State Tort Claims for Climate Change Damages Are Not Preempted by Federal Law

Introduction

In In Re County Commissioners of Boulder County and City of Boulder v. Suncor Energy USA, Inc., et al., 2025 CO 21, the Colorado Supreme Court resolved a critical threshold question: whether local governments may pursue state‐law tort claims—public and private nuisance, trespass, unjust enrichment, and civil conspiracy—against major oil and gas producers for harms allegedly caused by the defendants’ fossil fuel production and resulting climate change, or whether those claims are preempted by federal law. Plaintiffs Boulder County and the City of Boulder (collectively “Boulder”) seek monetary damages to offset costs of flood control, wildfire mitigation, drought management, and related climate impacts. Defendants Exxon Mobil Corporation and several Suncor Energy entities contend federal common law and the Clean Air Act (CAA) preempt these state‐law remedies. The Supreme Court granted original jurisdiction under C.A.R. 21 to decide this novel and nationally significant issue.

Summary of the Judgment

Justice Gabriel, writing for the majority (Chief Justice Márquez and Justices Hood, Hart, and Berken Kotter), held that:

The federal common law of interstate air and water pollution has been displaced by statute (notably the Clean Air Act and its companion, the Clean Water Act).
Once displaced, federal common law “plays no part” in preemption analysis; instead one must examine whether the statute itself preempts state tort claims.
The CAA contains no express preemption provision barring common‐law tort actions for climate‐related harms. Field preemption fails because states retain implementation and enforcement roles (SIPs) and the CAA’s savings clauses preserve state remedies at least as stringent as federal standards. Conflict preemption (impossibility or obstacle) does not apply: it is not impossible to comply with both the CAA and a state nuisance judgment, and a money judgment seeking remediation costs does not obstruct federal regulatory goals. Foreign affairs preemption is likewise inapplicable absent any actual conflict with an express federal treaty or policy.
Accordingly, Boulder’s state‐law claims may proceed; the Court discharged the C.A.R. 21 order to show cause and remanded for merits proceedings, explicitly reserving judgment on the ultimate viability of those claims.

Analysis

Precedents Cited

Erie R.R. Co. v. Tompkins, 304 U.S. 64 (1938): “There is no federal general common law.” State law ordinarily governs unless Congress has legislated otherwise. (§I)
Illinois v. City of Milwaukee (“Milwaukee I”), 406 U.S. 91 (1972): Recognized a narrow federal common law for “suits by one State to abate pollution emanating from another State.” The Court stressed the need for a uniform federal rule in interstate pollution disputes. (§I)
City of Milwaukee v. Illinois (“Milwaukee II”), 451 U.S. 304 (1981): Following enactment of the Clean Water Act, the Court held that Congress had displaced the federal common law of water pollution nuisance. (§I)
American Electric Power Co. v. Connecticut, 564 U.S. 410 (2011): Confirmed that the Clean Air Act displaces any federal common law nuisance remedy for carbon‐dioxide emissions. (§I)
United States v. Locke, 529 U.S. 89 (2000): Where federal law traditionally governs, the proper question is whether the state law “is consistent with the federal statutory structure,” not whether it is merely preempted. (§II)
City & County of Honolulu v. Sunoco, 537 P.3d 1173 (Haw. 2023): A state supreme court applied AEP’s displacement rule, then performed a CAA preemption analysis, holding there was no express, field, or conflict preemption of damages‐only tort claims. (§II)
City of New York v. Chevron Corp., 993 F.3d 81 (2d Cir. 2021): The Second Circuit held that federal common law had been displaced and the CAA does not affirmatively authorize state‐law tort claims for climate harms. (§III)

Legal Reasoning

The Court’s opinion follows a two‐step framework:

Displacement of Federal Common Law: Under Erie and Milwaukee I–II, federal courts once fashioned a federal common law of nuisance for interstate pollution. AEP held the Clean Air Act displaced that remedy for greenhouse‐gas emissions. Once federal common law is displaced, courts “look to whether the [statute] preempts state law.” The majority rejected any lingering role for federal common law in the preemption inquiry. (§¶27–32)
Preemption Analysis Under the CAA: The Supremacy Clause allows Congress to override state law, but courts start with a presumption against preemption where states historically legislated. The CAA contains no express preemption of state torts. Field preemption fails—Congress did not completely occupy the field of air‐pollution regulation; states implement, maintain, and enforce federal standards through SIPs, and two savings clauses (§§7416, 7604(e)) preserve state and private‐party rights to impose stricter limits or seek relief. Conflict preemption fails on both impossibility and obstacle grounds—no irreconcilable conflict with federal obligations, no demonstration that a money judgment interferes with the CAA’s balanced regulatory scheme. Finally, foreign affairs preemption requires an actual conflict with federal treaties or policy, which defendants did not establish. (§¶33–68)

Impact

This decision significantly expands the potential fora for climate‐change litigation. Local governments in Colorado (and potentially beyond) can now pursue state‐law damages claims against fossil‐fuel producers for costs incurred from climate impacts.

It invites a wave of similar suits—several other counties in Colorado and municipalities across the country have pending or contemplated actions alleging the same torts.
It underscores that, absent explicit congressional preemption, states retain a crucial role in addressing environmental harms through traditional tort remedies.
It may spur higher stakes in settlement negotiations, as damages awards for climate‐related mitigation costs could reach into the billions.
It highlights the interplay—and potential friction—between federal regulatory schemes and state common‐law powers. Policymakers in Congress and state legislatures may revisit express preemption clauses or statutory relief mechanisms to reduce uncertainty.

Complex Concepts Simplified

Federal Common Law: A narrow body of judge‐made law governing subjects of uniquely federal interest (e.g., disputes between states). Erie v. Tompkins abolished “general” federal common law in favor of state law, except where federally mandated.
Displacement: When Congress enacts a comprehensive statute in an area once covered by federal common law, it “displaces” that common law remedy (Milwaukee II; AEP).
Express, Field, and Conflict Preemption:
- Express: Congress explicitly says “no state law.”
- Field: Federal regulation so pervasive Congress intended to occupy the field exclusively.
- Conflict: State law is physically impossible to follow alongside federal law (impossibility) or stands as an obstacle to federal objectives (obstacle).
SIPs and Savings Clauses: States must submit State Implementation Plans to enforce federal air quality standards; two clauses in the CAA (§§7416, 7604(e)) expressly preserve state and private rights to impose stricter requirements or seek relief under state law.
Foreign Affairs Preemption: The federal government’s exclusive power over foreign affairs can preempt state laws—especially when those laws have more than an incidental effect on diplomacy. Absent any direct conflict or intrusion on federal‐government prerogatives, a court will not preempt a state common law claim.

Conclusion

The Colorado Supreme Court’s decision in In Re County Commissioners of Boulder County and City of Boulder establishes that, in the absence of explicit federal preemption, local governments may pursue state‐law tort claims against major fossil‐fuel producers for climate‐change damages. By rejecting any residual preemptive force of federal common law and applying a standard statutory preemption analysis to the Clean Air Act, the Court carved out a clear path for damages‐only remedies under traditional state tort principles. This landmark ruling crystallizes the tension between federal environmental statutes and state common‐law powers and will reverberate through future climate litigation and legislative responses at both the federal and state levels.