May 11, 2011

New United Nations world population projections based on UW research

News and Information

The world population could reach 10.1 billion people by the year 2100, according to a United Nations report released last week. Thats 12 percent more than the previous projection of 9 billion, the level at which world population was expected to stabilize by 2050.

Instead of holding steady, the world population is likely to keep growing.

The big leap in the population estimate is based on improved statistical methods for projecting future fertility rates. The methods were developed by UW researchers led by Adrian Raftery, a UW professor of statistics and sociology.

The 2010-2015 median total fertility projection based on a statistical model developed by UW statisticians.

United Nations

The 2010-2015 median total fertility projection based on a statistical model developed by UW statisticians.

Raftery and his colleagues at the UW Center for Statistics and the Social Sciences developed a probabilistic method using Bayesian statistics, an approach that allows for estimates to be made and updated even if some data are missing. This means that even though future fertility rates are unknown, because they havent been collected yet, projections of future population can still be made based on other relevant data.

In the new model, the UW statisticians incorporated historic fertility trends in each country and the past experience of other countries at similar levels of fertility. “This gives a large number of trajectories of future fertility decline,” Raftery said.

The model produced 100,000 trajectories for fertility in each country during the next 90 years, from 2011 to 2100. Then the median trajectory was used to plot future fertility paths in each country.

The 10.1 billion population projection is partly due to fertility in some poor countries not declining as quickly as previously expected and small fertility increases in some wealthier countries.

“The new method projects that the decline of fertility in much of Africa will be slower than the UN previously expected,” Raftery said. “As a result, the UN is now projecting the world population to reach 10 billion by the end of the century.”

For China, the worlds most populous country with about 1.8 billion people, the new projection indicates that fertility may decline over the next 15 years to about 1.5 children per woman. Then fertility will likely increase slowly for the rest of the century, Raftery said.

The projections are used by international organizations, governments and researchers. The United Nations Intergovernmental Panel on Climate Change, for instance, uses the population projections to forecast future climate change due to human activity.

The UW statisticians did not include the impacts of future climate change on the human population in their model. “Even though future population is hard to project 90 years into the future, it’s still easier to do than to forecast future climate change,” Raftery said.

The world population is about 6.9 billion now. It reached 6 billion in 1999.


For more information, contact Raftery at or 206-543-4505.