Ognen Teofilovski / Reuters

Can unexpected weather make a war or a failed state more likely? It’s a question that could define the 21st century.

A new study, published Thursday in Science, finds a link between temperature variation and forced migration.

When unusually hot or cold weather strikes the growing region of an agricultural country, more people living in that country seek asylum protection in the European Union. Those people are then, in turn, more likely to be accepted as permanent residents by the EU.

Because asylum applicants must be fleeing conflict or persecution—and because their acceptance seems to validate the severity of their claims—the study’s authors say they’ve found an underlying connection between weather, agriculture, and failed governance.

“It’s pretty much like a medical trial of a new drug. There are many impacts that affect health, just like there are many impacts that affect asylum applications. But we’ve set up the trial and those are not correlated, so I have faith that we’ve established a relationship between weather and conflict,” said Wolfram Schlenker, an environmental economist at Columbia University. Schlenker and Anouch Missirian, another environmental economist at Columbia, conducted the study.

The study began by examining two databases: asylum applications to the EU between 2000 and 2014, and average temperatures across 103 countries.

The authors omitted asylum data from 2015 and 2016, when refugees were fleeing the Syrian Civil War and other conflicts. More than 1 million people applied for asylum annually during those two years, a spike well above the average 350,000 annual applicants from 2000 to 2014.

Schlenker and Missirian found an early correlation between weather and migration, but they waded through the data, trying to account for as much statistical noise as possible. They removed one-year shocks from events like the onset of the global financial crisis. They also factored in the difference between hot and cold countries, as a naturally colder country might be able to deal with a few extra degrees more easily than a hot country.

Ultimately, they found that the entire effect in asylum increases was attributable to temperature shocks in maize-growing countries that hit during the growing season, in the area where crops are grown.

Though the research seems to examine the relationship between climate change and migration, it’s actually getting at the deeper question of climate and governmental collapse or oppression. That’s because of the definition of forced migration: Only refugees from conflict or persecution can apply for permanent asylum in another country. The 1951 UN Refugee Convention, which every nation in the EU has signed, does not define a refugee as someone fleeing a country for economic reasons.

If a country suffered a temperature shock, then its refugees were three times more likely to be accepted for asylum than the average applicant to the EU. To Schlenker, this points to a deep connection between climatic crises and military conflict or persecution.

“The mechanism through which this works is conflict,” he told me. “We don’t know why they get accepted or not, but people in destination countries in the EU find them to be worthy of protection.”

The paper also projected its findings forward, by examining when various climate models believe weather shocks in growing regions could become more likely. They found modest increases. In a world that gets carbon-dioxide pollution under control and holds global warming to roughly 2 degrees Celsius, the number of asylum applications to the EU could rise by about 28 percent.

But if carbon pollution continues unabated, and global temperatures rise by about 5 degrees Celsius, annual applications could rise by 188 percent by 2070. More than 650,000 people could seek protection in the European Union annually.

Schlenker made it clear that these numbers should be used as tools for thinking and not final projections. “This is the best estimate we can do with current data, but there’s lots of asterisks,” he said. The projections could err too high, because they don’t account for global adaptation to warmer weather. But they could also still be too low, because they can’t anticipate political tipping points like civil wars or regime change.

“If a country switches from a democracy to a dictatorship—which would cause many more asylum applications—that would have a huge effect and we don’t account for it in the model,” he said. Schlenker noted that while these would be huge increases, the number of migrants who fled the Syrian Civil War was “much larger than the predicted impact we’re seeing with climate change.”

Researchers who study the intersection of climate and politics said that the historical connection was an extremely sturdy finding, but they shared in Schlenker’s skepticism about the future projections.

“This is a very strong study showing a robust relationship between temperature and forced migration,” said Claire Adida, a political scientist at the University of California at San Diego, by email. “The authors were meticulous in their analysis.”

“They show that the relationship holds only when you look at temperature deviations over the growing season and in the crop area ... This, by the way, is consistent with a lot of other research in agricultural economics showing that climate change and conflict are likely to be related via the effect of climate change on agricultural income,” she said.

Elizabeth Chalecki, a political scientist at the University of Nebraska at Omaha, agreed that the core finding was strong, but she identified a few more variables that she said could be related. Forced migration could also depend on whether commodity crops were available to relieve a food shortage or whether there were broader non-climatic events—like a genocide—happening in the region.

“The researchers admit that their correlation is ceteris paribus, meaning all things being equal, but that never is the case in real life,” she said. “If nationalist politics are on the rise across the EU and elsewhere, then doing something about climate change might be good national-security policy, not just good environmental policy.”

Giovanni Bettini, a political theorist at Lancaster University, said it was important to be careful about discussing climate migration as fait accompli.

“The inference that correlation equates causation is very problematic,” he said, adding that the link between climate change and political mobility was a “political open question.”

And he said it was important to treat projections not as settled science but as one of many tools for thinking about the future. It was crucial, too, he said, to talk about migrants with nuance and care not to fall into racial tropes. “Climate refugees are never white, in the discourse,” he said. “The dangerous flocks of people who are set to be uprooted in the future, creating dangerous security threats for ‘us,’ are always somewhere else.”

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.