EPA’s authority to regulate greenhouse gas emissions is in peril. On the first day of his second term, President Trump issued an executive order directing EPA Administrator Lee Zeldin to reconsider “the legality and continuing applicability” of findings made in 2009 that greenhouse gases are air pollutants that endanger public health and welfare, on which basis they must be regulated. Consequently, EPA published a proposed rule in August 2025 that would rescind the endangerment finding made sixteen years prior and repeal greenhouse gas emission standards for new motor vehicles and engines. A final rule was issued in February 2026 to do substantially the same. If it goes into effect, not only could this deregulatory effort inflict drastic economic losses of billions of dollars annually, but it could increase emissions by over seven billion metric tons of carbon dioxide equivalent, with global ramifications.
This final rule will be challenged on several grounds. When interpreting EPA’s authority to regulate greenhouse gas emissions under the Clean Air Act, courts should apply the Charming Betsy canon, which instructs judges to read statutes so as “never . . . to violate the law of nations if any other possible construction remains.” Based on customary international law and structural commitments in the statute, courts should not only invalidate the EPA rule as arbitrary and capricious; they should affirmatively interpret the Clean Air Act to require greenhouse gas regulation.
I. Greenhouse Gas Regulation Under the Clean Air Act
The Clean Air Act regulates air pollution “to promote the public health and welfare,” with several independent provisions each intended to help achieve this purpose. Among them, § 202(a)(1) directs the EPA Administrator to set emission standards for all air pollutants emanating from new vehicles that “in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” In 2009, the Obama Administration issued an endangerment finding under §202(a)(1) for greenhouse gases. This declaration came on the heels of Massachusetts v. EPA, in which the Supreme Court had “little trouble concluding” that carbon dioxide may be categorized as an air pollutant and determined that, “[j]udged by any standard, U.S. motor-vehicle emissions make a meaningful contribution to greenhouse gas concentrations and hence . . . to global warming.” Following the endangerment finding, federal emission standards were issued in 2010, and EPA continued to set new standards through 2024.
At the sitting President’s urging, EPA reversed course in its new rule, rescinding its 2009 endangerment finding and repealing subsequent emission standards on three independent alleged bases: a lack of statutory authority to issue the 2009 endangerment finding, the absence of clear congressional authorization to regulate greenhouse gases, and the futility of the emission standards.1
First, EPA determined that the best reading of § 202(a)(1) permits regulation only of “emissions that cause or contribute to air pollution that endangers public health or welfare through local or regional exposure.” According to EPA, the ordinary meaning of pollution necessitates some “dangerousness and contamination,” which is satisfied only if an air pollutant itself endangers public health or welfare. The Agency further observed that air pollutants explicitly identified elsewhere in § 202 harm human health through direct exposure either to the pollutant itself (e.g., CO) or to air pollution that it precipitates (e.g., acid rain). As for causation, EPA argued that the statutory language of “cause,” “contribute,” and “reasonably be anticipated” should be read to incorporate a background legal principle of proximate cause. Without this limiting interpretation, EPA suggested that water vapor would be regulable — an absurd result. Because greenhouse gases pose harm “only on a global scale and through an attenuated and indirect causal chain,” they do not meet the standard for “air pollutant” identified by EPA.
