The EPA’s decision to focus its recent approval of California vehicle emissions regulations on traditional air pollutants instead of greenhouse gases could complicate any moves by an incoming Trump administration to undo the action, legal scholars say.

The Environmental Protection Agency’s justification for granting the two waivers—which allow California to supercharge zero-emission vehicle deliveries and strictly curb tailpipe emissions—mentioned climate-warming greenhouse gases but largely hinged on reducing soot and smog.

“The Biden administration tried to be very meticulous in drafting the waiver decisions in a way to best withstand judicial attack,” said Michael Gerrard, founder and faculty director of Columbia Law School’s Sabin Center for Climate Change Law.

Pollutants like particulate matter, nitrogen oxides, and hydrocarbons—the precursors to ground-level ozone—aren’t as “politically sensitive” as greenhouse gases, he said.

The Clean Air Act generally forbids states from regulating motor vehicle emissions. But the law lets California, which has some of the nation’s worst air quality, apply for special permission in the form of a waiver to regulate emissions more strictly than the EPA. A handful of other waivers, including an overarching Advanced Clean Fleets regulation, are still pending.

“EPA continues reviewing additional waiver requests from California and is working to ensure its decisions are durable and grounded by law,” Remmington Belford, EPA’s press secretary, said in an emailed statement.

Other states can choose to follow federal standards or opt into California’s standards. To date, about a dozen states have adopted California’s most-recent passenger vehicle rule, known as Advanced Clean Cars II. The EPA’s decision to grant the ACCII waiver allows California standards to be enforced in those other states.

Legal Challenges Loom

If President-elect Donald Trump keeps his promise to revoke the waivers, he’ll need to lay out a legal reasoning for doing so. In this case, “that’s actually trickier than one might think,” because the EPA decided that California primarily needs these waivers to curb traditional pollution sources, not greenhouse gases, said Ann Carlson, founding director of the Emmett Institute on Climate Change and the Environment at UCLA School of Law.

The country doesn’t yet have a judicial opinion on whether EPA is legally required to grant the waivers, Carlson said.

Trump rescinded California’s Advanced Clean Cars waiver during his first term, citing consumer costs. President Joe Biden reinstated it shortly after taking office, before a court could hear arguments challenging Trump’s decision. Biden’s support for the waiver is still tied up in litigation.

“The American people re-elected President Trump by a resounding margin giving him a mandate to implement the promises he made on the campaign trail, including stopping attacks on gas-powered cars,” Trump-Vance transition spokesperson Karoline Leavitt said in an emailed statement. “When he takes office, President Trump will support the auto industry, allowing space for both gas-powered cars AND electric vehicles.”

The Supreme Court recently decided to hear a case about whether fuel producers have standing to sue over California’s vehicle pollution rules. But it denied another petition by 17 red states, who asked the high court to decide whether giving California special power over vehicle emissions was unconstitutional. Congress violated states’ “equal sovereignty” by “playing favorites,” the states, led by Ohio, said in their July petition for writ of certiorari.

More than likely, a “well armed, well funded” group of red states will challenge EPA’s waiver decision this time, too, said Mary Nichols, who was chair of the California Air Resources Board during Trump’s first term.

But if courts look at climate change as different from the air pollution that was the original topic of the Clean Air Act, then it’s helpful that the EPA’s recent waiver decisions are grounded in California’s need to achieve National Ambient Air Quality Standards, she said.

“Any court that isn’t looking to deliberately try to overturn the waivers is going to be more inclined” to keep them, Nichols said.

Agency Deference and Loper Bright

Concerns about courts diminishing agency expertise, especially in the environment space, are front and center as the year closes out.

The US Supreme Court this summer overturned the longstanding Chevron doctrine, which empowered agencies to successfully defend their rulemaking in court. Ironically, parts of that landmark decision, Loper Bright Enterprises v. Raimondo, could help California if the waiver issue winds up in front of judges, Carlson said.

Loper Bright found that courts can interpret ambiguous statutory language—but also that they should look to longstanding agency interpretations, and whether those interpretations have been consistent over time, when deciding what a law means, Carlson wrote in a recent blog post for Legal Planet.

The EPA has a long history of granting California’s Clean Air Act waivers. The first waiver pertained to vehicle model year 1969, and the agency has granted California special permission to regulate its own emissions almost every time since due to the state’s “extraordinary” conditions that make pollution worse than in other parts of the country.

But even with the Biden EPA’s “legally solid” defense, success isn’t guaranteed if the matter reaches the high court, Carlson said.

“The Supreme Court is pretty hostile toward environmental regulation,” so it’s “not foolproof,” she said.