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The United Nations forecast Thursday that the world's population will increase from 7.2 billion today to 8.1 billion in 2025, with most growth in developing countries and more than half in Africa. By 2050, it will reach 9.6 billion.

India's population is expected to surpass China's around 2028 when both countries will have populations of around 1.45 billion, according to the report on "World Population Prospects." While India's population is forecast to grow to around 1.6 billion and then slowly decline to 1.5 billion in 2100, China's is expected to start decreasing after 2030, possibly falling to 1.1 billion in 2100, it said.

The report found global fertility rates are falling rapidly, though not nearly fast enough to avoid a significant population jump over the next decades. In fact, the U.N. revised its population projection upward since its last report two years ago, mostly due to higher fertility projections in the countries with the most children per women. The previous projection had the global population reaching 9.3 billion people in 2050.

John Wilmoth, director of the Population Division in the U.N.'s Department of Economic and Social Affairs, said the projected population increase will pose challenges but is not necessarily cause for alarm. Rather, he said, the worry is for countries on opposite sides of two extremes: Countries, mostly poor ones, whose populations are growing too quickly, and wealthier ones where the populations is aging and decreasing.

"The world has had a great experience of dealing with rapid population growth," Wilmoth said at a news conference. "World population doubled between 1960 and 2000, roughly. World food supply more than doubled over that time period."

"The problem is more one of extremes," he added. "The main story is to avoid the extreme of either rapid growth due to high fertility or rapid population aging and potential decline due to very low fertility."

Among the fastest-growing countries is Nigeria, whose population is expected to surpass the U.S. population before the middle of the century and could start to rival China as the second-most populous country in the world by the end of the century, according to the report. By 2050, Nigeria's population is expected to reach more than 440 million people, compared to about 400 million for the U.S. The oil-rich African country's population is forecast to be nearly 914 million by 2100.

The report found that most countries with very high levels of fertility — more than 5 children per women — are on the U.N. list of least-developed countries. Most are in Africa, but they also include Afghanistan and East Timor.

But the average number of children per woman has swiftly declined in several large countries, including China, India, Indonesia, Iran, Brazil and South Africa, leading to a reduction in population growth rates in much of the developing world.

In contrast, many European and eastern Asia countries have very low fertility levels.

"As a result, these populations are aging rapidly and face challenges in providing care and support to their growing ranks of older persons," Wilmoth said.

Wilmoth cautioned that "there is a great deal of uncertainty about population trends." He said projections could change based on the trajectories of three major components — fertility, mortality and migration.

Still, population growth until 2050 is all but inevitable.

The U.N. uses the "medium-variant" projection, which assumes a substantial reduction in the fertility levels of intermediate- and high-fertility countries in the coming years. In the "high-variant" — if women on average had an extra half of a child — the world population would reach 10.9 billion in 2050. In the "low-variant" — if women on average had half a child fewer — the population would be 8.3 billion in 2050.

Among the notable findings in the report:

• The population in developing regions is projected to increase from 5.9 billion in 2013 to 8.2 billion in 2050. In contrast, the population of developed countries is expected to remain largely unchanged during that period, at around 1.3 billion people.

• Africa's population could increase from 1.1 billion today to 2.4 billion in 2050, and potentially to 4.2 billion by 2100.

• The number of children in less-developed regions is at all time high at 1.7 billion. In those regions, children under age 15 account for 26% of the population. In the poorest countries, children constitute 40% of their populations, posing huge challenges for providing education and employment.

• In wealthier regions, by contrast, children account for 16% of the population. In developed countries as a whole, the number of older people has already surpassed the number of children, and by 2050 the number of older people will be nearly twice the number of children.

• Low-fertility countries now include all of Europe except Iceland plus 19 countries in Asia, 17 in the Americas, two in Africa and one in Oceania.

• The populations of several countries are expected to decline by more than 15% by 2050, including Belarus, Bulgaria, Croatia, Cuba, Georgia, Latvia, Lithuania, Republic of Moldova, Romania, Russia Serbia, and Ukraine.

• Life expectancy at birth for the world as a whole rose from 47 years in 1950-55 to 69 years in 2005-2010 and is projected to reach 76 years in 2045-2050 and 82 years in 2095-2100.