Second, EPA concluded that it lacks “clear congressional authorization . . . to decide the Nation’s policy response to global climate change concerns,” even if greenhouse gases are an air pollutant under § 202(a)(1). Invoking the major questions doctrine, the Agency drew parallels to West Virginia v. EPA and Utility Air Regulatory Group v. EPA, each of which invalidated regulations implemented under other sections of the Clean Air Act and which broadly declared that decisions of “vast economic and political significance” must rest on more than “a merely plausible textual basis.” EPA warned that, like the Clean Power Plan invalidated under West Virginia, “launching a GHG emissions program under this authority would result in an unprecedented expansion of regulatory power with profound adverse effects on the economy and American households.2
Third, EPA asserted that it should not regulate an air pollutant if regulation “would have no meaningful impact on the identified dangers.” Based on an assessment that even complete elimination of GHG emissions from new vehicles “would not address the risks attributed to elevated global concentrations of GHGs,”3 the Agency reasoned that any emission standard for greenhouse gases would have a de minimis impact, which “as a matter of text and structure” precludes regulation under § 202(a)(1). EPA acknowledged that while regulation “need not fully ameliorate the identified harms,” it “must be capable of having at least a material impact on the identified danger.” If not, then the emissions in question do not “contribute” to the dangerous air pollution. EPA resolved that futility is sufficient grounds to repeal emission standards, because it is “unreasonable and impermissible” to inflict “immense costs while providing no material value” under § 202(a)(2).4
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Following the Court’s recent declaration in Loper Bright Enterprises v. Raimondo that “statutes . . . have a single, best meaning,” EPA must defend its interpretation of § 202(a)(1) as not only sufficient, but supreme. That single, best meaning is determined “after applying all relevant interpretive tools,” which include both linguistic and substantive canons of construction. In its final rule, EPA emphasized one such canon, the major questions doctrine. But courts should apply all relevant canons to discern the best meaning of § 202(a)(1). The Charming Betsy canon is relevant and must be considered.
II. The Charming Betsy Challenge
While the Charming Betsy canon earns its name from an 1804 opinion delivered by Chief Justice Marshall, it predated both the Court and the Constitution. The canon reflects a strong presumption against violating international law, directing judges to reject an interpretation of a statute that would put the United States in transgression of international law if any other “possible construction” could be found in the alternative. When applying similar substantive canons, the Court has required a clear statement from Congress to overcome the substantive presumption. Clear statement rules sometimes obligate courts to “strain against the statutory language.” The single, best meaning of a statute may not align with the ordinary meaning of its isolated terms.
Substantive canons must have some basis to justify deviation from ordinary meaning. The Charming Betsy canon has at least three: It represents the presumed intentions of Congress, it commits the United States to international obligations, and it preserves the separation of powers by precluding the judiciary from violating international law, which should only ever occur at the command of the political branches.
Though the Charming Betsy canon extends to any statute, it is particularly relevant to the Clean Air Act, which contemplates transboundary ramifications of air pollution and appears structured to adhere to international law. Two sections of the Clean Air Act address respectively the prospect of foreign emissions impacting the United States and domestic emissions impacting the rest of the world, reflective of reciprocal obligations.
Under § 179B, the EPA Administrator must approve state implementation plans (SIPs)5 if they would achieve required emission levels, “but for emissions emanating from outside of the United States.” This structure incorporates a presumption that domestic governments should not be held responsible for foreign emissions. Such an implication might invite curiosity as to whether foreign governments should be held responsible for domestic emissions.
Felicitously, § 115 clarifies the scope of that reciprocal relationship. It requires revision of a SIP if the EPA Administrator “has reason to believe” that domestic emissions from that state “may reasonably be anticipated to endanger public health or welfare in a foreign country,” so long as that country “has given the United States essentially the same rights . . . as is given . . . by this section.” A recent opinion in the Second Circuit emphasizes that “the Act contemplates the need for reciprocal protections,” and it describes § 115 as a “carefully balanced scheme of international cooperation on a topic of global concern.” These sections reveal an embedded principle of reciprocity in the Act in line with customary international law.
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In an executive order withdrawing from the Paris Agreement in 2025, the President declared that by withdrawing, the United States would also withdraw from “any attendant obligations.” However, even if the United States has left the party, the party rages on. The United States is still obligated to adhere to customary international law regarding air pollution.
Last July, the International Court of Justice (ICJ) issued an advisory opinion that extended longstanding principles of avoiding transboundary harm — elaborated in the no harm principle and the polluter pays principle — to climate change. The court determined that all nations must “ensure the protection of the climate system . . . from anthropogenic greenhouse gas emissions.” More specifically, every nation has “a duty to prevent significant harm to the environment by acting with due diligence and to use all means at their disposal to prevent activities . . . from causing significant harm to the climate system.” Even if an activity “is environmentally insignificant in isolation,” a nation must regulate that activity so long as “the possible cumulative effects of their acts and the planned activities under their . . . control” would risk significant harm.
