How Many People Can the Earth Support?
The Carrying Capacity Briefing Book
Fertility rates are falling fast enough in most parts of the world that it is at least possible that a child born today will be living when human population reaches its peak. The United Nations regularly estimates a high, low, and “middle series” projection for population growth—the most recent “middle series” shows our numbers, currently about 5.8 billion, essentially stabilizing at 10.4 billion sometime late in the next century. A recently completed series of computer models prepared by the International Institute for Applied Systems Analysis in Austria predicts with 60 percent confidence that the planet’s population will not double again, most likely topping out at just over 11 billion.
This outcome is not certain. As Carl Haub, demographer at the Population Reference Bureau in Washington, D.C., points out, if you exclude China women in the world’s developing countries now average four children apiece, down from six a generation ago; to stabilize the population, that number must drop to two. If we stalled at current fertility rates, the population would reach the absurd size of 700 billion by the year 2150. Other than extrapolation from historical experience, there is no strong reason to believe that the rate will continue to drop quickly. If the two-child target is missed by even a small margin we will continue to grow forever; for instance, if each of the world’s women has 2.5 children, the population would reach an only slightly less absurd 28 billion by 2150.
One reason the outcome is so difficult to predict is that no one knows precisely why fertility fell in the past. What evidence there is supports a tangle of interwoven and occasionally contradictory explanations, ranging from increased economic development to better education to more widespread availability of birth control. Other data seem to show, though, that in some places birth rates fall fastest when times are hard.
Still, whatever the reason, for those of us who grew up with the vague and dark impression that the world’s population would increase infinitely, at an ever steeper rate, until our great-grandchildren stood shoulder to shoulder on their assigned square meter of the planet’s surface, the news of this downward trend must seem reassuring. According to the UN the number of people added to the planet in 1995 was about 81 million, down from 86 million annually in the late 1980s. So maybe the tide has turned. Perhaps this is a special moment in history—the trend in human numbers since Adam and Eve, or Lucy, has been generally up, rising more steeply in this century, but now it may eventually reach a plateau.
Of course, there is another way of looking at this. By many measures—food supply, environmental disruption, species extinction—the earth already strains to support six billion people. We are apparently altering the planet’s very climate, for instance, by burning fossil fuels and forests. And now we are poised to nearly double the earth’s population. You could argue that the camel has six billion straws on its back and we’re about to dump five billion more on top of them. Perhaps this is a special moment in history in an altogether different way—the moment when we run out of margin.
To puzzle out this issue, we must consider the controversial topic of “carrying capacity,” a subject that most academic demographers never go near. According to Joel Cohen, the director of Rockefeller University’s Laboratory of Populations, none of the more than two hundred symposia at recent meetings of the Population Association of America dealt with the subject. It is the quintessential back-of-the-envelope discipline, often left to zealots: “Demographers fear to tread where ecologists rush in,” Cohen writes. Happily, Cohen is an exception to the rule—both a respected demographer and a fine writer and historian, he has tackled directly the central question posed in his title, How Many People Can the Earth Support?
The book begins with a clear description of the history of human population—in particular the extremely slow growth in numbers that has marked our evolution as a species. For most of the history of human civilization—from 8000 BC until roughly 1750 AD—it took between 1,400 and 3,000 years for the globe’s population to double. As the period of “local agriculture” gave way to the era of “global agriculture,” which lasted from about 1750 until World War II, the pace speeded up—world population size began to double in little over a century.
During the “public health” era that began after the war, as modern medical practices spread to the Third World, there was a further acceleration: global population size doubled in only thirty-six years. This was the period when the fertility rate stayed high, but vaccinations and cleaner water meant that infant mortality declined and life expectancy soared. Now, in what seems to be the start of the era of falling fertility rates, doubling time has slowed slightly to forty years or more.
Those statistics make it clear just how weird, demographically, the last few decades have been. If the world’s population had increased by the same number each year throughout its history as it increased in 1994, then, thinking backward from the current total, the population would have to have started from zero in 1932 and Adam and Eve could have voted for FDR. Alternately, if there had been only one couple 12,000 years ago, at the end of the last Ice Age, and they had multiplied at the current rate of 1.6 percent a year, the world’s population would be 5.3 x 1082. “Finding matter to construct this number of people would be a problem,” Cohen writes, “because the number of charged particles in the entire known universe is approximately 1080, or 100 times smaller.”
