Suncor v. Boulder Supreme Court Argument: Eight Justices Weigh Whether Climate Tort Suits Survive

Suncor v. Boulder Supreme Court argument: what is at stake on Monday

The Supreme Court will hear Suncor Energy (U.S.A.) Inc. v. County Commissioners of Boulder County, No. 25-170, at 10 a.m. on Monday, October 5, 2026, to decide whether federal law bars a Colorado city and county from suing Exxon Mobil and Suncor under state tort law for the local costs of climate change. Only eight justices will sit, because Justice Samuel Alito recused on September 28, so a 4-4 split would leave intact the Colorado Supreme Court's 2025 ruling that Boulder's claims can go forward, while a five-vote majority for the companies could end more than two dozen similar suits nationwide.

The Suncor v. Boulder Supreme Court case is the first argument of the October 2026 Term and the most consequential preemption fight the Court has taken up since it confined pesticide warning claims in Monsanto Co. v. Durnell in June. It is not an emissions-regulation case in the ordinary sense. Boulder is not asking a court to cap anyone's emissions; it is asking a Colorado jury to award damages for flood control, wildfire mitigation, heat response and other adaptation costs, on the theory that the defendants sold fossil fuels for decades while concealing what they knew about the risks. That framing, product sales plus deception plus local harm, is the same architecture that drove opioid, tobacco and lead paint litigation. Whether it survives federal preemption is the question that every municipal Mass Tort plaintiff, every defense coordinating counsel and every litigation funder with exposure to public-entity cases is now waiting on.

The Court did not agree to answer only the merits. When it granted certiorari on February 23, 2026, it added a question of its own: whether it has statutory and Article III jurisdiction to hear the case at all, given that the Colorado Supreme Court's decision came at the motion-to-dismiss stage. That added question means the case can end in at least four different ways, and only one of them is the clean ruling for the industry that the petitioners and the federal government are seeking.

What is Suncor v. Boulder about?

Suncor v. Boulder is a suit by the Board of County Commissioners of Boulder County and the City of Boulder against Exxon Mobil Corporation and Suncor Energy entities, filed in Colorado state court in April 2018, seeking money damages for climate-related harms to local property and public infrastructure. The plaintiffs plead public nuisance, private nuisance, trespass, unjust enrichment and civil conspiracy under Colorado law; the Sabin Center's climate litigation database also lists a claim under the Colorado Consumer Protection Act among the causes of action originally pleaded.

The core allegation is that the companies knowingly contributed to the alteration of the climate by producing, promoting, refining, marketing and selling fossil fuels at levels they knew were dangerous, while concealing or misrepresenting those dangers. Boulder seeks compensation for past and future costs to analyze, mitigate, abate and remediate local climate impacts. It is a damages case, not an injunction case: the plaintiffs do not ask the court to order any change in how Suncor's Commerce City refinery operates or how much oil Exxon produces.

That distinction is the hinge of the entire dispute. The companies say that any state-law damages award premised on harms caused by worldwide greenhouse gas emissions is regulation of interstate and international emissions in disguise, which the Constitution's structure and the Clean Air Act reserve to the federal government. Boulder says it is suing over product marketing and deception that injured Colorado property, which is the ordinary business of state tort law and which no federal statute displaces.

The question presented in the companies' petition, as the Court accepted it, is whether federal law precludes state-law claims seeking relief for injuries allegedly caused by the effects of interstate and international greenhouse gas emissions on the global climate. The companies' word choice, "effects of interstate and international greenhouse-gas emissions", frames the case their way; Boulder's briefing frames the same claims as deception-based product claims. Much of Monday's argument will be a contest over which description is accurate.

When is the Suncor v. Boulder oral argument, and who argues?

The Suncor v. Boulder oral argument is scheduled for 10 a.m. Eastern on Monday, October 5, 2026, the opening day of the October 2026 Term, and it is the first case the justices will hear. Boulder County and the City of Boulder both announced the hearing date, and wire coverage on October 1 and 2 previewed it as the term's marquee case.

The argument will run longer than the standard hour's split between two sides. On July 31, 2026, the Solicitor General moved for leave to participate as amicus curiae supporting the petitioners and for divided argument, and the Court granted the motion. Under that arrangement, the petitioners have 20 minutes, the United States 10 minutes ceded from the petitioners' time, and the respondents 30 minutes.

