I. INTRODUCTION II. MASSACHUSETTS v. EPA AND ITS AFTERMATH A. The 2007 Opinion B. Regulatory Aftermath III. AMERICAN ELECTRIC POWER V. CONNECTICUT A. Impact on EPA's Clean Air Act Authority B. Potential Impact on Proposals to Amend Away EPA Authority IV. WHAT NEXT? REDUCING GREENHOUSE GASES THROUGH STATE PUBLIC NUISANCE ACTIONS V. CONCLUSION I. INTRODUCTION Despite increasingly dire warnings from scientists, the United States Congress has been unable to pass comprehensive legislation to reduce climate altering greenhouse gases. In the absence of new federal legislation, states, cities, and nonprofits have petitioned and sued administrative agencies to regulate pursuant to existing environmental laws. Under former President Bush, executive agencies largely rebuffed these efforts, prompting litigation by environmental plaintiffs. In response to judicial determinations and citizen petitions, as well as by virtue of its own policies, the Environmental Protection Agency under President Obama began regulating greenhouse gases pursuant to the Clean Air Act. Political and legal attacks followed swiftly, with opponents charging that the Agency had exceeded its mandate and politicians proposing legislation to incapacitate the Agency. Meanwhile, plaintiffs have also sued power producers in state and federal common law tort actions characterizing greenhouse gas emissions as a nuisance. This cause of action--which creates liability for an unreasonable interference with a right common to the general public (1)--was used to abate pollution long before the advent of the administrative state. (2) Although nuisance lawsuits are often viewed as a distinct predecessor to regulation under modern federal environmental statutes, this article argues that these common law actions can significantly influence development of a federal regulatory regime for greenhouse gases. In the midst of the battle over climate change regulation, the Supreme Court issued its 2011 decision in American Electric Power v. Connecticut (AEP) (3) holding that the federal common law nuisance cause of action relied upon by the plaintiffs had been displaced by the Clean Air Act. Many headlines touted the decision as a win for utilities, highlighting the Court's rejection of states' and environmentalists' claims against the five largest electricity generators in the United States. (4) Yet, such attention to the formal outcome missed the real import of the case. By strongly reaffirming the Court's 2007 decision in Massachusetts v. EPA, Justice Ginsburg's brief opinion strengthened the Environmental Protection Agency's (EPA's) authority to regulate greenhouse gases under the Clean Air Act. Moreover, because the Court held that displacement of nuisance claims under federal common law hinges on EPA's authority to regulate greenhouse gases, the decision impedes congressional Republicans' efforts to obstruct EPA's climate change efforts by amending the Act. Meanwhile, the Court did not decide whether or not the Clean Air Act preempts nuisance actions brought under state law, leaving intact another possible tort avenue for plaintiffs. By bolstering EPA's nascent efforts to regulate greenhouse gases, the decision appreciably advances the development of a federal regulatory regime to address climate change. Other aspects of the case--less a focus of this discussion--support environmental plaintiffs more broadly. By affirming the Second Circuit on jurisdictional questions, the American Electric Power opinion solidified the Court's 2007 holding in Massachusetts v. EPA that the injuries caused by climate change and their incremental redress provide an appropriate basis for standing to sue. In addition, the affirmance undermined claims (beginning to gain traction in some courts) that the political question doctrine prevents courts from reaching the merits of climate change suits. …