Wolfgang Lutz, of the International Institute for Applied Systems Analysis, recently published the results of his institute’s forecast of future population growth. The forecast received a lot of media attention in part because its forecast is significantly more optimistic than the United Nations’ forecasts about the rate at which grow will come to a halt.
The main difference between the IIASA’s forecast and the United Nations’ forecast is their differing assumptions about fertility. Lutz and his fellow researchers argue that the UN model underestimates the rate at which fertility is declining worldwide and thus overestimates future populations.
Warren Sanderson, an economist who worked on the IIASA model, told ABCNews.Com that the UN model was flawed because it assumes that fertility will stay above the replacement level of 2.1. Sanderson argued that this is unrealistic.
“The evidence shows women are already not having enough children to replace themselves,” Sanderson said. “We may as well wake up and smell the coffee and begin focusing on how to live sustainability with the number of people we will have in the next century.”
And how many people will that be? In a letter published in Nature, Lutz, Sanderson, and Sergei Scherbov write that,
Improving on earlier methods of probabilistic forecasting1, here we show that there is around an 85 per cent chance that the world’s population will stop growing before the end of the century.
By the end of the century they predict an 80 percent chance that the world’s population will be 8.4 billion. The United Nations, in contrast, forecasts that under the most likely scenario world population will be roughly 9 billion by 2100, and possibly still growing (though at the extremely small rate of one-tenth of one percent per annum).
Sources:
The end of population growth. Wolfgang Lutz, Warren Sanderson & Sergei Scherbov, Nature, 412, 543-545, August 2, 2001.
Population to peak. Amanda Onion, ABCNews.Com, August 2, 2001.