The United States formally exited the Paris Agreement yesterday, fulfilling an old promise by President Donald Trump to withdraw the world's second-largest greenhouse gas emitter from the global pact to fight climate change. 

But the outcome of the tight US election contest will determine for how long. Trump's Democratic rival, Joe Biden, has promised to rejoin the agreement if elected.

"The US withdrawal will leave a gap in our regime, and the global efforts to achieve the goals and ambitions of the Paris Agreement," said Patricia Espinosa, executive secretary of the UN Framework Convention on Climate Change (UNFCCC).

The United States still remains a party to the UNFCCC. Espinosa said the body will be "ready to assist the US in any effort in order to rejoin the Paris Agreement."

Trump first announced his intention to withdraw the United States from the pact in June 2017, arguing it would undermine the US economy. The administration formally served notice of the withdrawal to the United Nations on Nov 4, 2019, which took one year to take effect.

The departure makes the United States the only country of 197 signatories to have withdrawn from the agreement, hashed out in 2015.

Current and former climate diplomats said the task of curbing global warming to safe levels would be tougher without the financial and diplomatic might of the United States.

European and US investors with a collective $30 trillion in assets on Wednesday urged the country to quickly rejoin the Paris Agreement, and warned that the country risked falling behind in the global race to build a low-carbon economy.