to natural resources, though even less well documented, seems to have destroyed a thriving early civilization in the Indian subcontinent, now the Thar Desert in Pakistan. Deforestation, followed by floods and droughts and other environmental changes in densely populated areas, was also recorded by the ancient Chinese; they are problems that plague China to this day. And some scientists think that intensive cultivation of erosion-prone tropical soils was a major factor that led to the collapse of the Mayan civilization in Central America.13

Processes of environmental deterioration and degradation of the natural-resource base, which in various forms are known today as "desertification," evidently have occurred locally numerous timesin~human history. A human tendency toward overexploitation of renewable resources seems to have been established almost as early as agriculture -- if not before. The current human predicament, in which this pattern has become worldwide and is threatening to go out of control, can thereby be seen to be an outgrowth of our history.14

Despite some setbacks (such as the bubonic-plague epidemic, which reduced the population of medieval Europe by at least a quarter) and the rise and fall of various civilizations, the human population as a whole continued to grow throughout the historical period, reaching 500 million around 1650. From the advent of agriculture some ten thousand years earlier, the human population had increased about 100-fold, doubling its size approximately every 1,500 years.

After 1650, the growing human dominance of the planet became even more obvious. The New World had been "discovered" and was settled by more numerous and agriculturally more advanced Europeans, who displaced the indigenous societies. The occupation of all the habitable continents by increasingly efficient agricultural societies was only the beginning, however. As European forests shrank in the late Middle Ages, first peat and then coal were discovered and put to use as fuels, and water was harnessed as a source of power. The stage was set for the Industrial Revolution and a new surge in human population growth.


By sometime around 1800, the world population had grown to a billion, having doubled again in well under two hundred years. The Industrial Revolution was under way in western Europe and North America by then; in a multitude of ways, it transformed the world over the next century. Generally improving living conditions, including better housing and food along with advances in sanitation, accompanied industrialization in the West. These changes led to declines in death rates, especially among infants and small children, many more of whom survived their early years than before.

Annual birthrates of around 40 to 45, and death rates of 38 or more, per 1,000 in the population are characteristic of agrarian societies without modern sanitation and medicine. Such rates were typical in eighteenth-century Western Europe and North America, as well as the rest of the world. During the nineteenth century in some Western nations, however, death rates crept downward to 30 per 1,000 and below. The widening gap between the persisting high birthrates and falling death rates led to an acceleration of population growth in those countries, and growth rates climbed to previously unknown levels of 1.5 percent (15 per 1,000) per year or more in the late nineteenth and early twentieth centuries. Led by this population growth spurt in the industrializing West, the population worldwide doubled in a little over one hundred years to 2 billion in 1930.15

But, a generation or two following the onset of declining death rates in the West, birthrates too began to fall slowly. This even more remarkable change was apparently the result of individual couples perceiving that more of their offspring were surviving and that large numbers of children were an economic burden in industrializing societies,16 and therefore limiting their families. Other factors, such as later marriage -- thus reducing the years of a married woman's reproductive activity -- and moderately high rates of nonmarriage, also played a part. Later, the feminist movement and the rising participation of women in employment outside the home probably helped to reduce birthrates even further.

Although the precise causes of the decline in fertility are still being debated by demographers, it occurred in nation after nation in the industrializing West and came to be called the "demographic transition."17 By the 1930s, both birth and death rates in most European countries, the United States, and Canada had reached unprecedentedly low levels. For a few years during the Great Depression, birthrates in some industrial nations fell well below 20 per 1,000, with death rates around 12-15, producing growth rates well under one percent per year. Indeed, demographers of the time worried about an end to growth and the prospect of population shrinkage.

Despite the lowering of population growth rates in the West, however, the average worldwide rate of growth continued to rise after 1930 as the benefits of industrialization -- especially modern medicine and the control of insect-borne diseases -- reached societies far from the West. With the prosperity of the post-World War II years, birthrates in the industrialized West rose again, particularly in the English-speaking nations such as the United States, Canada, and Australia: the famous postwar baby boorn.

