Amicus Curiae Brief — Suncor Energy (U.S.A.) Inc., et al., Petitioners v. County Commissioners of Boulder County, et al.

No. 25-170

In the Supreme Court of the United States

SUNCOR ENERGY (U.S.A.) INC., ET AL.,

PETITIONERS

v.

COUNTY COMMISSIONERS OF BOULDER COUNTY, ET AL.

ON PETITION FOR A WRIT OF CERTIORARI

TO THE SUPREME COURT OF COLORADO

BRIEF FOR THE UNITED STATES

AS AMICUS CURIAE SUPPORTING PETITIONERS

SARAH M. HARRIS

Deputy Solicitor General

Counsel of Record

ADAM R.F. GUSTAFSON

Acting Assistant

Attorney General

CURTIS E. GANNON

Deputy Solicitor General

ROBERT N. STANDER

Deputy Assistant

Attorney General

FREDERICK LIU

Assistant to the

Solicitor General

ROBERT J. LUNDMAN

KYLE GLYNN

Attorneys

Department of Justice

Washington, D.C. 20530-0001

SupremeCtBriefs@usdoj.gov

(202) 514-2217

QUESTION PRESENTED

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.

(I)

STATEMENT OF COMPLIANCE WITH

SUPREME COURT RULE 37.2

The counsel of record for all parties received timely

notice of the United States’ intent to file this amicus curiae brief on August 29, 2025. This brief is being filed

earlier than ten days before the due date.

(II)

TABLE OF CONTENTS

Page

Interest of the United States ........................................................ 1

Introduction ..................................................................................... 2

Statement ......................................................................................... 3

A. Boulder’s state-court complaint ........................................ 3

B. Petitioners’ attempt to remove to federal court ............. 5

C. The state trial court’s denial of the motion to

dismiss ............................................................................... 6

D. The Colorado Supreme Court’s exercise of original

jurisdiction ........................................................................... 7

Argument ......................................................................................... 9

A. This Court has jurisdiction to review the decision

below ................................................................................... 10

B. The decision below is incorrect ....................................... 12

1. The Constitution precludes Boulder’s state

common-law clams...................................................... 12

2. The Clean Air Act preempts Boulder’s state

common-law claims .................................................... 16

C. The decision below warrants this Court’s review ......... 19

Conclusion ...................................................................................... 23

TABLE OF AUTHORITIES

Cases:

Alabama v. California, 145 S. Ct. 130 (2024) ........................ 1

Alden v. Maine, 527 U.S. 706 (1999) ..................................... 13

American Elec. Power Co. v. Connecticut,

564 U.S. 410 (2011) ........................................... 2, 9, 15, 17, 18

American Ins. Ass’n v. Garamendi,

539 U.S. 394 (2003) ............................................................... 15

Atlantic Richfield Co. v. Christian,

590 U.S. 1 (2020) ............................................................. 10 ,11

BMW of N. Am., Inc. v. Gore, 517 U.S. 559 (1996) ............. 13

(III)

IV

Cases—Continued:

Page

BP p.l.c. v. Mayor & City Council of Baltimore,

593 U.S. 230 (2021) ................................................................. 5

Burnet v. Brooks, 288 U.S. 378 (1933) .................................. 12

Cameron v. Vandegriff, 13 S.W. 1092 (Ark. 1890) .............. 14

City & County of Honolulu v. Sunoco LP,

537 P.3d 1173 (Haw. 2023), cert. denied,

145 S. Ct. 1111 (2025) ........................................................... 22

City of Denver v. Mullen, 3 P. 693 (Colo. 1884) .................. 17

City of New York v. Chevron Corp.,

993 F.3d 81 (2d Cir. 2021) .......................................... 3, 19, 20

Commonwealth v. Macloon, 101 Mass. 1 (1869) ................. 14

Cox Broad. Corp. v. Cohn, 420 U.S. 469 (1975) ................... 12

Fisher v. District Court, 424 U.S. 382 (1976) ...................... 10

Franchise Tax Bd. v. Hyatt, 587 U.S. 230 (2019) ..... 8, 12, 13

Fuld v. Palestine Liberation Org.,

606 U.S. 1 (2025) ......................................................... 2, 13, 15

Hoery v. United States, 64 P.3d 214 (Colo. 2003) ............... 18

International Paper Co. v. Ouellette,

479 U.S. 481 (1987) ......................................................... 16, 21

Lewis v. Lewis, 189 P.3d 1134 (Colo. 2008) .......................... 19

MacPherson v. Buick Motor Co.,

111 N.E. 1050 (N.Y. 1916) ................................................... 14

Merrick v. Diageo Americas Supply, Inc.,

805 F.3d 685 (6th Cir. 2015) ................................................ 16

National Pork Producers Council v. Ross,

598 U.S. 356 (2023) ............................................................... 13

Public Serv. Co. v. Van Wyk, 27 P.3d 377 (Colo. 2001) ...... 18

Shell PLC v. City & County of Honolulu:

144 S. Ct. 2627 (2024) ............................................................ 1

145 S. Ct. 1111 (2025) .......................................................... 22

State v. Lord, 16 N.H. 357 (1844) .......................................... 14

V

Cases—Continued:

Page

State Farm Mut. Auto. Ins. Co. v. Campbell,

538 U.S. 408 (2003) ............................................................... 13

Suncor Energy (U.S.A.) Inc. v. Board of County

Comm’rs of Boulder County, 143 S. Ct. 78 (2022) ............. 2

Sunoco LP v. City & County of Honolulu:

144 S. Ct. 2627 (2024) ............................................................ 1

145 S. Ct. 1111 (2025) ...................................................... 9, 22

World-Wide Volkswagen Corp. v. Woodson,

444 U.S. 286 (1980) ............................................................... 12

Young v. Masci, 289 U.S. 253 (1933) ..................................... 14

Zschernig v. Miller, 389 U.S. 429 (1968) .............................. 16

Constitutions, statutes, and rules:

