The first shot has been fired in the legal war over the Environmental Protection Agency’s rollback of its “endangerment finding,” which had been the foundation for federal climate regulations.

Environmental and health groups filed a lawsuit Wednesday morning in the U.S. Court of Appeals for the District of Columbia Circuit, arguing that the EPA’s move to eliminate limits on greenhouse gases from vehicles, and potentially other sources, was illegal. The suit was triggered by last week’s decision by the EPA to kill one of its key scientific conclusions, the endangerment finding, which says that greenhouse gases harm public health. The finding had formed the basis for climate regulations in the United States.

The lawsuit claims that the agency is rehashing arguments that the Supreme Court already considered, and rejected, in a landmark 2007 case, Massachusetts v. EPA. The issue is likely to end up back before the Supreme Court, which is now far more conservative.

In the 2007 case, the justices ruled that the EPA was required to issue a scientific determination as to whether greenhouse gases were a threat to public health under the 1970 Clean Air Act and to regulate them if they were. As a result, two years later, in 2009, the EPA issued the endangerment finding, allowing the government to limit greenhouse gas emissions, which cause climate change.

“With this action, EPA flips its mission on its head,” said Hana Vizcarra, a senior lawyer at the nonprofit Earthjustice, which is representing six groups in the lawsuit. “It abandons its core mandate to protect human health and the environment to boost polluting industries and attempts to rewrite the law in order to do so.”

There was some debate about escalating a legal fight to the Supreme Court, given its ideological orientation. But William Piermattei, the managing director of the Environmental Law Program at the University of Maryland Francis King Carey School of Law, said in an interview that environmentalists “must challenge this.”

If they don’t, “then you are agreeing that we should not regulate greenhouse gases under the Clean Air Act, full stop,” Piermattei said. “I don’t think any environmentalist, I think a majority of the public, does not agree with that statement at all.”

If the environmentalists, and the Democratic states that have vowed to file similar suits, lose, the onus would be on Congress to pass a national law to regulate greenhouse gases.

The idea that Congress should act was one of the arguments the EPA made in its final rule on the finding, announced at the White House on Thursday by the agency’s administrator, Lee Zeldin, and President Donald Trump. Zeldin has said that since 2007, two different Supreme Court decisions have changed the legal calculus about the endangerment finding.

One ruling found that Congress must weigh in on “major questions” that have significant political and economic implications, as the EPA’s rules do. Another found that the courts do not have to show so much deference to the decisions of federal agencies.

The EPA declined to comment, saying it had a long-standing practice of refraining from making statements on pending litigation.

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This article originally appeared in The New York Times.

By Karen Zraick/Mark Abramson
c. 2026 The New York Times Company