Reaching the Limit

How Many People Can the Earth Support?

by Joel E. Cohen
Norton, 532 pp., $14.95 (paper)

The Carrying Capacity Briefing Book

by the Carrying Capacity Network
2,600 pp. (two volumes) pp., $43.00 (2000 P Street NW, Suite 240, Washington, D.C. 20036)


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…

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