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

Supreme Court briefMay 21, 2026

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No. 25-170

Supreme Court of the United States

──────────────────────────

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

Petitioners,

COUNTY COMMISSIONERS OF BOULDER COUNTY;

CITY OF BOULDER,

Respondents.

──────────────────────────

ON WRIT OF CERTIORARI

TO THE SUPREME COURT OF COLORADO

──────────────────────────

BRIEF OF ALABAMA, WEST VIRGINIA,

AND 24 OTHER STATES AS

AMICI CURIAE IN SUPPORT OF PETITIONERS

──────────────────────────

Steve Marshall

Attorney General

John B. McCuskey

Attorney General

A. Barrett Bowdre

Solicitor General

Counsel of Record

Michael R. Williams

Solicitor General

Robert M. Overing

Principal Deputy

Solicitor General

OFFICE OF THE ALABAMA

ATTORNEY GENERAL

501 Washington Avenue

Montgomery, AL 36130

(334) 242-7300

Barrett.Bowdre@

AlabamaAG.gov

OFFICE OF THE WEST VIRGINIA

ATTORNEY GENERAL

State Capitol Complex

Building 1, Room E-26

Charleston, WV 25306

(304) 558-2021

mwilliams@wvago.gov

Counsel for Amici States

TABLE OF CONTENTS

TABLE OF AUTHORITIES....................................... ii

INTEREST OF AMICI STATES................................. 1

SUMMARY OF ARGUMENT ..................................... 2

ARGUMENT ............................................................... 3

Federalism and Precedent Foreclose StateLaw Claims Based on Interstate Emissions. ..... 3

A. State law can regulate emissions within

state borders but not beyond them. .............. 6

B. The displacement of federal common

law does not permit state law to

regulate interstate emissions. ..................... 20

C. This case is about interstate emissions. ..... 25

II. If No Federal Rule Applies, the States Will

Suffer. ................................................................ 28

CONCLUSION .......................................................... 31

LEGAL REPRESENTATIVES OF THE STATES ... 31

TABLE OF AUTHORITIES

Cases

Am. Elec. Power v. Connecticut, (AEP)

564 U.S. 410 (2011) ................................ 1, 6, 11, 21-24

Anne Arundel County. v. BP p.l.c.,

No. C-02-CV-21-565 (M.D. Cir. Ct.) .................... 11

Ariz. State Leg. v. Ariz. Indep. Redistricting Comm’n,

576 U.S. 787 (2015) .............................................. 16

Baldwin v. G.A.F. Seelig, Inc.,

294 U.S. 511 (1935) .............................................. 15

Banco Nacional de Cuba v. Sabbatino,

376 U.S. 398 (1964) .............................................. 21

Bell v. Cheswick Generating Station,

734 F.3d 188 (3d Cir. 2013) ................................. 23

Blankenship v. Gen. Motors Corp.,

406 S.E.2d 781 (W. Va. 1991) .............................. 27

BMW of N. Am., Inc. v. Gore,

517 U.S. 559 (1996) .................................... 3, 25, 28

Bonaparte v. Appeal Tax Ct. of Baltimore,

104 U.S. 592 (1881) .............................................. 15

Bond v. United States,

564 U.S. 211 (2011) .............................................. 18

Bristol-Myers Squibb Co. v. Superior Ct. of Cal.,

582 U.S. 255 (2017) .............................................. 12

Brown v. Fletcher’s Estate,

210 U.S. 82 (1908) .................................................. 4

iii

Bucks County v. BP P.L.C.,

No. 2024-01836 (Pa. Ct. Com. Pl.) ....................... 11

Burton’s Lessee v. Williams,

16 U.S. 529 (1818) .................................................. 3

C&A Carbone, Inc. v. Town of Clarkstown,

511 U.S. 383 (1994) .............................................. 15

California v. Exxon Mobil Corp.,

CGC-23-609134 (S.F. Super. Ct.) ........................ 10

Chevron Corp. v. Donziger,

974 F. Supp. 2d 362 (S.D.N.Y. 2014 .................... 30

Cipollone v. Liggett Grp.,

505 U.S. 504 (1992) .............................................. 26

City of Annapolis v. BP p.l.c.,

No. C-02-CV-21-250 (Md. Cir. Ct.) ...................... 11

City of Charleston v. Brabham Oil Co.,

2020-CP-10-03975 (S.C. Ct. Com. Pl.) ................. 11

City of Chicago v. BP P.L.C.,

No. 2024CH01024 (Ill. Cir. Ct.) ........................... 10

City of Hoboken v. Chevron Corp.,

No. HUD-L-3179-20 (N.J. Super. Ct.) ................. 11

City of Honolulu v. Sunoco LP,

No. 1CCV-20-380 (Haw. Cir. Ct.) ........................ 11

City of Imperial Beach v. Chevron,

No. MSC17-1227 (Contra Costa Super. Ct.) ....... 10

City of Milwaukee v. Illinois, (Milwaukee II)

451 U.S. 304 (1981) .......................................... 2, 21

City of New York v. Chevron Corp.,

993 F.3d 81 (2d Cir. 2021) ....... 7, 10, 11, 21, 27, 28

City of Oakland v. BP P.L.C.,

No. CJC-24-5310 (S.F. Super. Ct.) ...................... 11

City of Phila. v. New Jersey,

437 U.S. 617 (1978) ................................................ 9

City of Richmond v. Chevron,

No. MSC18-55 (Contra Costa Super. Ct.) ........... 10

Connecticut v. Exxon Mobil Corp.,

No. HHDCV206132568S (Conn. Super. Ct.) ....... 11

Connecticut v. Massachusetts,

282 U.S. 660 (1931) ...................................... 5, 8, 19

County of Marin v. Chevron,

No. CIV-17-2586 (Marin Super. Ct.) ................... 10

County of Maui v. Chevron U.S.A. Inc.,

No. 2CCV-20-283 (Haw. Cir. Ct.) ........................ 10

County of Multnomah v. Exxon Mobil Corp.,

No. 23CV25164 (Or. Cir. Ct.)............................... 10

County of San Mateo v. Chevron,

No. 17-CIV-3222 (San Mateo Super. Ct.) ............ 10

County of Santa Cruz v. Chevron,

No. 17-CV-3242 (Santa Cruz Super. Ct.) ............ 10

Coyle v. Smith,

221 U.S. 559 (1911) ................................................ 4

Dean Foods Co. v. Brancel,

187 F.3d 609 (7th Cir. 1999) ................................ 12

Delaware v. B.P. Am., Inc.,

No. N20C-09-97 (Del. Super. Ct.) ........................ 11

Dep’t of Rev. of Ky. v. Davis,

553 U.S. 328 (2008) .............................................. 27

District of Columbia v. Exxon Mobil Corp.,

No. 2020 CA 2892 (D.C. Super. Ct.) .................... 11

Edgar v. MITE Corp.,

457 U.S. 624 (1982) .............................................. 30

Fed. Mar. Comm’n v. S.C. State Ports Auth.,

535 U.S. 743 (2002) .............................................. 13

Franchise Tax Bd. of Cal. v. Hyatt,

587 U.S. 230 (2019) .................................... 3, 12, 25

Gade v. Nat’l Solid Wastes Mgmt. Ass’n,

505 U.S. 88 (1992) ................................................ 21

Georgia v. Tenn. Copper Co.,

206 U.S. 230 (1907) ........................ 1, 2, 5, 7, 14, 15

Gibbons v. Ogden,

22 U.S. 1 (1824) ...................................................... 4

H. P. Hood & Sons, Inc. v. Du Mond,

336 U.S. 525 (1949). ....................................... 16, 18

Hawaii v. BP P.L.C.,

No. 1CCV-25-717 (Haw. Cir. Ct.) ........................ 10

Healy v. Beer Inst.,

491 U.S. 324 (1989) .......................................... 9, 26

Hencely v. Fluor Corp.,

146 S. Ct. 1086 (2026) ............................................ 6

Hinderlider v. La Plata River & Cherry Creek Ditch

Co.,

304 U.S. 92 (1938) .............................................. 5, 6

Hoyt v. Sprague,

103 U.S. 613 (1880) .............................................. 28

Huron Portland Cement Co. v. City of Detroit,

362 U.S. 440 (1960) .............................................. 14

Illinois v. City of Milwaukee, (Milwaukee I)

406 U.S. 91 (1972) ...................... 2, 5, 6, 8, 9, 15, 23

Illinois v. City of Milwaukee, (Milwaukee III)