The advisory opinion further declared that nations must “put in place a national system, including legislation, administrative procedures and an enforcement mechanism necessary to regulate the activities in question, and . . . exercise adequate vigilance to make such a system function efficiently” such as “regulatory mitigation mechanisms that are designed to achieve . . . deep, rapid, and sustained reductions of GHG emissions.” To reach its conclusions, the opinion observed that the causal link between greenhouse gas emissions and “significant harm to the climate system is indisputably established,” and that all nations, no matter how much they emit, share responsibility for a “quintessentially universal risk” engulfing Earth.
An advisory opinion from the ICJ is a gold standard for customary international law, and it follows years of several scholars making comparable arguments on comparable grounds. EPA argued that ICJ advisory opinions are not legally binding, but that is irrelevant to the Charming Betsy canon. While the advisory opinion would not supersede a statute in irreconcilable conflict, it nevertheless provides a filter for interpretation. Statutes should be read where possible to respect customary international law on climate change, which requires greenhouse gas emissions reduction through a national system that regulates activities with cumulative effects on anthropogenic climate change.
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Duties identified in the ICJ’s advisory opinion are procedurally and substantively breached by EPA’s final rule. The reinterpretation of § 202(a)(1) strips the Agency of authority to regulate greenhouse gas emissions through the Clean Air Act, which frustrates the nation’s obligation to regulate private sector emissions with vigilance. Emissions from the domestic transportation sector are indeed significant. But even if that were not the case, a nation’s duty to regulate across sectors is triggered by the cumulative effects of its greenhouse gas emissions. Otherwise, no country could be held responsible for any of its particularized greenhouse gas emissions, which would disintegrate nations’ collective duty to protect the climate system. EPA’s interpretation of § 202(a)(1) is incompatible with the nation’s international obligations.
III. Properly Interpreting the Clean Air Act
Because EPA’s final rule would place the United States in contravention of international obligations, courts applying the Charming Betsy canon must reject the rule’s interpretation if “any other possible construction remains.” In turn, courts should invalidate the rule’s rescission and repeals as arbitrary, capricious, or otherwise not in accordance with law under § 307(d)(9). One can reasonably construct alternative interpretations of the Clean Air Act that respect international law.
The simplest construction would emulate the pre-rule legal consensus: Greenhouse gases comprise an “air pollutant” under § 202(a)(1), which the EPA Administrator must regulate through new motor vehicle emission standards if the air pollutant is reasonably anticipated to endanger public health or welfare, regardless of the standards’ capacity to reverse climate change.
First, greenhouse gases are unambiguously air pollutants in the context of § 202(a)(1). For nearly twenty years, the Court’s holding to that effect in Massachusetts has stood on solid ground, affirmed by the Court itself. Even in UARG, where the Court distinguished Massachusetts to exclude greenhouse gases from air pollutants under § 165, it summarized greenhouse gas regulation through § 202(a)(1) as “not . . . contrary to common sense” and “[a]t most . . . [a] modest step.” If not commanding as subject to statutory stare decisis, this intuitive and precedented construction must still be possible.
Second, there is a reasonably anticipated threat to public health or welfare. While the EPA Administrator makes the judgment, the Court has held that this language does not confer unlimited discretion. And the D.C. Circuit has interpreted § 202(a)(1) as “requir[ing] a precautionary, forward-looking scientific judgment,” not “proof of cause and effect.” The final rule’s elevated threshold for an endangerment finding diverges from doctrine and is not the single, best meaning of § 202(a)(1). Stringency manifests in a duty to regulate emissions contributing to reasonably anticipated harm, not in a duty to prove that harm. If the best available science indicates a threat to public health or welfare, the EPA Administrator must regulate emissions to mitigate that threat.
