The Environmental Protection Agency (EPA) has long regulated both mobile (cars, trucks and aircrafts) and stationary (industry, buildings,) sources of air pollution under the Clean Air Act (CAA) of 1970. Widely regarded as the nation’s most influential environmental law, the CAA established the EPA and set the legal foundation for major reductions in air pollution. Early successes included regulating tailpipe emissions, introducing technologies like catalytic converters, and mandating fuel changes such as unleaded gasoline.

However, greenhouse gases (GHGs)—such as carbon dioxide and methane—were not regulated by the EPA until after the April 2007 Supreme Court decision in Massachusetts v. EPA. Before this decision, EPA’s regulatory focus was limited to pollutants causing smog, soot, and other pollutants with immediate health impacts, not the heat-trapping gases scientifically linked to global warming. In the landmark case of Massachusetts v. EPA, the Supreme Court held that EPA does have the authority to regulate greenhouse gases. Further, since Section 202(a)(1) of the CAA requires the EPA Administrator to set emission standards for air pollutants from motor vehicles if they’re judged to endanger public health or welfare, the ruling directed EPA to formally assess whether GHG emissions do endanger public health or welfare.

In response to this finding, the EPA under President George W. Bush spent about six months conducting intensive analysis and, according to EPA staff disclosures to Congress, EPA Administrator Stephen Johnson signed off on his agency’s positive endangerment finding and transmitted the endangerment finding for White House review. Despite the directive from the Supreme Court and the positive finding from the EPA that greenhouse gases endanger public welfare, the Bush Administration refused to release the endangerment finding. Instead the Administration released an Advance Notice of Proposed Rulemaking seeking comment on their concern that the CAA was designed and intended “to control regional pollutants that cause direct health effects,” and that therefore using the law to regulate GHG emissions “would inevitably result in a very complicated, time-consuming, and, likely, convoluted set of regulations” that would have a “potentially damaging effect on jobs and the U.S. economy.”

After the election of President Barack Obama, the EPA again undertook an extensive public comment process and ultimately issued the Endangerment Finding in 2009, finding that greenhouse gases endanger both public health and welfare. This determination required the EPA, for the first time, to regulate carbon emissions from new vehicles and engines. The Endangerment Finding underpins significant federal climate action such as clean car and truck standards targeting GHG-driven warming, providing the legal and scientific basis for limits on transportation emissions.

On Tuesday July 29, in response to a Day One Executive Order from President Trump, and after signaling his intent since March, the EPA Administrator Lee Zeldin formally proposed to rescind the Endangerment Finding. Along with reversing the finding on the effect of greenhouse gases on public health and welfare, the proposed rule also would eliminate all resulting GHG emission standards for new motor vehicles and engines, including the light-duty, medium-duty, and heavy-duty vehicle and engine standards for model years (MY) 2012 to 2027 and beyond.

Responses and Legal Hurdles

Administrator Zeldin was joined at the announcement only by one sitting member of Congress, Representative Jim Baird (R-IN), and by Governor Mike Braun. While the move is likely to be generally welcomed by Republican members of Congress, Senate Environment and Public Works Committee Chair Shelley Moore Capito (R-WV) “applauded the Administration’s efforts” but also acknowledged that it will certainly be “tested in the courts and probably go all the way to the Supreme Court.”

Indeed the legal hurdles to finalize this proposal to rescind the endangerment finding will be significant and recent changes to agency deference may cut against the agency’s efforts. EPA’s first task will be to accept public comments, the docket for which is open until September 15. The agency will also have virtual public hearings on the proposal on August 19 and 20. Given the significance of the finding, the agency could anticipate hundreds of thousands of comments, if not millions. (The record for number of comments on a rulemaking was 22 million comments submitted to the 2017 proposed rule on net neutrality from the Federal Communications Commission.) The task of responding to these comments while avoiding arbitrariness will be made more challenging by the agency’s cutting the size of its workforce by at least 23% and closing its Office of Research and Development.