U.S. Const.:

Art. I, § 8, Cl. 3 (Commerce Clause) ................................ 6

Amend. XIV (Due Process Clause) ................................ 12

Colo. Const. Art. VI:

§ 2 ....................................................................................... 11

§ 3 ....................................................................................... 11

Mont. Const. Art. VII:

§ 2(1) .............................................................................. 10, 11

§ 2(2) .............................................................................. 10, 11

Clean Air Act, 42 U.S.C. 7401 et seq. ...................................... 1

Clean Water Act, 33 U.S.C. 1251 et seq. ........................ 16, 17

28 U.S.C. 1257(a) ......................................................... 10, 11, 22

28 U.S.C. 1441(a) ....................................................................... 5

28 U.S.C. 1442 ............................................................................ 5

Colorado Consumer Protection Act,

Colo. Rev. Stat. § 6-1-105(1) et seq. ...................................... 4

Sup. Ct. R. 10(b)....................................................................... 20

VI

Rules—Continued:

Page

Colo. App. R.:

Rule 21 ............................................................................ 7, 12

Rule 21(a)(1) ................................................................... 7, 11

Rule 21(a)(2) ....................................................................... 11

Rule 21(h) ........................................................................... 10

Rule 21(o) ........................................................................... 10

Mont. R. App. P.:

Rule 14(1) ..................................................................... 10, 11

Rule 14(3) ..................................................................... 10, 11

In the Supreme Court of the United States

No. 25-170

SUNCOR ENERGY (U.S.A.) INC., ET AL.,

PETITIONERS

v.

COUNTY COMMISSIONERS OF BOULDER COUNTY, ET AL.

ON PETITION FOR A WRIT OF CERTIORARI

TO THE SUPREME COURT OF COLORADO

BRIEF FOR THE UNITED STATES

AS AMICUS CURIAE SUPPORTING PETITIONERS

INTEREST OF THE UNITED STATES

This case presents the question whether the Constitution or the Clean Air Act, 42 U.S.C. 7401 et seq., precludes claims seeking to apply one State’s law to the activities of energy companies around the world to hold

those companies liable for injuries allegedly caused by

global climate change. The United States has a substantial interest in the proper interpretation of the federal

constitutional and statutory provisions involved. This

Court has previously called for the views of the Solicitor

General in cases involving similar state-law claims—

including in this very case, on the issue of whether the

claims could be removed to federal court. See Alabama

v. California, 145 S. Ct. 130 (2024) (No. 158, Orig.); Sunoco LP v. City & County of Honolulu, 144 S. Ct. 2627

(2024) (No. 23-947); Shell PLC v. City & County of Honolulu, 144 S. Ct. 2627 (2024) (No. 23-952); Suncor Energy

(1)

2

(U.S.A.) Inc. v. Board of County Comm’rs of Boulder

County, 143 S. Ct. 78 (2022) (No. 21-1550). And the

United States has recently brought suit to block similar

attempts by several States to impose state-law liability

for what they identify as the effects of climate change.

See United States v. Michigan, No. 25-cv-496 (W.D.

Mich.); United States v. Hawaii, No. 25-cv-179 (D. Haw.);

United States v. New York, No. 25-cv-3656 (S.D.N.Y.);

United States v. Vermont, No. 25-cv-463 (D. Vt.).

INTRODUCTION

In this case, local governments in Colorado seek to

hold various energy companies liable for the alleged

consequences of global climate change by applying Colorado common law to the companies’ production and

marketing of fossil fuels around the world. But under

the Constitution, “[s]tate sovereign authority is bounded

by the States’ respective borders.” Fuld v. Palestine

Liberation Org., 606 U.S. 1, 14 (2025). Colorado therefore may not apply its law to the companies’ conduct

outside the State. And even if it could, allowing Colorado

to deem the effects of the companies’ worldwide conduct

tortious “cannot be reconciled with the decisionmaking

scheme Congress enacted” in the Clean Air Act, which

precludes any such role for a single State. American

Elec. Power Co. v. Connecticut, 564 U.S. 410, 429 (2011).

Both the Constitution and the Clean Air Act thus bar

the state-law claims in this case.

The Colorado Supreme Court nevertheless allowed

the suit to proceed. That decision warrants this Court’s

review twice over. It is manifestly wrong on a question

of vast nationwide significance. And it concededly conflicts with a decision of the Second Circuit, which held

that the Clean Air Act bars similar claims brought by

the City of New York. See Pet. App. 19a-20a (discussing

3

City of New York v. Chevron Corp., 993 F.3d 81 (2d Cir.

2021)). The need for this Court’s review is especially pronounced because respondents’ suit is just one of many

that have been filed by States and local governments

across the country, each proceeding on similar theories

of state-law liability. If, as the Colorado Supreme Court

held, those theories are consistent with federal law,

then every locality in the country could sue essentially

anyone in the world for contributing to global climate

change. Because the decision below is contrary to the

Constitution and to the Clean Air Act, and because it

conflicts with the decision of a court of appeals on a frequently recurring issue of exceptional importance, the

petition for a writ of certiorari should be granted.

STATEMENT

A. Boulder’s State-Court Complaint

In April 2018, the City of Boulder, Colorado, and its

surrounding county (together, Boulder) sued petitioners in Colorado state court. Pet. App. 49a-50a. Petitioners are various energy companies that engage in “fossil

fuel activities”—namely, the production, promotion, refining, marketing, and sale of fossil fuels, such as coal,

oil, and natural gas. Am. Compl. ¶ 14.1

Boulder alleges that petitioners’ fossil-fuel activities

around the world have caused others to “use” petitioners’ fossil fuels. Am. Compl. ¶ 15; see id. ¶¶ 61, 81. And

Boulder alleges that fossil-fuel use occurring throughout the world has caused the “emission[]” of greenhouse

The plaintiffs originally included San Miguel County, but the

trial court transferred that county’s claims to a different venue. Pet.

