The world's population will peak at 9 billion over the next 70 years before beginning a decline into the 22nd century, Austrian researchers predict in a new study.
The world population now stands at 6.1 billion, and the Austrian study projects that most of the new growth will continue to occur in developing countries.
It also predicts some demographic changes. For example, it says the number of people aged 60 or older will more than quadruple by 2100.
The Austrian study, published in the latest issue of the journal Nature, was greeted with skepticism in many quarters. Some researchers argue the predictions are misleading because of unforeseeable changes in everything from air quality to food supply.
However, the study's authors at the International Institute for Applied Systems in Laxenburg, Austria, say they have developed a statistical computer model that considers uncertainties in migration, mortality and birth rates.
The result: There is an 85 percent chance the world's population will stop growing by the next century.
"Everybody thinks quite correctly, 'You can't predict the future,"' said Warren Sanderson, co-author of the study and a professor at the State University of New York at Stony Brook. "We have much more confidence in predicting a range."
The Austrian model predicted that in 2070, the world population would peak at 9 billion people. By 2100, the population would dip to 8.4 billion. Both represent the midpoint of the projected population range.
The probabilities were determined from historical demographic data. The researchers also asked outside experts to predict key outcomes, such as the number of children born to each woman.
The study also expects the world's population to become older. It predicts an increase from the current 10 percent to about 22 percent of those 60 or older by 2050.
At least one population researcher says the Austrian study falls short.
David Pimentel, of Cornell University, says there are so many young people in the world today that the population will increase for the next 70 years, even if world fertility drops to 2 children per woman.
Pimentel predicts the world population will hit 12 billion before declining. He attributes the slowing to high mortality rates from shortages of food, fuel and other resources.