The world’s population, now 7.3 billion, is expected to reach the 11 billion mark by 2100, according to revised population projections released yesterday by the United Nations (UN) Population Division.

“According to models of demographic change derived from historical experience, it is estimated the global population will be between 9.5 and 13.3 billion people in 2100,” UN Population Division Director John Wilmoth said August 10 at the 2015 Joint Statistical Meetings.

“In the United States, the population is projected to add 1.5 million people per year on average until the end of the century, pushing the current count of 322 million people to 450 million.”

The primary driver of global population growth is a projected increase in the population of Africa.

The continent’s current population of 1.2 billion people is expected to rise to between 3.4 billion and 5.6 billion people by the end of this century. The growth is due to persistent high levels of fertility and the recent slowdown in the rate of fertility decline.

The total fertility rate (TFR) has been declining on the continent over the past ten years, but has been doing so at one-quarter of the rate at which it declined in Asia, Latin America and the Caribbean in the 1970s.

In some African countries, the TFR decline appears to have stalled. For instance, in Nigeria – the continent’s most-populous country – the high fertility rate would result in a more than fourfold projected increase in total population by 2100, from 182 million to 752 million people.

“Although there is considerable uncertainty about these future trends, there is a 90 percent chance Nigeria’s population will exceed 439 million people in 2100, which is nearly 2.5 times its current size,” Wilmoth said.

Asia, with a current population of 4.4 billion, is likely to remain the most populous continent, with its population expected to peak around the middle of the century at 5.3 billion, and then to decline to around 4.9 billion people by 2100.

The report also examines the level of population aging in different countries. One such measure is the potential support ratio (PSR), which is equal to the number of people aged 20 to 64 divided by the number of people aged 65 or over and is frequently considered the number of workers per retiree. Japan currently has the lowest PSR at 2.1, followed by Italy at 2.6.

In the United States, where the median age of the population is expected to increase from today’s 38.0 years to 44.7 years in 2100, the PSR is projected to decline from 4.0 to 1.9.

Other countries that will experience sharp declines in their PSR by 2100 are Germany, China, Mexico, and Bangladesh.

Only five countries are projected to have a PSR above 5.0 in 2100: Niger, Somalia, Nigeria, Gambia and Angola. Niger is expected to have the highest PSR by the end of the century at 6.5.

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John Wilmoth. Population Projections by the United Nations. 2015 JSM: Joint Statistical Meetings, August 10, 2015