App. 49a n.1. The defendants originally included Suncor Energy,

Inc., but the trial court dismissed Boulder’s claims against that defendant for lack of personal jurisdiction. Id. at 75a-87a.

1

4

gases into the Earth’s atmosphere. Id. ¶ 15. Through

that chain of causation, Boulder asserts that petitioners

are “responsible” for “billions of tons of [greenhousegas] emissions” globally—making petitioners among the

“largest sources” of greenhouse-gas emissions in the

world. Id. ¶¶ 62, 82.

Boulder further alleges that global greenhouse-gas

emissions have injured Boulder’s “property” and “the

health, safety and welfare of [Boulder’s] residents.” Am.

Compl. ¶ 1. The alleged mechanism of injury is global

“[c]limate change”: According to Boulder, the emission

of greenhouse gases from fossil-fuel use has “increased

the concentration of those gases in the atmosphere,

trapping heat in the climate system, and warming the

planet.” Id. ¶ 123. Boulder alleges that global “climate

change,” in turn, has caused Boulder to experience “more

(and more serious) heat waves, wildfires, droughts, and

floods.” Id. ¶ 3; see id. ¶ 4.

Boulder asserts five Colorado common-law claims

against petitioners: public nuisance, private nuisance,

trespass, unjust enrichment, and civil conspiracy. Am.

Compl. ¶¶ 444-488, 501-530. Boulder seeks “past and

future damages” for its injuries caused by global “climate change.” Id. ¶ 532 (emphasis omitted). It also

seeks “remediation” or “abatement of the hazards” of

global climate change “by any other practical means.”

Id. ¶ 534.2

Boulder also asserted a statutory claim against petitioners under the Colorado Consumer Protection Act, Colo. Rev. Stat. § 6-1105(1) et seq. Am. Compl. ¶¶ 489-500. Unlike Boulder’s commonlaw claims, that claim sought to hold petitioners liable only for conduct (namely, “deceptive trade practices”) “in Colorado.” Id. ¶ 490.

The state trial court dismissed the statutory claim without preju2

5

B. Petitioners’ Attempt To Remove To Federal Court

In June 2018, petitioners removed the case to federal

court pursuant to the general removal statute, 28 U.S.C.

1441(a); the federal-officer removal statute, 28 U.S.C.

1442; and other statutes. See 405 F. Supp. 3d 947, 955,

975. The district court ordered the case remanded to

state court. Id. at 954. The court of appeals affirmed,

after concluding that it had jurisdiction to review only

whether the case could be removed under the federalofficer removal statute. 965 F.3d 792.

Petitioners sought review from this Court, which

granted a writ of certiorari, vacated the court of appeals’ judgment, and remanded for further consideration in light of the intervening determination in BP p.l.c.

v. Mayor & City Council of Baltimore, 593 U.S. 230, 234

(2021), that a court of appeals may “review any issue in

a district court order remanding a case to state court

where the defendant premised removal in part on” the

federal-officer-removal statute, id. at 1536. See 141

S. Ct. 2667.

On remand, the court of appeals again affirmed the

district court’s order remanding the case to state court,

after reviewing “all grounds for removal addressed in

[that] order.” 25 F.4th 1238, 1246. Petitioners sought

review of the removal issue, and this Court called for

the views of the Solicitor General. 143 S. Ct. 78. The

Solicitor General filed a brief taking the position that

the case was not removable to federal court and that

further review was unwarranted. U.S. Cert. Amicus Br.

at 6-7, Suncor Energy (U.S.A.) Inc. v. Board of County

Comm’rs of Boulder County, 143 S. Ct. 1795 (2023) (No.

21-1550). The brief explained that Boulder’s claims did

dice, on the ground that Boulder had not pleaded it with the requisite particularity. Pet. App. 133a-136a.

6

not present a federal question under the well-pleadedcomplaint rule, id. at 7-11, and that they could not be

recharacterized as claims arising under federal common

law, id. at 11-16. The brief emphasized, however, that

the removability question is distinct from the question

whether the Clean Air Act preempts Boulder’s claims,

which was not presented. Id. at 13-15. This Court denied

review. 143 S. Ct. 1795.

C. The State Trial Court’s Denial Of The Motion To Dismiss

In state court, petitioners moved to dismiss Boulder’s complaint for failure to state a claim. Mot. to Dismiss Am. Compl. for Failure to State a Claim (Dec. 9,

2019) (Mot.). As relevant here, petitioners contended

that Boulder’s claims are preempted by the Clean Air

Act. Mot. 14-16. Petitioners also contended that Boulder’s claims violate the Constitution. Mot. 16. Specifically, petitioners argued that “[b]y seeking to punish

[petitioners’] worldwide production and sale of fossil

fuels, [Boulder’s] claims would have the ‘practical effect’

of controlling [petitioners’] ‘conduct beyond the boundaries’ of the State of Colorado,’ ” in violation of the Commerce Clause. Mot. 19 (citation omitted). Petitioners

added that Boulder’s efforts to punish petitioners’

“worldwide” conduct impair the federal foreign-affairs

power and violate due process. Mot. 16; see Mot. 19.

In June 2024, the trial court denied petitioners’ motion to dismiss Boulder’s common-law claims. Pet. App.

