Hangzhou: The world's two biggest polluters, the United States and China, have formally ratified the Paris agreement to curb climate change in a joint announcement ahead of the G20 summit in Hangzhou.

The joint declaration, made by US President Barack Obama and Chinese President Xi Jinping in what is likely their final meeting before a new president enters the White House early next year, is a major stride toward the global accord being put in force by the end of the year.

"This is not a fight that any one country no matter how powerful can take alone," Mr Obama said alongside Mr Xi just hours after touching down in China on Saturday afternoon.

"Some day we may see this as the moment that we finally decided to save our planet."