Party	Position	Argument time	Counsel of record / team (from filings)
Suncor Energy (U.S.A.) Inc., Suncor Energy Sales Inc., Exxon Mobil Corporation	Petitioners: federal law precludes the claims	20 minutes	Kannon K. Shanmugam (counsel of record)
United States	Amicus supporting petitioners	10 minutes	Office of the Solicitor General
Board of County Commissioners of Boulder County; City of Boulder	Respondents: claims are not preempted; Court lacks jurisdiction	30 minutes	EarthRights International; Singleton Schreiber LLP (Kevin Hannon); Russell & Woofter LLC (Kevin K. Russell); Law Office of Marco B. Simons

Boulder's team mixes a human-rights litigation nonprofit, a Supreme Court boutique and a plaintiffs' personal injury and mass tort firm. That last element matters for the personal injury bar: the municipal climate cases are among the largest public-entity matters in which private plaintiffs' firms have a stake, and their value turns entirely on the answer to this preemption question.

What did the Colorado Supreme Court decide in the Boulder climate case?

The Colorado Supreme Court held on May 12, 2025, by a 5-2 vote, that Boulder's state-law claims are not preempted by federal law and that the trial court correctly refused to dismiss them. The opinion, County Commissioners of Boulder County v. Suncor Energy (U.S.A.) Inc., 2025 CO 21, 586 P.3d 161, was written by Justice Richard Gabriel and joined by Chief Justice Monica Márquez and Justices William Hood, Melissa Hart and Maria Berkenkotter. Justice Carlos Samour dissented, joined by Justice Brian Boatright.

The majority's reasoning came in two steps.

First, the court addressed federal common law. The companies relied on a line of cases, culminating in the Second Circuit's April 1, 2021 decision in City of New York v. Chevron Corp., holding that suits over global emissions arise under a federal common law of interstate pollution that leaves no room for state law. The Colorado majority accepted that federal common law once governed interstate air pollution, but held that the Clean Air Act displaced that body of federal common law, as American Electric Power Co. v. Connecticut recognized in 2011. Once displaced, the court reasoned, federal common law could not itself preempt state law. Displacement, in the majority's account, operates horizontally between Congress and federal courts; it does not carry a residual preemptive force against the states.

Second, the court asked whether the Clean Air Act preempts Boulder's claims directly. It found no express preemption, no field preemption and no conflict preemption. Leaning on International Paper Co. v. Ouellette, 479 U.S. 481 (1987), in which the Court held that the Clean Water Act barred an affected state's nuisance law against an out-of-state source but preserved claims under the source state's law, the majority treated Boulder's Colorado-law claims against companies that do business in Colorado as the kind of state remedy federal environmental statutes leave in place.

The dissent saw the case as the companies do. Justice Samour wrote that "state law remains incompetent to regulate interstate and international air pollution," and that Boulder therefore could not prosecute its claims. That sentence previews the petitioners' constitutional-structure argument almost word for word.

The case was in an original proceeding, a petition to the Colorado Supreme Court from the trial court's denial of the companies' motion to dismiss. The Colorado court discharged its order to show cause and remanded for further proceedings. No discovery-heavy record or verdict exists, which is precisely why the jurisdictional question is live.

Why did Justice Alito recuse himself from Suncor v. Boulder?

Justice Alito recused because, as he explained on September 30, after looking more deeply at the case and at "the particular arguments that were made on both sides," he concluded that recusal was "the prudent step," even though the Court's legal office had advised that disqualification was not required. He called the decision a "difficult judgment call." The initial notice, a one-sentence letter from the Clerk of Court released on September 28, gave no reason.

The financial-interest background is public. According to Alito's 2025 financial disclosure and a report from the advocacy group Consumer Watchdog published earlier in September, he holds individual stock in ConocoPhillips and Phillips 66. He does not hold Exxon or Suncor stock, the two companies actually before the Court. But Consumer Watchdog reported that every climate case it identified as stayed pending the outcome of Suncor v. Boulder names ConocoPhillips, Phillips 66 or both as defendants, including suits brought by California, New Jersey, Delaware, the City of Hoboken and eight California cities and counties. A ruling in Boulder would therefore move the value of companies in which the justice holds shares, even though those companies are not parties here.

