Warning: The 21st century may get a lot more crowded than previously thought.

In a paper published Thursday in Science, demographers from several universities and the United Nations Population Division conclude that instead of leveling off in the second half of the 21st century, as the UN predicted less than a decade ago, the world's population will continue to grow beyond 2100. (Read "Population Seven Billion" in National Geographic magazine.)

And for the first time, through the use of a "probabilistic" statistical method, the Science paper establishes a range of uncertainty around its central estimate-9.6 billion Earthlings in 2050, 10.9 billion by 2100. There's an 80 percent chance, the authors conclude, that the actual number of people in 2100 will be somewhere between 9.6 and 12.3 billion.

View Images NG STAFF. SOURCE: UN

That range "is the truly innovative part," says John Wilmoth, head of the UN Population Division and one of the authors of the Science paper. "It's a much more plausible analysis of uncertainty—but we may still be off by two billion."

According to other demographers, the UN has missed the mark by just about that amount. In a paper in press at Global Environmental Change and in a forthcoming book, Wolfgang Lutz and his colleagues at the International Institute for Applied Systems Analysis (IIASA) in Vienna, Austria, use a very different method—one that involves canvassing a large group of experts—to argue that population is likely to peak at 9.4 billion in 2075 and fall to just under 9 billion by 2100.

The UN team estimates there's no more than a 5 percent chance of that rosier scenario coming to pass.

Both groups foresee India becoming the world's most populous country, with its numbers peaking around 2070 and declining to around 1.5 or 1.6 billion by 2100. Where they differ most is in their estimates of the coming population decline in China and of the coming population explosion in Africa south of the Sahara—where most of the world's growth is going to occur.

According to the UN, the population in that region could quadruple, from less than one billion to nearly four billion. Africa in 2100 would be as densely populated as China is today.

"These are not predictions," says Wilmoth. "These are projections of what will happen if current trends continue. There is still an opportunity to intervene."

View Images Hindu pilgrims bathe in the Ganges River in Allahabad, India, which is projected to become the world's most populous country. Photograph by Stefano De Luigi, VII

A Change of Heart

A decade or so ago, the UN's demographers had a more optimistic view. The message back then, says Hania Zlotnik, who was chief of the UN Population Division at the time, was that "the population problem is essentially over, because fertility is coming down automatically. We know now that was wrong."

A country's total fertility—the number of children the average woman bears in her lifetime—is the key variable in the "demographic transition" that every developing country is expected to go through. At the outset of the transition, high death rates and low life expectancies are balanced by high birth rates, as women bear many children, knowing that some will die.

As the country acquires modern agriculture, sanitation, and medicine, its mortality rate can fall rapidly, especially in children. But it takes at least a generation for couples to adjust to the new reality and have fewer children. In that interval, the population booms—and the size of the boom depends on how long it takes for fertility to fall.

When Western countries went through this transition in their pasts, both mortality and fertility fell gradually over a period of a century or more. Beginning in the 1960s, demographers were shocked to see how fast the transition was happening in Asia and Latin America. "It was a total surprise," says Gilles Pison of INED, the French National Institute of Demographic Studies.

In countries like China and Brazil, it took just three or four decades for the fertility level to plunge from more than six to less than two—the number of kids it takes for a couple to replace itself, and for a country as a whole to maintain a stable population.

The experience in Asia and Latin America, says Pison, led demographers to expect a similarly rapid transition in Africa. Now they've been surprised again—unpleasantly this time.

Over the past decade or two, it has become clear that fertility is falling much more slowly in some countries in sub-Saharan Africa than it did on other continents. That realization has come gradually because vital statistics in Africa are so poor. As new data come in, researchers revise their estimates not only of the future, but also of the recent past.

Between 2010 and 2012, for instance, UN demographers raised their estimate of how many babies African women had recently been having by a quarter of a child per woman. Population grows like a bank account with compound interest; a quarter child more per woman today generates 600 million more Africans in 2100, according to the UN projections reflected in the Science paper.

Nigeria is a crucial country: Its population, already Africa's largest at 174 million, will more than quintuple by 2100, to more than 900 million. Of the people added to the planet in this century, according to the UN, one in five will be Nigerian.

"I think everybody has had trouble facing the reality of that many people in that quantity of land," says Wilmoth. "It's hard to imagine. People have trouble wrapping their heads around it."

A Range of Possible Futures

The small adjustments that demographers made in 2012 to Nigeria's recent fertility data added some 180 million people to the projection for that country—suggesting that the statistical model used is quite sensitive to such adjustments.

The model uses fertility data from all the world's countries going back to 1950 to generate a number of projections "that reflects the range of possible experiences," explains Adrian Raftery of the University of Washington, Seattle, one of the model's creators and a co-author of the Science paper.

By randomly generating tens of thousands of projections for each country, the model attempts to capture the full range of uncertainty about the future—barring a meteorite impact or a nuclear holocaust. (There are no examples of those in the demographic data since 1950, so the model can't imagine them.)

In contrast, Lutz and his colleagues from the IIASA attempt to model uncertainty in a completely different way: by surveying hundreds of demographers to get their subjective but expert answers to targeted questions about the demographic future of each country. The range of opinions gives a different set of possibilities.

There are three main reasons why the IIASA's projections, unlike the UN's, suggest the world's population will stabilize before 2100, Lutz explained by email. First, the UN assumes that China's fertility level, which has already fallen to 1.6 or so, is going to start rising again—an assumption, Lutz says, "that has no scientific basis, nor do trends point in this direction."

Second, the IIASA projects that Nigeria's population will triple but not quintuple in this century. The UN projections "on some African countries ... are simply too high," Lutz says.

And finally, the IIASA model incorporates data on a crucial variable that's not contained in the UN or other demographic models: the level of education in a given population. Educating girls in particular has been found to be one of the best ways of bringing down fertility, at least in the long term.

Lutz believes that improvements in education that are already happening-in Nigeria, he points out, half the women aged 20 to 24 have had a high school education, compared with a quarter of the women aged 40 to 44—will help keep the world's population from reaching 11 billion in 2100.

On the other hand, people will still have to figure out how to feed nine billion—a threshold that both the IIASA and the UN agree we'll cross before 2050.

View Images New apartment blocks are rapidly being built in Beijing, China. Currently the world's most populous country, China is projected to experience population decline this century. Photograph by Marcus Bleasdale, VII

Choosing a Path

So who's right about 2100? "We'll find out in a hundred years," says Gilles Pison. He created an ingenious simulator, posted on the INED website, that allows you to become a godlike demographer, choosing your own population path for each country, and for the world as a whole, by dialing fertility and mortality up and down at will.

The UN's Zlotnik doesn't doubt the importance of educating women, but she questions Lutz's faith that it will keep us off the population trajectory traced by the UN. "No one knows, neither he nor us, what's going to happen," Zlotnik says. "In order to get to a different future, you have to change the now." Education is slow, she says: What governments in Africa and elsewhere need to do is make contraception more widely available, now.