731 F.2d 403 (7th Cir. 1984) .......................... 22, 23

Int’l Paper Co. v. Ouellette,

479 U.S. 481 (1987) ........................................ 10, 24

Iowa v. Illinois,

147 U.S. 1 (1893) .................................................... 8

Kansas v. Colorado,

185 U.S. 125 (1902) .............................................. 15

Kansas v. Colorado,

206 U.S. 46 (1907) ............................. 2-6, 18, 19, 23

King County v. BP P.L.C.,

No. 18-2-11859 (Wash. Super. Ct.) ...................... 11

Kurns v. R.R. Friction Prods. Corp.,

565 U.S. 625 (2012) ........................................ 25, 26

Leon v. Exxon Mobil Corp.,

No. 25-2-15986-8 (Wash. Super. Ct.) ................... 10

Lessee of Marlatt v. Silk,

36 U.S. 1 (1837) ...................................................... 5

Lochner v. New York,

198 U.S. 45 (1905) ................................................ 15

Louisiana v. Texas,

176 U.S. 1 (1900) .................................................. 25

Maine v. BP P.L.C.,

No. PORSC-CV-24-442 (Me. Super. Ct.) ............. 10

Massachusetts v. EPA,

549 U.S. 497 (2007) .............................................. 20

vii

Mayor & City Council of Baltimore v. B.P. PLC,

No. 11, Sept. Term, 2025, 2026 WL 809501,

(Md. Mar. 24, 2026)................................................ 7

Mayor of Baltimore v. BP p.l.c.,

31 F.4th 178 (4th Cir. 2022) ................................ 21

Mayor of Baltimore v. BP p.l.c.,

No. 24-C-18-4219 (Md. Cir. Ct.)........................... 11

McIlvaine v. Coxe’s Lessee,

8 U.S. 209 (1808) .................................................... 3

Metro v. Exxon Mobil Corp.,

No. 3:24-cv-19 (Or. Cir. Ct.) ................................. 10

Minnesota v. Am. Petroleum Inst.,

63 F.4th 703 (8th Cir. 2023) ................................ 16

Minnesota v. API, (API)

No. 62-CV-20-3837 (Minn. Dist. Ct.) ............... 6, 27

Missouri v. Illinois,

180 U.S. 208 (1901) ................................................ 4

Missouri v. Illinois,

200 U.S. 496 (1906) .............................. 3, 4, 5, 7, 15

Municipality of San Juan v. Exxon Mobil Corp.,

No. 3:23-cv-1608 (D.P.R.) ..................................... 10

Muns. of Puerto Rico v. Exxon Mobil,

No. 3:22-cv-1550 (D.P.R. 2022) ............................ 10

Murphy v. Nat’l Collegiate Athletic Ass’n,

584 U.S. 453 (2018) ................................................ 3

N.Y. Life Ins. Co. v. Head,

234 U.S. 149 (1914) ........................................ 12, 25

Nacional de Cuba v. Sabbatino,

376 U.S. 398 (1964) .............................................. 21

viii

Nat’l Audubon Soc’y v. Superior Ct.,

658 P.2d 709 (Cal. 1983) ...................................... 15

Nat’l Pork Producers Council v. Ross,

598 U.S. 356 (2023) ........................................ 12, 25

Native Vill. of Kivalina v. ExxonMobil Corp.,

696 F.3d 849 (9th Cir. 2012) ................................ 23

Nevada v. Hall,

440 U.S. 410 (1979) .............................................. 13

New Jersey v. New York,

283 U.S. 336 (1931) .......................................... 5, 14

New State Ice Co. v. Liebmann,

285 U.S. 262 (1932) .......................................... 1, 16

New York v. JBS USA Food Co.,

No. 450682/2024 (N.Y. Sup. Ct. filed

Feb. 28, 2024) ....................................................... 30

North Carolina ex rel. Cooper v. Tenn. Valley Auth.,

615 F.3d 291 (4th Cir. 2010) ........................... 9, 11

Nw. Austin Mun. Util. Dist. No. One v. Holder,

557 U.S. 193 (2009) ................................................ 4

Nw. Laundry v. City of Des Moines,

239 U.S. 486 (1916) .............................................. 14

Oneida Indian Nation of N.Y. v. Oneida County,

414 U.S. 661 (1974) .............................................. 24

Oregon v. Ice,

555 U.S. 160 (2009) .............................................. 16

Pennoyer v. Neff,

95 U.S. 714 (1877) ................................................ 14

Platkin v. Exxon Mobil,

No. MER-L-1797-22 (Super. Ct. N.J.) ................. 10

Puerto Rico v. Exxon Mobil Corp.,

No. SJ2024CV06512 (T.P.I. San Juan) ............... 11

Ray v. Atl. Richfield Co.,

435 U.S. 151 (1978) .............................................. 21

Rhode Island v. Massachusetts,

37 U.S. 657 (1838) .............................................. 3, 4

Rhode Island v. Shell Oil Prods. Co.,

35 F.4th 44 (1st Cir. 2022) ................................... 21

Rhode Island v. Shell Oil Prods. Co.,

No. PC-2018-4716 (R.I. Super. Ct.) ..................... 11

Rodriguez v. Fed. Deposit Ins. Corp.,

589 U.S. 132 (2020) ................................................ 7

S. Pac. Co. v. Ariz. ex rel. Sullivan,

325 U.S. 761 (1945) ........................................ 10, 18

Sable Commc’ns of Cal., Inc. v. FCC,

492 U.S. 115 (1989) .............................................. 16

San Diego Bldg. Trades Council v. Garmon,

359 U.S. 236 (1959) .............................................. 26

Sherrer v. Sherrer,

334 U.S. 343 (1948) .............................................. 17

Sosa v. Alvarez-Machain,

542 U.S. 692 (2004) ................................................ 6

Sporhase v. Nebraska ex rel. Douglas,

458 U.S. 941 (1982). ............................................. 14

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

538 U.S. 408 (2003) .............................................. 12

Strassheim v. Daily,

221 U.S. 280 (1911) .............................................. 17

Texas Indus. v. Radcliff Materials, Inc.,

451 U.S. 630 (1981) .......................................... 6, 21

United States v. Morrison,

529 U.S. 598 (2000) ................................................ 8

United States v. Standard Oil,

332 U.S. 301 (1947) .............................................. 22

Vermont v. Exxon Mobil,

No. 21-CV-2778 (Vt. Super. Ct.) .......................... 10

Virginia v. Maryland,

540 U.S. 56 (2003) .................................................. 7

Virginia v. Tennessee,

148 U.S. 503 (1893) ................................................ 8

Watson v. Emps. Liab. Assur. Corp.,

348 U.S. 66 (1954) ................................................ 15

Wheeldin v. Wheeler,

373 U.S. 647 (1963) ................................................ 8

Wisc. Dept. of Ind. v. Gould Inc.,

475 U.S. 282 (1986) ................................................ 9

World-Wide Volkswagen Corp. v. Woodson,

444 U.S. 286 (1980) ................................................ 3

Statutes

42 U.S.C. § 7426(b) .................................................... 19

Ala. Code §6-5-127 ..................................................... 17

Ala. Code §9-1-6(a) .................................................... 17

Ala. Code §9-2-2 ......................................................... 17

Ala. Code §9-17-1, et seq. ........................................... 17

Ala. Code §22-23-47 ................................................... 17

Ala. Code §22-28-3 ..................................................... 17

Cal. Gov’t Code §7513.75(a)(3) (West) ...................... 16

Cal. Pub. Res. Code §25000.5(a) (West) ................... 16

Clean Air Act ................................................... 2, 20, 23

Clean Water Act .................................................. 20, 24

2009 Greenhouse Gas Endangerment Finding, 91

Fed. Reg. 13,088 (Feb. 18, 2026) ......................... 20

H.F. 2527, 91st Gen. Assemb., Reg. Sess.

