The United States has joined China to formally ratify the Paris agreement to curb climate-warming emissions, the world's two biggest economies said on Saturday.
The move could help put the pact into force before the end of the year.
US president Barack Obama and Chinese president Xi Jinping submitted their plan to join the agreement to UN secretary-general Ban Ki-moon, who is in China to witness the announcement.
Senior Obama adviser Brian Deese said the joint declaration should push other countries to formally join the agreement.
“The signal of the two large emitters taking this step together and taking it early, far earlier than people had anticipated a year ago, should give confidence to the global communities and to other countries that are working on their climate change plans, that they too can move quickly and will be part of a global effort,” Mr Deese told reporters .
India is also poised to join the agreement this year, Mr Deese said, adding that Mr Obama was expected to meet Indian prime minister Narendra Modi on the sidelines of a Group of 20 nations meeting in Hangzhou, China, this weekend.
Mr Obama and Mr Xi committed to cooperate on two other global environmental agreements this year - an amendment to the Montreal Protocol to phase down air-conditioning refrigerants and on a market-based measure to reduce carbon emissions from aviation.
‘Irreversible’
"Today's announcement is the strongest signal yet that what we agreed in Paris will soon be the law of the land," said Mattlan Zackhras, minister-in-assistance to the president of the Marshall Islands.
“With the two biggest emitters ready to lead, the transition to a low-emissions, climate-resilient global economy is now irreversible.”
Saturday's joint statement could spur further ratifications by the likes of Brazil and Canada.
“We expect a surge of ratifications around the U.N. Climate week later in September,” said Bill Hare, chief executive of Climate Analytics.
In Paris last December, nearly 200 countries agreed on a binding global compact to slash greenhouse gases and keep global temperature increases to “well below” 2 degrees Celsius.
Experts have said the temperature target is already in danger of being breached, with the UN’s weather agency saying 2016 is on course to be the warmest year since records began.
‘Light speed’
While 180 countries have now signed the agreement, 55 nations - covering at least 55 per cent of global emissions - need to formally ratify the treaty to put it into legal effect.
Before China and the United States, 23 countries had ratified - including North Korea - but they collectively accounted for just 1.08 percent of global emissions, according to the UN Framework Convention on Climate Change.
China represents just over 20 percent of global emissions while the United States accounting for 17.9 per cent, Russia 7.5 per cent and India 4.1 per cent.
The announcement is a major diplomatic achievement for the US president, who ends his term in January.
But the ability of the United States to achieve its Paris targets could be affected by the outcome of a federal court hearing this month, in which 27 US. states are trying to block the federal Clean Power Plan that slashes CO2 from power plants, the largest source of U.S. greenhouse gas emissions.
The US Republican Party Platform has also questioned the legality of the executive order used to ratify the Paris deal, saying it will need the consent of the Senate before it becomes binding.
Reuters