Delegates from more than 165 states are at the United Nations in New York to sign an historic deal to slow down climate change.
It comes four months after the agreement was made at COP 21 in Paris.
A signature from all the states represented will break the record for the most countries to endorse a UN international accord on day one.
Many nations still need to hold a parliamentary vote to formally ratify the pact. To enter into force, it needs the approval of 55 percent of states.
The signing, which coincides with Earth Day, commits the world’s largest economies to keeping temperature rises below two degrees and to maintaining them as close as possible to 1.5 degrees.