But the State Department's announcement doesn't formally start the process of the U.S. getting out of the voluntary agreement. That's still to come.
Still, the department described its communication as a "strong message" to the world, following President Donald Trump's decision in June to leave the accord.
"The State Department is telling that U.N. what the president already told the world on June 1 and it has no legal effect," said Nigel Purvis, who directed U.S. climate diplomacy during the Bill Clinton and George W. Bush administrations.