The United States has officially re-entered the Paris climate agreement, with US President Joe Biden describing climate change as a "global existential crisis".
It is nearly one month after Biden took office and pledged to re-enter the deal in one of his first moves as president.
US Secretary of State Antony Blinken said: "Now, as momentous as our joining the agreement was in 2016 - and as momentous as our rejoining is today - what we do in the coming weeks, months and years is even more important."
"Climate change and science diplomacy can never again be 'add-ons' in our foreign policy discussions," Blinken's statement added.
The Paris agreement was negotiated during Barack Obama's final term in office, when Biden was vice president.
Former president Donald Trump took the US out of the accord in 2017, arguing that it put too many limits on what US businesses could do.
Still, the Paris agreement's long lag time meant that the US did not formally exit the deal until November 2020.
The US is the only country out of the 196 signatories to have left the agreement.
It is also the world's largest contributor of carbon dioxide emissions.
Speaking via video at the Munich Security Conference on Friday, Biden argued that the whole world will suffer the consequences if it fails to address climate change.
"We can longer delay, or do the bare minimum, to address climate change," Biden said.
"This is a global existential crisis."
Biden called for countries to rapidly curb emissions and hold one another accountable.
UN Secretary-General Guterres welcomed the return of the United States as restoring the "missing link that weakened the whole" but warned "this is the race of our lifetimes".
"We must go much faster, and much farther," he said.
Australian Associated Press
