The two biggest greenhouse gas emitters in the world have formally joined the Paris climate agreement.

Shortly after China adopted the agreement, U.S. President Barack Obama today made the announcement that the U.S. had followed suit while he was in Hangzhou, China, ahead of this weekend’s G20 summit. Together, the U.S. and China are responsible for some 38.76 percent of global emissions.

“This is the single-best chance that we have to deal with a problem that could end up transforming this planet in a way that makes it very difficult for us to deal with all the other challenges that we may face,” said Pres. Obama in his remarks.

The two actions bring the Paris Agreement significantly closer to reaching the 55 percent emissions threshold required for the pact to enter into force. Fifty-five countries must formally join for it to take effect, as well. Once the agreement has entered into force, it will become binding international law, and countries that have formally joined will be subject to its provisions.

Last December, nearly 200 countries signed the Paris Agreement, which commits its signatories to keeping global warming below 2 degrees Celsius (3.6 degrees Fahrenheit) and to making their best efforts to limit warming to an aspirational goal of 1.5 degrees Celsius (2.7 degrees Fahrenheit). It also requires countries to submit increasingly ambitious emissions reduction plans every five years.

“The United States and China’s decision to help bring the Paris climate agreement into force is a profoundly important step forward that signals to all the other nations that this ground-breaking global deal will deliver,” William Ruckelshaus, who served as EPA administrator under Presidents Nixon and Reagan, said in a statement. “It ensures a significant reduction and slowdown in emissions from the two biggest economies and accelerates their transition to a clean energy economy. More must be done, but this is a leap forward to protecting ourselves from the biggest threat we face to our environment, our economy, and our ways of life.”

Fiji, Palau, and the Marshall Islands were the first three countries to ratify the agreement. A total of two dozen countries, representing 1.1 percent of emissions, had joined as of September 1, 2016. The Paris Agreement will enter into force 30 days after it reaches the twin thresholds of 55 countries joining the agreement and of their emissions comprising at least 55 percent of total global emissions.

It is projected that at least 58 countries accounting for 59.88 percent of global emissions will have deposited their articles of ratification for the Paris Agreement with the UN by the end of 2016. Once it has entered into force, countries that joined the agreement cannot withdraw for at least three years.

There is likely to be some confusion in the U.S. as to the legality of President Obama’s decision to join the Paris Agreement through an executive order, rather than through the treaty ratification process, which would require the approval of two-thirds of a senate controlled by Republicans who reject even the most basic facts of climate science.

But as ThinkProgress reports, the announcement by Obama “is well within the purview of executive authority and is in keeping with longstanding practice in the United States. Moreover, the authority for U.S. participation in the Paris Agreement stems in no small measure from a treaty signed by a Republican president and approved by the Senate with strong bipartisan support.”

Executive orders have been used by Democratic and Republican presidents alike for  more than 94 percent   of internationally binding agreements in the United States, as ThinkProgress notes.

The U.S. joined the umbrella treaty under which the Paris Agreement falls, the U.N. Framework Convention on Climate Change (UNFCCC), during the administration of President George H.W. Bush. That means that, under the constitutional foreign affairs powers of the president and also because the Senate has already ratified the umbrella treaty, President Obama has the the full legal authority to issue an executive order stating that the U.S. will be joining the Paris Agreement.

In fact, the U.S. joined the UNFCCC in 1992 with bipartisan support from the Senate. “I am pleased to support this fine agreement. I congratulate President Bush on his courageous leadership on the issue of global climate change,” Senator Mitch McConnell said at the time. McConnell is now Senate majority leader and has actively worked to defeat the Paris Agreement and all attempts to tackle the climate crisis.

No new domestic legislation will be required for the U.S. to meet its obligations under the Paris Agreement, which is consistent with existing U.S. laws, such as the Clean Air Act.

Environmentalists applauded the dual actions by the U.S. and China to formally join the agreement, though there is much work that is left to be done.

“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 at the Union of Concerned Scientists, said in a statement.

“Logistically, negotiations on the agreement’s detailed rules will likely take another year or two to finalize, and all countries will need to raise the ambition of their commitments under the agreement if we’re to avoid the worst impacts of climate change and reach a goal of net zero global warming emissions by mid-century. But this is an important step forward that reinforces the U.S. and China’s continued leadership in building a robust, durable international climate framework.”

Li Shuo, a senior climate policy adviser with Greenpeace East Asia, said that Chinese President Xi Jinping and Pres. Obama should seize the opportunity this weekend to urge the other G20 countries — the world’s twenty wealthiest nations — to join the Paris Agreement as well.

Pressure on the G20 nations is mounting. Last week, a group of 130 insitutions controlling $13 trillion in investments called on the group of countries to ratify the Paris Agreement and help accelerate investment in clean energy technologies. And earlier this week, three of the world’s largest insurers called on G20 leaders to eliminate subsidies for fossil fuels by 2020.

“Touting the Paris ‘triumph’ while handing out money to the fossil fuel industry is simply not compatible with the Paris Agreement,” Greenpeace’s Li Shuo said in a statement. “Governments must keep coal, oil, and gas in the ground and urgently focus on a just transition to renewables. This G20 meeting must be the moment when leaders lay out a timeline for eliminating fossil fuel subsidies, a practice which the Paris Agreement has made all the more hypocritical.”  

Main photo: U.S. President Barack Obama and Chinese President Xi Jinping in 2014 as they announced historic targets for reducing greenhouse gas emissions from both nations. Photo credit: Official White House Photo by Chuck Kennedy