The United States, the world's largest historic greenhouse gas emitter, would become the only country outside the accord, a decision President Donald Trump promised to boost U.S. oil, gas and coal industries.

"The U.S. is proud of our record as a world leader in reducing all emissions, fostering resilience, growing our economy, and ensuring energy for our citizens. Ours is a realistic and pragmatic model," Pompeo said on Twitter.

The State Department letter to United Nations Secretary General Antonio Guterres started the clock on a process that would be completed one day after the 2020 U.S. presidential election, on Nov. 4, 2020.

Environmental groups said they hoped Trump would be defeated in 2020 by a rival who would re-join the agreement with bold new targets.

"The next president will need to rejoin the accord immediately and commit to the rapid, wholesale clean-energy transformation the climate emergency demands,” said Jean Su, energy director with the Center for Biological Diversity.

The Obama administration signed the United States onto the 2015 pact, promising a 26-28% cut in U.S. greenhouse gas emissions by 2030 from 2005 levels.

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Trump campaigned on a promise to rescind that pledge, saying it would hurt the U.S. economy while leaving other big polluters like China to increase emissions. But he was bound by U.N. rules to wait until Nov. 4, 2019 to file exit papers.

"What we won’t do is punish the American people while enriching foreign polluters,” Trump said at a shale gas industry conference in Pennsylvania on Oct. 23.

All the top Democratic presidential hopefuls seeking to unseat Trump in next year's election have promised to re-engage in the Paris Agreement if they win. But Trump’s withdrawal from Paris could still leave a lasting mark, said Andrew Light, a senior fellow at the World Resources Institute and former adviser to the U.S. climate envoy under President Barack Obama.

"While it serves the political needs of the Trump administration, we will lose a lot of traction with respect to U.S. influence globally,” he said, adding it could take time for the international community "to trust the U.S. as a consistent partner."

Until its formal exit, the United States will continue to participate in negotiations over the technical aspects of the agreement, represented by a small team of career State Department officials.

The United States and China , the world’s two largest carbon emitters, have recently been leading negotiations of the Paris "rule book" that outlines transparency and reporting rules for signatories.

WASHINGTON - The Trump administration has formally notified the United Nations that it will withdraw the United States from the Paris Agreement, the first formal step in a one-year process to exit the global pact to fight climate change, U.S. Secretary of State Mike Pompeo confirmed on Monday.