The Obama administration spent years crafting a plan to curb carbon dioxide emissions from coal-fired power plants—by far the country’s number one source of greenhouse gases. It built its case for climate action at the U.N. climate talks in Paris on that plan, getting buy-in from China and India largely on the promise that the U.S. would cut its own emissions.

It took the Supreme Court two paragraphs—five sentences—to put the future of the Clean Power Plan, and the federal government’s efforts to combat climate change, into question. The 5-4 decision granted the coal industry and 29 states a stay on implementation of the regulations until legal challenges to the rule are resolved.

The decision by the Court’s four conservatives and swing voter Justice Anthony Kennedy has stunned legal observers. The Court has never granted a request of this kind.

“They’ve opened up a completely new role for the Court that has heretofore never been invoked,” says Scott Oostdyk, an attorney for McGuireWoods who represents Murray Energy, one of the plaintiffs in the case. “It’s the ultimate uncommon. It’s the first time in the history of the Court it ever happened.”

LISTEN: “Supreme Court Puts the Brakes on Clean Power Plan”

Ann Carlson, UCLA law professor and co-director of the Emmett Institute on Climate Change and the Environment, called the decision “outrageous.” Part of the reason proponents of the plan are so flummoxed by the stay is that the Supreme Court, in 2007, directed the EPA to regulate carbon dioxide under the Clean Air Act.

“EPA did what it was told,” Carlson says. “And now the court is saying, ‘Actually what you did, we don’t really like and we’re going to put a stay on it.'”

What impact will the stay have? For starters, it means that states will no longer have to meet a September deadline to submit plans to the EPA for how they will comply with the Clean Power Plan. Oostdyk says the earliest the Court would make a ruling on the regulation is fall 2017.

Some states, like Pennsylvania, will continue to develop their proposals. “We are still preparing a plan to present to EPA once the legal issues are resolved,” says Neil Shader, a spokesman for Pennsylvania’s Department of Environmental Protection. Other states will simply put the brakes on crafting plans to cut carbon. Kentucky and Georgia have already done so.

The Obama administration has called the decision a “bump in the road,” and the EPA says it believes the plan will stand up on appeal. “EPA firmly believes the Clean Power Plan will be upheld when the merits are considered, because the rule rests on strong scientific and legal foundations,” the agency said on its website.

But legal experts say the EPA shouldn’t be too confident. That’s because in order to grant a stay, the Court needed to decide that the plaintiffs have a good chance of winning their case.

“That means there are at least five justices on the Court who think, ‘You know, the EPA’s case is not that strong, and the opponents of the rule are onto something,'” Carlson says. “It’s not a good sign (for the EPA) that the court took it upon itself to prevent the rule from going into effect.”

The plan also has implications for the historic Paris climate accords, signed in December by over 190 countries. That agreement included pledges by every country to cut emissions of climate-warming gases. The Supreme Court’s ruling may rattle that already fragile global consensus around cutting fossil fuel emissions.

“Basically, what the court just did was take out the heart of the U.S. plan that is part of this global agreement and sent a signal to the rest of the world that the U.S. cannot live up to its end of the commitment,” Carlson says.

It’s not clear why the Supreme Court—and in particular swing-voter Justice Kennedy—simply didn’t allow the plan to proceed. The Court didn’t provide any explanation in its one-page order.

But Oostdyk thinks the reason has to do with another regulation the EPA imposed on the coal industry—the Mercury and Air Toxics Standard (MATS). That rule sought to reduce harmful air pollution and forced dozens of coal plants around the country to install costly air pollution filters.

The rule was finalized in 2011, and since then, coal plants have had to decide whether to comply or shut down.

“Basically what the court just did was take out the heart of the U.S. plan that is part of this global agreement and sent a signal to the rest of the world that the U.S. cannot live up to its end of the commitment.”

The EPA estimated that compliance with MATS cost the industry $9.6 billion and will raise electricity rates by 3 percent. The rule led to dozens of coal plant retirements. In 2015 alone, 4 percent of the U.S. coal fleet was retired, according to one analysis by SNL Financial.

But last year, the Supreme Court sent the rule back to the EPA for a re-write, saying the agency didn’t properly consider the cost of implementing it.

Oostdyk thinks that situation is what drove the Court to grant a stay on the Clean Power Plan.

“They don’t want the (MATS) situation, whereby by the time the case gets through the Court, all the harm would have been done.”

It’s also unknown how the Supreme Court will ultimately decide the fate of the Clean Power Plan once it gets the case. The answer could depend on an obscure legal ‘mistake’ made by Congress when it revised the Clean Air Act in 1990.

The House and Senate each submitted different versions of section 111d—the clause the EPA is using to regulate carbon dioxide from existing power plants. In a legislative oversight, the versions were never reconciled by the two chambers and each was written into the law.

Now, 25 years later, this mistake could determine one of the biggest environmental cases in U.S. history.

The EPA is choosing to use the Senate version, which is more regulation-friendly. In the past, the Court has let agencies interpret laws when legislation has been vague.

But Oostdyk says the plaintiffs in the case are arguing that the EPA shouldn’t get to pick and choose which version of the law it wants.

“The Supreme Court could be saying, ‘We don’t believe Congress intended what you, the agency, say they did, because they would have said it a lot clearer than they did,'” Oostdyk says. “‘If they can’t say what they meant, we’re not gonna allow you to say.'”

Carlson agrees that the EPA’s case will be hard to prove.

“The EPA plan is ambitious—there’s no doubt about it,” Carlson says. “And they’re doing it with a statute designed 45 years ago, so I actually think it’s a hard legal case. This is not a frivolous lawsuit.”

Of course, the Court could approve the rule in the end. It will first have to let the DC Circuit Court of Appeals take a crack at the plan. A hearing there is scheduled for June 2.

The presidential election, of course, could also play a huge role in deciding the fate of the plan.

“If a Republican is elected to office, the Clean Power Plan, I think, is dead,” Carlson says. “It probably would have been anyway, but this makes it all the easier.”