The possibility that the world’s population will climb to 11 billion by the end of the century is gaining traction now that demographers are using probability methods for their projections.
A paper published online on Thursday in the journal Science details new methodology that shows that most of the world’s anticipated growth is in Africa, where population is projected to quadruple from about 1 billion today to 4 billion by 2100.
“For the last 20 years, prevailing opinion was that world population would go up to 9 billion and level off in the middle of the century and maybe decline,” said Adrian Raftery, one of the paper’s lead authors and a professor of statistics and sociology at the University of Washington. “Population is going to keep growing. We can say that with confidence.”
Not only with confidence but with an exact percentage. There is a 70 percent probability that world population, now at 7 billion, will not stabilize this century, according to the research.
Countries in sub-Saharan Africa are adding people more rapidly than expected, said John Wilmoth, director of the United Nations’ Population Division and a co-author of the paper. The U.N. has been using the new probability model in its most recent projections.
“Fertility levels turn out to be higher today than was expected 10 years ago,” he said. “There’s been a worldwide reduction in fertility, even in sub-Saharan Africa over the last two decades. It’s falling, but slower than expected and more slowly than in other countries in Asia and Latin America.”
Earlier projections “took what happened in other countries, where birth rates came down and applied that across the board,” said Carl Haub, a demographer at the Population Reference Bureau in Washington, D.C. “The big issue is with Africa. It had not gone down very fast.”
Latest projections show that there is an 80 percent chance that the population in Africa will be 3.5 billion to 5.1 billion by 2100. Other regions are expected to see less growth. Asia, now at 4.4 billion, is projected to peak at about 5 billion in 2050 and then drop. In North America, Latin America and the Caribbean, population is projected to stay below 1 billion each.
Before, projections were strictly based on scenarios, said Patrick Gerland, a U.N. demographer and another lead author of the paper. One scenario was that women would have, on average, 0.5 children more or less than experts’ forecasts. The result was a yawning range of estimates.
“This work provides a more statistically driven assessment that allows us to quantify the predictions and offer a confidence interval that could be useful in planning,” Gerland said.
Projections are based largely on two factors: births and deaths. Earlier models relied on expert opinion to project trends. The new forecast uses Bayesian statistics, combining all available data, from government statistics to expert forecasts on mortality rates, fertility rates and international migration.
“We can answer questions about future population growth using standard principles of statistical inference, which has never really been done before,” Raftery said.
Rising population affects climate change, water and energy use, infectious disease rates and poverty. Continued high fertility rates in Africa are of major concern.
“It’s a major policy issue that there’s a relatively high level of unmet need for contraception,” Wilmoth said. The need is not met when women say they want to stop or delay childbearing but are not using any contraception.
“There’s a behavioral inconsistency,” he said. “Either they don’t have access to them or access is not acceptable to them. They may be concerned about side effects.”
Contraception and education of girls and women are two things that decrease fertility rates, and Africa could benefit greatly by acting now to lower fertility, Raftery said.