Leading climate scientists are reeling in the aftermath of Donald Trump’s victory over Hillary Clinton Tuesday, and scrambling to understand how his presidency will affect the struggle against climate change.

Trump has promised to be a fierce ally of the fossil fuel industry, has called climate change a Chinese hoax and has vowed to end federal spending on climate change initiatives and pull the U.S. out of the climate agreement reached in Paris last year.

While it would likely take more than a single term in the White House for Trump to withdraw from the Paris Agreement, the possibility has left climate scientists grasping for answers about what the future holds.

“I don’t think anyone knows what this means for U.S. policy on climate science or emissions reductions,” Andrew Dessler, a professor of atmospheric sciences at Texas A&M University, told Carbon Brief, a U.K. news site that interviewed 21 climate scientists about their reactions to Trump’s election.

“To quote James Hansen, I fear this may be game over for the climate,” said Dr. Michael E. Mann, a professor of atmospheric science at Pennsylvania State University.

One way Trump could immediately send a message about how his administration will handle climate change is with his appointment of an Environmental Protection Agency administrator. Myron Ebell, a prominent climate change denier, is heading up the EPA transition work and is rumored to be a top choice for the role of EPA administrator.

Climate scientists around the globe were shocked by Trump’s win. Jean-Pierre Gattuso, a professor of biological oceanography at the French Centre National de la Recherche Scientifique, told Carbon Brief that a Trump presidency could threaten the very stability of the Paris Agreement.

“The Paris Agreement is a construct that was many years in the making and is, therefore, extremely fragile,” he said. “Even though the U.S. cannot formally leave the agreement in the next four years, not having the U.S. on board and pushing for the full implementation of the Paris Agreement may well affect billions of people for hundreds of years.”

Others said Trump’s victory Tuesday suggests that Europe and Asia may have to take the leading role.

“The world has now to move forward without the U.S. on the road towards climate-risk mitigation and clean-technology innovation,” said Hans Joachim Schellnhuber, the director of the Potsdam Institute for Climate Impact Research in Germany. “The U.S. de-elected expertise and will likely show a blockade mentality now, so Europe and Asia have to pioneer and save the world.”

Others sounded a slightly more optimistic note. Piers Forster, the director of the Priestly International Centre for Climate at the University of Leeds in the U.K., said it’s possible Trump might not oppose climate change efforts as aggressively as he promised to during the campaign.

“What he said to get elected may not be what he does,” Forster said. “I think we need to redouble our efforts to show that climate change is [scientific] fact, not a belief, and proactively addressing it needs international collaboration.”