Pres. Donald Trump’s administration has announced plans to dismantle an array of federal efforts to fight global warming, including a program to reduce carbon emissions from coal-fired power plants, a rule limiting methane gas leaks and a mandate that aggressively boosts auto emissions standards.

But Trump officials face a major roadblock in their efforts, legal scholars say. It is the U.S. Environmental Protection Agency’s 2009 formal “endangerment finding,” which states carbon dioxide and five other greenhouse gases emitted from smokestacks and other man-made sources “threaten the public health and welfare of current and future generations.” This agency rule, supported by two Supreme Court decisions, legally compels the government to do exactly what its new leaders want to avoid: regulate greenhouse gases. Although EPA Administrator Scott Pruitt publicly doubts a connection between human-produced carbon emissions and global warming, any attempt to undo this rule “would be walking into a legal buzz saw,” says Michael Gerrard, faculty director of the Sabin Center for Climate Change Law at Columbia University. Endangerment is “the linchpin for everything—all of the carbon regulation under the Clean Air Act,” says Patrick Parenteau, a professor of environmental law at Vermont Law School.

The rule’s fundamental power is exactly why Pruitt has to remove it, says Myron Ebell, who oversaw the Trump transition team at the EPA. “You can’t just take out the flowers—you have to take out the roots—starting with the endangerment finding,” says Ebell, a senior fellow at the conservative think tank the Competitive Enterprise Institute. “You can undo the Obama climate agenda on the surface by reopening the Clean Power Plant rule, the methane rule, rescinding the [auto emissions] standards and so on. But the underlying foundation remains.” The conservative Web site Breitbart, read widely among Trump’s supporters and still tied to its former publisher, White House adviser Steve Bannon, has attacked Pruitt as a political careerist for reportedly resisting pressure to revoke the finding.

The rule rests on a 2007 Supreme Court decision in the case Massachusetts v. EPA, which determined the agency has the authority under the Clean Air Act to regulate greenhouse gases. When the finding itself was later challenged, the Court upheld it. The endangerment finding prevents Pruitt from ignoring climate change or eliminating greenhouse gas regulations outright. The EPA can attempt to water down these standards and regulations, perhaps substantially. But Pruitt “would have to come up with a scientific basis for saying that greenhouse gas emissions do not in fact pose a threat to public health and welfare,” Gerrard says. “That would be a very difficult finding, considering every court that has addressed the issue of the science of climate change has found there to be a solid factual, scientific basis for it.”

To begin to remove the endangerment rule, the EPA would have to go through a formal rule-making process. That means inviting public comments, reviewing available evidence and scientifically justifying every point. Formulating and then defending such a document in court would be a big challenge, given it cuts against the legal and scientific consensus linking carbon to climate change. Even Ebell concedes this is a formidable obstacle. “That’s why a lot of people on our side say it’s not worth the trouble,” he says. “The people who disagree with me are not nuts—they are making substantial arguments for why we should not do it.”

The endangerment finding has its roots in the waning days of the Clinton administration, when then–EPA General Counsel Jonathan Cannon drafted a legal memo stating the agency had the authority to regulate carbon emissions. At the time this was a novel and counterintuitive idea. CO2 is a ubiquitous, naturally occurring gas, essential to photosynthesis and other basic processes of life on Earth. It’s not poisonous like smog and other dangerous pollutants targeted by the Clean Air Act. “CO2 is a different sort of pollutant than many that the EPA regulates,” says Cannon, now a professor at the University of Virginia School of Law. “Its effects are felt over time through the climate system, not as immediate effects on one’s lungs or physical systems.

But the Clean Air Act “has a very broad definition of what a pollutant can be and what harm a pollutant causes,” says George Kimbrell, legal director of the International Center for Technology Assessment and the Center for Food Safety, two related groups among a coalition of environmental organizations that formally petitioned the EPA to regulate carbon in 1999. The law defines “air pollutant” as "any air pollution agent or combination of such agents, including any physical, chemical, biological, radioactive...substance or matter, which is emitted into or otherwise enters the ambient air.” According to Kimbrell, “The breadth of that language suggested greenhouse gas emissions would qualify under the statute.”

The language prompted a lawsuit from states and small environmental groups, during the George W. Bush administration, to sue the EPA to force it to regulate carbon. The result was the Supreme Court’s 5–4 2007 Massachusetts decision. Following that ruling, the endangerment finding then spelled out the legal rationale and the scientific basis for regulation.

What can the Trump administration do to get out of this regulatory box? It could push Congress to amend the Clean Air Act to explicitly exclude carbon dioxide and other greenhouse gases from the list of air pollutants. But even if it passed the Republican-dominated House, Parenteau notes, such a bill could be effectively opposed by Democrats in the Senate, who have enough votes to hold up or change legislation. . The EPA could also target climate rules not based on the endangerment finding, such as procedures for monitoring and reporting greenhouse gases, according to Gerrard.

The most likely outcome, legal scholars say, is a series of incremental battles in which the administration and Congress try to weaken individual climate rules and enforcement—while those efforts are repeatedly challenged in court by states and environmental groups hoping to run out the clock on the Trump administration. “One reason the endangerment finding is important,” Cannon says, “is that, should administrations change, it provides the basis for further climate initiatives.”