By the early 1960s, there were spectacular declines in death rates in the less developed nations of Asia, Africa, and Latin America, caused largely by the use of.antihiotics and of synthetic pesticides against malarial mosquitoes. Helped a little by the Western baby boom, the plummeting death rates (accompanied by no change in the high birthrates) produced a global population explosion.18 Growth of the world population peaked in that decade at an average rate of about 2.1 percent per year.

Although falling birthrates in a majority of countries since then have led to a slackening of the average growth rate, the annual population increment in 1990 is at an all-time high -- some 95 million people. In contrast, twenty years earlier the population was increasing by only some 75 million per annum, in spite of the higher growth rate. The reason, of course, is that 1990's lower rate is applied to a much larger population base; then it was only 3.5 billion, now it's past 5.3 billion.


The Industrial Revolution brought improved conditions of life in many ways, leading to a longer life expectancy for the average person. Similarly, discovery of an entire new category of resources, the fossil fuels, underwrote the huge twentieth-century expansion of the human population. But that also marked the change from human dependence primarily on renewable resources, constantly replenished by nature (even with human manipulations), to an enormously enhanced dependence on nonrenewable resources. By 1900, coal and peat had been supplemented by petroleum, an even more convenient fuel, and natural gas. The availability of these cheap, apparently abundant energy sources led to an acceleration in the extraction and use of metals as well. And the fossil fuels subsidized far more intensive agricultural practices, because of their use in the manufacture of fertilizers and pesticides and to fuel farm machinery.

By the 1980s, the depletion of accessible reserves of many nonrenewable resources -- notably, but not exclusively, petroleum -- was becoming more and more evident. Expansion and intensification of agriculture were approaching their limits. Both processes were increasing the damage to soils and depletion of groundwater reserves. Natural communities of plants, animals, and microbes were vanishing or being impoverished as humanity took over more and more of the planet's land surface, converted natural ecosystems to human-dominated ones, and coopted their net primary production. As a result, the life-supporting services performed by natural ecosystems have been impaired or lost. Human beings now occupy and use, at one level or another, some two thirds of the planet's land surface, and are striving to find ways to exploit the remaining inhospitable third.

As we have seen, in the decades since World War II humanity has undeniably become a global force. The assault that we are carrying out upon the environment and resources of the planet is not just a matter of brute numbers of people. Rather, it is what those people do; it is their impact on the things we care about -- on each other, on nonrenewable resources, and above all on the environmental systems that sustain us.

The impact of any human group on the environment can be usefully viewed as the product of three different factors. The first is the number of people. The second is some measure of the average person's consumption of resources (which is also an index of affluence). Finally, the product of those two factors -- the population and its per-capita consumption -- is multiplied by an index of the environmental disruptiveness of the technologies that provide the goods consumed. The last factor can also be viewed as the environmental impact per quantity of consumption. In short, Impact = Population x Affluence x Technology, or I = PAT.19

The I = PAT equation is the key to understanding the role of population growth in the environmental crisis. It tells us why, for example, rich nations have such serious population problems (because the A and T multipliers for each person are so large). That is why it is so important that those nations begin shrinking the size of their populations by lowering birthrates until they are below death rates. It also tells us why a little development in poor nations with big populations like China can have an enormous impact on the planet (because the P multiplier on the A and T factors is so large).

Note that the total impact of a society can be lowered by decreasing any of these three factors, as long as the others are not increased so as to offset the reduced factor. In the case of the attack on the ozone layer by chlorofluorocarbons (CFCs), the impact eventually could be made negligible by operating on the technology factor alone -- that is, by banning the use of the offending CFCs, which might result in a slight decrease in affluence if substitutes were more expensive or less convenient.

But the injection of the major greenhouse gases carbon dioxide (CO2) and methane into the atmosphere, which threatens to change the climate and, among many other things, wreck agricultural production, is not so easily corrected. The atmospheric concentrations of these gases are tightly tied to population size. Consequently, there is no practical way to achieve the necessary reduction in greenhouse emissions without population control.

