Five weeks after the Environmental Protection Agency, EPA, revoked its own rule for regulating greenhouse gas emissions, states are pushing back in court with Connecticut helping to lead the charge.

A coalition of two dozen states and a dozen cities and counties on Thursday filed suit against the EPA and its administrator Lee Zeldin contending that rescinding the so-called endangerment finding is unlawful.

“The Trump EPA has ignored the law and ignored the science in its reckless rush to fulfill the wishes of the fossil fuel industry. Rescinding the Endangerment Finding means bigger profits for the world’s biggest polluters, while the rest of us are left more vulnerable to extreme weather, extreme heat and rising sea levels,” said a statement by Connecticut Attorney General William Tong, who is now also the president of the National Association of Attorneys General. Connecticut, along with California, New York and Massachusetts, are the lead plaintiffs.

“We’re going to fight with everything we’ve got,” Tong said.

The endangerment finding refers to a determination made by the EPA in 2009 that emissions of the greenhouse gases carbon dioxide, methane and four others endangered public health or welfare. Therefore, under the Clean Air Act (CAA), these emissions had to be regulated in motor vehicles in addition to the standard pollutants EPA had been regulating since 1970.

The endangerment finding followed a ruling by the U.S. Supreme Court two years earlier in the case known as Massachusetts v. EPA. It affirmed the meaning of the Clean Air Act’s language, even though greenhouse gases were not specifically mentioned. The finding has since been used to regulate emissions emanating from other fossil fuel-burning sources.

Greenhouse gas emissions are known to be the leading cause of the global warming that has led to the pronounced climate changes and weather extremes occurring world-wide. Recent studies show those effects to be progressing faster than previously anticipated. In the U.S., transportation is the largest greenhouse gas-emitting sector.

Battles over whether the Supreme Court decided the original case properly have continued in the intervening years and it is widely believed that challenges to the EPA recision, such as this latest one, will eventually land there again. A coalition of environmental groups, including the New England-based Conservation Law Foundation, filed a similar suit less than a week after the recision became final on February 12.

While three of the four justices who voted against it originally are still on the high court — Chief Justice John Roberts, Clarence Thomas and Samuel Alito — none of the five justices who voted for it remain and three more conservative justices are now in place.

Also notable is that the legal and environmental landscapes have changed dramatically in the last 20 years. The court has issued a number of rulings that may make challenges to the endangerment revocation more difficult. The Chevron decision in June of 2024 and, in 2022, the court’s use of the so-called major questions doctrine both generally now restrict what agencies like the EPA and the Energy Department can do without specific direction from Congress.

That is a large part of what the EPA relied on in revoking endangerment.

“For years, unelected bureaucrats twisted the Clean Air Act into something it was never meant to be,” Zeldin said in announcing the revocation. “We looked at the Clean Air Act; we looked at what the highest court in the land had said, and we used a very simple metric – if Congress didn’t authorize it, EPA shouldn’t be doing it. Congress never voted for these climate mandates. If Congress wants EPA to regulate the heck out of greenhouse gases emitted from motor vehicles, then Congress can clearly make that the law, which they haven’t done.”

At the time of the Chevron ruling, in particular, the court said it was not license to review or undo previous regulations.

In an emailed statement on Thursday, a spokesman for the EPA, called Thursday’s lawsuit “clearly motivated by politics,” and again pointed to the earlier Supreme Court decisions.

“EPA carefully considered and reevaluated the legal foundation of the 2009 Endangerment Finding, the text of the CAA, and the Endangerment Finding’s legality in light of subsequent legal developments and court decisions,” the statement read in part.

“The CAA does not provide EPA statutory authority to prescribe motor vehicle emission standards for the purpose of addressing global climate change concerns. In the absence of such authority, the Endangerment Finding is not valid, and EPA cannot retain the regulations that resulted from it. EPA is bound by the laws established by Congress, including under the CAA. Congress never intended to give EPA authority to impose GHG regulations for cars and trucks.”

Revoking endangerment has been in Trump’s sights going back to his first administration, though it was never attempted. But it was a stated goal of Project 2025 in the runup to the 2024 election and almost exactly a year ago was specifically slated for review in a package of some 30 policy rollbacks and goals announced by EPA on a single day.

In July, EPA announced endangerment would be rescinded, citing as justification a climate report by the Department of Energy, conducted in secret by five known climate skeptics, if not outright deniers, handpicked by Energy Secretary Chris Wright.

That report was shown to be riddled with errors and falsehoods, generating a massive pushback from climate scientists and nearly 60,000 comments — most in opposition.

In January, a U.S. district court in Massachusetts determined the report process was illegal.

When the repeal rule — which had garnered some 572,000 public comments, most in opposition — came out in February, that scientific report was not part of the reasoning.

At the time, Dan Esty, Connecticut’s first Department of Energy and Environmental Protection, DEEP, commissioner and now the Hillhouse Professor at Yale University serving in both Yale Law School and the School of the Environment, told the Connecticut Mirror that not citing a scientific basis in the repeal could be a legal vulnerability for the Trump administration.

“There was an extensive scientific review undertaken prior to the 2009 endangerment finding being issued,” Esty said. “[The Trump administration is] not free to ignore the science and say ‘we have a different view.’ They actually have to grapple with the science, and should they not do so in a serious way, then there would be a charge, and there will be a case brought in this regard that the decision is arbitrary and capricious.”

Earlier this month, the Environmental & Energy Law Program at Harvard Law School published an analysis of each argument by the EPA.

“Congress has had ample opportunity to address the 2009 Endangerment Finding and has declined to limit EPA’s authority. Additionally, since Massachusetts v EPA, the Supreme Court has never suggested that EPA lacked the authority to regulate greenhouse gases under the Clean Air Act. Rather, the Court has only focused on how EPA set regulations predicated on the finding,” a summary of the analysis stated.

In a statement, DEEP Commissioner Katie Dykes, who also trained as a lawyer, called the endangerment finding a well-reasoned, scientifically sound decision.

“Climate change is impacting our public health and safety today while imposing significant and increasing costs on Connecticut residents,” Dykes said. “To deny that reality increases the vulnerability of our state to these impacts today, and for future generations. Wildfires, historic flooding, cold snaps, and heat waves — these are no longer rare events; they’re our new normal.”

The lawsuit is filed in federal district court in Washington, D.C.