Two other details shaped the coverage. Alito participated when the Court granted certiorari in February 2026, and he had rejected earlier calls to step aside. A Court spokesperson also told NBC News in May that his recusal in 2023, when Suncor and Exxon first unsuccessfully asked the Court to review the Boulder case, had been inadvertent. The September 28 recusal therefore reversed a stated position less than a week before argument, after the briefing was complete.

What happens if the Supreme Court splits 4-4 in Suncor v. Boulder?

If the eight participating justices divide 4-4, the Colorado Supreme Court's judgment is affirmed by an equally divided Court, Boulder's suit returns to the Boulder County District Court, and no nationwide precedent is set. An affirmance by an equally divided Court binds only the parties; it does not resolve the issue for the more than two dozen other climate suits pending in state and federal courts, and the companies could bring the same question back in another case once a full Court is available.

Post-recusal coverage, including Daily Caller's "Could Alito's Recusal Change The Outcome" analysis, treated the recusal as removing a probable vote for the companies, which is why it changes the arithmetic. With nine justices, the petitioners needed five of nine; with eight, they still need five of eight to win, while Boulder needs only four to preserve its Colorado judgment. Newsweek's coverage after the recusal flagged the 4-4 scenario explicitly. Whether the Court would want to spend its first argument of the term producing a non-precedential affirmance is a separate question; it could also hold the case or decide it on the jurisdictional ground, which requires only a majority of those sitting.

The two questions presented: preemption and jurisdiction

The Court is deciding two questions, and the order in which it takes them matters.

Question 1 (from the petition): whether federal law precludes state-law claims seeking relief for injuries allegedly caused by the effects of interstate and international greenhouse gas emissions on the global climate.

Question 2 (added by the Court): whether the Court has statutory and Article III jurisdiction to decide the case.

Could the Supreme Court dismiss Suncor v. Boulder for lack of jurisdiction?

Yes. The Supreme Court could dismiss Suncor v. Boulder without reaching preemption if it concludes that the Colorado Supreme Court's ruling is not a "final judgment" reviewable under 28 U.S.C. § 1257(a). That statute limits the Court's review of state-court decisions to final judgments of a state's highest court, and the Colorado decision only refused to dismiss the complaint, leaving the case to go forward to discovery and trial.

Boulder argued at the certiorari stage that the ruling was interlocutory and therefore not reviewable. The companies rely on Cox Broadcasting Corp. v. Cohn, 420 U.S. 469 (1975), which recognized four categories of state-court decisions treated as final even though further proceedings remain. The relevant one, the "fourth category," covers cases in which the federal issue has been finally decided in the state courts, the party seeking review might prevail on nonfederal grounds later (making the federal question unreviewable), and refusing immediate review would seriously erode federal policy. The companies say a preemption defense that would end the litigation entirely, in a field touching foreign affairs and interstate commerce, fits that category. Boulder also raises threshold Article III objections, which is why the Court framed its added question to cover both statutory and constitutional jurisdiction.

Because the Court added the question itself, the justices clearly regard it as serious. A dismissal for want of jurisdiction would leave the Colorado ruling standing exactly as a 4-4 split would, but for a different reason, and it would invite the companies to return after a final judgment in Boulder.

Does the Clean Air Act preempt state climate change lawsuits?

As of today, no court of last resort for the nation has said so: the Colorado Supreme Court held that the Clean Air Act does not preempt Boulder's state-law climate claims, and the Supreme Court will decide in this case whether that is right. The companies and the United States argue that it does, either directly or through constitutional limits on state power, while Boulder argues the statute leaves state-law damages remedies untouched.

The petitioners' case

Exxon and Suncor make three overlapping arguments, according to their briefs and the summaries of them in the Congressional Research Service primer and the Legal Planet guide.

1. Constitutional structure. Interstate and international air pollution was historically governed by federal common law because the Constitution's structure, including the equal sovereignty of the states and limits on extraterritorial regulation, does not permit one state to apply its law to emissions occurring in other states and countries. That structural principle survives even if the Clean Air Act displaced the federal common-law cause of action. State law, in the dissent's words, "remains incompetent" in the field.
2. The Clean Air Act. Congress gave the Environmental Protection Agency, not state juries, authority over domestic greenhouse gas emissions. State tort damages for emissions-driven harms conflict with that scheme and with the Act's limited preservation of state authority over in-state sources.
3. Foreign affairs. Climate policy is negotiated among nations, and state-law liability for worldwide emissions intrudes on the federal government's exclusive role in foreign relations.

