That would mean two billion more people on Earth than expected by 2100, largely due to high birth rates in Africa, said the report in US journal Science.
    
"The consensus over the past 20 years or so was that world population, which is currently around seven billion, would go up to nine billion and level off or probably decline," said co-author Adrian Raftery, professor of statistics and of sociology at the University of Washington.
    
"We found there's a 70 percent probability the world population will not stabilize this century," he added.
    
The study is uses the most recent United Nations population data, released in July, and employs Bayesian statistics to combine all available information to generate better predictions.
    
Previous estimates typically relied on expert opinions about how life expectancy and fertility rates would change over time.
    
"Earlier projections were strictly based on scenarios, so there was no uncertainty," said UN demographer Patrick Gerland.
    
"This work provides a more statistically driven assessment that allows us to quantify the predictions, and offer a confidence interval that could be useful in planning,” he added.
    
The bulk of the growth was anticipated in Africa, where the population is expected to rise from one billion today to four billion by the end of the century.
    
"There is an 80 percent chance that the population in Africa at the end of the century will be between 3.5 and 5.1 billion people," said the study.
    
While Africa is expected to boom in size, some populations are expected to peak or shrink, the study said.
    
Asia, now at 4.4 billion, is projected to peak at around 5 billion people in 2050 and then start to decline.
    
North America, Europe, and Latin America and the Caribbean are all expected to remain under one billion each.
    
More people on Earth will likely exacerbate problems such as climate change, infectious diseases and poverty.

"Population, which had sort of fallen off the world's agenda, remains a very important issue," said Raftery.
    
Two methods that have been shown to decrease population are wider access to birth control and the education of girls and women.

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