The world's population will peak at 9 billion over the next 70 years before beginning a decline into the 22nd century, researchers predict in a new study.
The world population currently stands at 6.1 billion, and the study projects that most of the new growth will continue to occur in developing countries.
It also predicts some demographic changes. For example, the authors say, the number of people aged 60 or older will more than quadruple by 2100.
The 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 computer 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 researchers also expect the world's population to become older. The study predicts the portion of the population age 60 or older to increase from the current 10 percent to about 22 percent by 2050. It's expected to increase to about 34 percent in the next 100 years.
Nico Keilman of the University of Oslo, Norway, praised the study's forecast. But he warned that using information from historical predictions may be inaccurate and that overconfident experts could give very narrow prediction ranges.
United Nations officials stuck by their 1998 assessment, in which they predicted an increase of 3 billion over the next 50 years to 9 billion. It would be the second largest population jump in history. From 1950 to 2000, world population increased by 3.5 billion.
``The first most rapid growth is over, but we're still growing,'' said Joseph Chamie, director of the United Nations' Population Division. ``It's like a huge freighter. You can't say, 'Stop the boat!' and expect it to stop immediately. It has to come to a slow stop.''
But Sanderson believes the United Nations' estimates are off because its calculations only allow fertility rates to fall to 2.1 children per woman. Sanderson said fertility rates are already below that level in many countries. Since those rates typically do not recover, population growth should decline, he said.
At least one population researcher says the study falls short.
David Pimentel, of Cornell University, said 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.