The US has formally served notice that it is withdrawing from the Paris Agreement that was drawn to fight climate change.
Secretary of State Mike Pompeo announced on Monday that the US sent the notification to the UN starting the year-long process of ending Washington's participation in the climate deal.
The US, which put out 5 billion tonne of carbon dioxide (CO2) last year, ranks behind China as the second biggest producer of the gas that is responsible for global warming.
But on a per capita basis, Americans are the world's biggest polluters, putting out 15.3 tonnes of the gas each.
The pullout from the agreement fulfils President Donald Trump's election pledge to his base which is wary of international agreements.
Pompeo said, "President Trump made the decision to withdraw from the Paris Agreement because of the unfair economic burden imposed on American workers, businesses, and taxpayers by US pledges made under the Agreement."
Monday was the third anniversary of the Paris Agreement coming into force after it had been ratified by the required number of countries.
Significantly, the withdrawal will come into force the day after the US presidential election on November 4, 2020, a year after the formal notice is served.
Former Vice President Al Gore, who was awarded the Nobel Peace Price for his climate activism, said the decision to return to the agreement "is ultimately in the hands of the voters" because they can elect a president, who supports it next year.
"It would take just 30 days for a new president to get us back in," he said.
Unlike Trump, all the Democratic Party candidates for the presidential nomination are strong supporters of the Paris Agreement, which committed the US to cut its carbon emissions from the 2005 level by 26 per cent by 2025.