The agency will be defending their proposal in the context of the Supreme Court’s 2024 decision in Loper Bright Enterprises v. Raimondo—which overturned the Chevron doctrine that stated that a court must defer to the agency’s interpretation of an ambiguous statute. Importantly, the Supreme Court in Massachusetts v. EPA concluded that “The statute is unambiguous” with regard to the question of whether greenhouse gases are air pollutants, stating that such gases “are without a doubt ‘physical and chemical … substances which are emitted into … the ambient air’” and therefore meet the statutory definition in the CAA. In this case, the EPA will have to argue that the facts have changed since their 2009 finding, regarding the relationship between greenhouse gases and public health and welfare. The NPRM notes that “the Administrator has serious concerns that many of the scientific underpinnings of the Endangerment Finding are materially weaker than previously believed” and that the “Administrator no longer has confidence in the assumptions, methodology, and conclusions in the Endangerment Finding in light of the scientific record”.

In substantiating this claim, the EPA points to a report released this week by the Department of Energy entitled “A Critical Review of Impacts of Greenhouse Gas Emissions on the U.S. Climate” that argues that elevated concentrations of CO2 will enhance agricultural productivity and that the costs of mitigating climate impacts will exceed the benefits. (The report is written by five academics well known for their long denial of the relationship between greenhouse gases and climate change, and has been fact checked by multiple sources and skewered by climate scientists as misrepresenting and cherry-picking data.) Despite this report, in legal proceedings the agency will be confronted with data compiled by the Intergovernmental Panel on Climate Change which has found that in the years since 2009 the impacts of climate change on public health and welfare have become more significant and that the relationship between greenhouse gases and climate change has become more certain.

The Endangerment Finding is often described as the legal “on-switch” for modern U.S. climate policy—analogous to how Miranda rights transformed police procedure or how Title IX reshaped gender equity in education and sports. It has fundamentally influenced not just regulatory standards but also the kinds of vehicles Americans drive, and the way manufacturers design vehicles. This itself may constitute a barrier to repeal, as the agency will have to consider the effect of repeal on the hybrid and electric vehicle manufacturers that will be harmed by the new finding. EPA’s NPRM notes that they seek “comment on the nature and extent of any reliance interests that may have arisen from our assertion of regulatory authority over GHG emissions from new motor vehicles and engines and is committed to assessing any such interests, determining whether they are significant, and weighing such interests against competing rationales, as required by law.”

One perennial concern with changes to emissions standards is the impact on federal preemption. The proposed rule indicates that this “repeal also would not impact Federal preemption of emission standards for new motor vehicle and engine emission standards.” Per the proposed rule: “Because new motor vehicles and engines currently subject to GHG emission standards would remain subject to Title II of the CAA, the statute would continue to preempt “any” State or local “standard relating to the control of emissions.” However, California has specific authority under the CAA Section 209(b) to obtain a waiver of the federal preemption therefore the effect on California and other states’ ability to set their own GHG regulations of mobile sources will likely continue to be a subject of debate and litigation. (In any case though, the waiver request would have to be approved by the EPA, which is unlikely in the current administration.)

The EPA’s proposed rollback of the light duty vehicle standards will not automatically affect the Corporate Average Fuel Economy standards, which have recently been weakened by DOT Secretary Sean Duffy. CAFE, regulated by the National Highway Traffic Safety Administration (NHTSA) under the Energy Policy and Conservation Act (EPCA) of 1975, is primarily aimed at reducing oil consumption by improving fleet fuel economy. By contrast, the EPA’s authority focuses on cutting GHG emissions under the Clean Air Act. Despite some overlapping enforcement mechanisms, the legal bases, primary metrics (GHG emissions vs. fuel consumption), and technology incentives differ. In the recently enacted budget reconciliation law, Congress removed the penalties for violations of both CAFE and emissions standards, undermining the enforcement of both regulations. Recent policy moves by both agencies are widely viewed as steps to disincentivize electrification within the transportation industry.

Revoking the Endangerment Finding would sharply curtail federal climate regulation for transportation, reduce incentives for cleaner and electric vehicles, and significantly hinder future efforts to cut carbon pollution nationwide. Existing regulations could become vulnerable to legal challenges and future administrations would lose crucial tools to reduce transportation emissions, which are the country’s largest source of climate pollution. In addition to public health and welfare effects, the change would also impact the pace of automotive innovation, the industry’s ability to compete internationally, and the nation’s role in global climate leadership.