In relation to the Clean Air Act ("CAA" or "Act") of 1963, an Endangerment Finding is an analysis of whether changes to the atmosphere endanger the public. The Act includes a requirement for EPA Administrators to issue rules to set (and from time to time revise) standards for new motor vehicles or new motor vehicle engines, to regulate emissions of any air pollution which, in the Administrator's opinion, may reasonably be anticipated to endanger public health or public welfare. After the IPCC Second Assessment Report of 1995 described increasing evidence of human caused climate change, petitioners asked the United States Environmental Protection Agency (EPA) to rule that greenhouse gases (GHGs) endanger public health and welfare.42 U.S.C. § 4521(a)(1)
Its refusal, under the George W. Bush Administration, led to a series of court cases culminating in the United States Supreme Court's 2007 Massachusetts v. Environmental Protection Agency decision that the plaintiff had standing, and the act required the EPA to make the analysis. It took another 2 years, until incoming EPA administrator Lisa Jackson decided the so called Endangerment Finding in 2009, which formed the basis for regulation on January 2, 2011. Standards for mobile sources have since been established pursuant to Section 202 of the CAA, and GHGs from stationary sources have been controlled under the authority of Part C of Title I of the Act. In June 2012, the basis for regulations was upheld in the United States Court of Appeals for the District of Columbia .Court Backs E.P.A. Over Emissions Limits Intended to Reduce Global Warming June 26, 2012D.C. appeals court upholds EPA regulations to fight global warming by Darryl Fears, June 26, 2012
In 2025, EPA administrator Lee Zeldin under the second Trump administration announced intent to deregulate greenhouse gases, and in July proposed to undo the endangerment finding. Trump and Zeldin announced revocation of the finding on February 12, 2026,McGrath, M., Trump revokes landmark ruling that greenhouse gases endanger public health, BBC News, published on 12 February 2026, accessed on 13 February 2026 and the "Final Rule: Rescission of the Greenhouse Gas Endangerment Finding and Motor Vehicle Greenhouse Gas Emission Standards Under the Clean Air Act" was published that day on the EPA website, to become effective 60 days after publication in the Federal Register. 1.37 MB, pre-publication, signed February 12, 2026, "DATES: This final action is effective on [INSERT DATE 60 DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER]".
Various regional climate change initiatives in the United States have been undertaken by state and local governments, in addition to federal Clean Air Act regulations.
History Initial petition 1999 and initial denial 2003 Section 202(a)(1) of the Clean Air Act requires the Administrator of the EPA to establish standards "applicable to the emission of any air pollutant from... new motor vehicles or new motor vehicle engines, which in [her] judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare" (emphasis added). On October 20, 1999, the International Center for Technology Assessment (ICTA) and several other parties (petitioners) petitioned the EPA to regulate greenhouse gases from new motor vehicles under the Act.International Center for Technology Assessment. (1999). Petition for Rulemaking and Collateral Relief Seeking the Regulation of Greenhouse Gas Emissions from New Motor Vehicles Under [Section] 202 of the Clean Air Act. Retrieved April 9, 2011 from http://www.icta.org/doc/ghgpet2.pdf The petitioners argued that carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and hydrofluorocarbons meet the definition of an air pollutant under section 302(g) of the Act, and that statements made by the EPA, other federal agencies, and the United Nations Intergovernmental Panel on Climate Change (IPCC) amounted to a finding that these pollutants are reasonably anticipated to endanger public health and welfare. Based on these factors, the petitioners asserted that EPA had a mandatory duty to regulate greenhouse gases under section 202 of the Act, and asked the agency to carry out that duty.
On September 8, 2003, EPA denied the ICTA petition on the ground that it did not have authority under the CAA to promulgate regulations to address global climate change and that CO2 and other GHGs therefore could not be considered air pollutants under the provisions of the CAA, including section 202.68 Fed. Reg. 173 (8 September 2003) pp. 52922, 52928 The agency further stated that even if it had the authority to regulate GHGs from motor vehicles, it would decline to do so as a matter of policy. The agency maintained that regulating motor vehicle GHG emissions would neither address the global problem effectively, nor be consistent with President Bush's policies for addressing climate change, which centered on non-regulatory efforts such as voluntary reductions in GHGs, public-private partnerships aimed at reducing the economy's reliance on fossil fuels, and research to probe into scientific uncertainties regarding climate change.68 Fed. Reg. 52929-52932
Supreme Court requires regulation, 2006 The agency's action on the ICTA petition set the stage for a prolonged legal battle, which was eventually argued before the Supreme Court on November 29, 2006.
