Over the course of the last 15 years, the U.S. Environmental Protection Agency (EPA) has developed a suite of regulations under the Clean Air Act (CAA) built upon EPA's finding – originally made in 2009 – that greenhouse gas (GHG) emissions in the atmosphere endanger public health and welfare. At President Trump's direction, EPA recently initiated a process for "formal reconsideration of the 2009 Endangerment Finding in collaboration with the Office of Management and Budget (OMB) and other relevant agencies." As part of this review, EPA "also intends to reconsider all of its prior regulations and actions that rely on the Endangerment Finding." On April 21st, EPA Administrator Lee Zeldin stated that he "anticipate[s]" that "there will be a rulemaking process" with "public comment" for reconsideringthe 2009 Endangerment Finding. To help shed light on this reconsideration process and potential outcomes, our team of experienced environmental attorneys prepared a list of nine questions for 2009 Endangerment Finding reconsideration.
Q: When does the Clean Air Act require EPA to make "endangerment" findings?
A: Several provisions of the CAA require EPA to make a finding that a source category contributes or significantly contributes to air pollution "which may reasonably be anticipated to endanger public health or welfare" prior to regulating emissions from that source category. Section 202(a) requires EPA to regulate air pollutants from new motor vehicles if the Agency finds that they "cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare." Similarly, Section 111(b) requires EPA to establish new source performance standards for stationary source categories that "cause[], or contribute[] significantly to, air pollution which may reasonably be anticipated to endanger public health or welfare." EPA must make an endangerment finding pursuant to these provisions before EPA can adopt pollutant-specific standards for these sources.
Q: What did the U.S. Supreme Court decide inMassachusetts v. EPA?
In 2007, the Supreme Court addressed EPA's failure to make an endangerment finding decision for GHGs emissions pursuant to Section 202(a) in response to a petition filed by Massachusetts and other states. See Massachusetts v. EPA, 549 U.S. 497 (2007). EPA argued that it did not have the power to regulate GHGs under the CAA because they do not meet the CAA's definition of air pollutant. In response, the Court held that the CAA's definition of "air pollutant" in Section 302(g) unambiguously permits EPA to regulate GHGs, emphasizing that the CAA's broad language confers flexibility for EPA to address harms that Congress may not have specifically anticipated when it enacted the provision, and further found that EPA could not defer making an endangerment finding based on potential uncertainties in scientific evidence related to climate change and its effects. However, Chief Justice Roberts along with Justices Thomas, Scalia, and Alito vigorously dissented in Massachusetts v. EPA, calling into question the petitioners' standing to challenge EPA's failure to act on climate change-related issues. These justices also argued that EPA could reasonably find that the CAA excludes GHGs from regulation as air pollutants or that scientific uncertainty prevented the Agency from making an endangerment finding. The Trump Administration is likely to lean heavily on these dissents in its reconsideration process.
Q: What is the 2009 Endangerment Finding?
A: In December 2009, following the Massachusetts v. EPA decision, EPA published a final rule finding that the mix of six directly-emitted, long-lived GHGs in the atmosphere—carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride—threaten public health and welfare. This 2009 finding was made pursuant to the motor vehicle emissions provision found at CAA Section 202(a). EPA pointed to current observations and projected risks of climate-related impacts associated with these emissions, including changes in air quality, increases in temperatures, changes in extreme weather events, and increases in food- and water-borne pathogens. While EPA acknowledged that new motor vehicles do not emit all six of these greenhouse gases, the Agency took the position that an endangerment finding requires only that air pollution associated with any well-mixed GHGs be reasonably anticipated to endanger public health and welfare, and that a domestic source category—in this case, new motor vehicles—causes or contributes to this air pollution.
Q: Has EPA issued other climate-specific endangerment findings since 2009?
A: EPA has made at least three additional endangerment findings for well-mixed GHGs since 2009:
- In October 2015, EPA made an endangerment finding pursuant to Section 111(b) for new, modified, and reconstructed electric utility generating units. In this rulemaking, EPA took the position that Section 111(b) does not require a pollutant-specific endangerment finding, but rather allows EPA to regulate all pollutants emitted by a listed source category. EPA nevertheless made a pollutant-specific finding for well-mixed greenhouse gases, referring to the 2009 Endangerment Finding and scientific developments since. In making this finding, EPA emphasized that "fossil fuel-fired [units] are by far the largest emitters of GHGs among stationary sources in the U.S., primarily in the form of CO2."
- In June 2016, EPA made an endangerment finding for greenhouse gases (in particular, methane), volatile organic compounds, and sulfur dioxide, pursuant to Section 111(b) for new, modified, and reconstructed facilities in the oil and gas sector. EPA cited to the 2009 Endangerment Finding and more recent scientific developments for public health and welfare impacts associated with GHG emissions, while also pointing to non-climate related impacts from methane emissions as a precursor to ground-level ozone formation.
- In August 2016, EPA made an endangerment finding pursuant to Section 231(a)(2)(A) for certain classes of aircraft engines. EPA placed "considerable weight" on the 2009 Endangerment Finding and concluded that more recent scientific assessments "strengthen and further support the judgment that GHGs in the atmosphere may reasonably be anticipated to endanger the public health and welfare of current and future generations."
Q: How did the Trump Administration address this issue in its first term?
