Climate change will lead to a surge in refugee problems around the world, a former George W. Bush adviser said on Thursday, raising American national security concerns for the next presidential administration.

“Climate change and a lot of other economic dislocations have put a lot of people out of work,” Stephen Hadley, who was a national security adviser for Bush from 2001 to 2009, said at a Politico event on Thursday. He cited the refugee crisis in Europe as one example.

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“They are on the move and they have no place to go, and it means they are recruiting grounds for terrorists and extremists and potential refugee flows that will tax Europe even more.”

Republicans have derided President Obama and his administration for calling climate change a national security issue, as well as similar claims from other Democrats.

The White House included climate change in a national security strategy released last February, and Obama has warned against its impact on refugee crises, as well.

“As human beings are placed under strain, then bad things happen,” he said in a December interview with CBS News.

“And, you know, if you look at world history, whenever people are desperate, when people start lacking food, when people are not able to make a living or take care of their families, that's when ideologies arise that are dangerous.”

Republicans reject the claim, with GOP presidential nominee Donald Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE last year calling Obama’s climate change warnings “one of the dumbest statements I’ve ever heard in politics.”