The population of the world will continue to increase through the end of this century, not level off as some experts have predicted, and could reach a total of as much as 13 billion people alive on the planet, a new projection says.

Four billion will be living in Africa, thanks to higher-than-expected birth rates in the continent's sub-Saharan regions, the projection from scientists says.

The researchers say their analysis is the first to be based on modern statistical methods rather than relying on expert best-guess estimates of coming birth rates.

Earlier opinion had held that population growth rate might slow over the coming decades.

"Previous forecasts did indeed forecast a leveling off of the world population around 2050, and in some cases a decline," says University of Washington sociologist and statistician Adrian Rafferty, who conducted the analysis along with United Nationas demographer Patrick Gerland.

That was based on expert expectations that sub-Saharan African birth rates would decrease, something not supported by the new analysis, Rafferty says.

An opposite trend, based on historical and real-time data showing how the fertility rate is changing over time in different regions of the world, was "not as clear when previous forecasts were made," he says.

"Experts are pretty good at knowing where things generally stand with these rates," he acknowledges. "But what they don't seem to be good at is integrating the newest data into future estimates in the right way."

The new analysis, based not on assumptions or expert estimates but directly on verifiable data, has resulted in a significant change in overall projections of population growth, the researchers say.

Reporting their study in the journal Science, they conclude the world well see a steady increase in human numbers through to the end of the 21st century, rather than a leveling off in midcentury as previously forecast.

Global population hit 1 million in the first part of the 19th century, reached 2 billion by the 1920s and stood at 6 billion in the 1990s.

In 2011, the world was home to 7 billion people.

While there might not be a slowdown as some had thought, the rate of increase will be nothing like what's been happening in recent years, other experts point out.

"We're still talking about much slower population growth than we just came through," says population researcher David Lam at the University of Michigan. "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," he says.

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