The era of expanding human populations may be ending, according to a new study, with major implications for societies, the economy and the environment.
The world's population may top out at roughly 9.7 billion around 2064, up from about 7.8 billion today, before shrinking to 8.8 billion by the end of the century, according to estimates published in the journal The Lancet.
The IHME estimate contradicts the benchmark forecast from the United Nations Population Division, which expects the population in 2100 to be 10.9 billion and rising.
U.N. Population Division chief John Wilmoth calls the projection "extreme."
If it proves to be correct, however, the consequences of a shrinking population would be far-reaching.
"I think this is one of the more profound changes that's faced humanity ever," said study co-author Chris Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington.
It would be great news for the environment. Fewer people would generate fewer greenhouse gases and other pollutants, for example. It would lower demand for food, reducing the pressure that agriculture puts on land and water.
But it would turn the economy on its head.
Declining populations mean fewer workers, which means lower GDP. It also means fewer consumers, the bedrock of the global economy.
"What happens when you don't have young people buying their first house, buying their first refrigerator, buying the first car?" said Darrell Bricker, co-author of the book, "Empty Planet: The Shock of Global Population Decline." Bricker was not involved in the study.
Also, populations age as fertility rates drop.
"This is actually more serious than just simple population decline," said Brown University sociology professor Zhenchao Qian, who was not involved with the research.
A smaller workforce would be supporting a larger elderly population, Qian noted, raising political and fiscal questions about how to pay for health care and social security systems.
Population declines could also have geopolitical consequences. Fewer workers also mean fewer potential soldiers, Murray noted.
"The balance of power between nation-states has always been related in some ways to the size of the working-age population," he said.
Fertility rates have been declining worldwide mainly because women are getting more education and better access to birth control.
"It's really a story about female empowerment," Bricker, the "Empty Planet" author, said.
The main difference between the new forecast and the U.N.'s is what they expect to happen after fertility rates bottom out.
Populations hold steady when women have an average of just over two children each. Demographers call this the replacement rate. Across wealthier countries, the average fertility rate currently is more like 1.6. In Japan, Singapore and Italy, it is just 1.3.
The U.N. assumes fertility rates will rise again over time to 1.75.
"We see no sign of that," he said. Instead, "we think that the phenomenon that we're starting to observe in places like Japan and Korea and Singapore and parts of Europe already will just become a general phenomenon."
By 2100, his group estimates that nearly every country in the world would be below the replacement rate. Populations in 23 countries, including Japan, Thailand and Spain, would be half what they were in 2017. Another 34 would lose between a quarter and a half of their citizens. China is expected to lose 48%.
That is assuming Murray and colleagues are right.
The U.N.'s Wilmoth notes that both groups are basing their assumptions on "what's still early experience in the lives of a few countries. So, I have to confess there's great uncertainty about that," he said.
"We will know much more about that in 10 or 20 years," he added. "But for now, we're both guessing to some extent."
If populations do shrink, Murray said countries have three options to keep themselves afloat.
"One is to make it easier for women to work and have children," he said, including generous parental leave and support for working mothers with young children.
Most countries that have implemented these policies have found they can help, he added, "but they don't bring fertility back to (the) replacement (rate)."
The second option is to open their borders to immigration.
Public sentiment currently is against this option in parts of the West. The Trump administration, for example, has sought to curb immigration into the United States. Opposition to European Union migration policies helped drive Britain’s Brexit vote.
However, "if Murray and colleagues’ predictions are even half accurate, migration will become a necessity for all nations and not an option," Ibrahim Abubakar, director of the University College London Institute for Global Health, wrote in a commentary accompanying the Lancet article.
A third scenario, "one that we think is a real risk," Murray said, is countries would be "tempted to roll back women's reproductive health rights in order to put pressure on them to have more children for the sake of the nation-state," a situation he called "very undesirable."
The major exception to Murray's group's shrinkage predictions is sub-Saharan Africa.
"Education for women is growing, but it's still at a very low level in many countries, and it's growing slowly," he said.
There is a lot of uncertainty in the longer term, however.
"Birth rates are still really high," Bricker noted, "but they're coming down, and urbanization is starting to take place there, too, at a really rapid rate," which tends to lower fertility rates.
On the other hand, the U.N.'s Wilmoth noted, "we've consistently had to up the estimates. ... I worry about underestimating the future population of Africa, not overestimating it."