Fertility rates, for the variety of reasons I have mentioned, did finally begin to fall—sometime between 1965 and 1970, the world’s population growth rate peaked at about 2 percent annually and began to decline. But of course that did not mean that the world’s population was, or is, anywhere near peaking—only now is the total number of people added each year beginning to decline slightly, and even so we’re gaining a population the size of Mexico annually, of India each decade. Even if fertility rates continue to fall steadily we are, at best, decades away from any hope of stabilizing the size of the total population. And of course if we are interested in the real world, we need to figure out what it means to be a human being, beyond an integer on a statistical table. Even though population has been growing rapidly, our demands on the globe have been growing much faster.
Take, for example, energy use—one measure of our ability to alter the planet. Though global averages are fairly useless when people on some continents walk and those on others drive Ford Explorers, the numbers are still startling. The per capita use of inanimate energy (everything from waterwheels to fossil fuels to fusion reactors) climbed from less than 1 megawatt-hour per person in 1800 to 19 megawatt hours per person today. Not only are there many more of us than ever before, but our appetites are much, much larger. Which leads directly, of course, to questions of carrying capacity. How many of us, living at what level, can the planet support?
This question is not a new one. Cohen cites alarmed predictions about population from ancient Babylonian tablets, Chinese scrolls, and even the early Christian leader Tertullian. But the modern story begins in 1798, when Thomas Malthus, the English cleric, published his warning that population growth, being geometrical, would inevitably outstrip growth in food production. Malthus was actually answering the Marquis de Condorcet, an Enlightenment optimist who was convinced, as Cohen puts it, that the human mind was “capable of removing all obstacles to progress.” In particular, he thought that new technology would support more people, and that more education would cause birth rates to fall. Their debate has continued for two centuries now; at the moment, the Malthusian position is held most prominently by Paul Ehrlich, the Stanford professor responsible for The Population Bomb, while Condorcet—whose point about increased education reducing fertility is now widely accepted—has found a champion for his hope of technological progress in Julian Simon, who believes that people are “the ultimate resource” and that any talk of impending catastrophe is claptrap.
So far, obviously, the argument has gone to Condorcet. Malthus wildly underestimated the ability of human beings to transform the landscape so as to increase the production of food, and also perhaps their ability to someday control their numbers. Everyone now agrees that Malthus was wrong—wrong for eighteenth-century Europe; and wrong for the subsequent centuries of world history, when the increase in food production has outstripped even the growth in population. Wrong even in recent decades—it was only a generation ago that Ehrlich predicted extremely widespread famine for the 1970s. Malthus’s sense of inevitability was flawed; clearly we are now starting to control our numbers.
But are we starting to control them soon enough to prove Malthus wrong in the long run? Very few people who study this question believe that there is no meaningful limit to the number of people the planet can support. They may disagree about whether it will be food that will limit human numbers, or water, or environmental degradation; but for most who examine the possible limit there is a point at which Malthus would be proved a prophet. For us the question becomes: Will it be before we double the population once more and then reach a plateau?
Cohen devotes the heart of his book to examining the search for that answer. He analyzes in great detail eight different attempts to estimate carrying capacity, beginning with an 1891 paper by an Englishman who calculated six billion and a 1925 German effort that reckoned eight billion. A somewhat bizarre 1967 analysis that posited photosynthesis as the limiting factor concluded the earth might support a trillion people, although if each person insisted on having a third of a football field’s worth of land, the number would drop to 79 billion. This “highly abstract” exercise, as Cohen calls it, was followed shortly by the analyses of a Stanford geneticist who figured a billion humans could subsist at American levels of affluence, a Harvard oceanographer who suggested in 1970 that there might be food for 40 billion people, a Brown University researcher who thought 5.9 billion vegetarians might be viable, and an Australian economist, Colin Clark, who first guessed 28 billion and then a decade later changed his mind to 157 billion, mainly because he revised his estimates of the amount of arable land and the number of people each hectare could support.
Cohen discusses each of these attempts at length, and gives glancing attention to fifty-eight additional efforts over four centuries. By the time he is finished, one is no longer wondering why demographers, as opposed to soil scientists or agronomists or environmentalists, avoid such calculations. The variables in the calculations are so enormous that anyone who makes them takes a big risk.