The petitioners also lean on City of New York v. Chevron, where the Second Circuit held in 2021 that a city could not use state tort law to hold producers liable for global emissions, describing climate change as a uniquely international concern touching federalism and foreign policy. The Colorado decision is in tension with that ruling, which is the closest thing to a split the case presents.

Boulder's case

Boulder's response is that the companies are arguing against a lawsuit Boulder did not file.

1. Displacement is not preemption. Once the Clean Air Act displaced federal common law, as American Electric Power held, nothing remained to preempt state law except the Act itself, and the Act preserves state remedies. A preemption doctrine built from a now-displaced federal common law, Boulder argues, is a structural intuition without a constitutional text.
2. Ouellette preserves source-state and local claims. Under International Paper v. Ouellette, federal pollution statutes allow claims under the law of the state where a defendant operates. Exxon and Suncor sell and refine fuel in Colorado.
3. Deception claims regulate conduct, not emissions. The injury-producing conduct alleged is marketing and concealment, which states have always policed. Commentary on Boulder's side, including a recent Just Security analysis, argues that deception-based claims would survive even a ruling that limits pure emissions-based nuisance claims.

Nineteen attorneys general, in a coalition co-led by Colorado's Phil Weiser and California's Rob Bonta, filed in support of Boulder, stressing state police power over in-state injuries and consumer deception. A 26-state coalition led by Alabama and West Virginia filed in support of the companies, arguing that a single state's courts should not set energy policy for the nation. Inside Climate News reported that more than 40 states had signed amicus briefs on one side or the other, consistent with the 26 and 19 state counts in the two coalitions.

What is the Trump administration's position in Suncor v. Boulder?

The Trump administration supports Exxon and Suncor and is asking the Court to hold that federal law precludes Boulder's claims. The federal government's amicus brief argues that the claims contravene the Constitution's territorial limits on state authority and the federal government's exclusive role over foreign affairs, and that they conflict with the decision-making scheme Congress enacted in the Clean Air Act.

That position fits a broader federal campaign. In April 2025, President Trump signed an executive order titled "Protecting American Energy From State Overreach," directing the Justice Department to identify and act against state laws and actions burdening domestic energy production. On April 30 and May 1, 2025, the Justice Department sued Hawaii and Michigan to block planned state climate suits before they were filed, and it also sued New York and Vermont over their climate superfund statutes. Hawaii filed its suit anyway. In late September 2026, law firm alerts reported a federal court ruling that New York's Climate Change Superfund Act is preempted by the Clean Air Act, a decision that shows the same preemption theory gaining traction in lower federal courts even before the justices rule.

The executive branch's participation gives the petitioners an additional 10 minutes at the lectern and a voice that can speak to foreign-relations consequences with institutional authority. It also creates an unusual alignment: the federal government arguing that a federal statute it administers leaves no room for state damages, while the same statute's savings provisions are central to Boulder's reply.

Which climate lawsuits will Suncor v. Boulder affect?

A ruling for the companies on the merits would likely end or sharply narrow more than two dozen climate damages suits brought by states, counties and cities, including cases by California, New Jersey, Delaware, Hoboken and eight California cities and counties that are currently stayed pending this decision, as well as Honolulu's suit against Sunoco, Shell, Exxon Mobil and others. A ruling for Boulder would let those cases proceed toward discovery and trial in state courts.

The landscape, as reported by Inside Climate News, Harvard's Environmental and Energy Law Program and Bloomberg's October 2 preview, includes:

• State attorney general suits, including California, New Jersey and Delaware; Bloomberg's preview notes that similar suits have been filed against BP, Chevron, ConocoPhillips, Shell and Phillips 66.
• Municipal suits, including Honolulu, where the Court denied the companies' certiorari petition on January 13, 2025, allowing the case to proceed in Hawaii state court, and Hoboken in New Jersey.
• Hawaii's state-level suit, filed after the Justice Department sought to block it.
• Climate superfund statutes in New York and Vermont, which impose cost-recovery liability by legislation rather than through tort suits but face the same preemption arguments.

