President Barack Obama and his Chinese counterpart both committed their countries to the Paris climate change agreement. | AP Photo U.S., China formally join international climate deal

President Barack Obama and Chinese President Xi Jinping formally committed their countries to last year's Paris climate change agreement on Saturday, ratcheting up pressure on other nations to follow suit.

During a ceremony in the southeastern Chinese city of Hangzhou one day before the G-20 summit, the two leaders submitted documents to United Nations Secretary-General Ban Ki-moon that officially accept the terms of the Paris agreement.


"The challenge of climate change could define contours of future," Obama said, according to pool reports. "Some day we may see this as the moment when we decided to save our planet.

"History will judge today's efforts as pivotal."

Climate, Obama said, could "define the contours of this century more dramatically than any other challenge."

Xi called the Paris agreement a "milestone," adding, "Hopefully this will encourage other countries to take similar efforts."

The Obama administration is hoping that the Paris deal will enter into force before the president leaves office, a move that would simultaneously bolster his environmental legacy and make it more difficult for Donald Trump to withdraw from the accord if he wins the presidency. The U.N. is hosting a summit in on Sept. 21 in New York aimed at further pressuring countries to formally ratify the deal.

Saturday's announcement, which POLITICO earlier this week reported was in the works, comes as leaders of the world's largest economies are arriving in China for the G-20 summit. Xi and Obama are expected to lean on the other world leaders to formally endorse the agreement. Obama is expected to meet briefly with Indian Prime Minister Narendra Modi on the sidelines of the summit to encourage him to quickly join the Paris deal, according to a source briefed by the administration. White House climate adviser Brian Deese told reporters that he expects Modi and Obama will "touch base” about climate change during the G-20, but officials have not announced a formal bilateral meeting between the two leaders.

The Paris accord is one step closer to entering into force with China and the U.S. formally on board. Under U.N. rules, the agreement will take effect 30 days after 55 countries representing 55 percent of global emissions formally approve or ratify it. Together, China and the United States account for just under 40 percent of global emissions.

About two dozen other countries have already joined the agreement, accounting for about 1 percent of global emissions. More than 30 additional nations have indicated they plan to join this year. If those countries follow through, the deal should clear the threshold required for entry into force.

Climate change is a rare bright spot in the U.S. relationship with China, which has suffered amid tension over hacking and uncertainty about trade. After repeated failures to reach consensus at international global warming negotiations, U.S. and Chinese officials embarked on a years-long diplomatic push to find common ground on climate change. The campaign culminated in November 2014 when Obama and Xi jointly announced domestic plans to limit their emissions.

“Cooperation between the U.S. and China on climate change once unimaginable, now stands as the brightest spot in their relationship. In joining the Paris Agreement in tandem, these two leaders have reconfirmed their responsibility to lead by example," World Resources Institute President Andrew Steer said in a statement.

Nearly 200 nations negotiated the terms of the climate deal last year during a two-week summit in Paris that capped off decades of tense international talks. As part of the agreement, each nation designed domestic plans to curb carbon emissions with the goal of limiting the increase in global temperatures to well below 2 degrees Celsius. But the agreement does not carry the legal weigh of a treaty, a factor that allowed Obama to join the deal without submitting it to the Senate for ratification.

Scientists warn that unchecked global warming will lead to catastrophic temperature increases, sea-level rise, drought and ocean acidification. While the effects of the warming planet are already being felt around the world, experts say that countries must dramatically limit greenhouse gas emissions in the coming decades to prevent disastrous climate change.

Republicans have long opposed the Paris agreement and expressed outrage at the president's decision to join it.

“The president is again putting America's economy and jobs at an extreme disadvantage for an international agreement that China and other countries have no incentive to abide by," Sen. John Barrasso (R-Wyo.) said in a statement. "This questionable unilateral action by the president can and should be struck down as soon as possible.”

India is the world's third-largest greenhouse gas emitter, behind China and U.S., and its speedy decision to ratify the deal would help ensure it goes into effect this year. The country is also a key player in negotiations over an ambitious push to phase down hydrofluorocarbons, powerful greenhouse gases used in refrigerators and air conditioners that exacerbate climate change. International negotiators are scheduled to meet in Rwanda in October to finalize an HFC-phase-down amendment to the Montreal Protocol, a 1987 international agreement that helped eliminate gases that caused a hole in the ozone layer.

In addition to encouraging Modi to join the Paris deal this year, Obama is expected to press the Indian leader to support a stringent Montreal Protocol amendment, the source said. Success there could be one of the biggest steps toward meeting the goals set forth in Paris. While preliminary talks in Vienna made progress, India was seen by some outside observers as signaling its reluctance to key parts of the amendment proposals.

Top Obama administration officials have been in close contact with Indian officials in recent weeks to discuss climate-related issues. "The president has invested probably just as much on this issue with Modi as he has with Xi, but it’s in some ways been tougher to align perspectives," a person familiar with the issue told POLITICO. "India has been a little bit tougher to predict."

The U.S. and China, for their part, have made progress toward finding a compromise on an HFC amendment. The White House announced on Saturday that the two countries agreed to work toward an amendment with an "early freeze date" at which time countries must stop increasing the production and use of HFCs, while pledging to work toward an "ambitious phase down schedule, along with increased financial support to assist in implementation."

The two countries also pledged to work with other nations to come reach an agreement to reduce greenhouse gas emissions from international aviation. The U.S. and China pledged Saturday that they would volunteer to become early participants in any program to slash aviation emissions.

Despite objections from Republicans in Congress, there appears to be little they can do to undermine the deal, given how it was structured. Deese told reporters that the use of executive agreements instead of treaties is "well established, both legally and diplomatically," adding, "We’re very comfortable with its legal form.”

Trump has promised to "cancel" the Paris deal, but formal withdrawal will take years once it enters into force. Still, Trump could opt to simply ignore the agreement and turn his back on the domestic climate plan the Obama administration developed, which sets a goal of cutting greenhouse gas emissions 26-28 percent below 2005 levels by 2025.

Eric Wolff contributed to this report.