What happens on election day will to some degree determine how much hotter and nastier 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 agre...