by Roger Harrabin
The US and China – together responsible for 40% of the world’s carbon emissions – have both formally joined the Paris global climate agreement.
After arriving with other leaders of G20 nations 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.
Last December, countries agreed to cut emissions in a bid to keep the global average rise in temperatures below 2C.