If humans continue having babies at the rate that they do now for the rest of the century, and life expectancy rates hold as well, there will be 27 billion people on planet Earth by 2100.
"This is the 'constant fertility scenario,'" said Gerhard Heilig, chief of population estimates and projections at the United Nations, "but no one thinks this will be the case." Instead, the U.N. predicts that the population will hit 10 billion at the end of the century, and then stabilize.
This may sound like false optimism, but actually, it isn't. To arrive at their predictions for the future population, the U.N. uses complex models to project past and current population trends into the future.
The data compiled by the U.N. shows that the fertility rate — the average number of children born per woman — has been declining almost everywhere in the world for the past 50 years, and at a faster and faster pace. In 1950, women averaged five babies apiece, but today, they have just 2.5 babies. There is wide variation between countries, but in general, the rate seems to be converging to the "replacement level" — 2.1 children per woman, the rate at which children exactly replace their parents (with some extra births to make up for children who die young).
This rapid drop comes down to the growing use of contraceptives, more widespread gender equality and improvements in education, health care and standard of living.
The population is still increasing, as the fertility rate in most countries remains above the replacement level, but humanity's numbers will stabilize if current trends continue, and more and more countries modernize. "Fertility begins to decline slowly in most developing countries, and then it declines fast around three to four children, and then it slows down again," Heilig told Life's Little Mysteries.
In developed countries, the fertility rate typically dips below the replacement level and then rises back up. "We predict that in countries with increasing fertility, such as Spain, France and China, the rate is gradually increasing toward 2.1 [children per woman]," he said.
With fertility rates in developing countries approaching the replacement level from above, and modernized countries approaching it from below, U.N. projections see the rate leveling off.
However, there is great uncertainty in how things will turn out. "The uncertainty for 2100 is very, very large," Heilig said. "We cannot say 'This is the end of world population growth.'" So many variables are factored into his team's statistical models that the 10-billion-person projection is just the middle guess among a huge range of others, he explained. Because of the great many unknowns, "I strongly recommend using our 2100 projection just as an illustration of what would be the long-term consequences of the current trend," he said.
In fact, keeping the fertility rate declining along its current trajectory will take a great deal of effort. "What will happen to the world population is not carved in stone," said Joel Cohen, a population biologist at Columbia University, "but is subject to influence by how much we invest today in family planning programs, education and the status of women, and alleviation of poverty. Nobody knows when or at what number the human population will peak because it depends on what we and future people decide to do to improve human well-being."
But this much seems clear: If we decide to do nothing, then instead of stabilizing at 10 billion, the human population could multiply exponentially to 27 billion.
This story was provided by Life's Little Mysteries, a sister site to LiveScience. Follow Natalie Wolchover on Twitter @nattyover. Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook.