|U.S. President Barack Obama shakes hands with UN Secretary General Ban Ki-moon during a joint ratification of the Paris climate change agreement ceremony ahead of the G20 Summit at the West Lake State Guest House in Hangzhou, China, September 3, 2016. REUTERS/How Hwee Young/Pool
The world's two biggest emitters of greenhouse gases made the landmark announcement as heads of state from the Group of 20 biggest economies, or G20, arrived for a summit in the city of Hangzhou, parts of which resembled a ghost town as Chinese security locked down the area.
US President Barack Obama's last scheduled trip to Asia before leaving office however got off to an awkward start. Soon after Air Force One landed, a Chinese security official blocked National Security Adviser Susan Rice on the tarmac, speaking angrily to her before a Secret Service agent stepped between the two.
China has gone to great lengths to try to make the Sept 4-5 G20 summit a success, hoping to cement its standing as a global power, but a range of thorny diplomatic topics could overshadow the agenda.
G20 leaders are likely to renew their promises to use tax and spending policies to invigorate the sluggish world economy, although a new pro-growth push was unlikely.
Overcapacity in the global steel industry, a sore point for China as the world's largest producer of the metal, barriers to foreign investment and the risk of currency devaluations to protect export markets will also figure in the discussions.
The climate deal set a positive tone.
"Just as I believe the Paris agreement will ultimately prove to be a turning point for our planet, I believe that history will judge today's efforts as pivotal," Obama said after he and China President Xi Jinping handed ratified documents to UN Secretary-General Ban Ki-moon.
"We have a saying in America that you need to put your money where your mouth is. And when it comes to combating climate change, that's what we're doing. Both the United States and China, we're leading by example."
At a joint ceremony, Xi said it "speaks to the shared ambition and resolve of China and the United States in addressing global issues".
French President Francois Hollande said it was an important step that would pave the way for the implementation of the Paris agreement at the end of the year.