Earth is fast becoming a more crowded place — and it may become even more crowded than expected. According to a new projection of human population growth, there could very well be 12.3 billion people by century's end, up to 2 billion more than some estimates.

The number's not written in stone, but it's something to consider. Life's already pretty complicated with 7.5 billion people confronting environmental problems, food insecurity and spotty public health. Are we ready for more?

"A rapidly growing population with bring challenges, said statistician and sociologist Adrian Raftery of the University of Washington. "But I think these challenges can be met."

In a study published today in Science, Raftery and 13 other scientists analyzed new data provided by the United Nations on national trends in fertility, mortality, migration and age patterns.

The most recent UN estimates put the global population at 10.9 billion by 2100. Some demographers have, however, criticized that projection as excessively high. The projections also contained a great deal of uncertainty, with possible population scenarios of as many as 15.8 billion people, or as few as 6.2 billion.

That wide range was determined by calculating what would happen if women on average had 0.5 more or fewer children than expected. That's not a bad rough guide, but is statistically rudimentary, failing to account for how those numbers will vary from country to country.

Raftery's group took a finer grained look at the data, running population models on a country-by-country basis. "There is an 80 percent probability that world population, now 7.2 billion, will increase to between 9.6 and 12.3 billion in 2100," they concluded.

Within that total, populations will level off in Asia and South America, where fertility rates have slowed considerably, but follow a very different trajectory in Africa, where fertility has not dropped as fast as expected.

Projections of probable populations by continent. Rafter et al./Science

As a consequence, the continent's population "is projected to rise to between 3.1 billion and 5.7 billion with probability of 95 percent by the end of the century," concluded the researchers. Most of that growth would be concentrated in sub-Saharan Africa.

"What they're doing makes a lot of sense," said economist and demographer David Lam of the University of Michigan, who was not involved in the study. "It's more rigorous and scientifically valid" than previous population predictions.

Over the last several decades, fertility rates dropped rapidly in many parts of the world, a change attributed to a number of factors: expanded contraceptive use, women's education and economic opportunities, and also changes in mortality. As people become healthier and live longer, they tend to have smaller families.

Whether that will happen in Africa is unclear. There remains a tremendous need for education and family planning: Just half of sub-Saharan women can read, and 36 million women don't have access to contraception. Were those needs met, though, Africa might not follow the expected path.

Raftery's team cited research by demographers John Bongaarts and John Casterline, who noted in a 2012 Population and Development Review article that cultural differences play an important role. In particular, ideal family sizes in Africa tend to be larger than in Asia or Latin America.

Unless social norms change, Africa may simply end up with more people than previously expected. A hint of this dynamic comes from countries like Uganda, said Lam, where fertility rates have stayed high despite economic growth.

Lam did caution that predicted growth is not inevitable. While the analysis is solid, and suggests that a global population of at least 10 billion people can be expected, "no amount of statistical sophistication is going to solve the fact that we just don't know what is going to happen to fertility in Africa," he said.

However the predictions play out, it's quite possible that Earth will be home to at least a few billion more people by century's end, when populations are expected to stabilize. The essential question remains: What sort of lives will people have?

"Rapid population growth can exacerbate the challenges that African countries will have in the future: poverty, environmental problems, health problems and resource depletion," said Raftery. "And in an increasingly globalized world, the problems in one part can impact the rest."

Yet even if the scenarios look grim, says Lam, it's worth remembering that human populations doubled between 1960 and 1999. That tremendous growth spurt occasioned fears of widespread famine and societal collapse. On the whole, though, we made it through in decent shape.

"Everything is not rosy, but the average person in a developing country is much better off than in 1960 in terms of food, poverty, education and employment," Lam said.

Central to that transition were increases in agricultural productivity. Though a case can be made that the increases were often unsustainable, relying on high rates of fertilizer, pesticide and water use, they're at least a testament to the possibility of human ingenuity.

Further agricultural improvements will be needed to feed even 10 billion people, much less 12.3 billion, but it should be possible. As population professor Joel Cohen of Rockefeller University wrote in the New York Times, humanity already produces enough grain to feed 11 billion people. We just don't use it smartly.

Climate change is another urgent issue, as is resource use. According to Vaclav Smil, an energy, environment and food production researcher at the University of Manitoba, the future will be shaped largely by material considerations: how much meat we eat, how much concrete and metal we require, and our ability to uncouple economic growth from consumption.

It's not any specific population total that will matter in 2100, said Smil, "but the prevailing levels of consumption." That won't likely change as a consequence of technical innovation, Smil thinks. Rather, the motivation must be cultural.

Meeting the needs of billions more people while also making room for nonhuman lives is another challenge. Conservationists must take on new roles, said Janet Edmond, director of Conservation International's population and environment program. Not only must they work to protect nature; they'll also need to promote education, access to family planning and sustainable development.

Raftery hopes the new numbers will inform discussions about all these issues. "There are challenges and we should be concerned, but I wouldn't subscribe to the idea that they can't be solved," he said.