World population could be nearly 11 billion by 2100

The projected rise is mostly due to fertility in Africa, where the U.N. had expected birth rates to decline more quickly than they have.

"The fertility decline in Africa has slowed down or stalled to a larger extent than we previously predicted, and as a result the African population will go up," said Adrian Raftery, a University of Washington professor of statistics and of sociology.

The current African population is about 1.1 billion and it is now expected to reach 4.2 billion, nearly a fourfold increase, by 2100.

The new U.N. estimates use statistical methods developed by Raftery and his colleagues at the UW Center for Statistics and the Social Sciences. The group's improved fertility forecasting methods, combined with updated data collected by the U.N., were used to project the long-term consequences of the fertility change in Africa since the last population estimate two years ago.

New to this year's projection are finer-tuned statistics that anticipate the life expectancies of women and men across this century.

In other areas of the world, fewer major population changes are expected. Europe may see a small decline because of fertility continuing below replacement level, and other nations around the globe may see modest increases due to longer life expectancies, Raftery said.

There's no end in sight for the increase of world population, he added, yet the topic has gone off the world's agenda in favor of other pressing global issues, including poverty and climate -- both of which have ties to world population.

"These new findings show that we need to renew policies, such as increasing access to family planning and expanding education for girls, to address rapid population growth in Africa," Raftery said.

The UN gives high and low variants of its projections, assuming that women have an average of half a child more or less than the best projection. That leaves a large uncertainty, from 7 billion to nearly 17 billion, in the range for potential world population by the end of this century.

By contrast, the UW research group has developed probabilities of future population levels to be coupled with best forecasts. "Our probability intervals are much tighter, ranging from 9 billion to 13 billion in 2100," Raftery said.

Global population reached 7 billion in 2011. It passed 6 billion in 1999.