(Iowa 2026) ........................................................... 16

N.Y. Env’t Conser. Law §§ 76-0101 to -0103 ............ 30

S.B. 1439, 60th Leg., 2d Reg. Sess. (Okla. 2026) ..... 16

2026 Tenn. Pub. Acts ch. 756 .................................... 16

Tex. Water Code Ann. §7.257 (West) ........................ 16

Utah Code Ann. §78B-4-515 (West) .......................... 16

Vt. Stat. tit. 10, §§ 596-599c...................................... 30

W. Va. Code § 37B-1-2 ................................................. 6

Other Authorities

2 THE RECORDS OF THE FEDERAL CONVENTION OF

1787 28 (Max Ferrand ed., rev. ed. 1966)............ 19

4 The Founders’ Constitution 517 (Philip B. Kurland

& Ralph Lerner eds., 1987).................................... 4

19 Wright & Miller, Fed. Prac. & Proc., Juris.

§ 4514 (4th ed. 2022) .............................................. 9

xii

Donald H. Regan, Siamese Essays: (i) CTS Corp. v.

Dynamics Corp. of America and Dormant

Commerce Clause Doctrine; (II) Extraterritorial

State Legislation, 85 Mich. L. Rev. 1865, 1885

(1987) .................................................................... 12

John Scrudato IV, A Constitution Fit for A Nation:

The Influence of the Law of Nations on the

Virginia Plan and James Madison's

Constitutional Thought,

31 Yale J.L. & Human. 169 (2020) ...................... 13

Joseph Story, Commentaries on the Conflict of Laws

§ 20 (1834) ............................................................ 13

Josh Goodman, Big Cities Face Deficits: Should

States Worry?, Pew (Aug. 5, 2025),

https://tinyurl.com/2hs96pj9 ................................ 18

Katherine Florey, State Courts, State Territory, State

Power: Reflections on the Extraterritoriality

Principle in Choice of Law & Legislation,

84 Notre Dame L. Rev. 1057 (2009) .................... 13

Kevin T. Frazier, Extraterritorial Limits on States as

Laboratories of AI Policy, The Regulatory Review

(Aug. 25, 2025) ..................................................... 16

Matthew P. Cavedon, Federalism Limits on State

Criminal Extraterritoriality,

57 Ariz. St. L.J. 811 (2025) .................................. 14

Nat’l Caucus of Env’tl Legislators, 2025 Superfund

Legislation (Climate) (2025),

https://tinyurl.com/3enh4k2u .............................. 30

Office of N.Y. Att’y Gen., Attorney General James

Sues World’s Largest Beef Producer for

xiii

Misrepresenting Environmental Impact of Their

Products, Feb. 28, 2024,

tinyurl.com/28udz5pa .......................................... 29

Roderick M. Hills, Jr., Against Preemption: How

Federalism Can Improve the National Legislative

Process, 82 N.Y.U. L. Rev. 1 (2007) ..................... 17

Ruth Mason & Michael S. Knoll, Bounded

Extraterritoriality,

122 Mich. L. Rev. 1623 (2024) ............................. 12

The Federalist No. 22 (A. Hamilton)

(Cooke ed. 1961) ..................................................... 9

The Federalist No. 80 (Cooke ed. 1961)...................... 4

The Federalist No. 81 (A. Hamilton)

(Cooke ed. 1961) ................................................... 19

Tyler L. Shearer, Locating Extraterritoriality:

Association for Accessible Medicines and the

Reach of State Power,

100 B.U. L. Rev. 1501 (2020) ............................... 14

INTEREST OF AMICI STATES

Amici States are Alabama, West Virginia, Alaska,

Arkansas, Florida, Georgia, Idaho, Indiana, Iowa,

Kansas, Kentucky, Louisiana, Mississippi, Missouri,

Montana, Nebraska, New Hampshire, North Dakota,

Ohio, Oklahoma, Pennsylvania, South Carolina,

South Dakota, Texas, Utah, and Wyoming.

The City and County of Boulder, Colorado, assert

a power with no analogue in our Nation’s history and

no place in our federalist system: “the forcible

abatement of outside nuisances.” Georgia v. Tenn.

Copper Co., 206 U.S. 230, 237 (1907). Like disputes

over borders, disputes over cross-border emissions

cannot be settled by one State alone. Rather, as the

Court has recognized for over a century, “suits

brought by one State to abate pollution emanating

from another State” are “meet for federal law

governance.” AEP v. Connecticut, 564 U.S. 410, 421-22

(2011). Whether Congress or federal courts supply the

controlling law, our Constitution does not allow two

small localities like these to set national energy policy.

Suits like this one imperil Amici States’ ability to

achieve their policy prerogatives on energy production

and environmental protection. To be sure, States can

assign liability for conduct outside their borders in

limited contexts. But the theory that every State can

regulate every molecule that enters the atmosphere

anywhere in the world is unlimited—and unjustified.

After all, a “courageous” locality can “serve as a

laboratory,” but only “without risk to the rest of the

country.” New State Ice Co. v. Liebmann, 285 U.S. 262,

311 (1932) (Brandeis, J., dissenting). When one

State’s power grows so far beyond its proper sphere,

the rights of every other State risk withering away.

SUMMARY OF ARGUMENT

I. A State cannot apply its home-state law to

interstate emissions because of the “basic interests of

federalism.” Illinois v. City of Milwaukee, 406 U.S. 91,

105 & n.6 (1972) (Milwaukee I). The “cardinal rule,

underlying all the relations of the states to each other,

is that of equality of right.” Kansas v. Colorado, 206

U.S. 46, 97 (1907). No State can “enforce its own

policy” on the others, so either Congress or “interstate

common law” must provide “the rule which shall

control.” Id. at 95, 98.

The court below erred in two ways. First, because

applying state law is constitutionally “impossible,”

Tenn. Copper Co., 206 U.S. at 237, any displacement

by statute of earlier federal law is irrelevant to the

question presented. Contra Pet.App.9a-11a, 17a-20a.

The Clean Air Act did not and could not alter the fact

that “state law cannot be used.” City of Milwaukee v.

Illinois, 451 U.S. 304, 313 n.7 (1981) (Milwaukee II).

Second, the Constitution does not distinguish between

“an action against a pollution emitter to abate

pollution” and this action for “damages from upstream

producers” for the alleged cost of pollution.

Pet.App.17a. Boulder tries to dodge centuries of

precedent, but it can’t hide the source of its alleged

injury, the chain of causation it pleaded, and the likely

effect of the drastic remedies it demands.

II. Cases like this one threaten the availability of

affordable energy and the sovereignty of States. The

Court should recognize the supremacy of federal law

for resolving interstate disputes before local actors

can do serious damage to the Nation’s energy system

and more.

ARGUMENT

I. Federalism and Precedent Foreclose StateLaw Claims Based on Interstate Emissions.

By declaring independence, the Colonies laid claim

“to all the rights and powers of sovereign states.”

Franchise Tax Bd. of Cal. v. Hyatt, 587 U.S. 230, 23738 (2019) (citing McIlvaine v. Coxe’s Lessee, 8 U.S. 209,

212 (1808)). “A sovereign decides by his own will,

which is the supreme law within his own boundary.”

Rhode Island v. Massachusetts, 37 U.S. 657, 737

(1838). When sovereign wills conflict, they may settle

their differences by treaty or war. For example, if one

creates a “nuisance” “upon a navigable river like the

Danube, [it] would amount to a casus belli for a state

lower down, unless removed.” Missouri v. Illinois, 200

U.S. 496, 520-21 (1906).

But the Colonies joined the Union, and from the

origins of our federal system flow several basic tenets

of constitutional law. While the Constitution “did not

abolish the sovereign powers of the States,” it “limits

[their] sovereignty in several ways.” Murphy v. Nat’l

Collegiate Athletic Ass’n, 584 U.S. 453, 470 (2018).

Most obviously, “[t]he sovereignty of each State …

implie[s] a limitation on the sovereignty of all of its

sister States.” World-Wide Volkswagen Corp. v.

Woodson, 444 U.S. 286, 293 (1980).

Unlike “absolutely independent nations,” which

may resort to force, no State “can impose its own

legislation” or “enforce its own policy upon the

other[s].” Kansas, 206 U.S. at 95, 98; see BMW of N.

Am., Inc. v. Gore, 517 U.S. 559, 572 (1996). “[H]appily

for our domestic harmony, the power of aggressive

operation against each other is taken away.” Burton’s

Lessee v. Williams, 16 U.S. 529, 538 (1818). Every

State agreed to “stand[] on the same level with all the

rest,” Kansas, 206 U.S. at 97, to form “a union of

states, equal in power, dignity and authority.” Coyle v.

Smith, 221 U.S. 559, 567 (1911).“[A]nd the

independence of one [state] implies the exclusion of

power from all others.” Brown v. Fletcher’s Estate, 210

U.S. 82, 89 (1908) (citation omitted); see also Letter

from James Madison to Edmund Randolph (Mar. 10,

1784), in 4 The Founders’ Constitution 517 (Philip B.

