When the Supreme Court gavels in October Term 2026 on Monday, the first oral argument the justices hear will be in Suncor Energy v. Commissioners of Boulder County, a showdown that could deal a knockout blow to state climate lawfare against energy companies. The city of Boulder, Colorado, and the surrounding county brought tort claims in Colorado state court seeking to hold Suncor and ExxonMobil liable for past and future harm they allege stems from global climate change. The key question before the Court is whether federal law precludes state-law claims for injuries allegedly attributable to the climate effects of interstate and international greenhouse-gas emissions.
Boulder’s lawsuit is emblematic of a larger trend. Scores of state and local governments have brought similar suits invoking state tort law and seeking billions of dollars in judgments against energy companies for fuel production and sales around the world. Through these lawsuits (including Honolulu’s, which I previously covered), state and local governments are wielding state law as a tool to effectively set national climate policy and regulate conduct far beyond their borders.
In addition to being preempted by federal statutes, these state tort lawsuits are incompatible with our constitutional federalism. I recently wrote about three amicus briefs backing Suncor here, each making the historical case that the Constitution prevents one state from regulating conduct in the other forty-nine. I returned to the significance of the structural Constitution here. I expect those constitutional arguments to take center stage at oral argument and shape the justices’ thinking about this case.
In short, while states ceded certain elements of their power to the national government under the Constitution, they retained the attributes of sovereignty to govern matters reserved to state control. Within their appropriate spheres, states stand on an equal footing with one another. No state, big or small, has the authority to reach into another state and impose its regulations on conduct occurring there. This limitation, described in those amicus briefs as “horizontal federalism,” “territorial sovereignty,” and “territorial jurisdiction,” is an essential component of our constitutional order, which preserved states as distinct sovereigns under the umbrella of a national union.
Federalism is also the backbone of the petitioners’ own merits brief, which invokes over a century of precedent treating interstate pollution as a matter of federal rather than state law.
Federal common law long applied to pollution crossing state lines. When Congress passed the Clean Air Act (CAA) and Clean Water Act, it reinforced the notion that states lack the power to regulate out-of-state emissions. The CAA centralized federal regulatory power in the EPA and adopted a “Good Neighbor” framework with a petitioning process for upwind and downwind states. As the petitioners’ brief observes, the “need for uniform federal law” governing the ambient or interstate aspects of air and water “that flow without concern for political borders” applies even more “in the context of global climate change, where any claim necessarily implicates emissions released from countless sources spread across every State in the Nation—and indeed every nation in the world.” That the underlying lawsuit includes alleged injuries from international greenhouse-gas emissions further impedes Boulder’s claims as it undermines the federal government’s exclusive authority to conduct foreign policy.
The CAA does let a state set its own, stricter emissions standards, but only for sources inside that state, not for lawsuits that try to reach conduct in other states, as the Court held under a nearly identical Clean Water Act provision in International Paper Co. v. Ouellette (1987). New York City tried a similar workaround in its own suit against oil companies, arguing that once the Clean Air Act had displaced federal common law, its state nuisance claims could take that law’s place. The Second Circuit rejected the theory in City of New York v. Chevron Corp. (2021), holding that Congress replacing judge-made federal law with a statute doesn’t open the door for state courts to fashion their own conflicting regimes.
A member of Boulder’s legal team has admitted that the nationwide litigation strategy is a bid to impose a “carbon tax” that could bankrupt the energy industry. Disillusioned by their inability to persuade Congress to impose a forced energy transition on the nation, activists have turned to the state courts instead. Suncor presents the justices the opportunity to stop this destructive gamesmanship and reaffirm a core tenet of our constitutional federalism.