EPA’s final rule contended that emissions from new motor vehicles are regulable only if they themselves endanger regional public health or welfare. Applying a precautionary standard, the best available science — as well as the majority opinion in Massachusetts — demonstrate that greenhouse gases emitted from new motor vehicles in the United States are reasonably anticipated to endanger public health and welfare throughout the United States. This would necessitate regulation, averting infringement of customary international law.
But to ensure compliance with international obligations, courts should interpret § 202(a)(1) to compel regulation even if new motor vehicle emissions do not endanger public health or welfare, so long as aggregate greenhouse gases do. EPA must regulate “the emission of any air pollutant from . . . new motor vehicle engines, which . . . cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” The most natural reading mandates emissions regulation if the air pollution to which emissions contribute threatens public health or welfare, not the emissions themselves. Surely, this reading, which was adopted in the 2009 endangerment finding, is at least possible. Interpreting § 202(a)(1) to require sector-based regulation following a determination of broader harm better aligns with principles of customary international law.6
Third, alleged futility is insufficient to preclude regulation. The Court has already declared that EPA’s actions under § 202(a)(1) need not reverse climate change, but merely “take steps to slow or reduce it” — and has concluded that any “reduction in domestic emissions would slow the pace of global emissions increases, no matter what happens elsewhere.” Under established law, EPA may — and, based on the science, must — regulate greenhouse gases. The doctrinal status quo presents a possible construction of the Clean Air Act that would respect international duties to mitigate climate change with due diligence, mandating rejection of EPA’s interpretation.
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To avoid this outcome, the government would challenge use of the Charming Betsy canon, likely for at least two reasons: that the canon has reportedly never been invoked against the United States as a party, and that the major questions doctrine should predominate if the canons would lead a court to different interpretations.7Neither argument is compelling.
First, some courts have suggested that the Charming Betsy canon should not be invoked against the government’s interpretation of a statute, because the Executive takes “the lead role in foreign policy.” But the Court recently affirmed in Zivotofsky ex rel. Zivotofsky v. Kerry that “whether the realm is foreign or domestic, it is still the Legislative Branch, not the Executive Branch, that makes the law.” And just last month, three members of the Court in Learning Resources, Inc. v. Trump denied a foreign affairs exception to an interpretive presumption.8 The Executive cannot contravene the will of Congress, which is imputed through the Charming Betsy canon. Moreover, the Executive has sent mixed signals regarding its position on international law: The executive order issued by President Trump directing withdrawal from the Paris Agreement reaffirmed that “[t]he United States must . . . play[] a leadership role in global efforts to protect the environment.” From this language, the Executive still considers itself to have some obligations to other nations regarding environmental protection. Further, EPA did not invoke the Executive’s residual foreign affairs power in its rule, but “relie[d] entirely on power derived from Congress.” Thus, even if executive statements on foreign policy could displace the Charming Betsy canon, it would not do so here.
Second, the major questions doctrine should not override the Charming Betsy canon here for two reasons: its substantive grounding is inferior, and it is unfit for application to § 202(a)(1). The Charming Betsy canon has a richer history and enjoys comparatively stronger constitutional roots. When two substantive canons are in tension, the substantively weaker one should yield. Independently, the major questions doctrine is hardly an appropriate tool for § 202(a)(1). The Court already deemed the nascent doctrine irrelevant in Massachusetts, despite rhetoric describing climate change as “the most pressing environmental challenge of our time.”9 EPA’s argument in the final rule that the major questions doctrine has doctrinally evolved since Massachusetts demonstrates precisely why the canon’s present form is anachronistic to the 2009 endangerment finding and subsequent regulations. If applied to interpret § 202(a)(1), the superior Charming Betsy canon should prevail.
Conclusion
EPA’s final rule would not only dismantle an essential climate program; it would place the United States in violation of customary international law. When courts hear challenges to EPA’s interpretation of § 202(a)(1), they should apply the Charming Betsy canon to ensure that the United States does not breach its duty to regulate greenhouse gas emissions.