American Electric Power Co., Inc. v. Connecticut.

Plaintiffs (eight states, a city, and three land trusts) brought federal common law public nuisance claims seeking injunctive relief against five of the largest carbon emitters in the United States (four private power companies and the federal Tennessee Valley Authority). Plaintiffs alleged that by collectively providing 2.5% of anthropogenic carbon emissions worldwide, defendants were contributing to global warming, which, in turn, resulted in a "substantial and unreasonable interference with public rights." Plaintiffs argued that federal common law could not be displaced by the Clean Air Act (“CAA”) because the EPA had yet to create rules regulating CO2 emissions. The Supreme Court disagreed. Relying on its decision in Massachusetts v. Environmental Protection Agency, 549 U.S. 497 (2007), the Court found that the federal common law of nuisance was preempted by the CAA, which gave the EPA the authority and responsibility to regulate greenhouse gas emissions. It was not material that the EPA had yet to create rules, only that it had been delegated the power to do so by Congress. The Court held that Congress's delegation of the power to regulate greenhouse gasses to the Environmental Protection Agency "displaces federal common law" relating to the abatement of carbon dioxide ("CO2") emissions. The Supreme Court noted that displacement of plaintiffs’ claims is not dependent on the EPA actually exercising its authority by setting emission standards. Displacement occurs, the Court held, because Congress delegated to the EPA the authority to decide whether and how to regulate carbon-dioxide emissions, and that delegation alone displaces federal common law. The Court added that, as an expert agency, the EPA is better equipped than individual federal judges to address the issues raised by the regulation of carbon emissions.