In a 5–4 decision in Massachusetts v. Environmental Protection Agency, the Supreme Court held that "greenhouse gases fit well within the Act's capacious definition of 'air pollutant' " and that EPA therefore has statutory authority to regulate GHG emissions from new motor vehicles.549 U.S. 497 (2007); https://www.supremecourt.gov/opinions/06pdf/05-1120.pdf The court further ruled that "policy judgments have nothing to do with whether greenhouse gas emissions contribute to climate change and do not amount to a reasoned justification for declining to form a scientific judgment." In EPA's view, this required the agency to make a positive or negative endangerment finding under Section 202(a) of the CAA whether greenhouse gas emissions from new motor vehicles "cause or contribute to air pollution which may be reasonably be anticipated to endanger public health or welfare, or whether the science is too uncertain to make a reasoned decision" .74 Fed. Reg. 239 (15 December 2009) p. 66501
EPA endangerment findings, 2009 On December 7, 2009, EPA Administrator Lisa Jackson found that, under section 202(a) of the Clean Air Act, greenhouse gases threaten both the public health and the public welfare, and that greenhouse gas emissions from motor vehicles contribute to that threat. The two distinct findings were:
1) The Endangerment Finding, in which the Administrator found that the mix of atmospheric concentrations of six key, well-mixed greenhouse gases threatens both the public health and the public welfare of current and future generations. These six greenhouse gases are:
carbon dioxide (CO2) methane (CH4) nitrous oxide (N2O) hydrofluorocarbons (HFCs) perfluorocarbons (PFCs) sulfur hexafluoride (SF6)
These greenhouse gases in the atmosphere constitute the "air pollution" that threatens both public health and welfare.
2) The Cause or Contribute Finding, in which the Administrator found that the combined greenhouse gas emissions from new motor vehicles and motor vehicle engines contribute to the atmospheric concentrations of these key greenhouse gases and hence to the threat of climate change.
EPA hence set the greenhouse gas emission standards to light-duty vehicles proposed jointly with the Department of Transportation's Corporate Average Fuel Economy (CAFE) standards in 2009.
The advocacy groups Center for Biological Diversity and 350.org had requested, in December 2009, that the EPA set the NAAQS for carbon dioxide at no greater than 350 ppm. In addition to the six pollutants mentioned in the lawsuit, they proposed that nitrogen trifluoride (NF3) be added as a seventh regulated greenhouse gas.http://www.biologicaldiversity.org/programs/climate_law_institute/global_warming_litigation/clean_air_act/pdfs/Petition_GHG_pollution_cap_12-2-2009.pdf.
Inflation Reduction Act of 2022 In June 2022, the Supreme Court found in West Virginia v. EPA that Congress did not authorize the EPA to regulate "outside the fence" options such as introducing renewable sources for regulations of power plants as the EPA had proposed in the Obama administration's Clean Power Plan. The Court did still acknowledge that, as per Massachusetts, the EPA could still regulate carbon dioxide as a pollutant under the CAA. As a result, building upon an economic stimulus bill to support Joe Biden's policies, Congress passed the Inflation Reduction Act of 2022 in August of that year. In its language, the bill specifically identifies carbon dioxide and other greenhouse gases earlier defined by the EPA as regulated pollutants under the EPA's remit. The bill also gives the EPA more than $27 billion in funding for regulation under the CAA, through a green bank for carbon dioxide and direct grants for methane.
Second Trump administration, 2025 policy to repeal emission standards As advocated in Project 2025, policies of the second Trump administration rolled back the U.S. Global Change Research Program, instead instituting the National Energy Dominance Council. Executive Order 14154 "Unleashing American Energy", signed on inauguration day (January 20, 2025), directed the EPA to report "on the legality and continuing applicability" of the Endangerment findings. Trump had called climate change a "hoax" and the "green new scam". Lee Zeldin was appointed administrator of the EPA in January, on 19 February he sent the White House Office of Management and Budget (OMB) a nine-page memo saying developments in climate science, engineering, economics and law, including a 2024 Supreme Court ruling limiting agency discretion where statutory text was ambiguous, invalidated the endangerment finding and any EPA regulation or action that relied on it. On February 25 his recommendation to end the Federal position that greenhouse gases endanger the public was publicised.