Chinese President Xi Jinping, center, U.S. President Barack Obama, right, shake hands as U.N. Secretary-General Ban Ki-moon looks on during a joint ratification of the Paris climate change agreement at the West Lake State Guest House in Hangzhou in eastern China’s Zhejiang province, Saturday, Sept. 3, 2016. (How Hwee Young/Pool Photo via AP)

This story has been updated.

President Obama and Chinese President Xi Jinping formally joined the Paris climate accord on Saturday, handing over signed documents to United Nations Secretary General Ban Ki-Moon in Hangzhou, China and giving a major push toward bringing the emissions-cutting plan into force before the end of the year.

Though the international climate deal was sealed in Paris last December, it still needs to be signed and delivered by at least 55 countries representing at least 55 percent of global emissions to achieve a legal status called “entry into force.”

Obama is in Hangzhou for a meeting of the Group of 20 and Ban flew there so that the American and Chinese leaders could formally deposit their “instruments” of ratification or approval of the agreement with the United Nations. Together, the two nations account for a hair under 38 percent of global emissions.

Handing Ban a red folder, Xi said “hopefully this will encourage other countries to take similar efforts.” Handing Ban a black folder, Obama said “Some day we may see this as the moment when we decided to save our planet.” He added, “History will judge today’s efforts as pivotal.”

“As the world’s two largest emitters and economies, if we can come together we can help the world move forward on combating climate change,” Brian Deese, senior adviser to Obama, said earlier in a conference call.

The move represents a major development for the agreement itself and for Obama, because it makes it more likely the accord could enter into force sometime this year — while Obama is still president. Republican presidential candidate Donald J. Trump has vowed to scrap the deal if he is elected.

If Hillary Clinton is elected in November, she is expected to continue Obama’s climate policies and to begin implementing the Paris climate agreement immediately. The accord calls on each nation to live up to a pledge that it has submitted to the United Nations to reduce its emissions — in the U.S.’s case, by 26 to 28 percent below 2005 levels by the year 2025.

But Trump opposes the accord and has said he would “cancel” the agreement. If the accord has already entered into force, though, doing so would be difficult — and certainly unprecedented.

Once the agreement enters into force, its language in Article 28 states that a party cannot then withdraw for three years — and there is yet another year for the withdrawal to take effect. The length, in short, of a presidential term.

Trump, if elected, could simply ignore the agreement, and the international sanctions for doing so are not very severe, primarily calling on any wayward country to explain its lack of cooperation.

Some Republican critics of the accord say it is a treaty that should be submitted to the Senate for ratification, but the Obama administration says that the president has the authority to commit to the Paris agreement just as President George H.W. Bush did when he signed the United Nations Framework on Climate Change.

Deese said any effort to roll back the agreement would run counter to prevailing sentiment among American businesses. “Much of the change is driven not simply by government policy but by the private sector making investments,” he said.

However, a group of Republican state attorneys general has challenged the Clean Power Plan, a key part of Obama’s climate plan, and the case is awaiting a hearing in an appellate court.

Nearly two years ago, Obama and Xi reached an historic agreement on climate change. With the world’s two largest emitters jointly pledging to ratchet down their contributions to global warming, a path was laid for the Paris climate accord a year later. There negotiators finally bridged the longstanding gap between developed and developing nations and unified the world behind climate action.

Now, the challenge is to bring the agreement into force. Two dozen other nations with just over 1 percent of global emissions have also formally joined the agreement, either by ratifying it or by another official process (this can vary from country to country). The participation of the United States and China brings the 55-country and 55-percent-of-emissions thresholds much closer, but still falls well short of those goals.

Deese said the United States expects another 35 countries to join the agreement with the United Nations by the end of the year. He said Brazil, Argentina, South Korea and Japan were all in advanced stages of winning domestic ratification. All of those countries are significant emitters.

The White House estimates that the 35 countries can lift the total emissions covered to 55.83 percent — and that’s without counting India. Obama has pressed India’s Prime Minister Narendra Modi over the past year and a half  and will meet with him again  in Hangzhou.

“A Chinese saying goes, ‘Only commitment and decision will lead to great achievement,'” Xi said on Saturday. “When the old path no longer takes us far, we should make use of new methods,” he added. “Innovation should be given a fresh boost.”

“There’s an American saying,’ You need to put your money where your mouth is.'” Obama said. “That’s what we’re doing.”

The United States and China have continued to work together on climate issues despite sharp differences, most notably over Chinese military installations that have been built on reefs in disputed waters of the South China Sea. That has created tension for U.S. diplomacy with Xi.

“We have been very clear throughout this process that there are areas where we can work together,” Deese said, “but there are lots of areas where we have disagreements and we’ll be very clear and resolute.”

Except for the United States and China, many of the countries that have already joined the accord tend to be smaller ones who are particularly worried about rising seas — the Bahamas, the Maldives, the Marshall Islands — but don’t emit a lot of greenhouse gases. Thus, getting to 55 percent of emissions is considerably tougher.

Among the world’s larger emitters, those that have yet to formally join include Russia (7.5 percent of emissions), India (4.1 percent), Japan (3.79 percent), Germany (2.56), Brazil (2.48 percent), Canada (1.95 percent), South Korea (1.85 percent), Mexico (1.7 percent), the U.K. (1.55 percent), Indonesia (1.49 percent), South Africa (1.46 percent) and Australia (1.46 percent). (For a breakdown by the U.N. of how much each country contributes to global emissions, see here.)

Some observers are optimistic several will join soon. U.N. Secretary General Ban has invited the world’s leaders to the United Nations headquarters in New York on Sept. 21 to deposit their official instruments for ratification or acceptance and so to bring the Paris agreement into force as soon as possible.

“Today’s announcement, coupled with other key countries signaling intentions to take similar action, all but assures the Paris Agreement will take effect this year,” Alden Meyer, director of strategy and policy for the Union of Concerned Scientists, said in a statement.

Correction: An earlier version of story said in the title that the US had ratified the Paris agreement on climate change instead of joining it. Ratification generally applies to treaties that require approval by a two-thirds vote in the Senate.

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