HANGZHOU, China (AP) — President Barack Obama on Saturday said cooperation was "the single best chance that we have" to save the planet 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 a global economic summit, Obama and Xi, representing the world's two biggest carbon emitters, delivered documents to U.N. Secretary-General Ban Ki-moon. The papers certified the U.S. 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 occurred shortly after Obama arrived in the scenic Chinese city of Hangzhou for the annual summit of Group of 20 industrialized and emerging economies.