48a-139a. The court rejected petitioners’ Clean Air Act

preemption argument on the view that Boulder’s claims

are “not about regulating emissions.” Id. at 105a; see

id. at 99a-108a. The court also rejected petitioners’ reliance on the Constitution. Id. at 108a-115a. The court

took the view that neither the foreign-affairs power nor

the Commerce Clause precludes Boulder from seeking

7

“damages for conduct causing in-state injuries.” Id. at

112a; see id. at 108a-109a. The court also concluded that

Boulder’s claims “do not violate the Due Process Clause.”

Id. at 113a.

D. The Colorado Supreme Court’s Exercise Of Original

Jurisdiction

1. A month after the trial court’s decision, petitioner

Exxon Mobil Corp. filed a petition, which the other petitioners here later joined, invoking under Colorado Appellate Rule 21 the Supreme Court of Colorado’s “original jurisdiction” to exercise “superintending authority”

over the trial court. Colo. App. R. 21(a)(1); see Pet. for

Order to Show Cause (July 16, 2024); Pet. 10; Pet. App.

7a. The petition sought an order to show cause why the

trial court did not err in resolving two issues, including

“[w]hether federal law precludes the application of state

law to claims seeking redress for alleged in-state injuries from the effects of interstate and international

[greenhouse-gas] emissions on the global climate.” Pet.

for Order to Show Cause 9; see id. at 16, 27-37. The

Colorado Supreme Court issued an order to show cause

as to “[w]hether the [trial] court erroneously concluded

that [Boulder’s] claims could proceed under state law.”

Order and Rule to Show Cause 2 (July 29, 2024).

2. After briefing and oral argument, the Colorado

Supreme Court issued a decision in May 2025 concluding that “Boulder’s claims are not preempted by federal

law,” discharging the order to show cause, and remanding the case to the trial court for further proceedings.

Pet. App. 2a; see id. at 1a-47a.

The Colorado Supreme Court determined that “the

preemptive effect of federal law” on Boulder’s claims

presented an issue warranting the exercise of the court’s

“original jurisdiction” under Colorado Appellate Rule 21.

8

Pet. App. 8a. The court explained that “[w]hether [Boulder’s] claims may proceed against [petitioners] has important implications for Colorado and its citizens.”

Ibid. The Colorado Supreme Court also observed that

“other courts that have addressed similar questions have

reached differing conclusions.” Ibid.

The Colorado Supreme Court then held that the Clean

Air Act does not preempt Boulder’s claims. Pet. App.

11a-16a. The court reasoned that “litigating Boulder’s

claims would not upset any balance set by Congress because Boulder’s claims do not seek to impose liability

for activities that the [Act] regulates.” Id. at 15a. As

the court saw it, Boulder’s “claims do not seek compensation for any [greenhouse-gas emissions] by [petitioners] themselves but rather focus on [petitioners’] upstream production activities.” Id. at 21a.

The Colorado Supreme Court also held that “federalism concerns arising from the United States Constitution” do not “bar Boulder’s claims.” Pet. App. 16a.

The court found no support in Franchise Tax Board v.

Hyatt, 587 U.S. 230 (2019), for the proposition that “the

structure of the Constitution” precludes Colorado from

applying its own law to petitioners’ worldwide conduct.

Pet. App. 17a; see id. at 18a (concluding that petitioners

had “point[ed] to” no federal “constitutional text that

preempts Boulder’s state law claims”). The court was

also “unpersuaded” that “the federal foreign affairs

power bars Boulder’s claims.” Id. at 22a. The court reasoned that allowing Boulder’s claims to proceed would

not “impair the effective exercise of this country’s foreign policy” because “Boulder’s claims do not seek to

regulate [greenhouse-gas] emissions.” Id. at 24a.

Justice Samour, joined by Justice Boatright, dissented. Pet. App. 25a-47a. In their view, all of Boulder’s

9

claims should be dismissed because the Clean Air Act

preempts the claims’ “interstate aspect” while “the federal government’s primacy in foreign affairs” precludes

the claims’ “international aspect.” Id. at 27a-28a.

ARGUMENT

The Colorado Supreme Court rejected petitioners’

contention that federal law precludes Boulder’s attempt

to apply Colorado law to petitioners’ fossil-fuel activities around the world. That decision is incorrect; it conflicts with the decision of a court of appeals on a frequently recurring issue of exceptional importance; and

this case is a suitable vehicle for addressing whether the

Constitution or the Clean Air Act precludes state-law

suits like this one. Accordingly, this Court’s review is

warranted.

In an invitation brief filed last year in Sunoco LP v.

City & County of Honolulu, 145 S. Ct. 1111 (2025), the

United States addressed the validity of state-law claims

seeking to hold energy companies liable for the alleged

consequences of global climate change. That brief took

no position on whether the Constitution precludes such

claims. U.S. Cert. Amicus Br. at 6-7, Honolulu, supra

(No. 23-947). The brief did, however, express the view

that the Clean Air Act does not categorically preempt

them. Id. at 17-18. After the change in Administration,

the United States has reexamined its position on that

statutory issue and has determined that state-law claims

like those alleged here conflict with “the decisionmaking scheme Congress enacted” in the Clean Air Act.

American Elec. Power Co. v. Connecticut, 564 U.S. 410,

429 (2011) (AEP).

10

A. This Court Has Jurisdiction To Review The Decision

Below

This Court may review “[f ]inal judgments or decrees

rendered by the highest court of a State.” 28 U.S.C.

1257(a). The Colorado Supreme Court’s decision in this

case is the “final determination of [an] original proceeding in the [state] supreme court.” Colo. App. R. 21(h); see

Colo. App. R. 21(o); Pet. App. 24a. The decision therefore qualifies as a final judgment under Section 1257(a),

even though it contemplates further proceedings in the

trial court on Boulder’s common-law claims. See Pet.

App. 24a-25a; Fisher v. District Court, 424 U.S. 382, 385

n.7 (1976) (per curiam).

