The United States and China on Saturday formally joined the Paris climate agreement, placing the world’s two largest greenhouse-gas emitters—together responsible for roughly 40 percent of global emissions—inside the pact and sharply increasing the prospect that it will take legal effect this year.

President Barack Obama and President Xi Jinping deposited their countries’ instruments of acceptance with United Nations Secretary-General Ban Ki-moon before the opening of the Group of 20 summit in Hangzhou. The step converts months of joint diplomacy into formal commitments under the accord negotiated by 195 nations in December. The White House said the two countries’ action demonstrated that major economies can pursue growth while leading the transition to low-carbon energy.

“There’s an ongoing debate in the United States about whether it is possible to grow the economy and deal with climate change at the same time,” Secretary of State John Kerry said in remarks released Saturday. The agreement, he argued, was built around nationally determined plans precisely so countries could act at different stages of development without abandoning a common framework.

Two thresholds now move within reach

The agreement will enter into force 30 days after at least 55 parties representing at least 55 percent of global emissions have formally joined. The number-of-countries test is approaching rapidly; the emissions test has been the larger uncertainty. By adding the United States and China together, Saturday’s ceremony supplied the single biggest bloc toward the second threshold.

Ban called the joint action a powerful signal that the transformation envisioned in Paris is accelerating. In its account of the Hangzhou ceremony, the United Nations said the two countries’ participation would help mobilize other governments to complete their domestic procedures. Official United Nations photographs documented the deposit of the legal instruments, an administrative act with unusually broad economic consequences.

The speed matters. The accord was opened for signature in April, and governments have been working to secure entry into force before the end of 2016. Ban in July invited all member states to a September 21 event intended to accelerate ratification. The U.S.-China announcement gives that meeting momentum and puts pressure on other large emitters to declare when they will join.

A bilateral relationship drives a global bargain

The ceremony also underscored how much the climate accord has depended on cooperation between Washington and Beijing. In 2014, the two governments announced their post-2020 climate targets together, helping break a long-running stalemate between developed and developing nations. In March, Obama and Xi issued a joint presidential statement promising to sign the Paris agreement on its first day and to take the domestic steps needed to join as early as possible.

That partnership does not erase profound disagreements over trade, cybersecurity, maritime claims and human rights. But on climate policy, each country gives the other political cover. American officials can point to China’s participation when answering critics who say U.S. action would merely shift industry abroad. Chinese officials can portray cleaner growth as a development strategy supported by the world’s largest advanced economy rather than an external constraint imposed upon Beijing.

The accord itself avoids a single top-down emissions quota. Each country files a nationally determined contribution, reports its progress and returns every five years with a stronger plan. The United States has pledged to cut net greenhouse-gas emissions 26 to 28 percent below 2005 levels by 2025. China has pledged to peak carbon-dioxide emissions around 2030, make best efforts to peak earlier, and increase the share of non-fossil energy to around 20 percent by 2030.

Domestic policies will determine credibility

Formal acceptance is a diplomatic milestone, but implementation will depend on national policy. The Obama administration’s strategy includes carbon limits for existing power plants, stronger vehicle fuel-economy standards, controls on methane and expanded support for renewable energy. The White House fact sheet said wind generation had tripled and solar generation had increased thirtyfold since 2008, while fuel-economy rules were intended to roughly double new-vehicle efficiency by 2025.

China’s plan calls for a national carbon-emissions trading system beginning in 2017, lower carbon intensity and substantial expansion of wind, solar, nuclear and natural-gas capacity. China also faces immediate domestic pressure from dangerous air pollution, which has made reduced coal dependence a public-health and political priority as well as a climate goal.

Both governments still face skepticism. In the United States, opponents say administration regulations are costly and that the Paris framework should have been presented to the Senate as a treaty. The administration maintains that the agreement can be accepted under existing executive authority because its emissions targets are not internationally binding and because reporting obligations build on prior agreements.

In China, the challenge is less legal than structural. Coal remains central to electricity and heavy industry, provincial enforcement varies, and economic slowing can strengthen demands for traditional stimulus. The credibility of Beijing’s target will rest on whether national directives alter investment decisions across a vast, decentralized economy.

Attention shifts to implementation beyond Paris

The two presidents also pledged cooperation on related negotiations that could produce near-term emissions reductions. They backed an ambitious amendment to the Montreal Protocol to phase down hydrofluorocarbons, powerful warming chemicals used in refrigeration, and supported a global market-based measure for international aviation emissions. Those talks provide tests of whether the political alignment displayed in Hangzhou can produce detailed rules in other forums.

Contemporary coverage described the ceremony as both environmental policy and strategic theater. Time reported from Hangzhou that the leaders were seeking to demonstrate joint stewardship before a summit otherwise burdened by economic and security disputes. The visual message was unmistakable: the two countries with the greatest capacity to derail the Paris framework were instead presenting themselves as its anchors.

Joining the agreement does not by itself reduce a ton of carbon. It does, however, strengthen the legal and political architecture through which reductions are expected to occur. With the world’s two largest emitters now formally committed, the central question is shifting from whether Paris will survive to whether national policies can deliver the rapid, sustained transformation its temperature goals require.