epa04486040 US President Barack Obama (L) speaks as Chinese President Xi Jinping adjusts his headphones during a press conference at the Great Hall of the People (GHOP) in Beijing, China, 12 November 2014. (How Hwee Young/EPA)

CHINA AND the United States can make or break the global response to climate change, but until now it has not been clear whether the nations would ever shake hands on a big joint commitment. On Wednesday they did just that, sealing one of the most significant international climate deals ever struck.

President Obama and Chinese President Xi Jinping — the leaders of the world’s largest emitters of carbon dioxide, representing about 45 percent of global greenhouse-gas emissions — announced serious, achievable commitments on emissions over the next two decades. That undercuts the argument, favored by U.S. conservatives, that China won’t move to protect the climate even if the United States sacrifices.

According to the plan, the United States will double the pace of its carbon reduction efforts before 2025, by that point reducing emissions 26 percent to 28 percent from 2005 levels. The Obama administration says it can put the nation on this path under existing executive authorities, without further congressional approval. We say “further” because Congress gave the president formidable powers to combat air pollution in the Clean Air Act and elsewhere; as the Supreme Court has confirmed, Mr. Obama has strong statutory authority to act unilaterally on ­greenhouse-gas emissions. Of course, it would be easier and less expensive if Congress enacted a more efficient anti-carbon policy, such as a fully rebated carbon tax, to achieve similar or greater reductions. But that’s not likely to happen soon.

China, meanwhile, will cap its carbon dioxide output around 2030 and will try to halt its rising emissions sooner than that. Incoming Senate majority leader Mitch McConnell (R-Ky.) claimed Wednesday that the deal would allow China to “do nothing at all for 16 years.” In fact, achieving that cap will require significant new effort. China will meet its goal in part by ramping up its use of non-fossil-fuel energy, including nuclear, hydro, wind and solar power, to about 20 percent of its total energy use.

That commitment is astonishing in scope; the generating capacity of the ­clean-energy infrastructure China intends to build would nearly equal the installed capacity of the entire U.S. electrical system, the White House reckons. Already China has been experimenting with ­market-based carbon pricing programs and requiring its economy to waste less energy. Experts expect these efforts to expand.

There is some give in the various commitments: Each specifies imprecise targets or approximate amounts of time in which to reach them. The level at which China eventually caps its emissions is also crucial, and unstipulated. Wherever that is, it will take deeper reductions over longer periods of time by both countries to respond to climate change adequately.

But more important than the details is the fact that China and the United States are finally leading on global warming, connecting the two largest pieces of an international climate puzzle that has been an ugly mess. Other nations will have confidence that they, too, will not be sacrificing in vain if and when they cut their emissions. Also, big developing countries, such as India, that have done too little should find it harder to avoid acting.

It took Republican leaders almost no time to attack the agreement, and they might need to wait only a few years to roll it back, if they win the presidency in the next election. That would be a mistake even larger in scale than Mr. Obama’s achievement in Beijing.