In what will likely become a pivotal moment in the nation’s regulations of greenhouse gases, the U.S. Supreme Court ruled against the rights of six states, New York City and three private land trusts, holding that there is no federal common law of public nuisance for claims of injuries caused by the emissions of greenhouse gases. American Electric Power Co., et al. v. Connecticut, et al., No. 10-174 (“AEP”).

The plaintiffs in AEP filed suit in July 2004 against six major electric utilities arguing they had suffered climate change-related injuries caused by the utilities’ emissions, which emissions they argued constituted a “public nuisance” under state and federal common law. At the time the lawsuit was originally brought the argument made sense: The Bush administration had stated that the Clean Air Act did not permit the Environmental Protection Agency (EPA) to issue regulations addressing climate change and so plaintiffs would need to rely on common law.

However, after several appeals and much change in the political landscape, the case reached the Supreme Court where the court unanimously rejected the AEP plaintiffs’ federal common law argument.

On June 20, 2011, the Court ruled that the Clean Air Act (CAA), in connection with regulation by the Environmental Protection Agency, “displaces” the federal common law public nuisance claims, reversing the United States Court of Appeals for the Second Circuit, which had allowed such suits. The Supreme Court stated that federal common law exists to address “areas of national concern” in the absence of federal legislation, and that the plaintiffs’ federal common law claims “cannot be reconciled with the decision making scheme Congress enacted” by authorizing EPA to regulate greenhouse emissions under the CAA.

Although the Court did not express any view on the science behind climate change, the AEP court did note that EPA has determined that the consequences of excessive greenhouse gas emissions include more heat-related deaths, coastal flooding, more intense and frequent hurricanes, destruction of ecosystems supporting animals and plants, and potential significant disruptions of food production. The Court did, however, decline to address whether climate change claims could constitute “political questions” that are outside the court’s jurisdiction to decide.

The Supreme Court did not decide the state nuisance claim issue, instead remanding AEP back to the Second Circuit for it to consider whether the CAA also preempts the plaintiffs’ claims under state nuisance law.

As a result of the Supreme Court’s decision, it is very likely that other cases where plaintiffs assert a common law nuisance claim against emitters will be dismissed. It is also very likely to have an impact in Congress, where EPA’s regulation of greenhouse gases continues to be a hot button topic. The case may also serve as the motivation and reassurance EPA needs to propose new regulation, including establishing the first CAA limitations on greenhouse gas emissions from motor vehicles.