The new report released on Tuesday comes just ahead of a demographic milestone, with the world's population expected to pass seven billion in late October, only a dozen years after it surpassed six billion. Demographers called the new projections a reminder that a problem that helped define global politics in the 20th century, the population explosion, is far from solved in the 21st.
'Every billion more people makes life more difficult for everybody - it's as simple as that,' said Mr John Bongaarts, a demographer at the Population Council, a research group in New York. 'Is it the end of the world? No. Can we feed 10 billion people? Probably. But we obviously would be better off with a smaller population.'
The projections were made by the UN population division, which has a track record of fairly accurate forecasts. In the new report, the division raised its forecast for the year 2050, estimating the world would likely have 9.3 billion people then, an increase of 156 million over the previous estimate for that year, published in 2008.
Among the factors behind the upward revisions is that fertility is not declining as rapidly as expected in some poor countries and has shown a slight uptick in some wealthier countries including the United States, Denmark and Britain. The US is growing faster than many rich countries, largely because of high immigration and higher fertility among Hispanic immigrants. The new report projects that the US population will rise from today's 311 million to 478 million by 2100.
Growth in Africa remains so high that the population there could more than triple in this century, rising from today's one billion to 3.6 billion, the report said - a sobering forecast for a continent already struggling to provide adequate food and water.
Ms Hania Zlotnik, the director of the UN population division, said the world's fastest-growing countries, and the wealthy Western nations that help to finance their development, face a choice about whether to renew their emphasis on programmes that encourage family planning.
Although they were a major focus of development policy in the 1970s and 1980s, such programmes have stagnated in many countries, caught up in ideological battles over abortion, sex education and the role of women in society.
The report highlighted a converse problem in some developed countries: populations that are stagnant or even falling.
China, which has for decades enforced restrictive population policies, could soon enter the ranks of countries with declining populations. The report projects the Chinese population will peak at 1.4 billion in the next couple of decades, then begin falling, to 941 million by 2100.
But women in societies as diverse as Iran, Sri Lanka, Bangladesh, Mexico and Thailand - armed with information and access to birth-control methods - have chosen to have fewer children.
While about three-quarters of married US women use a modern contraceptive during their child-bearing years, the comparable proportions are a quarter of women in East Africa, one in 10 in West Africa and a mere 7 per cent in Central Africa, according to UN statistics.
'West and Central Africa are the two big regions of the world where the fertility transition is happening, but at a snail's pace,' said Mr John F. May, a World Bank demographer.
One message from the new report is that the Aids epidemic, devastating as it has been, has not been the demographic disaster that was once predicted.
Other factors have slowed change in Africa, experts said, including women's lack of power in their relationships with men, traditions such as early marriage and polygamy, and a dearth of political leadership.
NEW YORK TIMES
