The world's population is likely to peak at 9.7 billion in 2064 and then fall to about 8.8 billion by the end of the century as women have more access to education and contraception, a new study has found.
By 2100, 183 of 195 countries will not have fertility rates required to maintain the current population, with a projected 2.1 births per woman, researchers from the Institute for Health Metrics and Evaluation at the University of Washington's School of Medicine said.
Some 23 countries -- including Japan, Thailand, Italy, and Spain -- will see populations shrink by more than 50%, researchers said.
The modeling study, published Tuesday in The Lancet, also forecasts dramatic declines in working-age populations in countries including India and China.
But as fertility declines, researchers note that immigration could offset population shrinkage, particularly in countries with low fertility, such as the US, Australia and Canada.
"The world, since the 1960s, has been really focused on the so-called population explosion," Dr Christopher Murray, who led the research, told CNN. "Suddenly, we're now seeing this sort of turning point where it is very clear that we are rapidly transitioning from the issue of too many people to too few."
Using data from Global Burden of Disease Study 2017, researchers predicted that the fastest-shrinking populations will be in Asia and eastern and central Europe.
The report authors project that the population of Japan will shrink from around 128 million people in 2017 to 60 million in 2100, Thailand will see a shrink from 71 to 35 million, Spain from 46 to 23 million, Italy from 61 to 31 million, Portugal from 11 to 5 million, and South Korea from 53 to 27 million.
Murray said that not only will the population shrink, but society will generally be older, which would have a substantial impact on economic growth.
"There's more people needing to receive benefits from the government, whether that's social security or health insurance, and there's fewer people to pay taxes," he explained.
Researchers project that the population of sub-Saharan Africa could triple over the course of the century, from an estimated 1.03 billion in 2017 to 3.07 billion in 2100.
North Africa and the Middle East is the only other region predicted to have a larger population in 2100 than in 2017, with a predicted 978 million compared to 600 million.
The study also predicts major changes in the global age structure as fertility falls and life expectancy increases, with an estimated 2.37 billion people over 65 years globally in 2100, compared with 1.7 billion under the age of 20.
The global number of people older than 80 could increase sixfold, from 141 million to 866 million. Meanwhile, the number of children under the age of five is forecast to decline by more than 40% -- from 681 million in 2017 to 401 million in 2100.