Co-operation is "the single best chance that we have" to save the planet, US president Barack Obama said, as he stood with China's president Xi Jinping to formally enter their two nations into last year's Paris climate change agreement.
At a ceremony on the sidelines of the G20 summit in Hangzhou, Obama and Xi, representing the world's two biggest carbon emitters, delivered documents to UN secretary-general Ban Ki-moon.
The papers certified the US and China have taken the necessary steps to join the Paris accord that set nation-by-nation targets for cutting carbon emissions.
"This is not a fight that any one country, no matter how powerful, can take alone," Obama said of the pact. "Some day we may see this as the moment that we finally decided to save our planet."
Xi, speaking through a translator, said he hoped the announcement would spur more countries to take action.
"Our response to climate change bears on the future of our people and the wellbeing of mankind," he said.
The announcement means the accord could take force by the end of the year, a faster-than-anticipated timeline.
The ceremony took place shortly after Obama arrived in the scenic Chinese city of Hangzhou for the annual summit of G20 industrialised and emerging economies.