The United States and China on Saturday formally joined the Paris climate change agreement, with President Barack Obama hailing the accord as the "moment we finally decided to save our planet".
The move by the world's two biggest polluters is a major step forward for the 180-nation deal, which sets ambitious goals for capping global warming and funneling trillions of dollars to poor countries facing climate catastrophe.
Obama and his Chinese counterpart Xi Jinping handed ratification documents to UN chief Ban Ki-moon, who said he was now optimistic the agreement will be in force by the end of this year.
At a ceremony in the Chinese city of Hangzhou, Obama said climate change would "define the contours of this century more dramatically than any other challenge."
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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".
"There's an American saying, You need to put your money where your mouth is. That's what we're doing."
The Paris accord aims to limit global temperature increases to two degrees centigrade, and will be triggered after it is ratified by at least 55 countries, accounting for 55 percent of global greenhouse gas emissions.
The US and China are together responsible for some 40 percent of the world's emissions, so their participation is crucial.
China's Communist-controlled parliament ratified the agreement earlier Saturday, and President Xi said the Asian giant was "solemnly" committed to the issue.
"Hopefully this will encourage other countries to take similar efforts," he said in Hangzhou, where he is to host the G20 summit of the world's leading developed and emerging economies.
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Until Saturday only 24 of the signatories had ratified the accord, including France and many island states threatened by rising sea levels but who only produce a tiny proportion of the world's emissions.
Ban said there would be high-level talks in New York later this month to push more countries to sign up, and told the two leaders they had "added powerful momentum" to efforts to bring the accord into force.
"I am optimistic we can do it before the end of this year."
- 'Powerful signal' - Climate is one of the few areas where the world's two most powerful countries -- who are at loggerheads on issues ranging from trade disputes, cyberspying and the South China Sea -- are able to find common cause.
Campaigners welcomed the move, with WWF saying they had sent "a very powerful signal that there will be real global action on climate change."
But some environmental groups say that the Paris pledges by China, the US and others fall far short of what is needed to meet the goal of less than two degrees of warming.
"This moment should be seen as a starting point, not the finale, of global action on climate," said Greenpeace policy adviser Li Shuo.
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