What are the facts?
Eight States, the City of New York, and several land trusts sued five large power companies—American Electric Power Co., Southern Company, Xcel Energy, Cinergy, and the Tennessee Valley Authority—alleging that carbon-dioxide emissions from defendants' fossil-fuel-fired power plants substantially and unreasonably contributed to climate change, thereby creating a public nuisance under federal common law (and, in the alternative, state law). Plaintiffs sought an injunction capping and then reducing defendants' emissions on a court-ordered schedule. The district court dismissed on political-question grounds. The Second Circuit reversed, holding the case justiciable, recognizing Article III standing, and concluding that federal common-law nuisance governed interstate air pollution and had not been displaced because EPA had not yet regulated CO2 emissions from the defendant plants. The Supreme Court granted certiorari. While the case was pending, EPA issued several greenhouse-gas-related actions, and the Court had earlier held in Massachusetts v. EPA that greenhouse gases are air pollutants under the Clean Air Act (CAA) and that EPA has authority to regulate them.
What is the legal issue?
Does the Clean Air Act, and the EPA rulemaking authority it confers over greenhouse-gas emissions, displace federal common-law public nuisance claims seeking abatement of carbon-dioxide emissions from fossil-fuel-fired power plants?
What rule applies?
Federal common law may govern limited areas implicating uniquely federal interests, such as interstate pollution, but it is displaced when Congress has spoken directly to the question at issue by enacting a statutory scheme. Displacement does not depend on whether the responsible agency has actually exercised its regulatory authority; it occurs when Congress has delegated to the agency authority to regulate the field. When displacement exists, plaintiffs must pursue their claims through the statutory framework, with judicial review available under that statute and the Administrative Procedure Act.
What did the court hold?
Yes. The Clean Air Act and the EPA actions it authorizes displace federal common-law public nuisance claims seeking to cap and reduce carbon-dioxide emissions from power plants. The Court affirmed the Second Circuit's exercise of jurisdiction by an equally divided Court and remanded for consideration of any state-law nuisance claims, including issues of preemption.
What is the reasoning?
The Court began by acknowledging that in past cases it had recognized a narrow federal common law for interstate pollution (e.g., the Milwaukee cases), but emphasized that such judge-made law yields when Congress enacts a statute that speaks directly to the problem. Applying Milwaukee II's displacement standard, the Court held that the Clean Air Act addresses carbon-dioxide emissions from the exact sources at issue: fossil-fuel-fired power plants. Massachusetts v. EPA confirmed that greenhouse gases qualify as air pollutants under the CAA and that EPA possesses regulatory authority. The CAA delegates to EPA the task of setting emissions standards, including performance standards for stationary sources, and provides mechanisms for updating those standards in light of evolving science and technology. Displacement does not turn on the degree to which EPA has regulated; it suffices that Congress authorized EPA to regulate the emissions at issue. Plaintiffs' request for a court-devised cap-and-reduction schedule would require judges to make complex, policy-laden determinations—balancing costs, benefits, technological feasibility, and national economic considerations—tasks that the statute assigns to EPA, which has expertise, fact-finding tools, and procedural safeguards (notice-and-comment rulemaking) and whose decisions are reviewable in federal court under the statute and the Administrative Procedure Act. Equitable relief cannot preserve a displaced federal common-law cause of action; the question is whether the cause of action exists, not what remedy is sought. On threshold issues, the Court noted that four Justices would find standing and justiciability under Massachusetts v. EPA, while four would not; the Second Circuit's exercise of jurisdiction was therefore affirmed by an equally divided Court, carrying no precedential effect. Finally, the Court did not decide the fate of state-law nuisance claims, remanding for the lower courts to consider whether the CAA preempts such claims and which state's law, if any, applies.
Why is this case significant?
AEP v. Connecticut is a cornerstone in climate litigation and federal courts doctrine. It forecloses federal common-law nuisance suits seeking to regulate greenhouse-gas emissions from stationary sources, channeling disputes into the Clean Air Act's administrative process and statutory judicial review. The case clarifies the displacement standard—Congress displaces federal common law when it speaks directly to the issue, regardless of the agency's progress—and underscores institutional competence by reserving complex emissions policy to EPA. It also set the stage for subsequent cases, such as Kivalina v. ExxonMobil, which applied AEP to dismiss federal common-law damages claims for climate harms, and it shifted climate plaintiffs toward state-law theories, where preemption and choice-of-law questions loom large.
What is the difference between displacement and preemption, and which one did the Court apply?
Displacement and preemption both address conflicts between different sources of law, but they operate in different domains. Displacement concerns whether a federal statute supplants federal common law. Preemption concerns whether federal law overrides state law (statutory or common law). In AEP, the Court applied displacement: the Clean Air Act supplanted the federal common-law nuisance claim. The Court expressly left open preemption of state-law nuisance claims, remanding that question to the lower courts.
Did the Supreme Court decide whether the plaintiffs had Article III standing?
Not definitively. The Justices were evenly divided on standing and other threshold justiciability issues. As a result, the Second Circuit's determination that the case was justiciable was affirmed by an equally divided Court, which resolves the case for the parties but sets no nationwide precedent. The Court proceeded to decide the displacement question on the merits.
Why did the Court find the Clean Air Act displaced federal common-law nuisance even though EPA had not set CO2 limits for power plants at the time?
Because displacement turns on congressional authorization, not on the extent of agency action. Under Milwaukee II, a federal statute displaces federal common law if it speaks directly to the issue. The Clean Air Act authorizes EPA to regulate greenhouse-gas emissions from stationary sources; Massachusetts v. EPA confirmed that CO2 is an air pollutant. That delegation of authority was sufficient to displace federal common-law nuisance, irrespective of whether EPA had yet promulgated specific standards for the defendants.
What remedies remain available to address greenhouse-gas emissions after AEP?
Plaintiffs must use the statutory pathways: petition EPA for rulemaking, participate in notice-and-comment proceedings, and seek judicial review of EPA actions or inaction under the Clean Air Act and the Administrative Procedure Act. Citizen-suit provisions may also be available in appropriate circumstances. Plaintiffs may attempt state-law tort or nuisance claims, but those claims face potential obstacles such as federal preemption, choice-of-law complexities, justiciability, and removal to federal court.
How does AEP relate to Massachusetts v. EPA?
Massachusetts v. EPA held that greenhouse gases are air pollutants under the Clean Air Act and that EPA has authority to regulate them, subject to statutory preconditions. AEP builds on that foundation, using Massachusetts to conclude that Congress has spoken directly to the regulation of greenhouse gases, thereby displacing federal common-law nuisance claims. Together, the cases channel climate regulation into EPA's administrative process and attendant judicial review rather than judge-made nuisance law.
What happened to the state-law claims in AEP?
The Supreme Court did not decide the state-law nuisance claims. It remanded for the lower courts to consider whether the Clean Air Act preempts those claims and to resolve choice-of-law and other threshold issues. In related litigation after AEP, courts have frequently confronted whether state tort theories targeting nationwide emissions are preempted or otherwise barred.