The Trump administration formally notified the UN on Monday that it would withdraw the US from the Paris Agreement on climate change, leaving global climate diplomats to plot a way forward without the cooperation of the world’s largest economy.

The action, which came on the first day possible under the accord’s complex rules on withdrawal, begins a year-long countdown to the US exit and a concerted effort to preserve the Paris Agreement, under which nearly 200 nations have pledged to cut greenhouse emissions and to help poor countries cope with the worst effects of an already warming planet.

Secretary of state Mike Pompeo announced the notification on Twitter and issued a statement saying the accord would impose intolerable burdens on the American economy.

“The US approach incorporates the reality of the global energy mix and uses all energy sources and technologies cleanly and efficiently, including fossils fuels, nuclear energy, and renewable energy,” Pompeo said.

Though American participation in the Paris Agreement will ultimately be determined by the outcome of the 2020 election, supporters of the pact say they have to plan for a future without American cooperation. And diplomats fear that Trump, who has mocked climate science as a hoax, will begin actively working against global efforts to move away from planet-warming fossil fuels.

Keeping up the pressure for the kinds of economic change necessary to stave

off the worse effects of planetary warming will be much harder without the world’s superpower.

“Yes, there are conversations. It would be crazy not to have them,” Laurence Tubiana, who served as France’s climate change ambassador during the Paris negotiations, said in New York recently, adding, “We are preparing for Plan B.”

Negotiators spent the early months of the Trump presidency debating strategies for salvaging American support for the accord. Trump proved immovable.

President Trump had long held that the accord would cripple growth and intrude on American sovereignty. “It is time to put Youngstown, Ohio; Detroit, Mich.; and Pittsburgh, Pa., along with many, many other locations within our great country, before Paris, France,” he said in 2017 when he announced that the US would withdraw from the accord.

Around the world, a shift in diplomatic strategy has already begun. Making the accord work without the US will require other major polluters like China and India to step up. China, now the largest emitter of planet-warming pollutants, has made significant promises but Beijing’s ability to deliver is still in question.

Under UN rules, China and India are considered developing countries and are not obligated to curb emissions. They agreed to do so as part of the Paris Agreement in large part because the US was taking action. With the US out, other nations will have to press those emerging powers.

The EU held high-level meetings last year in Beijing to confirm the Paris commitment of both the European bloc and China. It also has provided millions of dollars to aid Chinese emissions-control efforts and worked with Canada and other countries to coordinate standards for trillions of dollars of private and public financial investment in clean energy technologies.

But so far China has resisted pledging to speed up its initial emissions-control targets, which foresee greenhouse gas emissions rising until 2030. Europe, which is divided itself over how far to scale back coal power, may not have the clout to win new concessions.

“The EU is the front line out here. That’s very obvious,” the President of Finland, Sauli Niinisto, said.