To illustrate how this interaction works, suppose that, by dint of great effort, humanity managed to reduce the average per-capita consumption of resources on the planet (A in the I = PAT equation) by 5 percent and improved its technologies (T) so they did 5 percent less damage, on the average. This would reduce the total impact (I) of humanity by roughly 10 percent. Unless population growth (P) were restrained, however, its growth would bring the total impact back to the previous level in less than six years.


Of course, the size of the human population is not under control. In 1989, the world population appeared committed to at least doubling its size -- lacking any concerted effort to accelerate reductions in reproductive rates or a significant rise in death rates. That commitment is based on "demographic momentum" -- the tendency of a previously growing population to keep expanding long after reproductive rates have been reduced. A supertanker requires several miles to come to a stop after reversing its propellers; only a nuclear torpedo (or an Alaskan reef!) could stop it in its tracks. Similarly, only the demographic equivalent of such a torpedo, a sudden and dramatic rise in the death rate, could produce instant ZPG in a fast-growing population.

The reason for demographic momentum is the youth of rapidly growing populations. In 1989, 40 percent of the population of the average less-developed nation was under fifteen years of age.20 Well over a billion young people in those nations have yet to enter their prime reproductive years (fifteen through thirty) and make their contributions to the birthrate. They will then live alongside their children and watch the births of their grandchildren. It will be a half century before they reach old age (over sixty-five) and start making large contributions to the death rate.

When the average couple has slightly more than two children, the population has reached "replacement reproduction." That means each couple will be replaced by just two descendants in the next adult generation. The "slightly more" than two is to compensate for the children who die before they reach reproductive age. In countries with high infant and child mortality rates, slightly larger completed family sizes are needed for replacement than in nations with lower death rates. For instance, in the United States, replacement reproduction is an average completed family size of 2.1 (we're significantly below that now at 1.9). In India, where infant deaths are considerably higher, replacement would be about 2.4.21 The average Indian family size in 1989 was 4.3.

Demographic momentum may seem complicated to understand at first, but it will become clear if you just remember that births take place primarily among young people and deaths primarily among the old. Thus if a population has a high proportion of young people, one must wait for the average age in the population to increase before death rates catch up with birthrates. The process normally takes about a life expectancy (some fifty to sixty years in most poor nations) from the time replacement reproduction is reached.

The bottom line on demographic momentum is simple: barring plunges in the birthrate that take family sizes well below replacement reproduction, or substantial rises in the death rate, it will take fifty to sixty years after a rapidly growing population reaches replacement reproduction to achieve ZPG. The exact length of time depends on the age composition of the population (that is, the proportions of people of various ages) at the time birthrates begin to fall, how long it takes to reach replacement reproduction, and what happens to family sizes afterward (do they stay just at replacement level or drop below?).22

In 1990, for instance, India had a population of some 850 million people. Suppose, over the next thirty to thirty-five years, India's average completed family size dropped from the 1990 level of about 4.3 to 2.4 (replacement level) and remained there, and death rates didn't rise. India's population would continue to grow for almost a century, and when it stopped there would be about 2 billion Indians -- as many people living in that one nation as populated the entire planet in 1930!

That's what demographic momentum is all about; that's why knowledgeable people always think of population control first when they think of solving the human problems related to overpopulation. To stop human population growth humanely, by limiting births, will take a very long time -- at least two generations, even if completed family sizes everywhere dropped substantially below two children in the next decade or so. By contrast, social behavior and economic systems can be modified in a few years.

Overall, the prospects for a birth-control solution to the human population dilemma do not appear good. In May 1989, demographers Carl Haub and Mary Kent stated:

Even to reach a stable world population size of 10 billion, double the current total, birth rates will have to begin a steady descent soon. Unless we see declines in high fertility rates in many African and Asian countries during the 1990s, the pros-\pect for world population to level out at less than 10 billion seems very dim. As far as ultimate world population size is concerned, the 1990s will truly be a decade of decision.23

Their gloomy statement was partly based on an apparent reversal in the late 1980s of the long-established slowdown of population growth. In the mid-1980s, the worldwide population rate of increase had fallen to 1.7 percent annually; by 1989 it had risen again to 1.8 percent, driven largely by a resurgence of fertility in China (about which more later). The continued failure to gain control over population growth increases the prospect that humanity will suffer large rises in death rates over the next fifty years.


