In the first oral arguments of the October 2026 term, the Supreme Court Justices grappled with one of the most significant climate cases to reach the Court in recent years. Justice Kavanaugh’s questions indicated that he believes tort claims over climate harms can only be brought under federal common law. But although he is correct that several cases establish the availability of federal common law, he is incorrect to conclude that they rule out the concurrent availability of state common law.

The dispute in Suncor Energy Inc. v. County Commissioners of Boulder County concerns a state tort suit brought against two fossil fuel companies—Exxon Mobil and Suncor—over their contributions to climate harms in Colorado. Boulder alleges that Exxon Mobil and Suncor’s conduct—namely the sale of oil and the spread of misinformation about climate change—has caused damage to their city. Other cities and states around the country have brought similar claims against fossil fuel companies under a variety of state tort theories, such as nuisance, failure to warn, and fraud. Fossil fuel companies are arguing that only federal common law is available for such claims, relying on both the U.S. Constitution and the federal Clean Air Act.

Several important questions came up during oral argument: are these suits like other product liability claims, or are they instead about regulating global greenhouse gas emissions? Does it matter that the Trump administration has disclaimed any authority under the Clean Air Act to regulate greenhouse gases? If the cases move forward under state law, can Colorado apply its own law, or would it be necessary to apply the tort law of the state where the defendant corporation is located?

For a thorough analysis of these issues, I would point readers to an essay I recently published in the Yale Law Journal Forum. The essay peels apart these questions in the context of existing Supreme Court precedent, and it argues that state common law should be available for most, if not all, climate tort claims in light of the Court’s recent statutory interpretation methods, traditional approaches to preemption doctrine, and states’ historic police powers over their natural resources and public health.

What I’d like to do in this blog post is address Justice Kavanaugh’s statements at oral arguments. Justice Kavanaugh asserted that Suncor’s lawsuit can be disposed of quickly and easily because four cases unequivocally establish that only federal common law governs interstate environmental threats—at least unless Congress “authorizes” such suits under state law. This is an argument first articulated by the U.S. Court of Appeals for the Second Circuit, and it is wrong. The four cases do not say what Justice Kavanaugh and the Second Circuit claim they do.

The four cases are Illinois v. Milwaukee, 406 U.S. 91 (1972); Milwaukee v. Illinois, 451 U.S. 304 (1981); International Paper Co. v. Ouellette 479 U.S. 481 (1987); and American Electric Power v. Connecticut 564 U.S. 410 (2011).

Let’s start with the first case, which is the most crucial to Justice Kavanaugh’s contention that only federal common law is available unless Congress authorizes state tort suits. Illinois v. Milwaukee concerned a legal battle between sovereign governments—Illinois and four cities in Wisconsin—over water pollution in Lake Michigan. To remedy the transport of sewage across state lines, Illinois brought a federal common law claim against the Wisconsin municipalities and sought to invoke the Supreme Court’s original jurisdiction since the dispute was between sovereign powers.

In a unanimous opinion known as Milwaukee I, the Supreme Court held that the case should proceed in federal district court first, and in so holding, the Justices found that federal common law was available for claims involving pollution of interstate or navigable waters. Though Congress had enacted federal legislation concerning water resources, it had not yet passed the modern version of the Clean Water Act.

Here is what the case did not address: Milwaukee I did not discuss the availability of state tort claims against private companies, including for transboundary pollution or other interstate environmental problems. Indeed, the only references the Justices made to the relationship between federal and state law concerned federal procedures Congress had put in place to adjudicate disputes between states over transboundary water pollution. Given the existence of these provisions in federal statutes, the Court believed that Congress would have wanted the dispute between Illinois and Wisconsin municipalities to proceed under federal common law.

Justice Kavanaugh made the same mistake that the Second Circuit did in a 2021 decision involving similar state law tort claims against fossil fuel companies over climate harms. The fact that the Justices in Milwaukee I found federal common law available for a dispute between state and city governments over interstate pollution does not resolve the question of whether state tort law is available for similar claims. This is a serious error of logic and legal history, as state and federal common law are not mutually exclusive. Nor does Milwaukee I support the sweeping argument that the Constitution only permits federal common law claims when suits involve air and water pollution. The sole Constitutional provision cited in the entire Milwaukee I opinion is Article III, § 2, cl. 3, which gives the Supreme Court original jurisdiction over disputes in which a state is a party. The Justices simply did not evaluate the question of whether claims involving transboundary environmental issues could be brought under state tort law. And they said nothing about whether Congress needs to authorize such claims.

A subsequent dispute between Milwaukee and Illinois over Lake Michigan water pollution, which reached the Supreme Court in 1981, likewise contains no support for the contention that only federal common law is available for claims that involve transboundary environmental processes. In that case—known as Milwaukee II—the Justices found that the modern version of the Clean Water Act had displaced Illinois’s federal common law claims against Wisconsin cities for poor sewage management. Passed in 1972, the Act provided clear mechanisms for managing the activities Illinois sought to abate with its lawsuit. A majority of the Justices therefore determined that Congress had displaced federal common law remedies.

But again, the case did not address the question of whether a plaintiff could bring a case under state tort law if the claim concerns a transboundary threat. Indeed, as Justice Rehnquist acknowledged in the majority opinion, the question of whether federal law supersedes state law involves a different analysis, which “requires evidence of a clear and manifest congressional purpose to pre-empt state law.”

Justice Kavanaugh’s invocation of International Paper Co. v. Ouellette fares no better. As discussed at length in my Essay in the Yale Law Journal Forum, the Supreme Court unanimously held in International Paper Co. that the Clean Water Act had not preempted state tort suits over interstate water pollution so long as those suits were brought under the law of the state where the discharger was located. The Justices did find the Act preempted the application of a recipient state’s common law because it would obstruct operation of the federal permitting program. Yet far from establishing that Congress must authorize state tort suits, the Justices engaged in a traditional preemption analysis in finding such claims were preempted. The opinion did not stipulate that Congress had to authorize state tort suits over transboundary environmental harms.

The final case, American Electric Power v. Connecticut, dealt with a similar issue to Milwaukee II. It concerned the displacement of federal common law claims against fossil fuel-fired power plants. The case began long before the Supreme Court found that greenhouse gases were subject to regulation under the Clean Air Act in Massachusetts v. EPA. When American Electric Power reached the Court several years after that decision, the Justices concluded that any federal common law claim, if it existed, had been displaced. Because the Clean Air Act provided the Environmental Protection Agency authority to set greenhouse gas emission standards for power plants, there was no room for federal courts to fashion a separate scheme. The Court explicitly reserved the question of whether state tort law might be available for similar claims, as that issue had not been briefed during the litigation.

How the Justices will ultimately rule in Suncor is difficult to predict. But it should be evident that the above cases do not stand for the things that Justice Kavanaugh and the petitioners seem to believe they do. The fact that prior Supreme Court cases found federal common law claims were available for disputes between state and city sovereigns over transboundary pollution does not mean only federal common law is available for all tort litigation involving chemical substances that cross state lines.

It is important to get this right not only because of the potential ramifications for climate tort suits. Many harmful substances are transported across state and national boundaries. Requiring Congress to clearly authorize state tort suits when those substances travel over a border and damage the environment or public health—rather than preempt them, as has long been the rule—could prove enormously disruptive to state tort law. Then again, perhaps a widespread disruption to state authority over environmental threats is what the petitioners, their allies, and Justice Kavanaugh, are all seeking.

Rachel Rothschild is an assistant professor of law at the University of Michigan Law School. Her scholarship sits at the intersection of environmental law, history, and policy.