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, 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.

"By the end of the century, Nigeria could start to rival China as the second most populous country in the world," the report said, forecasting Nigeria's population at 913.8 million in 2100.

John Wilmoth, director of the Population Division in the UN's Department of Economic and Social Affairs, which prepared the report, cautioned that "there is a great deal of uncertainty about population trends."

"Trends and future population will be affected by the trajectories of its three major components — fertility, mortality and migration — but especially by the future course of fertility," he said.

Fallen fertility rates

He said fertility has fallen rapidly, especially since the 1960s. 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.

But Wilmoth said the UN's projections of future population have been revised upward from those issued two years ago, based mainly on recently available data on fertility levels.

In 15 high-fertility countries of sub-Saharan Africa, the estimated average number of children per woman has been adjusted upwards by more than five per cent, he said. These include Angola, Cameroon, Congo, Ethiopia, Mozambique, Niger and Nigeria.

The report said population in developing regions is projected to increase from 5.9 billion in 2013 to 8.2 billion in 2050.

During that same period, it said, the population of developed countries is expected to remain largely unchanged at around 1.3 billion people.

In Africa, the report said, the population could increase from 1.1 billion today to 2.4 billion in 2050 and potentially 4.2 billion by 2100.