When it comes to the party that is planet Earth, we might need to plan for a few extra guests, according to scientists. A new statistical projection concludes that the world population is unlikely to level off during the 21st century, leaving the planet to deal with as many as 13 billion human inhabitants—4 billion of those in Africa—by 2100. The analysis, formulated by U.N. and University of Washington (UW), Seattle, researchers, is the first of its kind to use modern statistical methods rather than expert opinions to estimate future birth rates, one of the determining factors in population forecasts.

“The U.N. in the past has been criticized for not doing complete statistics on their data and now they’ve done it exactly right,” says demography researcher John Bongaarts, vice president of the Population Council in New York City, who was not involved in the new work.

Through the early 2000s, most researchers thought that the world population—which today hovers around 7 billion—would reach 9 billion by midcentury and then stop growing. But the projection assumed that birth rates in Africa—the highest in the world—would steadily drop as access to contraceptives and women’s education improved. Instead, birth rates in most African countries have remained stagnant or declined only slightly. This can be explained partially by smaller jumps in contraception and education than predicted, though most scientists don’t fully know why the rates have stalled so much.

Africa’s situation is only part of what’s led to the new numbers. Every few years in recent history, the United Nations has recalculated its population projections after consulting with individual demography and statistics experts who provide best-guess estimates of future fertility (birth rates) and mortality (death rates). But not all experts agree on the trends these numbers will take. And the United Nations couldn’t run advanced statistics on the forecasts, because there were no quantifiable levels of uncertainty associated with the projections.

“Experts are pretty good at knowing where things generally stand with these rates,” says statistician Adrian Raftery of UW, a senior author on the new paper. “But what they don’t seem to be good at is integrating the newest data into future estimates in the right way.”

Rather than rely on expert opinions for the newest population projections, the United Nations teamed up with Raftery and his colleagues, who developed statistical equations—based on historical and real-time data—that describe how the fertility rate is changing over time in different places around the world. This let them crunch the numbers in a new way, and—in addition to calculating a single estimate—determine the statistical probability of different events, such as the population leveling off.

“The combination of a new method that’s not based on assumption but is based directly on data, and also the new data on Africa, have combined to make quite a big change to the overall population projections,” Raftery says.

To wit, there’s a 95% chance the world population will be between 9 billion and 13.2 billion by the year 2100, the team concludes online today in Science. Much of that growth, it found, will likely take place in Africa, whose population is estimated to rise from 1 billion to 4 billion by the end of the century. And, unlike projections from last decade, the new graphs show a steady increase through 2100 rather than a midcentury leveling off.

The new numbers will be used in models created by economists, environmentalists, and governments who use population estimates to predict pollution and global warming levels; prepare for epidemics; determine road, school, and other infrastructure requirements; and forecast worldwide economic trends. All of these plans need to be altered if the population is going to grow an extra few billion. “There’s a need to put population back on the world agenda as a major issue,” Raftery says. Determining what’s caused Africa’s birth rates to stagnate could be a step toward dealing with this skyrocketing population, he says.

But there’s no guarantee that the world’s population is going to continue to rise at its current rate, points out economist and population researcher David Lam of the University of Michigan, Ann Arbor. “We’re still talking about much slower population growth than we just came through,” he says. “The world population doubled between 1960 and 1999 and we’re never going to do that again. The population is leveling off and it’s going to eventually level off under any of these scenarios, whether that’s before 2100 or after.”

And of course, the numbers are just projections, Bongaarts says. “It could very well be that we could have epidemics, or wars, or unrest that creates massive mortality. But to be honest, it would require something of a huge magnitude to alter this trajectory.”