This Court’s decision in Atlantic Richfield Co. v.

Christian, 590 U.S. 1 (2020), is directly on point. That

case involved a suit brought in Montana state court. Id.

at 9. The plaintiffs asserted trespass, nuisance, and

strict-liability claims under state common law, and the

defendant argued that a federal statute precluded the

plaintiffs’ claims for certain damages. Ibid. In ruling on

the parties’ summary-judgment motions, the state trial

court rejected the defendant’s reliance on the federal

statute, thereby allowing the plaintiffs’ claims to proceed to trial. Id. at 10. The defendant then invoked the

Montana Supreme Court’s original jurisdiction to issue

writs of supervisory control. Id. at 10, 12. The Montana

Supreme Court exercised original jurisdiction and affirmed the trial court’s ruling. Id. at 10-11.

This Court held that the Montana Supreme Court’s

decision qualified as “final” under Section 1257(a). Atlantic Richfield, 590 U.S. at 12. This Court explained

that “[u]nder Montana law, a supervisory writ proceeding is a self-contained case, not an interlocutory appeal.”

Ibid. (citing Mont. Const. Art. VII, § 2(1) and (2); Mont.

11

R. App. P. 14(1) and (3)). Accordingly, the Montana Supreme Court’s resolution of that proceeding was “final,”

even though it “allowed the case to proceed to trial.”

Ibid.; see ibid. (emphasizing that “[i]t is the nature of

the [state-court] proceeding, not the issues the state

court reviewed,” that determines finality).

This case is in the same posture as Atlantic Richfield.

Like the Montana Constitution, the Colorado Constitution grants the state supreme court original jurisdiction

to exercise supervisory control over lower state courts.

Compare Mont. Const. Art. VII, § 2(1) and (2) (granting

the state supreme court “original jurisdiction” and “general supervisory control”), with Colo. Const. Art. VI,

§§ 2, 3 (granting the state supreme court “original”

jurisdiction and “general superintending control”). Under Colorado law, as under Montana law, the state supreme court’s exercise of that jurisdiction constitutes a

self-contained proceeding, not an interlocutory appeal.

Compare Mont. R. App. P. 14(1) and (3) (distinguishing

the exercise of “original” jurisdiction from the “normal

appeal process”), with Colo. App. R. 21(a)(1) and (2)

(distinguishing the exercise of “original jurisdiction”

from the “relief available by appeal”). And the Colorado

Supreme Court’s decision in this case—like the Montana Supreme Court’s decision in Atlantic Richfield—

terminates such a proceeding. See Pet. App. 24a (discharging the order to show cause). The decision in this

case, like the one in Atlantic Richfield, is therefore a

final judgment under Section 1257(a), even though it

contemplates further proceedings on the plaintiffs’ claims

in the trial court.

Petitioners contend (Pet. 2, 10, 32-33) that the Colorado Supreme Court’s decision is final for a different

reason: because it is a ruling on interlocutory review

12

that should nevertheless be treated as final under Cox

Broadcasting Corp. v. Cohn, 420 U.S. 469 (1975). Regardless of whether the Colorado Supreme Court could

be deemed to have conducted “interlocutory review”

(Pet. 10) for purposes of a Cox Broadcasting analysis,

Exxon Mobil successfully invoked the Colorado Supreme

Court’s “original jurisdiction” under Colorado Appellate Rule 21, Pet. App. 7a, and that court’s decision is

final because it terminated that original proceeding.

B. The Decision Below Is Incorrect

The Colorado Supreme Court erred in allowing Boulder’s common-law claims to proceed. Under the Constitution, Colorado lacks authority to apply its common

law to petitioners’ out-of-state conduct. And even if it

had such authority, the Clean Air Act would preempt

Boulder’s claims.

1. The Constitution precludes Boulder’s state commonlaw clams

The Constitution “transform[ed]” the States “from a

loose league of friendship into a perpetual Union based

on the ‘fundamental principle of equal sovereignty

among the States.’ ” Franchise Tax Bd. v. Hyatt, 587

U.S. 230, 246 (2019) (citation omitted). “Each State’s

equal dignity and sovereignty under the Constitution

implies certain constitutional ‘limitations on the sovereignty of all of its sister States.’ ” Id. at 245 (brackets

and citation omitted); see Burnet v. Brooks, 288 U.S.

378, 401 (1933) (“The limits of State power are defined

in view of the relation of the States to each other in the

Federal Union.”). Some of those limitations find expression in specific constitutional provisions, like the

Fourteenth Amendment’s Due Process Clause. See, e.g.,

World-Wide Volkswagen Corp. v. Woodson, 444 U.S. 286,

13

294 (1980) (recognizing that “the Due Process Clause,

acting as an instrument of interstate federalism, may

sometimes act to divest [a] State of its power to render

a valid judgment”).

Other limitations are “implicit in [the Constitution’s]

structure.” Hyatt, 587 U.S. at 247. One of those limitations is the principle that “[s]tate sovereign authority is

bounded by the States’ respective borders.” Fuld v.

Palestine Liberation Org., 606 U.S. 1, 14 (2025). Under

the Constitution, each State retains “a residuary and inviolable sovereignty.” Alden v. Maine, 527 U.S. 706, 715

(1999) (citation omitted). At the core of that sovereignty

lies the power to prescribe rules governing conduct.

See State Farm Mut. Auto. Ins. Co. v. Campbell, 538 U.S.

408, 422 (2003) (“A basic principle of federalism is that

each State may make its own reasoned judgment about

what conduct is permitted or prescribed within its borders.”). But each State’s equal dignity and sovereignty

under the Constitution implies a limitation on that

power: A State generally may not prescribe rules that

govern conduct beyond its “territorial limits.” National

Pork Producers Council v. Ross, 598 U.S. 356, 376 n.1

(2023) (recognizing such limits “under the Constitution’s

horizontal separation of powers”); see, e.g., BMW of N.

