The two countries, which produce 38% of the world’s emissions between them, are now backing what is known as the Paris Agreement, after signing up as leaders of the G20 leading group of nations meet in China.
The 24 countries who had previously signed up to the deal accounted for around 1% of the world’s emissions, but that figure just rose to 39% with the move by the US and China, The Press Association reported.
It means the agreement drawn up in the French capital last December comes closer to being adopted.
It requires at least 55 countries representing 55% of the world’s climate emissions to ratify the deal to drive down greenhouse gases for it to come into force.
There have been calls for countries to ratify the deal as soon as possible, so it can be adopted earlier than the planned 2020 start date, to meet the urgent need to tackle climate change.
United Nations climate chief Patricia Espinosa said: “I would like today to thank China and the United States for ratifying this landmark agreement - an agreement on which rests the opportunity for a sustainable future for every nation and every person.
“The earlier that Paris is ratified and implemented in full, the more secure that future will become.”
Mohamed Adow, Christian Aid’s senior climate advisor, said: “This marks the moment the two biggest economies throw their weight behind the Paris Agreement.
“We now need the other countries to join in and ratify as soon as possible, including the UK.
“On Wednesday there will be a debate in the House of Commons at which the UK Government will have a chance to set out their timeline for ratification.
“If the UK intends to retain its climate leadership on the global stage it mustn’t show up late to the party.”
WWF-UK’s head of climate and energy Emma Pinchbeck said: “The new Prime Minister must continue British leadership on climate change.
“By quickly ratifying the Paris Agreement, and pushing for a strong global deal to limit aircraft emissions at the International Civil Aviation Organisation conference this month, the Government could show the world that the referendum vote hasn’t watered down any resolve to deliver a low-carbon word and meet our global commitments.”
Greenpeace UK chief scientist Dr Doug Parr said: “This move from the world’s largest carbon emitters takes the Paris climate deal much closer to the critical threshold for adoption. Now it’s the UK’s turn to help push it over the finishing line.
“Britain showed its climate leadership with the Climate Change Act and the crucial role it played in the Paris negotiations. It’s now time to show that leadership once again.
He added: “There are no excuses left for the UK Government to delay the ratification of the Paris deal, least of all Brexit. If anything, the UK has never been more in need to demonstrate it’s a reliable partner on its international commitments.”