China and the United States have formally joined the Paris climate deal, a giant step by the world’s two biggest polluters in confronting global warming.

At a ceremony on the eve of the G20 summit in China, the leaders of both countries handed ratification documents to UN chief Ban Ki-moon, raising hopes that the accord could come into affect by the end of the year.

History would show that the Paris deal would "ultimately prove to be a turning point", said President Barack Obama. “The moment we finally decided to save our planet".