Rapid global warming could almost triple the number of migrants reaching the European Union by 2100, adding to factors such as war and persecution that force people to leave home, scientists said on Thursday.
The study, criticized by some other researchers as exaggerated, said asylum applications to the EU from 103 nations tended to rise in the 2000-2014 period when temperatures at home were far hotter or colder than the ideal for growing maize.
It projected that applications could surge to 1.01 million a year by 2100 from an average 351,000 from 2000-14 under a scenario of a big rise in temperatures that would hit harvests.
Under a scenario of less warming, applications could rise 28 percent.
"A lot of things can happen by the end of the century - countries can become democracies, they can become dictatorships," senior author Wolfram Schlenker, a Columbia University professor of economics, told Reuters, referring to factors that cause migration.
Still, if weather trends from 2000-14 continue, "this is a first best estimate" for 2100, he said of the findings published in the journal Science and requested by the European Commission.
The report examined trends this century, before a migration surge in 2015 caused by Syria's civil war. Immigration has become a major political concern in the EU, the favored destination for many asylum seekers from nations such as Afghanistan or Iraq.
Some other scientists were doubtful about the findings.
"The evidence so far on the impacts of climate change on migration is still quite weak," said Jan Selby, a professor of international relations at the University of Sussex.
He said it was wrong to project that gradual warming would have the same effect on harvests as weather shocks.
"A sudden climatic shock may destroy a crop; a gradual increase in temperature over decades would not [instead farmers would change crops, etc]. We simply can't extrapolate from one to the other," he wrote in an e-mail.
The study published by Science found a strong link between temperature swings and maize, a widespread staple, but less for rice, wheat and soybeans.
"They note that their model only works for maize rather than other staples. Why? In any case, maize isn't a major crop in most of the EU's refugee-origin countries," said Mike Hulme, a professor of human geography at Cambridge University.
"I would have thought that civil war, political repression, weak civil institutions, low levels of educational attainment, etc, are more powerful predictors of asylum-seeking. But this is a question these authors don't ask. Yet it matters," he said.
On Monday, U.S. President Donald Trump, who doubts global warming is man-made, dropped climate change from a list of national security threats, aides said.
By contrast, Trump's predecessor Barack Obama said climate change posed "immediate risks" to national security as a "threat multiplier" aggravating everything from disease to terrorism.
Thursday's study said "our findings support the assessment that climate change, especially continued warming, will add another 'threat multiplier' that induces people to seek refuge abroad."