The U.S. Supreme Court on Monday heard nearly two hours of oral argument in Suncor Energy Inc. v. County Commissioners of Boulder County, weighing whether federal law blocks Boulder, Colorado's state-court lawsuit accusing Suncor and ExxonMobil of contributing to climate change. The justices sent mixed signals about which side was winning, leaving the ultimate outcome — potentially a 4-4 tie — uncertain.

Justice Samuel Alito recused from the dispute. A 4-4 outcome would leave intact a Colorado Supreme Court ruling that allowed Boulder's claims to proceed.

What did the energy companies argue?

Kannon Shanmugam, representing Suncor and ExxonMobil, called Boulder's claims "an unprecedented effort to use state law to regulate global conduct." Federal law governs interstate-pollution claims, he said, and a state cannot regulate out-of-state conduct "based solely on attenuated effects" within its borders.

Sarah Harris, principal deputy U.S. solicitor general, argued on behalf of the Trump administration in support of the companies.

Which justices pushed back?

Chief Justice John Roberts appeared skeptical of the energy companies' position. He asked Harris: "what makes this situation different from those" cases in which the court allowed out-of-state-conduct lawsuits to proceed in state court.

Harris answered that, unlike those lawsuits, Colorado would ban the production of fossil fuels anywhere in the country.

Roberts did not appear persuaded. "That's a question of relief" for Colorado, "and what might be available," he said. It did not, in his view, necessarily go to whether Colorado could bring the lawsuit at all.

Justice Elena Kagan also questioned Shanmugam. Boulder's "suit appears to be based on the old 1990s tobacco suits," she said, as well as recent opioid lawsuits. Were those also trumped by federal law?

Shanmugam distinguished them, arguing that "the injury at the center of Boulder's case is not suffered by the Colorado consumer in the traditional sense."

Kagan pressed: "Where," she asked, "is the[re] anything for that?"

What did Boulder argue?

Kevin Russell, representing Boulder, told the justices that states have long provided court remedies for in-state injuries even when the conduct occurred elsewhere. Applying state law, he said, has never projected that law nationwide. Congress can preempt state laws when it chooses.

Justice Brett Kavanaugh emerged as the justices' most vocal supporter of the energy companies. He cited Supreme Court precedent that, in his words, "make crystal-clear that interstate air and water pollution are matters for federal law" unless Congress specifies otherwise.

How would the ruling extend beyond Boulder?

Several justices raised floodgates concerns. Justice Clarence Thomas asked Russell to identify principles restricting whom state and local governments could sue. Russell cited jurisdictional limits and the dormant commerce clause, which bars discrimination against out-of-state defendants.

Thomas then asked whether state and local governments could also sue "large retailers" of fossil fuels.

Russell acknowledged that "[n]othing in our theory prevents that," adding that "when lots of people act together to cause widespread harm, it is not uncommon for multiple different defendants to be potentially liable."

Justice Amy Coney Barrett asked whether Colorado could pass a law capping greenhouse-gas emissions in the other 49 states. Russell agreed such a law would be barred, distinguishing the lawsuit. Boulder "doesn't prohibit them from engaging in the conduct," he said. "[I]t requires them to internalize a portion of the cost of their activities by paying for some of the damage that those activities inflict."

Shanmugam, in rebuttal, called "the breadth of Boulder's theory here" "astonishing." He suggested Boulder could sue him "for refilling my car and thereby contributing to global climate change." Under Boulder's theory, "[a]ny individual who has suffered the effects of climate change" could sue.

"Climate change," he concluded, "is certainly one of the most pressing issues of the day, but we would submit that this is exactly the wrong way to go about resolving it."

What happens next?

Justices Sonia Sotomayor and Ketanji Brown Jackson probed whether the Court has jurisdiction to consider the appeal — an off-ramp the justices flagged when they granted cert in February. The jurisdictional question received little traction elsewhere on the bench.

Why does the case matter for practitioners?

The suit, filed eight years ago, targets Suncor — a subsidiary of one of Canada's largest energy companies that operates the only two oil refineries in Colorado — and ExxonMobil, the largest U.S. energy company. Boulder seeks compensation for costs tied to alleged climate impacts, including protecting municipal property and residents.

A ruling for the companies would halt Boulder's case and likely chill similar municipal suits nationwide. A ruling for Boulder would clear the path for Colorado and potentially other state and local governments to pursue damages against fossil-fuel producers under state tort law.