Kurland & Ralph Lerner eds., 1987) (describing how

one State lacks the power to punish even its own

citizens for “transgressing within the pale of

another.”). This “historic tradition” has become a

pillar of American government. Nw. Austin Mun. Util.

Dist. No. One v. Holder, 557 U.S. 193, 203 (2009).

Relinquishing the powers of diplomacy and war did

not leave the States defenseless. What would have

been political fights among sovereigns became judicial

questions with answers in federal law. Rhode Island,

37 U.S. at 737-38, 743. By ratifying the Supremacy

Clause, the States “surrendered to congress, and its

appointed Court, the right and power of settling their

mutual controversies.” Id. at 737; see Kansas, 206 U.S.

at 95; Missouri, 200 U.S. at 518-20; Missouri v.

Illinois, 180 U.S. 208, 241 (1901); see also Gibbons v.

Ogden, 22 U.S. 1, 211 (1824). The Constitution thus

provided a structural solution for “bickerings and

animosities … that could not be foreseen.” The

Federalist No. 80 at 537 (Cooke ed. 1961). “Whatever

practices” that “tend[] to disturb the harmony

between the States are proper objects of federal

superintendence and control.” Id.

In areas ripe for interstate conflict, the Court has

maintained State equality and harmony by declining

to apply any one State’s law. See Kansas, 206 U.S. at

95; Missouri, 200 U.S. at 520; see also, e.g., New Jersey

v. New York, 283 U.S. 336, 342 (1931); Connecticut v.

Massachusetts, 282 U.S. 660, 670-71 (1931). Instead,

only federal law can govern matters that implicate

interstate relations. The doctrine extends even to

cases involving private parties, like this one. See, e.g.,

Hinderlider v. La Plata River & Cherry Creek Ditch

Co., 304 U.S. 92, 110 (1938); Tenn. Copper Co., 206

U.S. at 237; Lessee of Marlatt v. Silk, 36 U.S. 1, 22-23

(1837).

The Court had these principles in mind when it

decided Milwaukee I, which was also an interstate

nuisance case. Illinois alleged that Milwaukee had

polluted Lake Michigan, an interstate body of water.

Invoking the logic of federalism, the Court held

unanimously that Illinois could not force Milwaukee

to abate its activity, but neither could Illinois be asked

“to submit to whatever might be done.” 406 U.S. at

104. Pitting two sovereign wills against each other,

the “nature of the problem” created an impasse that

required a neutral arbiter, i.e., federal law. Id. at 103

n.5. Congress can legislate, or federal courts can apply

common law. Either way, state law cannot govern a

controversy that “touches basic interests of

federalism” or that needs “a uniform rule.” Id. at 105

n.6. “Certainly,” the pollution of Lake Michigan was

such a controversy. Id.

Likewise, the claims here cannot proceed under

state law. Boulder seeks to enact a global climate

policy—one that would interfere with the sovereign

power of every other State to regulate energy and the

environment within its borders. By pursuing

devastating damages against particular targets, the

city and county propose to squelch lawful conduct

occurring elsewhere—indeed, conduct that other

States actively encourage. See, e.g., W. Va. Code

§ 37B-1-2 (declaring it the public policy of West

Virginia to “[f]oster, encourage and promote

exploration for and development, production, and

conservation of oil, natural gas and their

constituents”). “This is, in effect, an interstate

dispute.” Minnesota v. Am. Petroleum Inst., 63 F.4th

703, 718 (8th Cir. 2023) (Stras, J., concurring) (API);

contra Pet.App.18a. Colorado law cannot resolve an

interstate dispute without breaking basic tenets of

federalism.

A. State law can regulate emissions within

state borders but not beyond them.

1. In cases that implicate the conflicting rights of

States that are not governed by a federal statute, the

Court has identified and applied “interstate common

law.” Kansas, 206 U.S. at 98; see API, 63 F.4th at 718

(Stras, J., concurring) (“The rule of decision in

[interstate disputes] has always been … what we now

know as the federal common law.”). “Erie did not in

terms bar any judicial recognition of new substantive

rules, no matter what the circumstances, and postErie understanding has identified limited enclaves in

which federal courts may derive some substantive law

in a common law way.” Sosa v. Alvarez-Machain, 542

U.S. 692, 729 (2004). So while federal general common

law may be “bygone,” Hencely v. Fluor Corp., 146 S.

Ct. 1086, 1098 n.3 (2026), “specialized” federal

common law remains in certain areas, AEP, 564 U.S.

at 421 (recognizing this “distinction”). See also Tex.

Indus. v. Radcliff Materials, Inc., 451 U.S. 630, 641 &

n.13 (1981); Milwaukee I, 406 U.S. at 105-06;

Hinderlider, 304 U.S. at 110.

Cases involving interstate emissions implicate the

conflicting rights of States—and thus constitute one

area warranting a federal rule. See Rodriguez v. Fed.

Deposit Ins. Corp., 589 U.S. 132, 136 (2020)

(recognizing that “certain controversies between

States” justify federal common law). That’s no

surprise, as State versus State conflict can arise

whenever one State’s use or regulation of natural

resources could “harm the other’s interest” in the

resource. Virginia v. Maryland, 540 U.S. 56, 74 n.9

(2003); accord Tenn. Copper Co., 206 U.S. at 237.

The federal judiciary has understood for well “over

a century” the need for federal resolution of such

interstate pollution disputes. City of New York v.

Chevron Corp., 993 F.3d 81, 91 (2d Cir. 2021)

(collecting cases); accord Mayor & City Council of

Baltimore v. B.P. PLC, No. 11, Sept. Term, 2025, 2026

WL 809501, at *10-18 (Md. Mar. 24, 2026) (same). For

example, Missouri once sued to enjoin the dumping of

sewage into an Illinois river, which it alleged would

deposit downstream into Missouri riverbeds and

poison Missouri water. Missouri v. Illinois, 200 U.S.

at 517. Applying principles “known to the older

common law,” not state law, the Court found that

Missouri’s claim failed for want of injury and

causation. Id. at 522.

Air pollution is no different. When Georgia sought

to enjoin a Tennessee company from “discharging

noxious gas” over state lines, Georgia law did not

govern. Tenn. Copper Co., 206 U.S. at 236. The Court

identified common-law principles to determine that a

State could be “entitled to specific relief” rather than

“give up quasi-sovereign rights for pay.” Id. at 237-38.

And the Court rejected a defense of laches. Id. at 239.

None of the analysis depended on state law but

instead a federal equity jurisprudence built for

interstate emissions cases.

In Milwaukee I, the Court recognized a general

rule that claims to protect “ecological rights” against

“impairment … from sources outside the State[]” have

their “basis and standard in federal common law.” 406

U.S. at 100. The dispositive fact was not that Lake

Michigan is “bounded … by four States,” one of which

was polluting. Id. at 104 n.6. When “deal[ing] with air

and water in their … interstate aspects,” the “basic

interests of federalism” demand the application of

federal law. Id. at 103 n.5, 104 n.6; see also Iowa v.

Illinois, 147 U.S. 1, 7-8, 13 (1893) (rejecting the views

of dueling state courts in favor of “equality” in river

rights); Connecticut, 282 U.S. at 669-70 (rejecting

“municipal law”); Virginia v. Tennessee, 148 U.S. 503,

523-24 (1893) (applying public law, international law,

and moral law). “The Constitution requires a

distinction between what is truly national and what is

truly local,” and air pollution is truly national. United

States v. Morrison, 529 U.S. 598, 617-18 (2000).

Alleging liability for emissions from sources

outside Colorado, the interstate dispute at hand

cannot be resolved under state law. Because Boulder’s

claims invoke only state and local law, those claims

fail as a matter of law and must be dismissed.

The interstate emissions cases support dismissal

for another reason: the “need for a uniform rule of

decision.” Milwaukee I, 406 U.S. at 105 n.6; accord

Wheeldin v. Wheeler, 373 U.S. 647, 651 (1963) (noting

how a federal common law can be justified “in view of

the desirability of a uniform rule”). Only federal law,

“not the varying common law of individual states,” can

serve as a “basis for dealing in uniform standard with

the environmental rights of [each] State.” Milwaukee

I, 406 U.S. at 108 n.9. After all, “the want of concert,

arising from the want of a general authority and from

clashing and dissimilar views in the States,” was one

of the very reasons that the Constitution came to be.

