Donald Trump has long questioned the science behind climate change, by turns calling it a Chinese deception designed to harm American manufacturing, an “expensive hoax,” and “bullshit.”

The consequences of climate change decisions made during the next presidency cannot be overestimated. In February, a group of scientists writing in Nature argued that the policy decisions made in the next few years would profoundly affect the global environment and human society for the ten next millennia and beyond.

Analysts say if we continue emitting the current levels of CO2 emissions, we’ll drive temperatures up to a point past the Paris agreement goal— 1.5 °C above pre-industrial levels—in just five years.

Yet ensuring global action on this front has been a challenge. After over 20 years of arguing about an international strategy to combat climate change, world leaders finally signed the Paris Agreement last year, in large part because of US leadership under President Barack Obama. The Paris Agreement, among 195 countries, aimed to limit the increase in global temperatures to 2 °C above pre-industrial levels, and to just 1.5 °C if possible.

Officials from nearly 200 countries are currently gathered in Marrakech, Morocco to discuss implementing the promises made in Paris; Trump’s victory has cast a pall of uncertainty on the summit, raising questions about whether the US would pull out of the historic climate deal.

At a campaign rally in North Dakota last May, Trump said that he would “cancel” the Paris accord, expand oil and gas production, and roll back President Obama’s executive actions on climate change that include plans to cut greenhouse gas emissions.

“We’re going to cancel the Paris climate agreement and stop—unbelievable—and stop all payments of the United States tax dollars to UN global warming programs,” he said.

Trump has also picked a climate skeptic, Myron Ebell, to head the head the Environmental Protection Agency’s transition team.

While 100 countries have ratified the Paris Agreement, there is still little clarity in how the goals of the accord will be achieved. The plan was to figure that out at the current summit in Marrakech.

On the eve of the summit and days before the election result, world leaders were resolute. “There is no possible turning back in the negotiation on what was agreed in Paris,” said Moroccan Foreign Minister Salaheddine Mezouar, who will preside over the talks. “We can only advance.”

Breaking from its traditional silence on foreign elections, China had something to say about the 2016 US presidential race. Xie Zhenhua, China’s top climate negotiator repudiated Trump’s position days before the election, saying that the world was moving towards a trend of balancing environmental protection with economic growth. “I believe a wise political leader should take policy stances that conform with global trends,” said Xie. “If they resist this trend, I don’t think they’ll win the support of their people, and their country’s economic and social progress will also be affected.”

When news of the US election results broke, stunned American climate activists in Marrakech “cried and embraced,” reports the Associated Press.

Under the terms of the Paris Agreement, there are safeguards in place to ensure the US government’s continued involvement for the full coming presidential term: the accord prohibits any formal exit for a period of three years, and countries must provide an additional year-long notice period.

But as president, Trump could disregard the Obama administration’s Paris pledge to decrease emissions in the US by 26-28% from 2005 levels by 2025. Under the agreement, countries define their own commitments and are not punished for failing to reach their targets.

Some are despairing. “The Paris Agreement and any U.S. leadership in international climate progress is dead,” Dana Fisher, director of the Program for Society and the Environment at the University of Maryland, told the New York Times.

Other climate activists are hoping that Trump’s remarks on climate change were just hyperbole, and that once he is president he will be forced to take a more reasonable position. Carbon Pulse, a independent digital publication that covers climate change policy, reached out to numerous climate activists to get their thoughts.

“Trump’s made some cavalier statements about climate change being a hoax,” said Alden Meyer, director of strategy and policy for the Union of Concerned Scientists. “The real question is, will a Trump president be the Trump we saw during the campaign or the more measured and responsible Trump we saw in his acceptance speech… If the US pulls out of Paris and goes rogue, this will impact everything else that Trump wants to accomplish with foreign leaders, and I hope he understands that.”

Some say they hope that once in the White House, Trump would act in accord with the majority of Americans who support climate action, or risk fanning fears of economic insecurity. “This was an election driven by economic insecurity and dislocation,” said Nathaniel Keohane, vice president of the Global Climate, Environmental Defense Fund. “And a decision by the next President to go backward on climate change would only exacerbate those concerns—because the biggest source of economic instability in the long run will be climate change.”

Others believe that during a Trump presidency, other nations will have to take the lead on global efforts to curb greenhouse gas emissions. “President-elect Donald Trump’s stance on global warming is well known,” said Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research. “Science cannot expect any positive climate action from him. The world has now to move forward without the US on the road towards climate-risk mitigation and clean-technology innovation.”

For US officials in the midst of Marrakech negotiations, Jeff Swartz, the director of international policy for the International Emissions Trading Association, has some pragmatic advice: pass the leadership baton to others, like Canada, Mexico, or the EU. “Start picking your friends that you want to pass your strategies onto, other countries who can take on that leadership role,” he told the Desmog Blog. “Because come this time next year [you] won’t be able to do that.”