The Clean Air Act prohibits states from enacting their own pollution regulations for mobile sources, such as cars and trucks. California, however, is allowed to request a waiver from the EPA to create its own, stricter rules, since the state was already regulating vehicle pollution prior to the law’s passage. Once EPA approves one of California’s waivers, other states can subsequently adopt the stricter rules without requesting the same federal dispensation.
At first California’s air quality regulations were focused on more traditional health-harming pollutants such as ozone and particulate matter. But in 2005, California created the world’s first greenhouse gas emissions standards for cars, beginning with model year 2009, and requested a waiver from the EPA to enforce them.
At the time the EPA did not have any national standards for greenhouse gas emissions, but a seminal court case would soon force it to create them. In 2007, the Supreme Court ruled in Massachusetts vs. EPA that greenhouse gases are pollutants, as defined by the Clean Air Act, and that the agency has a duty to regulate them if it finds that they endanger public health or welfare. In 2009, the EPA under President Obama issued its “endangerment finding,” determining under a mountain of evidence that yes, greenhouse gases threaten public health, and prompting the development of the first federal climate standards for vehicles.
Now the Trump administration is trying to reverse that finding and put an end to federal climate regulations for vehicles once and for all.
At the same time, Trump has approved a move by Congress to rescind California’s latest waivers — although the move was legally dubious and the state is challenging it in court. Congress revoked the waivers under the Congressional Review Act, a law that allows the legislative branch to undo recently-finalized agency rules with a simple majority, despite previous rulings from the Government Accountability Office and the Senate parliamentarian that the waivers are not “rules” as defined by the Congressional Review Act.
But if the EPA says the Clean Air Act does not require the agency to regulate greenhouse gas emissions, does California even need those waivers?
“If I were the state of California and the endangerment finding gets rescinded, I would argue that there are no federal standards,” Ann Carlson, a professor of environmental law at the University of California, Los Angeles, and a former Biden administration official, told me. “There is, in the view of EPA, no need to regulate, and therefore states shouldn’t be preempted. I don’t know if that’s a winner, but I think it’s worth a try.”
Eliminating the endangerment finding would give states a solid argument for being able to regulate greenhouse emissions themselves, Carlson told me. But what would make the argument a “slam dunk,” she said, was if the Supreme Court ultimately overturned Massachusetts vs. EPA, and ruled that greenhouse gases are not air pollutants under the Clean Air Act after all.
The road to that outcome would be long and could veer in a different direction if Democrats retake the White House in 2028. First, the EPA has to put out its proposal for public comments and issue a final decision. That process alone could take a year. Then states or environmental groups would challenge the decision in the D.C. Circuit Court of Appeals, which would likely take another year to reach a ruling, putting us into mid-2027 or so.
While we won’t know what EPA’s exact argument will be until it issues the final decision, the justifications it has put forward so far are weak, according to experts. The agency’s main claim in the proposal is that it can only regulate pollutants that endanger health through local or regional exposures — the global problem of climate change doesn’t count. “This is hard to square with the Supreme Court’s decision in Massachusetts vs. EPA,” Harvard Law School’s Jody Freeman told me, “but EPA claims that doesn’t settle it.”
Carlson said she thinks there’s a pretty good chance the D.C. court would strike down the EPA’s attempt to reverse the endangerment finding. But the Trump administration would presumably appeal that ruling to the Supreme Court, which would present an opportunity for the conservative majority to overturn Massachusetts vs. EPA. Chief Justice Roberts, along with Justices Alito and Thomas, dissented in the original 2007 decision, while Justice Brett Kavanaugh, who was confirmed in 2018, “has made clear his disdain for using the Clean Air Act to regulate greenhouse gasses,” Carlson said.
There are a lot of open questions about what would happen next. If the case is still ongoing by 2029, the next administration could decide to withdraw it, or simply to reinstate the endangerment finding.
Another wrinkle: The Inflation Reduction Act amended the Clean Air Act to explicitly define greenhouse gases as pollutants under new sections of the law. That could make it harder for the Supreme Court to overturn Massachusetts vs. EPA, although the court has previously held that different sections of the law may define “air pollutant” differently.
Finally, even if the case goes all the way to the point of reversing Massachusetts vs. EPA, there would probably still need to be additional litigation to clarify what states can do, Atid Kimelman, a clean vehicles attorney for the Natural Resources Defense Council, told me.
He noted that the federal government might argue that regardless of the fact that the EPA isn’t regulating greenhouse gases, states are still preempted, as the whole point of the preemption in the Clean Air Act is to make sure that the country doesn’t have 50 different standards for motor vehicles. Another hurdle might be that the federal Energy Policy Conservation Act, which authorizes the Department of Transportation to set fuel economy standards, also preempts states from adopting their own vehicle regulations.
“This is somewhat novel territory that hasn’t really played out in courts,” he said. “These are arguments that have to be tested.”
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