The Federalist No. 22 at 140 (A. Hamilton) (Cooke ed.

1961). That rationale applies a fortiori to claims based

on global emissions, which implicate every State, not

just those with claims to a specific river or lake. In

contrast to federal law, “local law will not be

sufficiently sensitive to federal concerns, … is not

likely to be uniform across state lines, and … will

develop at various rates of speed in different states.”

19 Wright & Miller, Fed. Prac. & Proc., Juris. § 4514

(4th ed. 2022). None of that will do.

Because “our economic unit is the Nation,” City of

Phila. v. New Jersey, 437 U.S. 617, 623 (1978) (citation

omitted), the Court should evaluate how state-focused

suits like Boulder’s “may interact with the legitimate

regulatory regimes of other States and what effect

would arise if not one, but “many or every, State

adopted similar [strategies],” Healy v. Beer Inst., 491

U.S. 324, 336 (1989). The logic here doesn’t hold up. It

justifies a world in which the same production and

sale of energy is subject to every State’s regulatory

and enforcement regime at once, creating

unpredictable and conflicting duties. See Wisc. Dept.

of Indus. v. Gould Inc., 475 U.S. 282, 286 (1986)

(“Conflict is imminent whenever two separate

remedies … bear on the same activity.” (citation

modified)). Such “balkanization of clean air

regulations and a confused patchwork of standards”

would harm “industry and the environment alike.”

North Carolina ex rel. Cooper v. Tenn. Valley Auth.,

615 F.3d 291, 296 (4th Cir. 2010) (Wilkinson, J.);

accord City of New York, 993 F.3d at 91; Pet.App.25a,

28a, 47a (Samour, J., dissenting). If every State were

to regulate the same conduct, energy producers would

face enormous “uncertainty,” and States would risk

“chaotic confrontation” with one another. Int’l Paper

Co. v. Ouellette, 479 U.S. 481, 496 (1987). “[T]he

confusion and difficulty with which interstate

operations would be burdened under the varied

system of state regulation and the unsatisfied need for

uniformity in such regulation … are evident.” S. Pac.

Co. v. Ariz. ex rel. Sullivan, 325 U.S. 761, 773-74

(1945).

Confusion and difficulty are already the order of

the day. Dozens of States, localities, and even private

parties are prosecuting actions like this one under the

aegis of state and local law.1 These suits threaten

1 See Leon v. Exxon Mobil Corp., No. 25-2-15986-8 (Wash. Super.

Ct.); Hawaii v. BP p.l.c., No. 1CCV-25-717 (Haw. Cir. Ct.); Maine

v. BP p.l.c., No. PORSC-CV-24-442 (Me. Super. Ct.); City of

Chicago v. BP p.l.c., No. 2024CH01024 (Ill. Cir. Ct.); Metro v.

Exxon Mobil Corp., No. 3:24-cv-19 (Or. Cir. Ct.); Municipality of

San Juan v. Exxon Mobil Corp., No. 3:23-cv-1608 (D.P.R.);

California v. Exxon Mobil Corp., CGC-23-609134 (S.F. Super.

Ct.) (coordinated with County of San Mateo v. Chevron, No. 17CIV-3222 (San Mateo Super. Ct.); County of Marin v. Chevron,

No. CIV-17-2586 (Marin Super. Ct.); County of Santa Cruz v.

Chevron, No. 17-CV-3242 (Santa Cruz Super. Ct.); City of

Imperial Beach v. Chevron, No. MSC17-1227 (Contra Costa

Super. Ct.); City of Richmond v. Chevron, No. MSC18-55 (Contra

Costa Super. Ct.)); County of Multnomah v. Exxon Mobil Corp.,

No. 23CV25164 (Or. Cir. Ct.); Municipalities of Puerto Rico v.

Exxon Mobil Corp., No. 3:22-cv-1550 (D.P.R. 2022); Platkin v.

Exxon Mobil Corp., No. MER-L-1797-22 (Super. Ct. N.J.);

Vermont v. Exxon Mobil Corp., No. 21-CV-2778 (Vt. Super. Ct.);

County of Maui v. Chevron U.S.A. Inc., No. 2CCV-20-283 (Haw.

Cir. Ct.); Connecticut v. Exxon Mobil Corp., No. HHDCV

ruinous liability for the energy industry. As cases

progress around the country, it becomes more and

more likely that one state court, interpreting one

State’s law, would “scuttle the nation’s carefully

created system for accommodating the need for energy

production and the need for clean air.” North

Carolina, 615 F.3d at 296. As this Court has

recognized, federal law “prescribe[s] [an] order of

decisionmaking” because “our Nation’s energy needs

and the possibility of economic disruption must weigh

in the balance.” Cf. AEP, 564 U.S. at 427 (emphasis

added). That balance can be struck—and disaster

avoided—only if this Court puts a stop to the idea any

one State can regulate all interstate emissions.

2. Even if controlling precedent did not mandate

dismissal, our constitutional structure would. Suits

like this one undermine federalism by interfering with

the right of “each State [to] make its own reasoned

206132568S (Conn. Super. Ct.); Delaware v. B.P. Am., Inc., No.

N20C-09-97 (Del. Super. Ct.); City of Hoboken v. Chevron Corp.,

No. HUD-L-3179-20 (N.J. Super. Ct.); District of Columbia v.

Exxon Mobil Corp., No. 2020 CA 2892 (D.C. Super. Ct.);

Minnesota v. Am. Petroleum Inst., No. 62-CV-20-3837 (Minn.

Dist. Ct.); City of Honolulu v. Sunoco LP, No. 1CCV-20-380 (Haw.

Cir. Ct.); Rhode Island v. Shell Oil Prods. Co., No. PC-2018-4716

(R.I. Super. Ct.); City of Oakland v. BP p.l.c., No. CJC-24-5310

(S.F. Super. Ct.); but see City of New York, 993 F.3d 81

(dismissed); City of Charleston v. Brabham Oil Co., 2020-CP-1003975 (S.C. Ct. Com. Pl.) (dismissed); Bucks County v. BP p.l.c.,

No. 2024-01836 (Pa. Ct. Com. Pl.) (same); Anne Arundel County.

v. BP p.l.c., No. C-02-CV-21-565 (M.D. Cir. Ct.) (dismissed;

appeal pending); City of Annapolis v. BP p.l.c., No. C-02-CV-21250 (Md. Cir. Ct.) (same); Mayor of Baltimore v. BP p.l.c., No. 24C-18-4219 (Md. Cir. Ct.) (same); Puerto Rico v. Exxon Mobil

Corp., No. SJ2024CV06512 (T.P.I. San Juan) (voluntarily

dismissed) King County v. BP p.l.c., No. 18-2-11859 (Wash.

Super. Ct.) (same).

judgment about what conduct is permitted or

proscribed within its borders.” State Farm Mut. Auto.

Ins. Co. v. Campbell, 538 U.S. 408, 422 (2003). It is

axiomatic that the Constitution places “territorial

limitations on the power of the respective States.”

Bristol-Myers Squibb Co. v. Superior Ct. of Cal., 582

U.S. 255, 263 (2017). So “a statute or regulation”—

including regulation by litigation—is invalid when it

“violates the extraterritoriality ban.” Dean Foods Co.

v. Brancel, 187 F.3d 609, 616 (7th Cir. 1999).

While federal common law might be said to

vindicate vertical federalism, this structural

recognition “is a question of horizontal federalism.”

Ruth Mason & Michael S. Knoll, Bounded

Extraterritoriality, 122 Mich. L. Rev. 1623, 1626

(2024). And “[t]his extraterritoriality principle is not

to be located in any particular clause.[2] It is one of

those foundational principles of our federalism which

we infer from the structure of the Constitution as a

whole.” Donald H. Regan, Siamese Essays: (i) CTS

Corp. v. Dynamics Corp. of America and Dormant

Commerce Clause Doctrine; (II) Extraterritorial State

Legislation, 85 Mich. L. Rev. 1865, 1885 (1987); see

also Katherine Florey, State Courts, State Territory,

State Power: Reflections on the Extraterritoriality

Principle in Choice of Law & Legislation, 84 Notre

2 That said, “[n]umerous provisions [of the Constitution] reflect

this reality.” Cf. Franchise Tax Bd., 587 U.S. at 245-48

(recognizing “implied” aspects of federalism). The Import-Export

Clause, Privileges and Immunities Clause, Tonnage Clause,

Commerce Clause, Full Faith and Credit Clause, and Due

Process Clause, among others, all presume territorial limits on

state power. Nat’l Pork Producers Council v. Ross, 598 U.S. 356,

376 (2023). Whatever specific provision it might derive from, the

principle is “obviously the necessary result of the Constitution.”

