When a group of researchers from China, Japan, Europe and the U.S. set out to answer that question, the results were so shocking they spent another year analyzing the numbers to make sure they were on the right track before publishing the results earlier this month.
The concerning results were the same: In 50 years as many as 3.5 billion people could be pushed to migrate away from their homes, because the places where they live will have become uncomfortably hot, outside the "niche" temperature range that humans have gravitated toward for the last 6,000 years.
For every degree centigrade of warming that's avoided, 1 billion fewer people would likely move. But even under the scenario with the most action by governments around the world to reduce greenhouse gas emissions and combat climate change, the researchers still estimate 1 billion people could be displaced 50 years from now.
That's staggering if you consider the political tensions that already exist with only a few hundred million people worldwide living somewhere other than their birthplace, explains Tim Kohler, an archaeology professor at Washington State University who helped look at human behavior over the last 6,000 years for the study.
"If 1 billion people is the best-case scenario, that's huge," Kohler says. "For a little bit of context, there's something on the order of 250 million people right now living outside the countries they were born in."
Most people don't want to leave the place they were born, Kohler says. But if you look at human history over the last several thousand years, most people have migrated to zones of the Earth that tend to have average temperatures of 11 to 15 degrees Celsius (52 to 59 degrees Fahrenheit), with most of the remainder living in places with an average of 20 to 25 degrees Celsius (68 to 77 degrees Fahrenheit).
"Migration is ordinarily not a first choice for populations. Normally they like to stay where they are and at least make do," Kohler says. "The more investment you have in a place, the harder it is to leave. People would mostly rather stay and adapt, but that’s not going to be such an easy job in most places."
It was somewhat surprising to Kohler that the temperature niche was so constant over time, whether people were hunter-gatherers or farmers with developed agriculture. That's not to say that all people live in that niche, but the vast majority do, he says, and importantly, the most successful civilizations tend to be in that range.
The researchers used potential warming scenarios outlined by the Intergovernmental Panel on Climate Change. The headline-grabbing figure — the one showing 3.5 billion people could be displaced — is based on the status quo, Kohler says. If governments don't take drastic action, most people will live in places that are 7.5 degrees Celsius warmer than pre-industrial averages. Under that scenario, nearly one-third of the world's people would live somewhere warmer than 29 degrees Celsius on average (84.2 degrees Fahrenheit).
But there's reason to believe that investments in clean energy and movement away from fossil fuels could already be putting us on the path to a somewhat smaller increase, Kohler says.
Unfortunately, while migration is already something that causes tension, Kohler says, another complicating factor is that the places that will see the largest increase in temperature are also the most likely to see a boom in population. And again, even in the best-case scenario in the study, four times as many people as currently live outside their homelands would likely be displaced.
"The very places that are going to be most difficult for people to remain in are the places right now where the population growth rates are the highest," Kohler says. "So you have this unfortunate collision between population and climate futures."