US President Barack Obama and Chinese President Xi Jinping entered their countries into the Paris climate agreement on Saturday, pushing forward global efforts to curb climate change.
Obama and Xi handed over notification that their countries had joined the agreement to UN Secretary General Ban Ki-moon on the sidelines of the Group of 20 (G20) summit in Hangzhou, China, and urged all other parties to follow suit.
China's parliament ratified the Paris climate agreement earlier Saturday, but no such action was necessary on the part of the US Congress.
Obama told reporters that the deal was an "example of what can happen when our two countries work effectively together."
"A Chinese saying goes: Only commitment and decision will lead to great achievement," Xi said in praising the effort to cut emissions that warm the planet.
The agreement, which was passed by nearly 200 countries in Paris last December, is the first universal action plan intended to mitigate the impacts of climate change and hold the rise in global average temperature to below 2 degrees Celsius, compared to pre-industrial levels.
The ratification of the agreement is the third step in implementing the deal agreed in Paris last year and signed by the US, China and hundreds of other nations earlier this year. The agreement will enter into force 30 days after 55 countries accounting for at least 55 per cent of global greenhouse emissions ratify it.
The move by the two top carbon dioxide emitters to ratify the treaty simultaneously is a major step forward in reaching that goal and continues cooperation between Washington and Beijing on climate that set the stage for the Paris agreement.
"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," said Patricia Espinosa, Executive Secretary of the UN Framework Convention on Climate Change (UNFCCC).
"The earlier that Paris is ratified and implemented in full, the more secure that future will become," she added.
Prior to the US and Chinese move, 24 nations accounting for just over 1 per cent of global emissions had ratified the agreement.
White House climate advisor Brian Deese said that because the US and China together represent nearly 40 per cent of global emissions, this will put the world on a path to full implementation of the agreement.
"The signal of the two largest emitters taking this step together and taking it early should give confidence to the global community and other countries that they too can move quickly," Deese said.
The US and China also agreed Saturday to work to reach a global agreement this year to phase out the global use of polluting hydrofluorocarbons and to support an international agreement to lower aviation emissions.
Obama had made climate protection a key part of what is expected to be his final trip to Asia to attend the G20 summit in the eastern Chinese city of Hangzhou.
Before departing for China, Obama visited a marine conservation area in the Pacific Ocean and met with the leaders of Pacific island nations most severely affected by climate change at a meeting in Hawaii.
In November 2014, Obama and Xi had announced new climate change targets following bilateral talks in Beijing.
Under the agreement, China's CO2 emissions would peak before 2030 and the US would cut its net greenhouse gas emissions to 26-28 per cent below 2005 levels by 2025.
However, environmentalists say both sides have yet to reach a game-changing climate relationship.
China's commitments have so far been "very ambiguous," the Mercator Institute for China Studies said in a 2015 report.
Beijing has also said it aims to reduce carbon intensity by 60 to 65 per cent from 2005 levels by 2020, but has not set a quantitative emissions cap.
Obama and Xi spent more than five hours in talks and at a dinner Saturday evening at Hangzhou's historic West Lake. They also discussed territorial disputes in the South China Sea, human rights, cybersecurity, trade and other issues.
The White House described discussions on the South China Sea as "candid" with Obama calling on China to abide by an international tribunal in July over a dispute with the Philippines that found Beijing did not have a territorial claim to part of the region.
He stressed the US commitment to its allies in the region and "reaffirmed that the United States will work with all countries in the region to uphold the principles of international law, unimpeded lawful commerce, and freedom of navigation and overflight."