A study published this week in the scientific journal Science says the world's population is likely to grow much higher this century than previous estimates have forecast, reaching nearly 11 billion people by century's end.

That's about 2 billion higher than most commonly cited forecasts from recent years, according to the study published Sept. 18 and titled "World Population Stabilization Unlikely This Century," which uses the latest numbers and an updated prediction model from the United Nations Population Division to reach its conclusions.

“The consensus over the past 20 years or so was that world population, which is currently around 7 billion, would go up to 9 billion and level off or probably decline,” said Adrian Raftery, a professor of statistics and of sociology at the University of Washington, and one of the study's authors.

“We found there’s a 70 percent probability the world population will not stabilize this century," he added in a news release. "Population, which had sort of fallen off the world’s agenda, remains a very important issue.”

Why? The study's authors say the cause lies in a few factors. The first is that fertility rates –the number of births per woman –haven't declined in Africa to the degree they have in places like Asia and Latin America, where fertility has declined rapidly since 1950.

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United Nations 2012 world population projection (solid red line), with 80 percent prediction interval (dark shaded area), 95 percent prediction interval (light shaded area), and the traditional U.N. high and low variants (dashed blue lines).

(SCIENCE)

While Asia is likely to remain the world's most populous continent this century, thanks largely to nations like China and India, its population will probably peak around mid-century and then decline after that.

The biggest population increases will come from Africa, the study says, noting that the ideal family size in much of sub-Saharan Africa is still very high by western standards at a median of 4.6 children per woman. In most countries this century, average family sizes are expected to be between 1.5 and 2 children per woman, the study notes.

"These projections indicate that there is little prospect of an end to world population growth this century without unprecedented fertility declines in most parts of Sub-Saharan Africa [which is] still experiencing fast population growth," the study says.

From today's population of about 1 billion, Africa will likely grow to a population of between 3.1 and 5.7 billion by 2100, the study adds.

Other causes for Africa's continued growth range from the fact that its deaths from HIV/AIDS have dropped off in a big way, as Vox.com notes, and contraception is still unavailable across much of the continent, as the "unmet need" for contraception –the difference between the demand for it and its use, the study says– hasn't changed much in two decades.

See the full study at Science Magazine.

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