WASHINGTON — For more than two years, President Trump has talked about pulling the United States out of the landmark Paris climate agreement. Starting today he finally can do something about it.

Even then, though, the withdrawal process takes a year and wouldn’t become official until at least the day after the 2020 presidential election.

In the Paris agreement, nearly 200 countries set their own national targets for reducing or controlling pollution of heat-trapping gases. It was negotiated in 2015 with lots of prodding by the U.S. and China and went into effect Nov. 4, 2016.

The terms of the deal say no country can withdraw in the first three years. So today is the first time the U.S. could actually start the withdrawal process, which begins with a letter to the United Nations. And it doesn’t become official for a year after that, which leads to the day after the election.

If someone other than Trump wins in 2020, the next president could get back in the deal in just 30 days and plan to cut carbon pollution, said Andrew Light, a former Obama State Department climate negotiator now at the nonprofit World Resources Institute.

Light and other experts say the withdrawal by the U.S., the second biggest climate polluter and world’s largest economy, will hurt efforts to fight global warming.

“Global objectives can’t be met unless everybody does their part and the U.S. has to play the game,” said Appalachian State University environmental sciences professor Gregg Marland, who is part of a global effort to track carbon dioxide emissions. “We’re the second biggest player. What happens to the game if we take our ball and go home?”

Someone else, probably the biggest polluter China, will take over leadership in the global fight, said MIT economist Jake Jacoby, who co-founded the MIT Joint Program on the Science and Policy of Global Change.