Sometime on October 31st, the world’s population will hit seven billion. The baby who does the trick will most likely appear in India, where the number of births per minute—fifty-one—is higher than in any other nation. But he or she could also be born in China—the world’s most populous country—or in a fast-growing nation like Nigeria or Guatemala or, really, anywhere. The idea that a particular child will on a particular day bring the global population to a particular number is, of course, a fiction; nobody can say, within tens of millions, how many people there are on earth at any given time. The United Nations Population Fund has picked October 31st as its best estimate. That this date is Halloween is presumably just a coincidence.

Illustration by Tom Bachtell

Depending on how you look at things, it has taken humanity a long time to reach this landmark, or practically no time at all. Around ten thousand years ago, there were maybe five million people on earth. By the time of the First Dynasty in Egypt, the number was up to about fifteen million, and by the time of the birth of Christ it had climbed to somewhere in the vicinity of two hundred million. Global population finally reached a billion around 1800, just a couple of years after Thomas Malthus published his famous essay warning that human numbers would always be held in check by war, pestilence, or “inevitable famine.”

In a distinctly un-Malthusian fashion, population then took off. It hit two billion in the nineteen-twenties, and was three billion by 1960. In 1968, when Paul Ehrlich published “The Population Bomb,” predicting the imminent deaths of hundreds of millions of people from starvation, it stood at around three and a half billion; since then, it has been growing at the rate of a billion people every twelve or thirteen years. According to the United Nations, it reached six billion on October 12, 1999. (A baby boy born in Sarajevo, Adnan Mević, was, for symbolic purposes, designated the world’s six-billionth person and greeted at the hospital by U.N. Secretary General Kofi Annan.) For large and slow-to-reproduce mammals like humans, such a growth curve is, to put it mildly, unusual. Edward O. Wilson has called “the pattern of human population growth” in the twentieth century “more bacterial than primate.”

Predicting where the numbers will go from here is, at least in the short term, pretty straightforward. Fourteen years from now, there will be eight billion people on the planet. At around the same time, India will overtake China as the most populous nation on earth. Most of the growth will occur in the world’s poorer countries. Proportionally, Europe’s population will decline, while Africa’s will increase.

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The further ahead you look, the trickier things become. This is partly a matter of birth rates; because the base is now so large, even relatively trivial changes produce enormous effects. In most European nations, and also in countries like Japan and China, birth rates have already fallen below replacement levels. Until quite recently, the U.N. was projecting that rates in other parts of the globe would follow a similar downward slope, so that sometime toward 2050 global population would level out at around nine billion. A few months ago, though, the U.N. announced that it was revising its long-term forecast. The agency now estimates that the number of people on earth in 2100 will be ten billion and still climbing. One reason for the upward revision is that birth rates in many developing countries, particularly in Africa, have remained unexpectedly high. Another is an uptick in births in wealthier countries, like the United States and Britain. (Last week, the Pew Research Center released a report showing that birth rates in the U.S. dipped during the recession that started in 2007; it is doubtful, though, that this will have much impact on long-term population estimates.) If families have, on average, just half a child more than the U.N. currently projects, by 2100 there will be sixteen billion people on the planet.

The other uncertainty is the world’s ability to provide for all these additional people. As many, including Bill Gates, have pointed out, just to keep per-capita food production constant in the coming decades will require a second “green revolution.” (The first one increased global grain yields by roughly two per cent a year from 1950 to 1990.) This will have to be accomplished under increasingly trying circumstances. An analysis in Science concluded that rising temperatures have already begun to depress global corn and wheat production. Another analysis, published in the Proceedings of the National Academy of Sciences of the United States, warned that, owing to global warming, corn and soybean crops in the U.S. could decline by as much as eighty per cent by the end of the century.

Part of what made the first green revolution possible was a sharp increase in the use of phosphorus-rich fertilizers. Thanks to this increased use, experts say, reserves of phosphorus are now being exhausted. Foreign Policy has called this “the gravest natural resource shortage you’ve never heard of.” Other essential commodities that could similarly run short include oil, water, and arable land. Jamais Cascio, a research fellow at the Institute for the Future, an offshoot of the RAND Corporation, put it this way recently on the Times Web site: getting to ten billion “would be a sign of successful navigation of this century’s problems.”

Which brings us back to Malthus. Obviously, many of the predictions that he made in his “Essay on the Principle of Population,” in 1798, proved to be wrong. But the premise of the work—that there must be some limit to population growth—is hard to argue with. The question of where that limit lies—of how many people the earth can support over the long or even medium term—remains, at this point, open. As we sail past the seven-billion mark toward eight, nine, or ten billion, we should, sooner or later, arrive at an answer. ♦