Governments have created "worldwide momentum" to slow climate change despite threats by U.S. President Donald Trump to pull out of the 2015 Paris Agreement to slash greenhouse gas emissions, the U.N.'s climate chief said on Friday.

"The Paris Agreement remains a remarkable achievement, universally supported by all countries when it was adopted," Patricia Espinosa, head of the U.N. Climate Change Secretariat in Bonn, wrote in a letter to staff.

Trump, who doubts that greenhouse gases from fossil fuels are warming the planet, began undoing former President Barack Obama's policies to slow global warming this week in a shift to favor the U.S. coal industry.

He expects to decide by late May whether to carry out a campaign promise to withdraw from the 2015 Paris Agreement, reached by almost 200 nations. It sets a goal of phasing out greenhouse gas emissions in the second half of this century.

Espinosa listed signs of "worldwide momentum" including that "solar power capacity globally grew 50 percent in 2016 led by the United States and China" and that ever more governments were passing laws to curb global warming.

"This governmental momentum continues to be underpinned by companies, investors, cities, regions and territories including now many oil majors whose CEOs have in recent weeks publicly spoken out in support of the Paris Agreement," she said.

She said the United States was still a member of the agreement and that "we look forwards to welcoming and working with its delegations" at meetings in 2017. Governments are trying to write a rule book to guide implementation.

Many countries, led by China and the European Union, have reaffirmed backing this week for the pact, meant to be the cornerstone of efforts to limit heatwaves, floods, extinctions of animal and plant species and a rise in sea levels.