What happens on election day will to some degree determine how much more hot and nasty the world’s climate will likely get, experts say.

The day after the presidential election, the United States formally leaves the 2015 Paris agreement to fight climate change. A year ago, President Donald Trump’s administration notified the United Nations that America is exiting the climate agreement. And because of technicalities in the international pact, Nov. 4 is the earliest a country can withdraw.

The U.S., the world’s second biggest carbon polluter, will be the first country to quit the 189-nation agreement, which has countries make voluntary, ever-tighter goals to curb emissions of heat-trapping gases. The only mandatory parts of the agreement cover tracking and reporting of carbon pollution, say U.S. officials who were part of the Paris negotiations.

Former Vice President Joe Biden has pledged to put the country immediately back in the Paris agreement, which doesn’t require congressional approval. Experts say three months — from November to the January inauguration — with the U.S. out of the climate pact will not change the world, but four years will.

If America pulls back from Paris and stronger carbon cutting efforts, some nations are less likely to cut back too, so the withdrawal’s impact will be magnified, said scientists and climate negotiators.

Because the world is so close to feared climate tipping points and on a trajectory to pass a temperature limit goal, climate scientists said the U.S. pullout will have noticeable effects.

“Losing most of the world’s coral reefs is something that would be hard to avoid if the U.S. remains out of the Paris process,” said climate scientist Zeke Hausfather of the Breakthrough Institute in Oakland, California. “At the margins, we would see a world of more extreme heat waves.”

If the U.S. remains out of the climate pact, today’s children are “going to see big changes that you and I don’t see for ice, coral and weather disasters,” said Stanford University’s Rob Jackson.

Because the two presidential candidates have starkly different positions on climate change policy, the election could have profound repercussions for the world’s approach to the problem, according to more than a dozen experts.

“That election could be a make or break point for international climate policy,’ said Niklas Hohne, a climate scientist at Wageningen University in Germany.