N.Y. Life Ins. Co. v. Head, 234 U.S. 149, 161 (1914).

Dame L. Rev. 1057, 1060 (2009) (suggesting the

extraterritoriality principle is “better understood as a

prohibition rooted in general structural principles of

horizontal federalism”). As the Court has often seen,

that constitutional structure—or “blueprint”—

matters. Fed. Mar. Comm’n v. S.C. State Ports Auth.,

535 U.S. 743, 751 (2002); see also, e.g., Nevada v. Hall,

440 U.S. 410, 433 (1979) (Rehnquist, J., dissenting)

(“[T]his Court has often relied on notions of a

constitutional plan—the implicit ordering of

relationships within the federal system.”).

A territorially bounded system brings with it many

salutary benefits. Among other things, this territoryfocused structure ensures that those imposing the

laws are accountable to those regulated by them. It

avoids dueling and inconsistent obligations. And it

prevents discrimination and discord that undermine

our union, while still granting States the degree of

independence that a non-centralized republic entails.

And this structure did not arise in a vacuum. Our

constitutional system was shaped by the law of

nations. John Scrudato IV, A Constitution Fit for A

Nation: The Influence of the Law of Nations on the

Virginia Plan and James Madison's Constitutional

Thought, 31 Yale J.L. & Human. 169, 193 (2020). But

a system in which individual States could regulate

across borders would either undermine the “national

jurisdiction” of the United States as a whole, id., or

violate the maxim that “no state or nation can by its

laws directly affect or bind property out of its own

territory, or bind persons not resident therein,”

Joseph Story, Commentaries on the Conflict of Laws

§ 20 (1834). In other words, whether the Framers saw

America as a single strong nation or a loose

confederation of sovereign entities, extraterritorial

regulation didn’t fit with either conception. So when

this Court began exploring the territorial limits on

States, it too grounded them in ‘principles of public

law’ that the Framers would have recognized.

Pennoyer v. Neff, 95 U.S. 714, 722 (1877); see also

Matthew P. Cavedon, Federalism Limits on State

Criminal Extraterritoriality, 57 Ariz. St. L.J. 811, 817

(2025) (canvassing the “historical rules of

territoriality that are incorporated into constitutional

federalism”). The Framers and other early

interpreters wanted to leave States free to pursue

their own interests while still “lean[ing] on the

common law and historical interpretation of the

nature of [international] state sovereignty” to place

guardrails on those pursuits. Tyler L. Shearer,

Locating Extraterritoriality: Association for Accessible

Medicines and the Reach of State Power, 100 B.U. L.

Rev. 1501, 1526-33 (2020).

At the same time, States have long pursued their

sovereign interests in reducing pollution through

regulation, litigation, and other means. See, e.g.,

Huron Portland Cement Co. v. City of Detroit, 362 U.S.

440, 442 (1960) (a law “designed to free from pollution

the very air that people breathe clearly falls within …

the police power”); Nw. Laundry v. City of Des Moines,

239 U.S. 486, 490-92 (1916) (“no doubt” that “emission

of smoke [was] within the regulatory power of the

state”). For every State has “real and substantial

interests” in the environment, New Jersey, 283 U.S. at

342, including “all the earth and air within its

domain,” Tenn. Copper Co., 206 U.S. at 237. So the

Court has sometimes been “reluctant” to “condemn”

state efforts to “conserve and preserve … vital

resource[s].” Sporhase v. Nebraska ex rel. Douglas,

458 U.S. 941, 956 (1982).

Preserving the environment and natural resources

is a traditional aim of regulation because by “the law

of nature these things are common to mankind.” Nat’l

Audubon Soc’y v. Superior Ct., 658 P.2d 709, 718 (Cal.

1983) (quoting the Code of Justinian). So it is no

surprise that a sovereign might complain of “outside

nuisances” and other “injuries [like] torts” emanating

from beyond its borders. Tenn. Copper Co., 206 U.S. at

237; see also Missouri, 200 U.S. at 520-21. Amici

States don’t deny that reality.

But the extraterritorial extension of state law (i.e.,

the use of force) is not a constitutional option for

dealing with outside nuisances. See id.; Milwaukee I,

406 U.S. at 103-05; Kansas v. Colorado, 185 U.S. 125,

140-41 (1902); cf. Bonaparte v. Appeal Tax Ct. of

Baltimore, 104 U.S. 592, 594 (1881). A locality cannot

“extend … [its] police power beyond its jurisdictional

bounds” just because it’s targeting conduct “it might

deem harmful to the environment.” C&A Carbone,

Inc. v. Town of Clarkstown, 511 U.S. 383, 393 (1994).

Seizing that power diminishes the power of every

other State “to promote the general welfare, or to

guard the public health, the public morals, or the

public safety” within its borders. Lochner v. New York,

198 U.S. 45, 67 (1905) (Harlan, J., dissenting). So one

State may not “project its legislation” into another.

Baldwin v. G.A.F. Seelig, Inc., 294 U.S. 511, 521

(1935); see also Watson v. Emps. Liab. Assur. Corp.,

348 U.S. 66, 70 (1954) (explaining that “a state is

without power to exercise ‘extra territorial

jurisdiction,’ that is, to regulate and control activities

wholly beyond its boundaries,” because of due

process). Were it otherwise, “the door [would] be[]

opened to rivalries and reprisals that were meant to

be averted by subjecting commerce between the states

to the power of the nation.” H. P. Hood & Sons, Inc. v.

Du Mond, 336 U.S. 525, 532 (1949).

Colorado law cannot govern the global atmosphere

because our federal system allows States to pursue

their own divergent policies. And they do. Compare,

e.g., H.F. 2527, 91st Gen. Assemb., Reg. Sess. (Iowa

2026) (enacted) (limiting liability for alleged climate

change due to emissions); S.B. 1439, 60th Leg., 2d

Reg. Sess. (Okla. 2026) (enacted) (similar); 2026 Tenn.

Pub. Acts ch. 756 (similar); Utah Code Ann. §78B-4515 (West) (similar); Tex. Water Code Ann. §7.257

(West) (similar) with Cal. Gov’t Code §7513.75(a)(3)

(West) (noting “the state’s broad[] efforts to

decarbonize”); Cal. Pub. Res. Code §25000.5(a) (West)

(declaring “overdependence on … petroleum based

fuels” to be “a threat”). Such variety reflects the

genius of American federalism, which lets “different

communities” live by “different local standards.” Sable

Commc’ns of Cal., Inc. v. FCC, 492 U.S. 115, 126

(1989); Ariz. State Leg. v. Ariz. Indep. Redistricting

Comm’n, 576 U.S. 787, 817 (2015); Oregon v. Ice, 555

U.S. 160, 171 (2009). Within its own domain, a State

may “serve as a laboratory; and try novel social and

economic experiments without risk to the rest of the

country.” New State Ice Co. v. Liebmann, 285 U.S. 262,

311 (1932) (Brandeis, J., dissenting).

But the laboratory model does not work if one State

can terminate another State’s experiment—or

dragoon another State’s citizens into its preferred

project. See Kevin T. Frazier, Extraterritorial Limits

on States as Laboratories of AI Policy, The Regulatory

Review (Aug. 25, 2025) (“There is also no exception [to

the extraterritoriality doctrine] for regulatory

experimentation under the guise of states’ acting as

laboratories of democracy if that experiment includes

out-of-state participants.”). And sometimes, “[t]he

whole point of the federal scheme is to suppress states’

creativity, which might consist only of creatively

achieving benefits for their own citizens at the

expense of nonresidents.” Roderick M. Hills, Jr.,

Against Preemption: How Federalism Can Improve the

National Legislative Process, 82 N.Y.U. L. Rev. 1, 5

(2007).

