In September, President Obama turned over the paperwork and made the proper handshakes to formalize the United States’s participation in the Paris climate accord. And on Nov. 4, the agreement formally came into force. But President-elect Donald Trump, who has called global warming a “hoax,” has vowed to “cancel” the Paris deal and has made it clear that his administration will be hostile to concerns over climate change.

Polls show that Americans of every political stripe are growing more worried about climate change, and a Reuters/Ipsos poll released in December found that a majority of Republicans support U.S. efforts to work with other countries to address the problem.

If he does move forward with his campaign promises, Trump can’t pull the U.S. out of the Paris agreement right away. Article 28 of the Paris climate agreement allows parties to the accord to withdraw, but only three years after it has entered into force, and even then, the withdrawal would not take effect until one year after official notice was given. That means the earliest that Trump could take the U.S. out of the agreement would be Nov. 4, 2020. (Suggestions that he might renegotiate the deal are just talk — the deal is done.) What Trump could do, however, is withdraw the U.S. from the 1992 United Nations Framework Convention on Climate Change, a treaty that committed participating nations to address climate change. The agreement was signed by President George H.W. Bush and approved by the Senate, but Trump could rescind the U.S. commitment to that convention, a move that would become final one year after notification.

Trump could undermine the Paris agreement without touching either climate deal. Since the Paris agreement has no binding targets, only voluntary ones, Trump’s administration could simply refuse to take the steps necessary to fulfill the U.S. pledge — to reduce emissions by 26 percent to 28 percent below 2005 levels by 2025. Obama’s plan for meeting these goals relies on actions administered through the Department of Energy and the Environmental Protection Agency. The centerpiece is the Clean Power Plan, an EPA program that reduces carbon pollution from power plants by partnering with states. The EPA sets the emissions reduction goals, and the states decide how to meet them.

Republican lawmakers almost immediately fought implementation of the Clean Power Plan, and this year, in response to a lawsuit filed by more than two dozen states, the Supreme Court issued a “stay,” preventing the EPA from enforcing the rules until the lawsuits are resolved.

Under Trump, the EPA will probably drop its support for the plan in court, given that Trump’s EPA transition team is led by Myron Ebell, a well-known climate change skeptic with the Competitive Enterprise Institute and chairman of the Cooler Heads Coalition, which questions “global warming alarmism” and opposes “energy rationing policies.” Ebell has called the Clean Power Plan the “Costly Power Plan.” Trump’s administration and the Republican Congress might also refuse to make good on the U.S.’s pledge to contribute $3 billion over four years to a fund to help poorer countries adapt to climate change.

What would this mean for the Paris accord and, more importantly, the climate? Blocking implementation of the Clean Power Plan (or undoing the plan altogether) might slow emissions reductions, but it isn’t likely to halt the move to cleaner power. Even with the Supreme Court’s stay, many utilities are moving forward with plans to cut emissions. One lobby group for investor-owned utilities told Wall Street executives in February that carbon dioxide emissions will continue to fall, whether the Clean Power Plan goes into effect or not, because the industry has already made strategic investments that will decrease carbon emissions. Some states, too, are already moving ahead with plans.

The U.S. is powerful, but it is not almighty. The Paris agreement has global participation, and the loss of one country is unlikely to stop other countries from pushing on with their plans to cut emissions, said Niklas Höhne, professor for mitigation of greenhouse gas emissions at Wageningen University and a founding partner of NewClimate Institute, a German nonprofit research center. Höhne points to China as an example, saying that the country is likely to continue reducing its coal consumption and developing renewable energy to address air pollution issues.

The problem is that time is running out, and as I wrote in December, the pledges made under the Paris agreement leave a scary gap between what’s needed to keep temperature increases from becoming catastrophic and what countries aspire to do. The U.S. emissions reductions pledge was already insufficient to meet the Paris accord’s target of limiting warming to below 2 degrees Celsius above pre-industrial levels. Trump now has the power to make things much worse. Even just four years of unbridled emissions could be a disaster because these emissions stay in the atmosphere for hundreds of years, Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research, told Carbon Brief. “The climate system doesn’t forget, and it doesn’t forgive,” he said. Trump can claim that climate change is a hoax, but science is likely to get the last word as impacts from climate change such as hurricanes, droughts and rising seas make climate an issue too pressing to ignore. By then, it could be too late to reverse course.