The U.S. Supreme Court will hear oral argument Monday in Suncor Energy Inc. v. County Commissioners of Boulder County, a closely watched dispute over whether a Colorado municipal climate suit can proceed against two of the world's largest fossil-fuel producers.

The case tests whether federal law — the Constitution, foreign-affairs doctrine, and the Clean Air Act — bars state-law claims by the city and county of Boulder against Suncor Energy (a subsidiary of one of Canada's largest energy companies that operates the only two oil refineries in Colorado) and ExxonMobil. Boulder seeks compensation for the costs of protecting local property and residents from the alleged effects of climate change.

What's at stake?

Boulder filed the suit in 2018. After the energy companies failed to remove it to federal court and failed to obtain dismissal in state trial court, the Colorado Supreme Court ruled that the claims could go forward under Colorado law. Suncor and ExxonMobil petitioned the U.S. Supreme Court in August 2025; certiorari was granted in February 2026.

Dozens of similar municipal and state-court actions now pending across the country will be shaped by the justices' decision. The energy firms contend the suit would "impose an enormous 'carbon tax' that could 'bankrupt[]' the energy industry." Boulder counters that the companies are "at the wrong time with the wrong arguments."

Why might the justices sidestep the merits?

Before the court can address preemption, it must first decide it has jurisdiction. The Supreme Court generally reviews only final state-court judgments, and Boulder argues the Colorado Supreme Court proceeding was an ordinary appeal, not a self-contained "original jurisdiction" matter.

The companies invoke Cox Broadcasting Corp. v. Cohn (1975), 420 U.S. 469, under which the justices can review state-court rulings that finally resolve federal-law questions whose reversal would terminate the underlying litigation. Boulder responds that the energy firms still have other federal-law defenses — including constitutional ones — that they could raise on remand.

A 4-4 tie is now a real possibility. Justice Samuel Alito, who owns stock in oil and gas companies, announced on September 28, 2026 that he would not participate. A tie would leave the Colorado Supreme Court ruling in place and produce no national precedent.

How do the companies justify preemption?

The petitioners advance three grounds:

• Constitutional structure: States gave up authority over "inherently transboundary issues such as global climate change," leaving federal law to govern disputes over ambient air and water.
• Foreign affairs: Greenhouse-gas emissions allegedly occur outside the U.S., and lawsuits like Boulder's "interfere with the federal government's extensive diplomatic efforts."
• Clean Air Act preemption: The statute creates a "comprehensive statutory scheme" giving the EPA primary responsibility for emissions standards.

How does Boulder push back?

Boulder counters on each front:

• No constitutional text forbids state-law claims, and courts routinely apply state law to cross-border harms.
• The foreign-affairs doctrine requires a clear conflict with an express federal policy embodied in "a statute, self-executing treaty, or executive agreement" — not "generalized diplomatic concerns."
• The Clean Air Act regulates "point-source emissions," not damages for downstream harm, and the EPA itself has said it lacks authority to directly regulate greenhouse gases under the statute.

Congress, Boulder notes, "is in fact currently considering legislation that would give the companies the immunity they seek."

Who else is weighing in?

The Trump administration filed an amicus brief supporting the energy companies. It argues that Boulder and similar plaintiffs seek "to address a global problem caused by global conduct with global effects by imposing open-ended liability on fossil-fuel producers for every far-downstream consequence in that State that a state court attributes to global warming." The brief warns that "[t]he Constitution rejects that butterfly-effect theory of state authority" and that "[o]ur federal system would disintegrate" if each state could force its regulatory prescriptions on the other 49.

The Washington Legal Foundation warned of "cataclysmic consequences," claiming judgments like the one Boulder seeks could "wipe out the corporate infrastructure" supporting "over 80 percent of world energy use and 83 percent of our domestic mix." Two former Arizona solicitors general urged the justices "to stop the use" of such suits "as an ideological, cross-border battering ram" in areas ranging "from climate to plastics to cars to guns."

On Boulder's side, climate economists told the court that "the economic evidence does not support the claim that" requiring compensation "would destabilize the American economy." The Natural Resources Defense Council argued that scientists "can identify and quantify human contribution to observed changes in the climate system." The Brady Center to Prevent Gun Violence and the Giffords Law Center countered that ordinary personal-jurisdiction and causation limits prevent open-ended liability for large out-of-state defendants.

What changes for practitioners?

A ruling for the energy companies would likely force dismissal of dozens of similar state-court actions and sharply narrow municipal recourse under state tort law. A ruling for Boulder — whether by majority or 4-4 affirmance — would keep those cases alive and intensify disclosure and litigation exposure for fossil-fuel issuers and their downstream customers, while leaving foreign-affairs and Clean Air Act defenses open for further development on remand.