With 7.5 billion inhabitants and growing, Earth is speeding towards the end of its ability to sustain humans. Yet it is still a surprise to most that a new study predicts that the world’s human population could grow to as high as 12.3 billion by 2100—2 billion more than many experts' previous projections.
“We found there’s a 70 percent probability the world population will not stabilize this century,” statistician Adrian Raftery says. “Population, which had sort of fallen off the world’s agenda, remains a very important issue.”
Raftery, a professor of statistics and sociology at the University of Washington and a research affiliate at the school's Center for Studies in Demography and Ecology, worked with colleagues to develop models of population growth that looked at fertility and mortality data country by country and predicted the most likely population trends. The findings, published in Science last week, are part of the first U.N. population report to use Bayesian statistics, a comprehensive method of statistics that incorporates more variables to better predict outcomes.
“Previous models worked in a very deterministic way” and were filled with uncertainty, says Patrick Gerland, a statistician with the U.N. Population Division and collaborator on the study. He explains that the new models look at multiple forms of data stretching back at least 60 years—basically a summation of each country’s past experience—and use those numbers to run nearly 10,000 future population simulations for each country. The researchers determined, with an 80 percent probability, that the world population would increase to between 9.6 billion and 12.3 billion by 2100.
The research group noticed separate trends in the developing world. Fertility rates in Asia and South America have slowed and will eventually level off. The same, however, cannot be said of Africa, where birth rates have hardly slowed down. Gerland and his group calculated a 95 percent probability that by 2100, Africa could be home to as many as 5.7 billion people, a staggering figure.
The study raises concerns over how to meet the energy needs of growing populations. And like all discussions before, the answer is rife with uncertainty. “There’s no real consensus as to how that will happen,” says Raftery.
“There’s a complexity that’s very hard to predict in the long term,” says Gerland. “Not everyone in the world consumes the same energy with the same purpose.” Roger Pielke Jr., a professor of environmental studies at the University of Colorado Boulder, cites differences in diet, health, and transportation among the slew of variables that must be considered.
Currently, sub-Saharan Africa maintains sustainability because its populace consumes very little energy. But because an increased population would require more economic development, a much larger energy infrastructure would need to be built to get power to everyone. And more people enjoying a better standard of living would cause sub-Saharan Africa’s energy needs to jump exponentially—well before it caused a reduction in fertility rates and would help stabilize population growth. According to a 2011 report (pdf) by Morgan Bazilian of the Joint Institute for Strategic Energy Analysis, “sub-Saharan Africa would need to increase its installed electricity capacity by 33 times to reach the level of energy use enjoyed by South Africans—and 100 times to reach that of Americans.”
Pielke thinks China may actually be an illustration of how African countries and developing nations elsewhere could accommodate the energy needs of a booming population. However, China's success at providing for its residents' energy needs has come at a tremendous environmental cost, especially due to the use of air-fouling coal-fired power plants. Pielke cautions that careful discussion is needed before policy makers decide how best to meet their countries' energy demands.
That discussion will have to come soon. There will be a great need for better access to energy in the future, and “there are a lot of challenges to face,” says Gerland.