Donald Trump has called climate change a “hoax” and said he plans to unburden American industries from Obama-era requirements to limit emissions of carbon dioxide and the other greenhouse gases causing the planet to warm.

In the crosshairs of his agenda — according to written statements the Trump campaign made shortly before the Nov. 8 election — is a 2009 determination by the U.S. Environmental Protection Agency that greenhouse gases are dangerous to human health and security, a ruling that serves as the foundation for the EPA’s efforts to curb climate-damaging emissions and that affects its governing of everything from automobile exhaust to power plants to refrigerators.

But overturning that rule — known as the “endangerment finding” — might prove difficult, if not impossible, to accomplish with anything like the simple administrative stroke of the pen, according to legal experts.

“Attempting to overturn the endangerment finding would be like running toward a machine gun,” said Michael Gerrard, director of the Sabin Center for Climate Change Law at Columbia University. “Scientific support was very strong when it was issued in 2009; it has become much stronger since then.”

The rule represented the direct evolution of a 2007 decision by the U.S. Supreme Court, one that determined that the EPA not only had the authority to regulate climate gases as pollutants under the federal Clean Air Act, but was obligated to do so. The court directed the EPA to examine the scientific evidence and determine if greenhouse gases posed a threat to the public.

The EPA did that — examining everything from the potential for more damaging hurricanes, to death rates due to ozone and heat exposure, to deadly exposure to pathogens — and concluded in unambiguous terms that there was “compelling” reason to believe the gases threaten the health of Americans, and that the threat would get worse.

The agency’s conclusion rested on thousands of pages of peer-reviewed research, from the Intergovernmental Panel on Climate Change, from the U.S. Global Climate Research Program, and from the National Research Council. The agency wrote its rules and subjected them to public criticism. The public submitted voluminous comments, all of which were reviewed by the EPA before it issued a final rule.

The original rule-writing process alone took 14 months, (or 10 years if you count the arguments leading up to the 2007 Supreme Court fight). After its completion, The American Chemistry Council and other groups petitioned the U.S. Court of Appeals in the District of Columbia to review the validity of the finding. In 2012, that court upheld the EPA’s endangerment rule, finding that the agency’s interpretation of its authority and of its obligation to regulate carbon dioxide “is unambiguously correct.”

In order to effectively eliminate the rule now, one former EPA attorney told ProPublica, the Trump administration would likely have to reargue the original decision, including the merits of the scientific evidence, and then build on it.

Per the Supreme Court’s 2007 decision, the new administration would have to use existing evidence to prove no risk is posed by climate gases. Then it would have to explain how new information that has emerged since 2009 — a period including the hottest years on record, some of the biggest storms and driest droughts, and destabilizing mass human migration — demonstrate how the EPA erred in its 2009 conclusion.

It’s a steep hill to climb, and, as Gerrard points out, would inevitably lead to a fresh wave of lawsuits both against the EPA and against polluters directly in courts across the country.

In the meantime, the EPA remains obligated to enforce the climate-related rules on record.

There is, however, another option, but not one the Trump administration can execute alone: Congress could render the Supreme Court’s 2007 decision moot by passing new legislation amending the Clean Air Act itself — changing the landmark 1970 environmental law so as to explicitly exclude its oversight of carbon dioxide, methane and the pollutants known to be driving climate change.

Such a step would inevitably invite filibuster resistance not only among Democrats, but among Republicans concerned about climate risks, said Gerrard.

David Goldston, director of public affairs at the Natural Resources Defense Council, said Trump and other Republicans may try and chip away at environmental enforcement or regulations that are based on the finding, but not likely the finding itself.

“There’s a reason why, despite all the ideological gnashing of teeth for 20 years at least, that Republicans haven’t actually been able to touch the Clean Air Act successfully,” Goldston said. “It passed by a wide bipartisan margin, and it turns out, it’s almost like an article of faith with the public.”