The US has begun the process of withdrawing from the Paris Agreement, as other countries expressed regret and disappointment at the move.
The Trump administration formally notified the United Nations on Monday of its intention to exit the deal.
The notification begins a one-year process of exiting the global climate change accord, culminating the day after the 2020 US election.
The agreement brought together 188 nations to combat climate change.
It committed the US and 187 other countries to keeping rising global temperatures below 2C above pre-industrial levels and attempting to limit them even more, to a 1.5C rise.
The decision to withdraw - taken by President Donald Trump - made the US the world's sole non-signatory and prompted high-level efforts by the European Union to keep the agreement on track.
The US issued its formal notification on the first day it was possible to do so.
Mr Trump had made withdrawing from the 2015 agreement one of his campaign pledges - but UN rules have meant it was not possible for the US to start the withdrawal process until 4 November 2019.
The withdrawal is still subject to the outcome of next year's US presidential election - if Mr Trump loses, the winner may decide to change course.
But scientists and environmentalists fear the effect the Trump administration will have on climate protections in the meantime.
A report issued in December 2018 by the Institute of International and European Affairs suggested President Trump's decision to leave had done "very real damage" to the Paris agreement, creating "moral and political cover for others to follow suit".
The report cited the examples of Russia and Turkey, which both declined to ratify the deal despite signing.-BBC
