The White House signaled withdrawal was likely, but Trump has been known to change his mind at the last minute on such major decisions.
Though he regularly criticized the climate deal on the campaign trail, President Trump has been mulling a decision on whether to withdraw or remain in the Paris climate deal for some time.
What impact would a USA withdrawal have?
The document confirms their commitment to the Paris accord and to "significantly intensify their political, technical, economic and scientific cooperation on climate change and clean energy".
New Orleans Mayor Mitch Landrieu, the group's vice-president, said withdrawal from the agreement was "shortsighted" and called climate change a grave threat to coastal communities, the nation and the world.
Brown has emerged as a leader in the fight against climate change by urging local governments throughout the world to agree to reduce greenhouse gas emissions 80 to 95 percent below 1990 levels by 2050.
Scientists say the planet is likely to reach more unsafe levels of warming sooner if the USA retreats from its pledge, because America contributes so much to rising temperatures.
The lobbying has been fierce both in and outside the White House.
White House aides have been divided on the question of staying or leaving the accord and had been deliberating on "caveats in the language" as late as Wednesday, one official said. The U.S. has held the mantle of climate leadership, with emerging economies such as China and India following the country's lead in renewable energy policy.
Mr Dathi told the Press Association: "I think Theresa May is herself someone who doesn't really believe in climate change, doesn't have a sound commitment to it, and is much more interested in profits, in the unfettered ability of companies to make huge amounts of profit".
A US pullout could have sweeping implications.
A secession from the Paris alliance would leave the US, which the European Commission cites as the second largest carbon dioxide-emitting nation on the planet, as one of only three nations out of 197 that are entitled to be in the pact but refuse to do so.
Antonio Tajani, the European Parliament president, said: "Climate change is not a fairy tale".
Just 45% of Americans, according to polling in April from Gallup, said they worry "a great deal" about global warming.
Republican Gov. Charlie Baker, who recently signed a letter urging Trump to keep the country committed to the treaty, told The Boston Globe that he hoped the reports weren't true. They said withdrawal would undermine United States credibility and its position as a global leader, empowering nations like China to drive the climate agenda and set worldwide standards while also reaping economic benefits from a growing clean energy sector. The carbon emission curbs put forward by countries under Paris are not legally binding but the framework of the accord, which includes a mechanism for periodically cranking those pledges up, is binding.