The study also shows that the world population is likely to be much older in the future, with 34 percent of the population over the age of 60 by the end of the century. Addressing the needs of an older population will require a rethinking of programs providing social security, health, and education, said Sanderson.
Before the world stabilizes, however, there will be a growing demographic divide, the IIASA study warns. On one side will be countries with shrinking populations, such as the European states that were formerly part of Soviet Union; on the other side, countries with growing populations, such as Nigeria.
This demographic divide is likely to result in enormous stresses both within countries and between countries over immigration and other issues, said Sanderson.
Lots of Uncertainty
Although probability-based forecasting is an improvement over population scenarios that do not take uncertainty into account, probability analysis itself is uncertain to some extent, Nico Keilman, an economist at the University of Oslo in Norway, points out in an accompanying article in Nature.
Critics of the IIASA study say that no mathematical model can accurately measure the socioeconomic factors that affect population growth. Fertility decline, for example, which is the major factor in a slowdown of population growth, can happen in a variety of settings for unpredictable reasons, Haub explained.
Eastern Europe always had a higher fertility rate than Western Europe, he noted. But when the former Soviet Union broke up, fertility rates in those former Soviet countries plummeted unexpectedly.
"The actual outcome of world population growth will depend on how people's social behavior changes," said Haub. "That is really very difficult to predict using any kind of mathematical method."
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