So far, we have discussed the population situation as if people always stayed put. But they don't; they have wandered since their days as hunter-gatherers, and they still pick up and move to better hunting grounds. Unequal access to resources is, of course, one major reason people move from place to place.24 When one is looking at global overpopulation, migration does not come into the picture, since we neither receive immigrants from space nor send emigrants to other planets.

In various regions of Earth, however, migration may be an important factor in population problems and may greatly influence how humanity uses its inheritance. Recently, more and more "ecological refugees" have been fleeing from areas where ecosystems are collapsing to seek better lives elsewhere. Much of this movement is toward cities, from rural environmental-disaster areas such as the Sahel. During the recent Sahel droughts, more than 250,000 people in Mauritania and nearly a million in Burkina Faso (about a sixth of the nation's population) migrated to cities.25

The Sahelian migrations probably will not have strong impacts on global systems -- they amount mostly to relocation of poor people who will remain poor and do relatively little environmental damage. On the other hand, refugees from deserti-fied northeastern Brazil are not just flooding into cities; they, along with migrants from southern Brazil, are moving into the Amazon basin, where they are helping to cut down the rain forest for farming. That deforestation, in turn, is an important factor contributing to global warming -- which may well reduce the carrying capacity of the entire planet.

Migration from poor to rich nations represents a very different kind of threat, however. To the degree that immigrants adopt the lifestyles of their adopted countries, they will begin consuming more resources per person and to do disproportionate environmental damage. Net immigration to rich nations is the rough equivalent of natural population increase (more births than deaths) in those nations.

The United States faces very serious and complex problems with immigrants from developing countries. The nation has traditionally said that it welcomed the "poor and downtrodden" of the world, but unhappily the "poor and downtrodden" are increasing their numbers by some 80 million people a year. Many of these, of course, would like to come to the United States or other rich countries and acquire the standard of living of the average American (in the process greatly increasing their use of Earth's resources and abuse of its life-support systems). The United States, therefore, must reexamine how many such people should be admitted legally and how illegal immigration can be curbed.

In particular, Americans must find a way to integrate migration policy with a comprehensive population policy. This is especially important because net immigration now makes up something on the order of 25 percent of the growth of the American population.26 If a national goal of ending and then reversing population growth is established, as it should be, then it will be important to decide how much of the input side of our population equation is to be made up by natural births, and how much by net immigration.

This is a particularly vexing problem because of our traditional welcoming attitude toward immigrants, because of many ethical and moral questions surrounding immigration policies, and because of the great difficulty of measuring the flow of illegal immigrants. If the United States is going to avoid even more serious problems of overpopulation, its people are inevitably trapped in a zero-sum game; every immigrant admitted must be compensated for by a birth forgone. This will require either a further lowering of American birthrates (which are already low enough to bring an end to natural increase eventually) or much more attention to the problems of restricting immigration in the future. Booming populations in desperately poor nations are bound to make the comparatively affluent United States a target for ever more migrants.

Much of the attention to immigration questions in the United States is focused around the flow of immigrants from Mexico. The United States is the only very rich nation that shares a long unfortified border with a poor nation.27 Through much of this century, America has used Mexico as a "labor pool of last resort," opening its borders when there were shortages, especially in farm labor, and sliding them shut again when labor was plentiful. There have been disgraceful incidents in which children born to Mexican citizens in the United States, and therefore American citizens, have been illegally expelled from the country. Furthermore, a detailed study of the Mexican immigration situation indicated that the benefits for the United States of Mexican immigration have far outweighed its costs.28 America has been able to profit greatly from the poverty of Mexico. In spite of that, the flow of immigrants into the United States should be damped, simply because the world can't afford more Americans. Because of the need for U.S. population shrinkage, immigration from Mexico and other nations must be held to a level that, added to a reduced number of natural births, keeps the number of births plus immigrants below the number of deaths plus emigrants.29

Achieving that goal will not be simple. And much of the complexity traces to the joint history of the United States and Mexico.30 In our view, no policy of forcible exclusion is likely to prevent a steady flood of Mexicans seeking work from coming into the United States. The border is too long, the ties across it are too tight, and the difference in average wages is too great for a "Great Wall" policy to succeed. The only way that the tide is likely to be stemmed is through creative policies that simultaneously help, Mexico to control its own population and substantially improve the standard of living of Mexicans within their homeland.