None of the municipal climate suits has reached trial. The Supreme Court had repeatedly declined to take the merits question, ruling only on procedural removal issues in BP p.l.c. v. Mayor and City Council of Baltimore in 2021, before agreeing to hear Boulder. That history explains why a merits ruling now would be the first nationwide answer on whether state law can carry these claims.

Four ways the case can end

Outcome	Votes needed	Effect on Boulder	Effect on other climate suits	Precedent
Reversal on preemption	5 of 8	Dismissed	Most state-law damages suits face dismissal; deception-only theories may be tested	Binding nationwide
Affirmance on the merits	5 of 8	Proceeds to discovery and trial	Stayed suits revive; defendants fall back on causation, standing and damages defenses	Binding nationwide
4-4 split	4 for Boulder	Proceeds	No change in law; stays likely lifted case by case	None
Dismissal for want of jurisdiction	Majority of those sitting	Proceeds; companies may return after final judgment	Lower courts continue to diverge	Only on § 1257 finality

A narrower middle path is also possible. The Court could hold that pure emissions-based nuisance and trespass claims are preempted while leaving deception-based theories for remand, or it could rest on constitutional structure without reaching the Clean Air Act. Either would produce a binding ruling but leave plaintiffs room to replead around it, much as Durnell left design defect claims open after foreclosing label-based Failure to Warn claims.

Timeline: from a 2018 complaint to a 2027 decision

Date	Event
April 2018	Boulder County and the City of Boulder sue Suncor entities and Exxon Mobil in Colorado state court
April 1, 2021	Second Circuit affirms dismissal in City of New York v. Chevron on federal-law grounds
2023	Supreme Court denies the companies' first certiorari petition in the Boulder case (removal posture); Alito's recusal later described as inadvertent
January 13, 2025	Supreme Court denies certiorari in the Honolulu climate case
April 2025	Executive order "Protecting American Energy From State Overreach"
April 30–May 1, 2025	Justice Department sues Hawaii and Michigan; also challenges New York and Vermont superfund laws
May 12, 2025	Colorado Supreme Court rules 5-2 that Boulder's claims are not preempted (2025 CO 21)
February 23, 2026	Supreme Court grants certiorari and adds jurisdiction question
July 31, 2026	Solicitor General moves for divided argument supporting petitioners; motion granted
August 2026	State amicus briefs: 19 for Boulder, 26 for the companies
September 28, 2026	Clerk announces Justice Alito's recusal
September 30, 2026	Alito calls recusal a "difficult judgment call" and "prudent"
October 5, 2026	Oral argument, 10 a.m., first case of the term
By end of June 2027	Expected decision, in line with the Court's practice of deciding argued cases by the end of the term

When will the Supreme Court decide Suncor v. Boulder?

A decision in Suncor v. Boulder is expected by the end of the October 2026 Term, which in practice means by late June or the first days of July 2027. The Court usually decides cases argued in October earlier than that, but major, closely divided cases often run to the end of the term, and a fractured eight-member bench with a jurisdictional threshold question is more likely to take longer than a unanimous one. A dismissal for want of jurisdiction or a 4-4 affirmance could come much sooner, sometimes as a one-line order.

The mass tort lens: preemption as the gateway defense

For mass tort practitioners, the Suncor v. Boulder Supreme Court case is best read alongside the Court's recent preemption work in product cases. In Monsanto Co. v. Durnell, decided June 25, 2026, the Court held 7-2 that FIFRA expressly preempts state Failure to Warn claims demanding a cancer warning different from the EPA-approved label, but it did not reach design defect claims, which Roundup plaintiffs are now trying in St. Louis County. The lesson defense counsel drew from Durnell is that preemption, framed tightly around a federal regulatory scheme, can dispose of a litigation's central theory. The lesson plaintiffs drew is that a well-drafted complaint can route around it.

Boulder sits at the far end of that spectrum. There is no federal label and no express preemption clause; the companies rely on implied preemption and constitutional structure, which the Court has historically applied sparingly. But the remedy Boulder seeks, compensation for harms flowing from global emissions, has no obvious limiting principle, which is why the Court may be receptive to a structural rule. The more the opinion speaks in terms of interstate externalities and foreign relations, the less it will matter to ordinary product cases; the more it speaks in terms of implied conflict with federal regulatory schemes, the more it will be cited in PFAS, plastics, and other environmental Mass Tort dockets where defendants argue that state juries are second-guessing federal regulators.

