The US and China – jointly responsible for 40% of the carbon emissions of the world – have now ratified the Paris global climate agreement.
After arriving with other leaders of G20 countries for a summit in the city of Hangzhou, Mr Obama said:
“History will judge today’s effort as pivotal.”
CO2 emissions are the driving force behind climate change.
Nations agreed to cut emissions enough to keep the global average rise in temperatures below 2C. last December
The Paris deal is the world’s first climate arrangement that is comprehensive. It’ll only come into force legally after it really is ratified by at least 55 nations, which between them generate 55% of global carbon emissions.
Members of China’s National People’s Congress Standing Committee adopted “the suggestion to review and ratify the Paris Agreement” on Saturday morning at the end of a week-long session.
This really is a huge step towards turning the Paris climate arrangement into reality.
Other countries will tussle over their own ratification, but this will put pressure on G20 countries over the weekend to go quicker with their assurance to phase out subsidies.
But even if enough other players step to make the Paris deal law, huge challenges lie ahead.
Before China made its announcement, the 23 countries that had so far ratified the agreement accounted for over 1% of emissions.
The UK has yet to ratify the Paris deal. A spokesman for the prime minister told BBC News that the government would ratify when possible – but gave no date.
The White House issued a statement on Saturday morning announcing the US ratification.