Am., Inc. v. Gore, 517 U.S. 559, 572 (1996) (explaining

that “principles of state sovereignty and comity” mean

“that a State may not impose economic sanctions on violators of its laws with the intent of changing the tortfeasors’ lawful conduct in other States”).

Boulder’s state common-law claims in this case far

exceed the territorial limits on Colorado’s authority. As

the complaint states, Boulder “bring[s] this lawsuit

against [petitioners] for the substantial role that their

production, promotion, refining, marketing and sale of

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fossil fuels played and continues to play in causing, contributing to and exacerbating alteration of the climate.”

Am. Compl. ¶ 2 (emphasis added). Petitioners, however, engage in those fossil-fuel activities across the

globe, see, e.g., id. ¶¶ 61-62, 77, 81-82, 397, and Boulder’s

common-law claims make no attempt to distinguish petitioners’ activities in Colorado from their activities

elsewhere, see id. ¶¶ 444-488, 501-530. To the contrary,

Boulder’s common-law claims seek to hold petitioners

responsible for all of their fossil-fuel activities, anywhere in the world—extending the reach of Colorado

common law well beyond Colorado’s territorial limits.

The Constitution therefore precludes Boulder’s state

common-law claims, unless they fall within an exception

to the principle that state law may not reach out-of-state

conduct. They do not. In some circumstances, this Court

has recognized that “a person acting outside the State

may be held responsible according to the law of the

State for injurious consequences within it.” Young v.

Masci, 289 U.S. 253, 259 (1933). But in such cases, there

has been a direct and traceable connection from the outof-state conduct to the in-state harm. See, e.g., id. at 256

(owner of a car in one State allowed someone else to

drive the car into another State, where the driver struck

a man with the car); Commonwealth v. Macloon, 101

Mass. 1, 4 (1869) (out-of-state infliction of injuries caused

a man to die from those injuries within the State); State

v. Lord, 16 N.H. 357, 359 (1844) (out-of-state dam construction caused flooding of a road within the State);

Cameron v. Vandegriff, 13 S.W. 1092, 1092-1093 (Ark.

1890) (out-of-state blasting of rock caused a rock to hit

a man within the State); MacPherson v. Buick Motor

Co., 111 N.E. 1050, 1051 (N.Y. 1916) (defect in a car man-

15

ufactured out of state caused the car to break down within

the State).

Here, no direct connection exists. Boulder alleges

that petitioners’ fossil-fuel activities all around the world

have caused effects everywhere in the world and hence

injured Boulder and its Colorado residents. Am. Compl.

¶¶ 1-4, 7-8, 13-15. The medium allegedly transmitting

that injury is literally the Earth’s entire atmosphere,

where “[g]reenhouse gases once emitted ‘become well

mixed’ ”—making it impossible to trace any particular

activity outside the State to any particular injury within

it. AEP, 564 U.S. at 422 (citation omitted). Indeed, Boulder cannot trace its asserted injuries to any particular

source of emissions—let alone to any particular conduct

by petitioners, even further down the alleged causal

chain. See ibid. (explaining that the mixing of greenhouse gases in the atmosphere means that “emissions in

New Jersey may contribute no more to flooding in New

York than emissions in China”). If such an indirect and

untraceable chain of causation were enough to justify

applying a State’s laws extraterritorially, any State

could reach virtually any out-of-state conduct. The Constitution precludes Boulder’s reliance on such a theory

to reach petitioners’ fossil-fuel activities in other States.

The Constitution likewise precludes Boulder’s attempt to reach petitioners’ worldwide fossil-fuel activities. For the reasons above, Boulder cannot identify

any direct connection between those activities and any

in-state injury. Moreover, “[t]he Constitution confers

upon the Federal Government—and it alone—both nationwide and extraterritorial authority.” Fuld, 606 U.S.

at 15; see American Ins. Ass’n v. Garamendi, 539 U.S.

394, 413 (2003) (emphasizing “the Constitution’s allocation of the foreign relations power to the National Gov-

16

ernment”); Zschernig v. Miller, 389 U.S. 429, 432 (1968)

(recognizing a “field of foreign relations which the Constitution entrusts to the President and the Congress”).

2. The Clean Air Act preempts Boulder’s state commonlaw claims

Even if Colorado had authority under the Constitution to apply its common law to petitioners’ nationwide

and worldwide conduct, the Clean Air Act would preempt

Boulder’s claims.

a. In International Paper Co. v. Ouellette, 479 U.S.

481 (1987), this Court addressed the preemptive scope

of a similar federal statute—the Clean Water Act, 33

U.S.C. 1251 et seq. The Clean Water Act establishes a

framework for addressing pollution in the Nation’s waters. See Ouellette, 479 U.S. at 489. Under that framework, the amount of pollution that is acceptable is a

matter for the U.S. Environmental Protection Agency

(EPA) and the State in which the pollution originates

(the source State) to decide. See id. at 489-490. Ouellette involved a suit brought under Vermont common

law against a source of pollution in New York, seeking

to deem the harm that the pollution caused in Vermont

a “nuisance.” Id. at 483. The Court held that the Clean

Water Act preempted the suit because any attempt to

apply one State’s law to pollution originating from another State would conflict with the scheme that the statute establishes for who decides the amount of acceptable pollution. See id. at 495.

The same principles govern preemption under the

Clean Air Act, which establishes a similar framework

for addressing air pollution. See, e.g., Merrick v. Diageo Americas Supply, Inc., 805 F.3d 685, 692 (6th Cir.

