The world's population will probably peak before 2100, and then may begin shrinking for the first time in several centuries, according to a new study. The finding is good news for the gloomy field of human population projection, but growth will have to slow substantially in developing countries if global numbers are to peak at an estimated 9 billion people.
The world's population doubled twice in the 20th century, hitting 6 billion in 1999. United Nations demographers have previously forecast that population will plateau in the second half of the 21st century at somewhere under 10 billion. To get this number, the UN assumes that all countries are converging toward a so-called fertility replacement rate of 2.1, or roughly two children for every couple. "This is a traditional assumption and it is convenient," says Wolfgang Lutz, a demographer at the International Institute for Applied Systems Analysis (IIASA) in Laxenburg, Austria. But he calls the use of a universal fertility rate "meaningless."
For the new study, Lutz's team used a spread of lower fertility rates that they say are more regionally realistic. They also used cutting-edge probability methods to simulate thousands of future population growth scenarios and attach a probability to various outcomes. The mathematical churning yielded an 85% chance that population growth will end by 2100, they report in the 2 August issue of Nature. On average, the simulations suggested that the world's population could peak as early as 2070 at around 9 billion. Unlike UN projections, the team expects a slow decrease in world population to 8.4 billion by 2100.
"This is a new kind of projection and I salute them," says Robert Engelman, vice president of research at the Washington, D.C.-based Population Action International. He praises the attempt to make population forecasts more sophisticated and realistic, but he cautions that the findings imply a level of mathematical precision that's just not possible for long-term demographics. Even short-term weather forecasters, he notes, joke that they are "not very good about forecasting, particularly when it involves the future."