Just observe how these concepts play out in the

energy and environment space. Alabama, for example,

highly values the production and use of traditional

energy. It is Alabama’s policy “that the extraction of

coal provides a major present and future source of

energy and is an essential and necessary activity

which contributes to the economic and material wellbeing of the state.” Ala. Code §9-1-6(a); see also id. §917-1, et seq. (governing the development of oil and

gas). Alabama has also enacted laws to protect air

quality, prevent water pollution, and conserve wildlife, see id. §§6-5-127, 9-2-2, 22-23-47, 22-28-3, but its

views on how to achieve those ends diverge sharply

from those of Boulder, Colorado. See, e.g., J.A. 84

(complaining that petitioners plan to “sell[] more fossil

fuels”); id. at 137-38 (seeking future damages and

“abatement of the hazards”). In Boulder’s world,

though, Boulder’s view would trump.

There can be no question that Boulder’s billiondollar carbon tax (e.g., id. at 114) would damage the

efforts of other States to promote affordable and

reliable energy. Yet there’s no suggestion that out-ofstate producers “intended to produce … detrimental

effects within [Boulder].” Strassheim v. Daily, 221

U.S. 280, 285 (1911). So Boulder can’t erase an out-ofstate, preferred industry by fiat. Cf. Sherrer v.

Sherrer, 334 U.S. 343, 355 (1948) (“If in its application

local policy must at times be required to give way,

such ‘is part of the price of our federal system.”).

And it’s especially pernicious to greenlight

lawsuits like this one by municipalities themselves.

Local budgets are facing serious pressures

nationwide, and these budget challenges often can get

passed to States. See Josh Goodman, Big Cities Face

Deficits: Should States Worry?, Pew (Aug. 5, 2025),

https://tinyurl.com/2hs96pj9. With a need to fill these

budget gaps, tort suits against deep-pocketed, out-ofstate interests can look like magic bullets. Localities

might thus begin to litigate and “legislate according to

[their] estimate of [their] own interests,” reinitiating

the very sort of “drift toward anarchy and commercial

warfare” that prompted the Framers to move toward

a strong central government in the first place. H.P.

Hood & Sons, Inc., 336 U.S. at 533 (1949) (citation

modified). “Climate compensation” would soon become

an escalating battle to shift monies and liabilities

among cash-strapped jurisdictions.

Our constitutional structure can happily tolerate

irreconcilable differences among States—but not if

every State is “bound to yield its own views” on

interstate gas emissions to those of Colorado (and pay

for Colorado’s liabilities along the way). Kansas, 206

U.S. at 97. “The federal balance is, in part, an end in

itself, to ensure that States function as political

entities in their own right.” Bond v. United States, 564

U.S. 211, 221 (2011). And the Court must be diligent

in preserving that balance because, “to the extent that

the burden of state regulation falls on interests

outside the state, it is unlikely to be alleviated by the

operation of those political restraints normally

exerted when interests within the state are affected.”

S. Pac. Co., 325 U.S. at 768 n.2 (1945). So too here.

At the end of the day, this constitutional design is

not merely negative; it does more than simply prohibit

extraterritorial regulation. It channels interstate

disputes into federal institutions where every affected

sovereign has a voice. See, e.g., 42 U.S.C. § 7426(b)

(providing avenue by which States can resolve

interstate air pollution nuisances through EPA).

Congress legislates on behalf of the whole Nation,

with representatives from producing and consuming

States alike. The Executive negotiates international

commitments, including decisions to enter into or

withdraw from agreements about emissions, with the

interests of all States in view. And this Court decides

interstate disputes under federal law, applying

principles of equality rather than the will of any single

sovereign. See Kansas, 206 U.S. at 97-98; Connecticut,

282 U.S. at 670-71.

Each of these institutions provides what a

Colorado courtroom does not: a forum in which

energy-producing States, energy-consuming States,

and the affected industry can all be heard before an

independent decisionmaker before a rule of general

applicability is imposed. See 2 THE RECORDS OF THE

FEDERAL CONVENTION OF 1787 28 (Max Ferrand .ed.,

rev. ed. 1966) (quoting Madison: “[c]onfidence” cannot

always “be put in the State Tribunals as guardians of

the National authority and interests”); accord The

Federalist No. 81 at 547 (A. Hamilton) (Cooke ed.

1961). In contrast, Boulder would prefer its claims to

be adjudicated by a local court, applying local law,

before a local jury drawn exclusively from the

community seeking billions of dollars from out-ofstate defendants who have no political representation

in Boulder and no voice in setting the legal standards

by which they will be judged. To describe that

circumstance is to indict it.

Federal law must provide the rule of decision for

claims about interstate emissions like Boulder’s.

B. The displacement of federal common law

does not permit state law to regulate

interstate emissions.

The court below and others around the country

have resisted federal law’s application on the ground

that the federal common law governing interstate

emissions “no longer exists” after the Clean Air Act

and Clean Water Act.3 Pet.App.17a; see also, e.g.,

3 As the Amici States have explained elsewhere, the Clean Air

Act has its own preemptive force in this context. But the Court

ultimately need not probe the scope of that preemption here, as

the federal common law and constitutional constraints decide the

question presented on their own. That separate force also

explains why the EPA’s recent rescission of the 2009 Greenhouse

Gas Endangerment Finding, 91 Fed. Reg. 13,088 (Feb. 18, 2026),

has no consequence here. The bar on state regulation of

interstate emissions does not depend on the vigor of any

particular federal regulatory program. Indeed, EPA’s rescission

rule recognized that the Clean Air Act “continues to preempt

state common-law claims and statutes that seek to regulate outof-state emissions, independently of CAA section 209(a)’s express

preemption provision for mobile-source emissions.” 91 Fed. Reg.

at 13,131. And the rescission only underscores what the Amici

States have long maintained: that the question of how and

whether to regulate interstate gas emissions is one for the

political branches of the federal government—not for state juries

applying local tort law. See Massachusetts v. EPA, 549 U.S. 497

533-34 (2007) (recognizing the Executive’s central role in

formulating climate policy). When Congress or the Executive

calibrates the scope of federal regulation—whether by expanding

it or by scaling it back—that calibration is itself a federal policy

choice that state-law liability schemes cannot override. See, e.g.,

Rhode Island v. Shell Oil Prods. Co., 35 F.4th 44, 55

(1st Cir. 2022); Mayor of Baltimore v. BP p.l.c., 31

F.4th 178, 206 (4th Cir. 2022). On this view,

displacement of federal common law allows “state law

… [to] snap back into action unless specifically

preempted by statute.” City of New York, 993 F.3d at

98. The “snap back” approach is misguided for several

reasons.

First, federal common law “exists … because state

law cannot be used.” Milwaukee II, 451 U.S. at 313

n.7. In the “enclaves” of federal common law, States

are not “free to develop their own doctrines.” Banco

Nacional de Cuba v. Sabbatino, 376 U.S. 398, 426

(1964). After displacement, “the need” for federal

common law “disappears,” Pet.App.10a, but only

because a different federal rule took its place. And

whatever form the federal law takes, it remains

equally “inappropriate for state law to control.” Tex.

Indus., 451 U.S. at 641; see also AEP, 564 U.S. at 422.

As the Second Circuit held, “state law does not

suddenly become competent to address issues that

demand a unified federal standard simply because

Congress … displace[d] a federal court-made standard

with a legislative one.” City of New York, 993 F.3d at

98; accord Pet.App.26a (Samour, J., dissenting). The

underlying deficiency of state law remains because,

among other things, the constitutional considerations

that support the need for a federal rule continue no

matter what a particular federal statute might say.

The Court addressed a similar issue in United

States v. Standard Oil, a damages action arising from

the collision of a truck with a U.S. Army soldier. 332

Gade v. Nat’l Solid Wastes Mgmt. Ass’n, 505 U.S. 88, 103 (1992);

Ray v. Atl. Richfield Co., 435 U.S. 151, 178 (1978).

U.S. 301, 302 (1947). The truck owner’s liability could

not “be determined by state law” because the matter

“vitally affect[ed] [federal] interests, powers, and

relations … as to require uniform national disposition

rather than diversified state rulings.” Id. at 305, 307.

“The only question” was “which organ of the

Government is to make the determination [of]

liability.” Id. at 316. Finding that decision best left for

“Congress, not for the courts,” the Court effectively

barred a remedy—much like AEP in the emissions

context. But the Court did not revisit its choice-of-law

holding as if state law might “snap back” in the

absence of federal common law.

Each of these cases is an “authority supporting the

proposition that once federal common law exists, the

structure of the Constitution precludes the

application of state law even when that common law

no longer exists.” Contra Pet.App.17a. More precisely,

federal common law has supplied a rule of decision in

cases where state law cannot. Repeating that federal

common law “in this area” no longer exists

(Pet.App.6a, 10a, 11a, 16a, 17a), the court below

blinded itself to decisive precedent. What does the

work is not the preemptive effect of federal common

law today, but the constitutional need for its creation.