Indeed, it is high time to strengthen the bonds of cooperation among the three large nations of North America. Almost 150 years of peaceful coexistence31 make the United States, Mexico, and Canada an ideal trio to show how cooperation could help, solve transnational environmental and economic problems.32 We should begin to think in terms of the carrying capacity of North America -- not merely of three separate and disparate nations that just happen to occupy the continent. Family sizes, consumption patterns, and technological choices made over the entire continent should be coordinated. And an economic unification should include the goal of raising the standard of living in Mexico while reducing the total impact of North Americans on the environment and resources of Earth. Needless to say, that's a big order. But the alternatives, such as trying to turn the United States and Canada into fortress states against the influx from the south, are big orders, too.

The dilemma of the United States and Mexico, of course, is embedded in the global one and in many ways is a microcosm of that situation. Movement of people from poorer to richer areas -- within or between nations -- is a natural response to the rich-poor gap. And as that gap continues to widen, and as environmental deterioration makes staying home less feasible, the numbers of migrants can be expected to rise, perhaps dramatically. The United States is not the only developed nation facing the problem of immigrants, and especially of refugees, both political and ecological. As in North America, the problems will have to be addressed in terms of the root causes and through solutions jointly sought between the developing and developed worlds.

Now that we've explored the history of humanity's rise to planetary dominance and despoliation, let us consider what all this means for continuing to provide that most basic of resources for our massive and still expanding population: food. If the food supply is not what it ought to be now, what are the implications for a population twice as large fifty years from now?


1. Some would not count our ancestors as having been truly human until the "cultural revolution" of some 35,000 years ago, when a highly and rapidly innovative modern Homo sapiens emerged. For a fine exposition of that point of view, see J. Diamond, "The Great Leap Forward," Discover, May 1989. We prefer to define humanity as beginning with the first fully upright, small-brained australopithecine hominids some 4 million years ago. It is, of course, just a question of definition; there is little dispute on the facts.

2. Births and immigrations are the "input" side of the demographic equation, while deaths and emigration are the "output" side. Here we are ignoring immigration, and we will ignore emigration in the discussion of output as well. On a global scale, of course, migration can be ignored. In the demography of some individual nations, such as the U.S., migration can be important and must be included in calculations of growth rates. In such situations, "rate of natural increase" is the difference between birth and death rates, while "growth rate" takes migration (a net gain or loss) into account.

More details on the mathematics of population growth call be found in P. Ehrlich, A. Ehrlich, and J. Holdren, Ecoscience: Population, Resources, Environment (Freeman, San Francisco, 1977). If you find dealing with the mathematical aspects of population and related issues daunting, we strongly recommend John Harte's Consider a Spherical Cow: A Course in Environmental Problem Solving (Wm. Kaufmann, Los Altos, Calif., 1985).

3. Once again, all 1989 population data are from the World Population Data Sheet 1989, produced by the Population Reference Bureau (PRB). Anyone interested in population issues should join this excellent organization and receive the wealth of information it supplies to members. In some cases, the PRB numbers have been rounded for simplicity, so our examples may not always match the data sheet exactly. Normally, the estimated midyear population is used as the divisor. Census statistics, especially in developing countries, are somewhat unreliable (those in the U.S. also contain significant errors, especially when compared with those from nations like Sweden). That is the reason for all the "weasel" words (e.g., "roughly," "about") accompanying population numbers. Note, however, that it makes no difference to the message of this book whether the midyear population in 1989 was 5.1 or 5.4 billion, or whether the birthrate was 26 or 29 per 1,000. Such uncertainties make little difference in the long run.

4. 146/5200= .028. Remember, a billion is 1,000 million.

5. 51/5200 =.010.