Practitioners should also watch how the justices treat the deception theory. Public nuisance and consumer-protection claims based on misleading marketing are the backbone of public-entity litigation against opioid distributors, social media platforms and e-cigarette makers. A majority opinion that describes Boulder's deception allegations as merely a repackaged emissions claim would be a tool defendants could deploy well beyond climate. A majority that separates deception from emissions, even while ruling for the companies on the emissions theory, would protect that model.

What it means for municipal plaintiffs, plaintiffs' firms and funders

For public-entity plaintiffs and their outside counsel. Many municipal climate suits are litigated by private firms on a Contingency Fee basis, alongside nonprofit co-counsel. A merits loss would wipe out years of unbilled work across multiple dockets at once; a win would make those dockets among the largest public-entity matters in the country, with discovery into decades of internal company documents. Counsel in stayed cases should be preparing both a post-decision motion to lift stays and an amended-pleading strategy that isolates deception and in-state marketing conduct in case the Court draws that line.

For Third-Party Litigation Funding providers. Funders with positions in public-entity portfolios should treat October 5 as a binary event with a long tail. Questions from the bench on whether deception-based theories are severable from emissions theories will be the best real-time signal of residual value. A jurisdictional dismissal or 4-4 split would preserve value but push any national answer years out.

For defendants and insurers. Energy defendants, and companies in other industries facing public-entity nuisance suits, should expect the opinion to be cited immediately in motions to dismiss. Insurers on the other side of coverage disputes over climate claims will read the case for its characterization of the conduct at issue, which bears on occurrence and expected-or-intended questions in liability policies.

For mass tort defense and MDL practitioners. If the Court adopts a constitutional-structure rule, expect attempts to extend it to other pollution-based Mass Tort claims involving interstate sources. If it rests narrowly on the Clean Air Act, its reach into product cases governed by other statutes will be limited. Either way, the Court's treatment of 28 U.S.C. § 1257 finality could make it easier or harder to seek Supreme Court review of interlocutory state-court rulings rejecting preemption defenses, a procedural question with large consequences for state-court coordinated proceedings that never pass through a federal Multidistrict Litigation (MDL).

For the plaintiffs' bar generally. The case is a reminder that the most valuable public-entity litigation of the last two decades, from tobacco to opioids, depended on state-law theories that survived federal preemption challenges at the threshold. A ruling that state law is "incompetent" to address harms with interstate causes would draw a new boundary around that model.

Frequently asked questions

What is Suncor v. Boulder about?

Suncor v. Boulder asks whether federal law bars the City of Boulder and Boulder County from suing Exxon Mobil and Suncor under Colorado tort law for local climate-change costs. Boulder alleges the companies sold fossil fuels while concealing their dangers and seeks damages for adaptation and remediation.

When is the Suncor v. Boulder oral argument?

Oral argument is at 10 a.m. Eastern on Monday, October 5, 2026, the first case of the Supreme Court's October 2026 Term. The petitioners have 20 minutes, the United States 10 minutes in support of the petitioners, and Boulder 30 minutes.

Why did Justice Alito recuse himself from Suncor v. Boulder?

Alito said recusal was "the prudent step" and a "difficult judgment call" after reviewing the parties' arguments, even though the Court's legal office said it was not required. He owns stock in ConocoPhillips and Phillips 66, which are defendants in climate suits stayed pending this case, though not parties to it.

What happens if the Supreme Court splits 4-4 in Suncor v. Boulder?

A 4-4 split affirms the Colorado Supreme Court's judgment by an equally divided Court, so Boulder's suit proceeds in state court. It sets no national precedent, leaving other climate suits to be decided court by court.

Does the Clean Air Act preempt state climate change lawsuits?

The Colorado Supreme Court held in 2025 that it does not, finding no express, field or conflict preemption, and the Supreme Court will now decide the issue. The oil companies and the federal government argue the Act and the Constitution's structure preclude such claims.

When will the Supreme Court decide Suncor v. Boulder?

A decision is expected by the end of the term in late June or early July 2027. A 4-4 affirmance or a dismissal for lack of jurisdiction could come sooner.

This article is analysis of a pending case and is not legal advice.