UNITED NATIONS - The world's population will hit 7.2 billion next month and 10.9 billion by 2100, with most of the growth a result of high birthrates in the developing world, the United Nations said Thursday.
The UN's latest "World Population Prospects" report said the number of people inhabiting the planet at the start of the next century could top 16.6 billion, or depending on the statistical model, could be as low as 6.8 billion.
In either case, the population in the world's poorest regions is anticipated to rise dramatically, the UN said.
The number of inhabitants in the world's least developed countries is projected to double, from 898 million inhabitants this year to 1.8 billion in 2050. The number will soar to 2.9 billion by 2100, the UN report said.
"Although population growth has slowed for the world as a whole, this report reminds us that some developing countries, especially in Africa, are still growing rapidly," Wu Hongbo, United Nations Undersecretary General for Economic and Social Affairs, said in a statement.
By contrast, population in the world's developed nations is expected to remain largely unchanged, inching upward from 1.25 billion this year to around 1.28 billion in 2100.
The report said the number of people living in the developed world would decline if not for immigration from poorer countries, which is projected to average about 2.4 million people a year from 2013 to 2050.
Much of the increase in world population between 2013 and 2050 -- when the number is expected to hit 9.6 billion -- is projected to take place in Africa.
The report said that half of all population growth between 2013 and 2100 is expected to be concentrated in just eight countries: Nigeria, India, Tanzania, the Democratic Republic of Congo, Niger, Uganda, Ethiopia and the United States.
The study also highlighted the fast-growing number of seniors -- and not just in rich regions.
In more developed parts of the world, 23 percent of the population is already 60 or older. Their percentage is projected to climb to 32 percent in 2050, and 34 percent in 2100.
Globally, the number of people 60 or older is expected to more than triple by 2100 to hover near 3 billion. The proportion of older citizens in developing countries is forecast to more than double by 2050 and triple to 27 percent by 2100.
Longevity also is on the rise, the United Nations said.
The number of people aged at least 80 is projected to spike almost seven-fold to 830 million by the start of next century, up from 120 million this year and 392 million in 2050.
Sixty-eight percent of those 80 and over are forecast to live in developing countries by 2050.
Even as the population is rising, the UN report said fertility is expected to fall globally, with a major drop projected for least developed countries -- from 4.53 to 2.87 children per woman in 2045-2050 and to 2.11 in 2095-2100.
The rest of the developing world is expected to see a dip to 2.09 from 2.40 in 2045-2050, and 1.93 in 2095-2100.
Most developing countries have had below-replacement fertility -- below 2.1 children per woman -- for several decades. That includes all of Europe except Iceland.
The largest so-called low-fertility countries are China, the United States, Brazil, Russia, Japan and Vietnam.
In other findings, the UN study said that India would surpass China's as the world's most populous country around 2028, when both nations will have about 1.45 billion people.
India will continue to grow for several decades after that to about 1.6 billion and then slowly slip to 1.5 billion in 2100.
China's population is expected to start decreasing after 2030 and could reach 1.1 billion in 2100.
The study also found that Nigerians are expected to outnumber Americans before 2050.
Europe's population, meanwhile, is projected to decline by 14 percent between 2013 and 2100.
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