2015) (“Clean Water Act precedents are persuasive with

respect to the Clean Air Act because many provisions in

17

the Clean Water Act—including the savings clauses—

were modeled on the Clean Air Act, so that the two acts

are often in pari materia.”). Under the Clean Air Act,

as under the Clean Water Act, the amount of acceptable

pollution is a matter for EPA and the source State to

decide. See AEP, 564 U.S. at 424, 427-428. Thus, any

attempt to apply Colorado law to emissions from out of

State would conflict with “the decisionmaking scheme

Congress enacted.” Id. at 429.

b. Given those principles, there can be no dispute

that if Boulder had brought the same common-law

claims against out-of-state emitters (such as the power

plants in AEP, see 564 U.S. at 418), the Clean Air Act

would have preempted those claims. Here, instead of

invoking Colorado law against out-of-state emitters,

Boulder has sued the companies that supply the emitters with fossil fuels. Am. Compl. ¶ 2. The question is

whether Boulder can evade the preemptive force of the

Clean Air Act by targeting others within the supply

chain. The answer is no.

Suing the suppliers instead of the emitters does not

avoid the conflict with the Clean Air Act’s decisionmaking scheme. Boulder’s public and private nuisance claims

illustrate the point. Am. Compl. ¶¶ 444-471. Those

claims allege that emissions from around the world—

nearly all of which originated out of State—have created a “nuisance” in Colorado. Id. ¶¶ 451, 460. But under the Clean Air Act, it is not for Colorado to say,

through its courts’ definition of “nuisance,” whether

emissions from outside Colorado have reached an unacceptable level; rather, the Clean Air Act reserves to

EPA and source States the authority to determine the

extent of appropriate regulation. See City of Denver v.

Mullen, 3 P. 693, 699 (Colo. 1884) (defining public nui-

18

sance as an “unreasonable” interference); Public Serv.

Co. v. Van Wyk, 27 P.3d 377, 391 (Colo. 2001) (same, for

private nuisance). Any finding that petitioners’ worldwide fossil-fuel activities have created a nuisance would

thus entail a judgment about the degree of acceptable

out-of-state greenhouse-gas emissions that the Clean

Air Act precludes Colorado from making. For similar

reasons, this Court has already concluded that a nuisance claim under federal common law could not “be

reconciled with the decisionmaking scheme Congress

enacted” because the claim would require that “individual federal judges determine, in the first instance, what

amount of carbon-dioxide emissions is ‘unreasonable.’ ”

AEP, 564 U.S. at 428-429 (citation omitted). Because

Boulder’s nuisance claims would require state judges to

make the same kinds of judgments, those claims cannot

be reconciled with the Clean Air Act’s decisionmaking

scheme either.

Boulder’s other common-law claims suffer from the

same problem. Boulder’s trespass claim alleges that

emissions from around the world, including from outside Colorado, have caused a “trespass” in Colorado;

that claim would allow Colorado to enforce, through its

definition of “trespass,” its own conception of the acceptable amount of out-of-state greenhouse-gas emissions. Am. Compl. ¶¶ 474-476; see Hoery v. United States,

64 P.3d 214, 217 (Colo. 2003) (defining trespass as “a

physical intrusion upon the property of another without

the proper permission”). Boulder’s unjust-enrichment

claim alleges that the combustion of fossil fuels around

the world, including outside Colorado, has conferred on

petitioners a “benefit” at Boulder’s “expense”; that claim

would allow Colorado to dictate, through its judgment

on whether it would be “unconscionable” for petitioners

19

“to retain that benefit,” what degree of out-of-state

greenhouse-gas emissions is acceptable. Am. Compl.

¶ 488; see Lewis v. Lewis, 189 P.3d 1134, 1141 (Colo. 2008)

(“A person is unjustly enriched when he benefits as a

result of an unfair detriment to another.”). And Boulder’s civil-conspiracy claim suffers from the same problem, because it simply alleges a conspiracy to commit

the other torts. Am. Compl. ¶¶ 501-530. The Clean Air

Act thus preempts each of Boulder’s common-law claims.

C. The Decision Below Warrants This Court’s Review

1. As the Colorado Supreme Court acknowledged

(Pet. App. 8a, 19a-20a), its decision conflicts with the

Second Circuit’s decision in City of New York v. Chevron Corp., 993 F.3d 81 (2021). Like this case, City of

New York involved a suit brought by a local government

against various energy companies, including one of the

petitioners here. Id. at 86. Like Boulder, the City of

New York alleged that the companies’ “production, promotion, and sale of fossil fuels” around the world had,

through global climate change, caused injuries to the

City and its residents. Id. at 88; see id. at 100. And the

City sought to recover damages for those injuries by asserting claims for public nuisance, private nuisance, and

trespass under New York common law. Id. at 88.

The Second Circuit affirmed the dismissal of the

City’s claims. City of New York, 993 F.3d at 86. As relevant here, the court observed that the Clean Air Act

makes EPA “the ‘primary regulator of domestic greenhouse gas emissions’ ” while reserving to States the

power “to create and enforce their own emissions standards applicable to in-state polluters.” Id. at 99 (brackets and citation omitted). But the City had not sought

to “take advantage of th[at] slim reservoir of state

common law.” Id. at 100. “Rather,” the court observed,

20

it sought “to impose New York nuisance standards on

emissions emanating simultaneously from all 50 states

and the nations of the world.” Ibid. The court therefore

held that the Clean Air Act did “not authorize the City’s

state-law claims, meaning that such claims concerning

domestic emissions [we]re barred.” Ibid.

Expressly disagreeing with the Second Circuit, Pet.

App. 19a-20a, the Colorado Supreme Court concluded

that the Clean Air Act does not bar state common-law

claims seeking to hold energy companies liable for injuries allegedly caused by global climate change. Id. at

11a-16a. The two courts have thus reached contrary conclusions on whether such claims may proceed.