6. Again, we're ignoring migration.

7. Technically, demographers say that when birth and death rates are equal, there is no "natural increase" and the population is "stationary." Zero population growth, however, has become much more widely used in the popular literature; it is also the name of the premier organization pushing to get population growth in the United States under control.

8. There is evidence, still being debated, that some groups of people had settled in the Western Hemisphere much earlier, but they may have been less effective hunters than the invaders that arrived around 12,000 years ago.

9. See P. S. Martin and R. G. Klein, eds., Quaternary Extinctions: A Prehistoric Revolution (Arizona Univ. Press, Tucson, 1984). The last word is not in, however, on the debate about Pleistocene extinctions.

10. For more details and references, see Ehrlich, Ehrlich, and Holdren, Ecoscience, chap. 5.

11. T. Jacobsen and R. M. Adams, "Salt and Silt in Ancient Mesopotamian Agriculture," Science, vol. 128, pp. 1251-58 (1958). The degree to which problems with the irrigation system contributed to the downfall of the civilization is controversial.

12. For a fine brief overview of the environmental impact of the Greeks and Romans, see J. D. Hughes, The Ecology of Ancient Civilizations (Univ. of New Mexico Press, Albuquerque, 1975). There are those, however, who doubt that ecological factors had much to do with the decline of either Greece or Rome; see, e.g., T. H. van Andel and C. Runnels, Beyond the Acropolis (Stanford Univ. Press, Stanford, Calif., 1987). Even those authors, however, concede that human activity has brought the Mediterranean basin to its present degraded state.

13. See, for instance, J. A. Sabloff, "The Collapse of the Classic Maya Civilization," in J. Harte and R. Socolow, Patient Earth (Holt, Rinehart and Winston, New York, 1971), pp. 16-27; T. P. Culbert, ed. The Classic Maya Collapse (Univ. of New Mexico Press, Albuquerque, 1973), especially chap. 15 (W. T. Sanders, "The Cultural Ecology of the Lowland Maya: A Reevaluation"); E. S. Deevey, D. S. Rice, H. H. Vaughan, M. Breener, and M .S. Flannery, "Mayan Urbanism: Impact on a Tropical Karst Environment," Science, vol. 206, pp. 298-306 (1979). Other scholars have cast doubt on this idea, citing evidence of a well-developed, sustainable soil-preserving agricultural system.

14. A more detailed discussion of the history of human population growth can be found in Ehrlich, Ehrlich, and Holdren, Ecoscience, chap. 5.

15. Since there are few reliable census statistics available for most of the world before this century, dates for when the total human population reached various levels tend to be informed guesses. We have long accepted 1850 as the approximate time that the population reached a billion (based on various sources); the PRB now uses 1800. They know more than we do, so we have switched to their estimate.

16. Or at least less of an asset than they were on the farm.

17. Some social scientists believe that a demographic transition is an automatic part of the demographic history of societies. They complacently assume that sooner or later today's less-developed countries will undergo such a transition and birthrates will fall to industrial levels, regardless of population policies. That assumption is supported by the recent gradual decline of birthrates in many nations. But this simple view is a dubious proposition, as we outline in Chapter 9. An example of an otherwise excellent analysis that tacitly accepts this assumption is John R. Weeks, "The Demography of Islamic Nations." Population Bulletin, vol. 43, no. 4 (December 1988).

18. Details in Ehrlich, Ehrlich, and Holdren, Ecoscience, chap. 5.

19. J. P. Holdren and P. R. Ehrlich, "Human Population and the Global Environment," American Scientist, vol. 62 (1974), pp. 282-92. This basic formulation was first published in P. R. Ehrlich and J. P. Holdren, "Impact of Population Growth," Science, vol. 171, pp. 1212-17 (1974). For a detailed discussion, see Ehrlich, Ehrlich, and Holdren, Ecoscience, chap. 12. Note that the formula is simplified, since the multiplicative factors are not entirely independent. "Consumption" is in some ways a more accurate term than "affluence," but PAT is a much handier acronym than PCT.

