The US has told the United Nations it has begun the process of pulling out of the landmark 2015 Paris climate agreement.
Secretary of state Mike Pompeo said he submitted a formal notice to the UN, starting a withdrawal process that does not become official for a year.
Mr Pompeo’s statement touted America’s carbon pollution cuts and called the Paris deal an “unfair economic burden” to the US economy.
Nearly 200 nations signed the climate deal in which each country provides its own goals to curb emissions of heat-trapping gases that lead to climate change.
“In international climate discussions, we will continue to offer a realistic and pragmatic model — backed by a record of real world results — showing innovation and open markets lead to greater prosperity, fewer emissions and more secure sources of energy,” Mr Pompeo said in a statement.
With a hand-delivered letter, the US is the first nation to pull out of the deal. Agreement rules prevented any country from pulling out in the first three years after ratification on November 4 2016.
President Donald Trump has been promising withdrawal for two years, but Monday was the first time he could actually do it.
His decision was condemned as a reckless failure of leadership by environmental experts, activists and critics such as former New York City mayor Michael Bloomberg.
“Donald Trump is the worst president in history for our climate and our clean air and water,” said Michael Brune, the executive director of the Sierra Club.
“Long after Trump is out of office, his decision to withdraw the United States from the Paris Agreement will be seen as a historic error.”
The agreement set goals of preventing another 0.5C to 1C of warming from current levels. Even the pledges made in 2015 were not enough to prevent those levels of warming.
The deal calls for nations to come up with more ambitious pollution cuts every five years, starting in November 2020. Because of the expected withdrawal, the US role in 2020 negotiations will be reduced, experts said.
Climate change, largely caused by the burning of coal, oil and gas, has already warmed the world by 1C since the late 1800s, caused massive melting of ice globally, triggered weather extremes and changed ocean chemistry.
Scientists say that depending on how much carbon dioxide is emitted, it will only get worse by the end of the century, with temperatures jumping by several degrees and oceans rising by close to 3ft.
Mr Trump has been threatening to pull out of the Paris deal since 2017, often mischaracterising the terms of the agreement, which are voluntary. In October, he called it a massive wealth transfer from America to other nations and said it was one-sided.
That is not the case, experts said.
For example, the US goal — set under Barack Obama — had been to reduce carbon dioxide emission in 2025 by 26% to 28% compared with 2005 levels. This translates to about 15% compared with 1990 levels.
The European Union’s goal was to cut carbon pollution in 2030 by 40% compared with 1990 levels, which is greater than America’s pledge, said Rob Jackson, a Stanford University professor and chairman of the Global Carbon Project. The UK has already exceeded that goal, he said.