The world's population is likely to peak at 9.7 billion in 2064, and then decline to about 8.8 billion by the end of the century, as women get better 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.
However, the population of sub-Saharan Africa could triple, allowing for just under half of the world's population to be African by the end of the century.
The modeling study, published Tuesday in The Lancet, also forecasts dramatic declines in working-age populations in countries including India and China, which will hurt economic growth and could have negative implications for labor forces and social support systems, researchers said.
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.
A further 34 countries -- including China -- are also predicted to see their population decline by up to 50%.
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.
"Because fertility will remain high for longer, the relative share of the world population that is African will go up very substantially. We will reach the point towards the end of the century, where just under half the world's population will be African on these trajectories," Murray told CNN.
Over-80s will outnumber under-fives
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.
Researchers said that these "dramatic declines" in working-age populations in countries such as India and China, will both obstruct economic growth and lead to shifts in global powers.
"It's already starting -- this is something that is not in the distant future. The number of working-age adults in China has already begun declining," Murray told CNN.
"The profound decline in working aged adults that will happen in a place like China means that they will not be able in the long term to sustain economic growth at the pace that they have," Murray told CNN.
The report authors say the new forecasts highlight the "huge challenges" that a shrinking workforce will pose to economic growth and the high burden that an aging population will pose to health and social support mechanisms.
The role of immigration
The authors suggest that population decline could be offset by immigration, and that countries with liberal immigration policies will be better able to both maintain population size and support economic growth -- even as fertility falls.
"If more people are dying and then are born, then the population will go into decline. And the only way to counteract that is with migration," Murray said.
Researchers suggest that population decline could be offset by immigration, predicting that countries with low fertility -- including the US, Australia and Canada -- will maintain working-age populations with immigration as fertility declines.
While the report authors note that fewer people would have "positive implications for the environment, climate change, and food production," an aging population can bring its challenges.
If it was just a question of the number of people and not all these other effects, you could make a very good case that for the planet -- it could be a good thing," Murray said.
"The problem is that it's the inverted age pyramid is a real issue for how societies are organized and how economies work, how taxes get paid," he said. "What we really need to figure out is how to transition from the state we're in now," he said.
text by Amy Woodyatt, CNN