WASHINGTON (AP) — For more than two years President Donald Trump has talked about pulling the United States out of the landmark Paris climate agreement . Starting Monday, 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 United States and China, and went into effect November 4, 2016.

The terms of the deal say no country can withdraw in the first three years. So Monday is the first time the US 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 non-profit World Resources Institute.

Light and other experts say the withdrawal by the United States, 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 US 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.

The penalty for the US “is not in economic loss. The penalty is in shame, in discrediting US leadership”, Jacoby said.

Asked what the US plans next, State Department spokesman James Dewey on Friday emailed only this: “The US position with respect to the Paris Agreement has not changed. The United States intends to withdraw from the Paris Agreement.”

The agreement set goals of preventing another 0.9 degrees (0.5 degrees Celsius) to 1.8 degrees (1 degree Celsius) of warming from current levels. Even the pledges made in 2015 weren’t enough to prevent those levels of warming.

The deal calls for nations to come up with more ambitious pollution cuts every five years, starting in November 2020 in at a meeting in Scotland. Because of the expected withdrawal, the US role in 2020 negotiations will be reduced, Light said.

Climate change, caused by the burning of coal, oil and gas, has already warmed the world by 1.8 degrees (1 degree Celsius) since the late 1800s, caused massive melting of ice globally, triggered weather extremes, and changed ocean chemistry. And scientists say, depending on how much carbon dioxide is emitted it will only get worse by the end of the century, with temperatures jumping by several degrees and oceans rising by close to three feet (1 metre).

Trump has been promising to pull out of the Paris deal since 2017, often mischaracterising the terms of the agreement, which are voluntary. In October he called it a massive wealth transfer from America to other nations and said it was one-sided

That’s not the case, experts said.

For example, the US goal — set by Barack Obama’s administration — had been to reduce carbon dioxide emission in 2025 by 26 per cent to 28 per cent compared to 2005 levels. This translates to about 15 per cent, compared to 1990 levels.

The European Union’s goal was to cut carbon pollution in 2030 by 40 per cent, compared to 1990 levels, which is greater than America’s pledge said Stanford University’s Rob Jackson, who chairs the Global Carbon Project, a group of scientists that tracks carbon emissions worldwide. The United Kingdom has already exceeded that goal, he said.