20. Excluding the People's Republic of China. If China is included, the figure is 37 percent.

21. Demographers usually consider only females when calculating reproductive rates. At any moment, one can picture these rates as being calculated by taking a hypothetical group of 1,000 newborn female babies and, using a computer, running the group through the current death and birth rates for each age group in the population. At first there are no births, but a few deaths occur; as the age of puberty arrives, the hypothetical survivors (still the vast majority) start producing babies. When the childbearing years are over, the computer adds up the number of female babies ever born to the original group of females, and divides it by 1,000. The resulting number is called the net reproductive rate (NRR). If 2,000 female babies had been produced, the population would be growing like a skyrocket, doubling every generation, and the NRR would be 2,000/1,000 = 2. If the original 1,000 female babies produced exactly 1,000 female babies, there would be replacement reproduction, or a net reproductive rate of one (1,000/1,000= 1).

The number usually quoted, however, is the current total fertility rate (TFR), which is simply the average number of babies of both sexes that would be born per woman in her lifetime if current age-specific fertility rates remained constant; or the average completed family size, as it is sometimes expressed.

22. Technically, the timing depends on the precise trajectories of age-specific birth and death rates, but those details need not concern us here. From a policy point of view, a year or two's difference in reaching replacement reproduction is of little consequence -- although a decade's difference can be very significant.

23. News release, "1990s a Crucial Decade for World Population Stabilization," Population Reference Bureau, Washington, D.C., May 24, 1989.

24. For an overview of human migration, with special attention to the relationship between Mexico and the U.S., see P. Ehrlich, L. Bilderback, and A. Ehrlich, The Golden Door: International Migration, Mexico, and the United States (Wideview Books, New York, 1981). The numerous difficult ethical questions that surround the immigration dilemma are explored in that book.

25. J. Jacobsen, Environmental Refugees: A Yardstick of Habitability, WorldWatch Paper 86 (Worldwatch Institute, Washington, D.C., 1988). This fine paper describes a horrifying worldwide problem.

26. This is assuming a net natural increase of 1.75 million (about 4 million births minus 2.25 million deaths) and (conservatively) a net immigration of 0.6 million, giving a total annual increase of 2.35 million people. The contribution of migration is then 0.6/2.35 = .255, or 25.5 percent. Immigration at this rate is sufficient to raise the 1989 U.S. population growth rate from 0.7 to 0.95 percent. For an interesting discussion of recent Census Bureau projections of the future U.S. population, see Leon Bouvier, "The Census Bureau's 1989 Projections of Future U.S. Population: Which Scenario Is Reasonable?" CIS Backgrounder (Center for Immigration Studies, 1424 Sixteenth St. NW, Washington, D.C., March 27, 1989).

27. We are not counting the USSR as a very rich nation, and the Soviet border with China is heavily militarized.

28. That study is Ehrlich, Bilderback, and Ehrlich, The Golden Door. Much of the material in this paragraph and the discussion that follows comes from this source.

29. Today, population growth in the U.S. has a component of natural increase (because of demographic momentum) and one of net immigration (surplus of immigrants over emigrants). If family sizes do not increase, the first component will gradually shrink to zero and then become negative. What will happen to net immigration (and whether it will increase and thereby keep the U.S. population growing, even in an era of natural decrease) we cannot predict.

30. Unfortunately, the U.S. has long interfered in the affairs of Mexico, doing so in a way that has helped to worsen our southern neighbor's problems. Our meddling goes back to the days of U.S. Minister Joel Poinsett in 1822, but was most blatant during the Mexican War (which the Mexicans call "the War of American Intervention") of 1846-47 and the dictatorship of Porfirio Diaz (1876-1911). It continues to this day in the form of trade and investment policies. Few Americans realize that our nation took more than half of Mexico's territory by force in the Mexican War and even considered making the entire nation part of the U.S., which certainly would have solved any "migration" problems.

31. We ignore here Woodrow Wilson's Mexican adventure just before World War I. The U.S., Mexico, and Canada have all been at peace with each other since the Mexican War.

32. A step in that direction occurred in October 1989 when the Canadian government announced that it was joining the Organization of American States (OAS) (reported on CNN World Report, Oct. 29).