A: During the first Trump Administration, EPA did not reconsider or alter the 2009 Endangerment Finding. Notably, in January 2021 during the final weeks of the first Trump Administration, EPA relied on the 2009 Endangerment Finding to finalize a "Significant Contribution Finding" (SCF) pursuant to Section 111(b) for new, modified, and reconstructed electric utility generating units, which applied a threshold of 3% of total U.S. GHG emissions to assess significance, while also considering secondary factors such as emission trends and relative contribution of domestic sources. However, the D.C. Circuit vacated and remanded this rule in accordance with a request by the Biden Administration. See California v. EPA, No. 21-1035 (D.C. Cir. Apr. 5, 2021).
Q: What is EPA currently considering?
A: On January 20, 2025, President Trump signed Executive Order 14154, "Unleashing American Energy," which directed agencies to review agency actions that may "impose an undue burden" on domestic energy resources. In particular, President Trump directed EPA to make a recommendation to the OMB Director within 30 days regarding the legality and continuing applicability of the 2009 Endangerment Finding.
On March 12, 2025, Administrator Zeldin announced that the Agency will reconsider the 2009 Endangerment Finding, as well as "regulations and actions that rely on that Finding," which likely include EPA's 2024 GHG performance standards for the electric utility sector, the 2024 methane performance standards for the refining sector, and various light-, medium-, and heavy-duty vehicle emission standards, among other actions.
Q: Did the Inflation Reduction Act address the 2009 Endangerment Finding?
A: The Inflation Reduction Act (IRA), which was enacted in 2021, added several new sections of the Clean Air Act defining GHGs as "air pollutants." For example, the IRA provides: "In this section, the term 'greenhouse gas' means the air pollutants carbon dioxide, hydrofluorocarbons, methane, nitrous oxide, perfluorocarbons, and sulfur hexafluoride." These definitions are section-specific, and it will be debated whether the IRA definition is controlling for purposes of Section 202(a) or Section 111(b). The Supreme Court has held that "air pollutant" may have different meanings in different sections of the CAA. See Utility Air Regulatory Group v. EPA, 573 U.S. 302, 320 (2014). One might anticipate arguments concerning whether the IRA definition makes it more or less challenging for EPA to adopt a narrow definition of "air pollutant."
Q: What are some of the other potential implications of repeal of the 2009 finding?
A: Some commentators have suggested that withdrawing the 2009 Endangerment Finding in its entirety might result in greater GHG regulation at the state level, due in part to the potential impact on arguments that federal law currently displaces state law on GHG emissions. Regulation of GHG emissions under the CAA has supported one line of preemption defenses to state-based GHG regulations, as well as a host of climate tort suits based in state common law brought against fossil fuel producers and power plants. Several state courts have dismissed the tort suits citing to preemption or displacement arguments. Other commentators have also noted potential implications for environmental permitting, among other things.
Various sectors could encounter greater short-term uncertainty as to GHG emission standards. For example, EPA has indicated that it intends to reconsider both the 2009 Endangerment Finding and the 2024 Power Plant GHG Rules, but it remains unclear whether EPA will issue a replacement Power Plant Rule or withdraw all GHG-related requirements for the source category. On the other hand, EPA has regulated methane emissions from the oil and gas sector based on non-climate related impacts as well as climate impacts. Thus, even in the absence of the 2009 Finding, EPA might continue to assert authority to regulate methane emissions from the refining sector separate and apart from the 2009 Endangerment Finding.
As to mobile sources (vehicles), under Section 209(a) of the CAA, states are prohibited from "adopt[ing] or attempt[ing] to enforce any standard relating to the control of emissions from new motor vehicles or new motor vehicle engines subject to this part . . ." California, however, has specific authority under Section 209(b) to obtain a waiver of this preemption under certain circumstances. (There is corollary language in Section 209(e) applicable to nonroad engines and nonroad vehicles.) It will be debated whether California would need a Section 209 waiver for its zero emission vehicle standards if EPA does not exercise its authority to regulate GHG emissions from motor vehicles, with some contending that Section 209(a) preemption is separate from the 2009 Endangerment Finding and would require a waiver in any event.
Q: What are some of the issues that the new Administration may address as EPA revisits the 2009 Endangerment Finding?
A: EPA's Press Release announcing its reconsideration action suggested that the following issues will play a key role in the Agency's upcoming reconsideration rulemaking:
- Definition of Air Pollutant: The 2009 Endangerment Finding defines "air pollutant" as the mix of six directly-emitted, long-lived, and well-mixed greenhouse gases. EPA indicated that its reconsideration action may take a narrower view of this definition.
- Level of Contribution: The 2009 Endangerment Finding evaluated climate-related impacts in the United States associated with worldwide emissions of greenhouse gases. However, EPA indicated that its reconsideration action may focus on direct impacts of emissions from domestic motor vehicles only.
- Scientific Evidence: EPA noted that there have been "major developments in innovative technologies, science, economics, and mitigation" since the 2009 Endangerment Finding that may warrant further evaluation.
- Supreme Court Precedent: EPA suggested that recent decisions by the Supreme Court may limit agency discretion to regulate GHG emissions, and that such regulation may implicate the major questions doctrine as articulated in West Virginia v. EPA. On the other hand, EPA would also need to grapple with the Supreme Court's currently binding precedent in Massachusetts v. EPA.
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