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 There is an 80 percent likelihood that the number of people on the
planet, currently 7.2 billion, will increase to between 9.6 billion
and 12.3 billion by 2100, the researchers said. They also saw an 80
percent probability that Africa's population will rise to between
3.5 billion and 5.1 billion by 2100 from about 1 billion today.
The study, led by U.N. demographer Patrick Gerland and University of
Washington statistician and sociologist Adrian Raftery and published
in the journal Science, foresees only a 30 percent chance that
earth's population will stop rising this century.
"Previous forecasts did indeed forecast a leveling off of the world
population around 2050, and in some cases a decline," Raftery said.
Raftery said the new projections arise from data that clearly
establishes that birth rates in sub-Saharan Africa have not been
decreasing as quickly as some experts had expected, a trend that was
"not as clear when previous forecasts were made."
 Raftery said the researchers used data on population, fertility,
mortality and migration from every country and then predicted future
rates using new statistical models. Some of the figures, such as the
median projection of the population hitting 10.9 billion by 2100,
mirror a U.N. report published in 2013.
U.N. demographer Gerland said sub-Saharan Africa countries already
with big populations and high fertility levels are expected to drive
population growth, including Nigeria, Tanzania, Democratic Republic
of the Congo, Niger, Uganda, Ethiopia, Kenya, Zambia, Mozambique and
Mali.
The world's population reached 1 billion in the early 19th century,
doubled to 2 billion in the 1920s and doubled again to 6 billion in
the 1990s. It hit 7 billion in 2011.
The findings underscore worries expressed for decades by some
experts about a planet growing more crowded and humankind exhausting
natural resources, struggling to produce enough food or cope with
poverty and infectious diseases.
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Raftery said African nations could benefit by intensifying policies
to lower fertility rates, with studies showing that greater access
to contraceptives and more education for girls and women can be
effective.
The researchers projected that Asia's population, now 4.4 billion,
will peak at around 5 billion people in 2050, then begin to decline.
They forecast that the populations of North America, Europe and
Latin America will stay below 1 billion each by 2100.
Among the experts who had predicted the global population rise would
peter out was a 2010 report by Austrian demographer Wolfgang Lutz.
He forecast it likely would reach 8 billion to 10 billion by 2050
but "population stabilization and the onset of a decline are likely"
in the second half of the century.
(Reporting by Will Dunham; Editing by Grant McCool)
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