The head of the Environmental Protection Agency (EPA) is fueling speculation that he could try to repeal the lynchpin of the federal government’s climate change regulations.

In an interview Thursday with CNBC, Scott Pruitt cast doubt on his own agency’s 2009 conclusion that greenhouse gases “endanger both the public health and the public welfare of current and future generations.”

The so-called endangerment finding was the backbone of the Obama administration’s climate change regulations. Under Obama, the EPA argued that the 2009 finding compelled it to issue greenhouse gas emissions limits for sectors like cars, trucks and power plants.

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But as Pruitt and President Trump look to unwind Obama’s major climate policies, the endangerment finding might be imperiled.

“You know what’s interesting about the situation with CO2, Joe, is we’ve had a Supreme Court decision in 2007 and then the endangerment finding that you’re making reference to in 2009,” Pruitt told CNBC host Joe Kernan, referring to the Supreme Court’s Massachusetts v. EPA decision — the court ruled that greenhouse gases are air pollutants under the Clean Air Act and the EPA has to determine whether they should be regulated.

“Nowhere in the continuum, nowhere in the equation, has Congress spoken. The legislative branch has not addressed this issue at all,” Pruitt said.

“The decision in 2007 was not that the EPA had to regulate. The decision in 2007 was they needed to make a decision.”

Pruitt also stated that “I would not agree that [carbon dioxide is] a primary contributor to the global warming that we see,” a comment that is at odds with the scientific consensus on climate change.

His comments contrast with what he told the Senate Environment and Public Works Committee at his January confirmation hearing.

“That is the law of the land,” he said of the Supreme Court ruling and the endangerment finding.

In March 2016, Trump said in a survey from the fossil fuel industry-funded American Energy Alliance that he would review the finding if elected.

Repealing the finding would undermine the basis for some of Obama’s environmental agenda and wipe out a major argument in favor of government regulation of such emissions.

But experts say rolling back the endangerment finding would be a difficult task. The EPA would have to undertake a lengthy rulemaking process — complete with notice and public comment — gather an extensive body of science to justify the change, and then face nearly certain court challenges.

“This would be a very steep climb, because what he said goes against decades of science showing that carbon dioxide is a greenhouse gas and is warming the planet,” said Noah Sachs, a law professor a the University of Richmond and member scholar at the Center for Progressive Reform.

Neither Pruitt nor Trump has committed to a review since the inauguration. But Pruitt’s Thursday comments are stirring hopes and fears.

“I think that Administrator Pruitt’s comments are suggestive of their thinking, and eventually they will decide that it is really necessary to reopen the finding,” said Myron Ebell, an outspoken climate change skeptic who leads energy and environment policy at the conservative Competitive Enterprise Institute. Ebell also led the transition efforts for Trump at the EPA.

“I would be hopeful that they do an honest assessment of it. The Obama endangerment finding is a joke,” said Rep. Joe Barton (R-Texas), vice chairman of the House Energy and Commerce Committee.

“It was shoddy, it was full of half-facts. So if Administrator Pruitt wants to go back and do it right, God bless him, and I’ll support him.”

Rep. Frank Pallone Jr. (N.J.), the Energy Committee’s top Democrat, said he wouldn’t be surprised if Pruitt, who battled the EPA in court as Oklahoma’s attorney general, went after the finding.

“I’m convinced that Scott Pruitt was sent to EPA in order to undermine all the environmental protections and international agreements however he thinks he can accomplish things,” he said.

“I don’t think anything’s off-limits for him. I don’t think he even believes in the EPA.”

To roll the finding back, the administration would likely have to provide scientific proof that greenhouse gases do not harm the public health and welfare, or that regulatory action could not mitigate that harm.

Carbon dioxide is not a pollutant in the same way that particulate matter or nitrogen oxides are, since breathing it in normal amounts is not dangerous.

But the Supreme Court found that the Clean Air Act had a “capacious” definition of “air pollutant,” and that due to climate change, carbon dioxide fit well within that category.

The finding has already withstood court scrutiny. A coalition of states and industry groups sued the EPA over the 2009 finding, and it was upheld by the Court of Appeals for the District of Columbia in 2012. The Supreme Court declined to take the case.

Separately, Sen. Lisa Murkowski Lisa Ann MurkowskiGOP ramps up attacks on Democrats over talk of nixing filibuster OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' House Democrats tee up vote on climate-focused energy bill next week MORE (R-Alaska) and Rep. Fred Upton (R-Mich.) tried unsuccessfully to have it overturned in Congress.

“It’s impossible to compile a record that would undo the endangerment finding, because it was based on sound science,” said Joanne Spalding, the Sierra Club’s head climate attorney.

“It is based on the scientific consensus that has been building and strengthening over the years since the endangerment finding was made,” she said. “Everything points to reaffirming that finding. There is no science out there that would allow the agency to withdraw it.”

Sachs said federal courts would scrutinize the scientific backing for any withdrawal.

“In administrative law, agencies are allowed to change their minds, that’s the natural flow of politics,” he said. “But they have to support their new positions with adequate scientific evidence. And I don’t think that the EPA can do that in this case.”

The Trump administration’s allies are confident that science is on their side.

Ebell said the 2009 finding was not compliant with the Information Quality Act because it used climate models that are not reliable.

“No matter how much they talk about how much the models are improving, it’s still true that the models cannot yield predictions of the future, because the climate is a nonlinear, chaotic system,” he said.

To Barton, the scientific questions have a simple answer: Carbon dioxide is not dangerous.

“I don’t think it’s a harm to humankind. CO2 is a naturally occurring compound. I am creating it as I’m talking to you,” he said. “We’ve got to believe all those esoteric theory about what’s going to happen 100 years from now to think it is.”