The Supreme Court’s decision this week to halt President Obama’s sweeping climate change regulation for power plants is causing environmentalists and experts to wonder whether they need a backup plan.

The Obama administration has repeatedly said, both before and after the judicial stay was ordered, that it does not have a Plan B if the Environmental Protection Agency’s (EPA) Clean Power Plan gets overturned.

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Officials insist that a backup plan isn’t necessary because once the high court hears the case, it will find that the rule is well within the boundaries of the Clean Air Act and the Constitution.

“We remain confident that when this is given its day in court, it’s going to be upheld on the merits,” White House spokesman Eric Schultz told reporters.

“Plan A’s a good one, and I don’t want anyone to think it isn’t,” EPA Administrator Gina McCarthy Regina (Gina) McCarthyOVERNIGHT ENERGY: Energy Department proposes showerhead standards rollback after Trump complaints | Interior memo scaling back bird protections is 'contrary to law,' court rules | Former EPA chiefs call for agency 'reset' Former EPA chiefs call for agency 'reset' Azar arrives in Taiwan amid tensions with China MORE said in January.

But the unexpected move by the Supreme Court nonetheless caused a jolt among environmentalists, reminding them that the nine justices at the Supreme Court might interpret the law differently than they do.

“The Supreme Court took unprecedented action, so of course it makes everyone pause and reevaluate,” said John Coequyt, global climate policy director for the Sierra Club.

“I would suspect that people at EPA have already been thinking about that question” of what to do if the rule is overturned, said Justin Pidot, a University of Denver law professor and former Justice Department environmental attorney.

“The prospect that the Supreme Court is going to ultimately invalidate all or some of the Clean Power Plan has been a real possibility for a while,” he said.

The court’s stay order came in 5-4 vote, likely reflecting a break between the liberal and conservative wings.

The order from the justices was brief, with no explanation provided.

It is the first time the high court has stayed a regulation after a lower court refused to do so, and the first time the justices have issued a stay before any court heard the merits of the case.

Now the EPA cannot enforce any parts of the rule until the litigation is over, a major win for the states and energy interests who argued that, if the rule were allowed to proceed and later be overturned, they would experience irreparable harm.

But since the Clean Power Plan is such a major piece of the administration’s climate policy, its downfall would make it difficult to achieve the emissions cuts needed to slow global warming and to meet the country’s pledge under last year’s Paris climate agreement.

Complicating the matter is the timing. The Court of Appeals for the District of Columbia Circuit has to hear the case and rule before the Supreme Court takes it up, putting any decision about the next steps in the hands of the next president.

Additionally, if the Supreme Court rules against the EPA, its decision could be narrow — allowing the agency to try to rewrite the rule — or it could be broad, potentially prohibiting any future greenhouse gas regulations for power plants.

Environmentalists were generally tight-lipped about what a Plan B might look like, instead repeating their confidence that the rule will stand.

“We always are working on additional plans. There’s always multiple approaches, and we’re always looking for new ways of moving forward,” said David Doniger, climate program director at the Natural Resources Defense Council, which has been credited with forming the general structure of the rule that the EPA adopted.

Coequyt said the federal government is likely to look at the Clean Air Act for future climate regulations.

“I think the only tools that exist for the administration to achieve large-scale reductions that are in front of us are the tools under the Clean Air Act and other administrative options,” he said.

Michael Burger, executive director of Columbia University’s Sabin Center for Climate Change Law, said predicting the backup plan is hard without knowing much about the case against the rule.

“It’s a really hard prediction, because the case has not been briefed on the merits yet at the D.C. Circuit Court of Appeals or at the Supreme Court,” he said. “And the order the Supreme Court issued doesn’t say on what basis it issued the stay.”

Burger and colleagues put out a paper recently arguing that the Clean Air Act gives the EPA wide authority to implement new climate rules based on international harms from greenhouse gases, something the Obama administration has not said it plans to pursue.

But there’s wide agreement that green advocates probably cannot turn to Congress for a solution to climate change if the court isn’t friendly.

“I wouldn’t say we have plans to pass new legislation in the coming Congress,” said Doniger.

“It just doesn’t seem like the Republicans are interested in solving climate change,” Coequyt added.

The rule’s opponents don’t think there should be a replacement, and they predicted that the Supreme Court’s ruling would make it impossible for that to happen.

“We definitely think that the Clean Air Act does not provide EPA with the authority to do what they tried to do with the Clean Power Plan,” said Dan Simmons, vice president of policy at the American Energy Alliance, a free-market group backed by fossil fuel companies.

“If the Clean Power Plan is overturned, we don’t think it should be replaced,” he said, adding that the Clean Air Act “simply wasn't made to do what the environmentalists and EPA want it to do.”