Nigeria's population is expected to surpass that of the US by 2050, according to new UN projections that predict the west African country could be the world's third most populous by the end of this century.
The UN report World population prospects: the 2012 revision, published on Thursday, predicts the world's population, now at 7.2 billion, will reach 8.1 billion in 2025. By mid-century, the world's population is expected to top 9.5 billion, reaching nearly 11 billion by 2100.
More than half of the growth predicted between now and 2050 is expected in Africa, where the number of people is set to more than double, from 1.1 billion to 2.4 billion. Africa's population will continue to rise even if there is a future drop in the average number of children each woman has, says the report, which predicts the number of people living on the continent could reach 4.2 billion (or more than 35% of total global population) by 2100.
Several other countries, including India, Indonesia, Pakistan and the Philippines, are also set for significant population increases.
India, currently the world's second-largest country by population, is expected to rival China in size by 2028, when both nations will each have about 1.45 billion citizens. After 2030, China is expected to shrink in numbers, reaching an estimated 1.1 billion by the end of this century, when India's population is projected to reach about 1.5 billion.
Some of the most rapid increases are expected in the world's 49 least-developed countries. The populations of Mali, Niger, Somalia, Tanzania and Zambia are expected to increase at least fivefold by 2100.
In contrast, the populations of 43 countries are expected to decrease over the next few decades. Belarus, Bulgaria, Cuba, Romania, Russia, and Ukraine are among the countries where populations are expected to drop by more than 15% by 2050.
"These future trends are not guaranteed," stressed John Wilmoth, director of the population division at the UN's department of economic and social affairs. Wilmoth, who said the future shape of the world will depend largely on what happens to fertility rates, underlined the continued unmet need for family planning in many countries.
"Demography is not destiny, [but] these new population projections highlight the pressures facing governments … population growth in some cases, and population ageing and decline in others," Wilmoth said.
Migration from developing to developed countries is projected to average roughly 2.4 million people each year from 2013-2050 before dropping to 1 million each year from 2050-2100. Without this, the populations of many European and other developed countries would be expected to decline significantly.
The report also anticipates a rise in the number of old people, with life expectancy projected to rise to 76 globally by 2050 and 82 by 2100.
Today, the average person is 29 years old. This is expected to increase to 36 by 2050, and to 41 by 2100. By the end of the century, the average person in Europe, the region with the oldest average age, will be 47.
Globally, the number of over-60s is expected to rise from 841 million in 2013 to nearly 3 billion by the end of the century. Two-thirds of the world's over-60s already live in developing countries. By 2050, this share is expected to reach nearly 80%.
"These figures are a wake-up call for all countries, including developing countries with young populations," said Silvia Stefanoni, interim head of HelpAge International.
"The generation that will retire in 2050 is the current youth bulge and, given the limited employment opportunities available to young people [now], many of them will not be able to retire [in the future]. Young and old depend on each other – our shared future for all people on our planetdepends on recognising and investing in all our age groups."
The number of people between 25 and 59 in developing countries is at an all-time high, and is projected to increase by more than 400 million over the next decade.
"These population trends point to the urgency of supporting employment creation in developing countries as part of any strategy to address the slow economic recovery that the world is experiencing," the report warns.
The figures are based on a comprehensive review of recent data on population, including a 2010 round of national censuses. They are higher than previous estimates, which put the world's population at 9.3 billion in 2050 and 10.1 billion in 2100, in part because UN demographers have found higher fertility rates in some countries than previously predicted.
Beyond providing new estimates of how the world will change over the next century, the figures also provide the basis for a number of key UN indicators to assess progress towards the millennium development goals.
The figures are based on the UN's "medium-variant" projections, which assume a decline in the number of children born per woman in many countries with high fertility rates. Under different assumptions, estimates of the world's population in 2050 range from 8.3 billion to 10.9 billion.
Average family sizes have dropped significantly over the past few decades. An estimated 48% of the world's people now live in countries where the average number of children per woman is fewer than 2.1. Another 43% live in countries where that figure is between 2.1 and five children.
Only 9% of the world's people live in the 31 countries where the average woman has more than five children. Twenty-nine of these are in Africa and two are in Asia (Afghanistan and Timor-Leste).