The United States and China on Saturday formally joined the Paris climate deal, with US President Barack Obama hailing the accord as the "moment we finally decided to save our planet".
The pair handed ratification documents to UN chief Ban Ki-moon, who said he was now optimistic the agreement on fighting global warming would be brought into effect by the end of this year.
At the ceremony in the Chinese city of Hangzhou Obama said that climate change would "define the contours of this century more dramatically than any other challenge".
History would show that the Paris deal would "ultimately prove to be a turning point", he said, "the moment we finally decided to save our planet".
China's parliament ratified the agreement earlier Saturday, and President Xi said the Asian giant was "solemnly" committed to the deal.
"We need to take an innovative approach to climate change," he said in Hangzhou, where he is to host the G20 summit of the world's leading developed and emerging economies.
Ban said there would be a "high level" meeting in New York in September to advance the 180-nation accord which sets ambitious goals for capping global warming.
"By formally joining the Paris agreement you have added powerful momentum" toward bringing the agreement into effect, he told the two leaders.
"I am optimistic we can do it before the end of this year."
Until Saturday only 24 of the signatories had ratified the deal, including France and many island states threatened by rising sea levels but who only produce a tiny proportion of the world's emissions.
China is responsible for almost a quarter of the world's emissions, with the US in second place on around 15 percent, so their participation is crucial.