China and the US have formally ratified a historic climate change agreement drawn up in Paris to cut emissions and fight climate change.
Presidents Barack Obama and Xi Jinping delivered documents to UN Secretary-General entering their countries into the pact during a ceremony on Saturday.
Mr Obama said the US was committed to being a global leader in the fight against climate change, hoping the Paris agreement would be remembered as the moment the world united to stop global warming.
“This is not a fight that any one country no matter how powerful can take alone,” he added. “Some day we may see this as the moment that we finally decided to save our planet.”
Mr Xi, the President of China, called the agreement a milestone that marks the “emergence of a global government system” for climate change.
“Our response to climate change bears on the future of our people and the wellbeing of mankind,” he said.
Erik Solheim, director of the United Nations’ environment programme, said the ratifications bring significant additional momentum to the drive against global warming.
“And by putting the well-being of our planet at the top of the agenda, the two largest economies in the world are also showing that our economic future is low-carbon and green,” he said.
“The fight against climate change remains difficult and urgent, but having heavy-hitters like China and the US on your side is extremely heartening.”
Andrew Steer, president and CEO of the World Resources Institute, said Saturday’s announcement “raised the bar” for other nations.
“Co-operation between the US and China on climate change was once unimaginable but now stands as the brightest spot in their relationship,“ he added.
“In joining the Paris agreement in tandem, these two leaders have reconfirmed their responsibility to lead by example.“
China and America’s move has increased pressure on the British Government, which has not yet signed the landmark deal.
Camilla Toulmin, a senior fellow at the International Institute for Environment and Development, said: “There’s no reason now for the UK not to ratify the agreement as soon as possible.”
It will only come into force after a threshold of 55 signatories producing more than half of emissions has been passed, possibly by the end of this year.
It came on the sidelines of the G20 summit in Hangzhou, China, where issues including the Syrian civil war, Ukraine conflict and refugee crisis are also on the table for discussion.
Tensions remain high between Beijing and Washington, with issues including cyber hacking, Chinese military expansion in the South China Sea and the planned deployment of an American anti-missile system in South Korea souring relations that had been improving.