We address in this opinion the question whether the plaintiffs (several States, the city of New York, and three private land trusts) can maintain federal common law public nuisance claims against carbon-dioxide emitters (four private power companies and the federal Tennessee Valley Authority). As relief, the plaintiffs ask for a decree setting carbon-dioxide emissions for each defendant at an initial cap, to be further reduced annually. The Clean Air Act and the Environmental Protection Agency action the Act authorizes, we hold, displace the claims the plaintiffs seek to pursue.