The report, The World Population Prospects: The 2017 Revision, by the Department of Economic and Social Affairs' Population Division, forecasts that the current world population of nearly 7.6 billion will increase to 8.6 billion by 2030, 9.8 billion in 2050 and 11.2 billion in 2100.
about 83 million people a year are added to the world population and the trend is expected to continue despite falling fertility rates since the 1960s.
"The population in Africa is notable for its rapid rate of growth, and it is anticipated that over half of global population growth between now and 2050 will take place in that region," said John Wilmoth, director of the population division. At the other extreme, he said, Europe's population will decline somewhat in the coming decades.
The report forecasts that until 2050 half the world's population growth will be concentrated in nine countries: India, Nigeria, Congo, Pakistan, Ethiopia, Tanzania, the United States, Uganda and Indonesia — listed in the order of their "expected contribution to total growth".
Twenty-six African countries are expected to at least double their population.
Nigeria, the world's seventh-largest country, has the fastest growing population of the 10 most populous countries and is projected to surpass the US shortly before mid-century.
China, which has 1.4 billion inhabitants, is expected to be replaced by India as the world's most populous country around 2024. India now has 1.3 billion inhabitants.
The report said fertility has been declining in nearly all regions in recent years.
Between 2010 and 2015, Wilmoth said, "the world's women had 2.5 births per woman over a lifetime — but this number varies widely around the world."
"Europe has the lowest fertility level, estimated at 1.6 births per woman in the most recent period, while Africa has the highest fertility, with around 4.7 births per woman," he said.
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