2. Whether federal law precludes state-law claims

like those asserted here is a frequently recurring issue

of exceptional importance, making this Court’s resolution of the conflict appropriate. See Sup. Ct. R. 10(b).

Boulder’s suit is just one of many materially similar

suits that have already been filed by States and local

governments across the country.3 That there are so many

See, e.g., In re Fuel Indus. Climate Cases, No. S288664 (Cal.);

Delaware v. BP Am. Inc., No. N20C-09-97 (Del. Super. Ct.); Hawaii

v. BP p.l.c., No. 1CCV-25-717 (Haw. Cir. Ct.); City & County of

Honolulu v. Sunoco LP, No. 1CCV-20-380 (Haw. Cir. Ct.); County

of Maui v. Sunoco LP, No. 2CCV-20-283 (Haw. Cir. Ct.); City of

Chicago v. BP p.l.c., No. 2024CH1024 (Ill. Cir. Ct.); Maine v. BP

p.l.c., No. PORSC-CV24-442 (Me. Super. Ct.); Mayor & City Council of Baltimore v. BP p.l.c., No. 24-C-18-4219 (Md. Cir. Ct.); Anne

Arundel County v. BP p.l.c., No. 02-CV-21-565 (Md. Cir. Ct.); City

of Annapolis v. BP p.l.c., No. 02-CV-21-250 (Md. Cir. Ct.); Minnesota v. American Petroleum Inst., No. 62-CV-20-3837 (Minn. Dist.

Ct.); Platkin v. Exxon Mobil Corp., No. MER-L-1797-22 (N.J. Super. Ct.); City of Hoboken v. Exxon Mobil Corp., No. HUD-L-317920 (N.J. Super. Ct.); City of New York v. BP p.l.c., No. 18-cv-182

(S.D.N.Y.); Town of Carrboro v. Duke Energy Corp., No. 24CV3385670 (N.C. Super. Ct.); County of Multnomah v. Exxon Mobil Corp.,

3

21

suits should come as no surprise, given that the theory

underlying each suit is that every locality in the United

States is entitled to recover damages for the localized

consequences of climate change. And they can be expected to multiply if the decision below is allowed to

stand.

The Colorado Supreme Court was therefore correct

to recognize that “this case presents substantial issues

of global import.” Pet. App. 1a. Indeed, the “significant

public importance” of the issues is what led the Colorado Supreme Court to exercise original jurisdiction in

the first place. Id. at 8a; see ibid. (noting the “important

implications for Colorado and its citizens”). And the implications of the decision below are profound: If, as the

Colorado Supreme Court held, suits like this one may

go forward, energy companies across the globe will be

subject not only to billions of dollars in damages, but also

to a multiplicity of rules governing their conduct in any

given location, as one city after another seeks to hold

the companies liable for fossil-fuel activities anywhere

in the world. This Court’s review is necessary to ensure

that such an “irrational system of regulation” does not

displace the framework established by the Constitution

and the Clean Air Act. Ouellette, 479 U.S. at 496.

3. Finally, this case is a suitable vehicle for resolving

the question presented. This Court recently denied petitions seeking review of a Hawaii Supreme Court deciNo. 23-cv-25164 (Or. Cir. Ct.); Bucks County v. BP p.l.c., No. 20241836 (Pa. C.P.); Rhode Island v. Shell Oil Prods. Co., No. PC-20184716 (R.I. Super. Ct.); City of Charleston v. Brabham Oil Co.,

No. 2020-CP-10-3975 (S.C.C.P.); King County v. BP p.l.c., No. 18-211859-0 (Wash. Super. Ct.); Municipality of Bayamón v. Exxon

Mobil Corp., No. 22-cv-1550 (D.P.R.); Municipality of San Juan v.

Exxon Mobil Corp., No. 23-cv-1608 (D.P.R.).

22

sion affirming the denial of a motion to dismiss climatechange-related state-law claims brought by Honolulu

against energy companies. See Sunoco LP v. City &

County of Honolulu, 145 S. Ct. 1111 (2025) (No. 23-947);

Shell PLC v. City & County of Honolulu, 145 S. Ct. 1111

(2025) (No. 23-952). But, for two reasons, this petition

is a better vehicle than either of those were.

First, the Colorado Supreme Court issued its decision

in this case at the end of a self-contained original proceeding, eliminating any doubt that the decision satisfies Section 1257(a)’s final-judgment rule. See pp. 10-12,

supra. In contrast, the Hawaii Supreme Court issued its

decision on interlocutory review of the denial of a motion to dismiss, raising the more difficult issue of whether

the decision was final under the fourth Cox category.

See City & County of Honolulu v. Sunoco LP, 537 P.3d

1173, 1185 (2023), cert. denied, 145 S. Ct. 1111 (2025).

Second, the issue of whether the Constitution precludes the plaintiffs’ state-law claims was raised and addressed below in this case. See pp. 6-8, supra. In contrast, that issue was not raised or addressed by the Hawaii Supreme Court in Honolulu. See 537 P.3d at 11811182, 1186-1187. This case therefore presents an opportunity to address both the constitutional and the Clean

Air Act issues.

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CONCLUSION

The petition for a writ of certiorari should be granted.

Respectfully submitted.

SARAH M. HARRIS

Deputy Solicitor General*

ADAM R.F. GUSTAFSON

Acting Assistant

Attorney General

CURTIS E. GANNON

Deputy Solicitor General

ROBERT N. STANDER

Deputy Assistant

Attorney General

FREDERICK LIU

Assistant to the

Solicitor General

ROBERT J. LUNDMAN

KYLE GLYNN

Attorneys

SEPTEMBER 2025

* The Solicitor General is recused in this case.