Again: the law of interstate emissions was

developed because “the basic scheme of the

Constitution so demands.” AEP, 564 U.S. at 421. “The

very reasons the Court gave for resorting to federal

common law in Milwaukee I are the same reasons why

… federal law must govern” even after any

displacement. Illinois v. City of Milwaukee, 731 F.2d

403, 410-11 (7th Cir. 1984) (Milwaukee III). Resolving

an interstate controversy under a single State’s law is

a violation of state sovereignty, see supra §I.A.2;

Kansas, 206 U.S. at 95, which no federal statute could

permit.

Likewise, because “uniquely federal” interests

demand “uniform federal standards,” state law can

never be conclusive. Milwaukee III, 731 F.2d at 410.

“[T]he state claiming injury cannot apply its own state

law to out-of-state discharges.” Id. Illinois squarely

argued in Milwaukee III that if federal common law

were “dissipated” by statute, then “Illinois law must

again control.” Id. at 406. But the Seventh Circuit

understood that under “the logic of Milwaukee I,” state

law could never apply to interstate pollution. Id. at

411. Whether common law or statute, “federal law

must govern.” Id.

So it must govern here. The federal interests are

the same or even stronger. As explained in AEP, trial

courts “issuing ad hoc, case-by-case injunctions” are

not well “suited to serve as primary regulator of

greenhouse gas emissions.” 564 U.S. at 428. If the

Clean Air Act was meant to be a better and more

uniform solution, it would make no sense for state law

to “snap back” and recreate the problem that better

federal law tried to solve. Certainly, Congress didn’t

contemplate that result; under both the Clean Air Act

and Clean Water Act, “source states, and not affected

states, play the primary role in developing the

regulations by which a particular source will be

bound.” Bell v. Cheswick Generating Station, 734 F.3d

188, 196 (3d Cir. 2013).

Put differently, “displacement of a federal common

law right of action” is a “displacement of remedies.”

Native Vill. of Kivalina v. ExxonMobil Corp., 696 F.3d

849, 857 (9th Cir. 2012). Whether a “federal common

law remedy [is] available” (Pet.App.10a) has no

bearing on the availability of another remedy under

state law. Nor can state law govern just because the

claim “may fail at a later stage” under federal law.

Oneida Indian Nation of N.Y. v. Oneida County, 414

U.S. 661, 675 (1974); cf. Ouellette, 479 U.S. at 499-500.

To reach a contrary view, the court below misread

AEP v. Connecticut. There, the Court left open the

possibility of certain state-law claims in view of an

express statutory savings clause, 564 U.S. at 429, but

not the claims here. The Court noted in dicta (because

it was not briefed) that after the Clean Water Act, a

plaintiff could still bring a “nuisance claim pursuant

to the law of the source State.” Id. (quoting Ouelette,

479 U.S. at 489). That fact does not help Boulder,

which brings claims under Colorado law, not that of

any source State. See Ouelette, 479 U.S. at 495. The

type of claim AEP left open (intrastate) never had to

be governed by federal common law in the first place.

By contrast, the type of claim here (interstate) has

never been resolved by state law alone.

In fact, AEP reaffirmed that “suits brought by one

State to abate pollution emanating from another

State” are “meet for federal law governance.” 564 U.S.

at 421-22. In such suits, “borrowing the law of a

particular State would be inappropriate.” Id. The

court below ignored these lines in AEP, Pet.App.9a11a, as well as this Court’s doubt that “a State may

sue to abate any and all manner of [interstate]

pollution,” 564 U.S. at 421-22. If federal law, despite

its virtues in this area, does not provide a judicial

remedy, id. at 422-23, AEP surely did not invite state

law to fill the void.

Finally, the lower court mistook petitioners to be

advancing a “brooding” and “vague federal interest.”

Pet.App.18a. But the horizontal separation of powers

is a fundamental tenet of constitutional law. See Nat’l

Pork Producers Council, 598 U.S. at 376. One does not

need “text” (Pet.App.18a) to know what is “so

obviously the necessary result of the Constitution.”

N.Y. Life Ins. Co. v. Head, 234 U.S. 149, 161 (1914).

For example, States lack the “raw power to apply their

own law” to “disputes over borders, water rights, or

the interpretation of interstate compacts.” Franchise

Tax Bd., 587 U.S. at 246 (citations omitted). So too the

fact that States may not regulate the global

atmosphere is “not spelled out in the Constitution” but

nonetheless “implicit in its structure and supported by

historical practice.” Id. at 247. Anything else is

“ahistorical literalism.” Id.

C. This case is about interstate emissions.

If a state statute imposed massive fines for oil and

gas sold in a neighboring State, no one would doubt

the violation of the neighbor’s sovereignty. Cf.

Louisiana v. Texas, 176 U.S. 1, 27-28 (1900) (Brown,

J., concurring). Boulder’s attempt to impose tort

liability to the same end is no different, for “State

power may be exercised as much by a jury’s

application of a state rule of law in a civil lawsuit as

by a statute.” BMW, 517 U.S. at 573 n.17. A damages

remedy that is calibrated to out-of-state production is

regulation of out-of-state production, whatever label

the complaint applies.

Styling a State’s effort “to impose its own policy

choice” as a tort action does not shield it from the basic

“principles of state sovereignty and comity.” Id. at 572.

Those principles would be “meaningless” if a State

could do indirectly what it could not do directly. See

Kurns v. R.R. Friction Prods. Corp., 565 U.S. 625, 637

(2012); Healy, 491 U.S. at 335-36 & n.13. Thus,

whether Colorado law regulates emissions through

the “power to give damages rather than to enjoin,” the

result is still “a potent method of governing conduct

and controlling policy.” San Diego Bldg. Trades

Council v. Garmon, 359 U.S. 236, 247 (1959); see also

Cipollone v. Liggett Grp., 505 U.S. 504, 548 (1992)

(Scalia, J., concurring). Especially given the size of the

potential award, this case plainly contemplates

“regulation … exerted through an award of damages.”

Kurns, 565 U.S. at 637.

Really, this case is more pernicious than an

ordinary tort. Boulder’s suit is not analogous to an

ordinary suit in which a plaintiff seeks damages for a

discrete injury traceable to a specific product or act of

a specific defendant. At least in those cases, the

connection between the defendant’s conduct and the

plaintiff’s injury cabins the regulatory effect of any

judgment. Here, by contrast, Boulder does not allege

that any particular defendant’s products caused any

particular injury in Colorado. Rather, it alleges that

the cumulative emissions from the global production

and combustion of fuels by all producers over decades

have altered the Earth’s climate. The alleged tort is

the entire industry’s existence. A judgment on that

theory would not regulate a defective product,

commercial advertising, or any discrete course of

conduct; it would impose a levy on the aggregate

output of a global industry, calibrated not to any

Colorado-specific harm but to each defendant’s share

of worldwide production. That approach—an ugly

form of the “market share liability” theory that has

been widely rejected—bespeaks a redistributive and

parochial policy imposed under a tort label. See, e.g.,

Blankenship v. Gen. Motors Corp., 406 S.E.2d 781, 783

(W. Va. 1991) (candidly acknowledging that a court in

a “small rural state” like West Virginia is “utterly

powerless” to make “interstate commerce more

rational,” so the court should focus instead on not

“punish[ing] [West Virginia’s] residents”).

The court below asserted that Boulder does “not

seek to regulate [greenhouse gas] emissions.”

Pet.App.21a. But taxing emissions—i.e., regulating

them—is the heart of the suit. Boulder complains that

petitioners “continue to conduct their fossil fuel

activities” and “produce a substantial amount of

[greenhouse gas] emissions.” J.A. 84-85, 114. Boulder

claims this is a nuisance precisely because of the

emissions. For the alleged harms caused by emissions,

Boulder seeks billions of dollars in damages, id. at

58-59, 114, and disgorgement, id. at 20, 24, 123.

Boulder also demands “future damages and costs” to

“mitigate, abate, and/or remediate the impacts of

climate change.” Id. at 136-37. In what way doesn’t

this case “implicat[e] the regulation of interstate

pollution”? Pet.App.17a.

There is no way Boulder can succeed without

impacting “any oil and gas operations or sales in

Colorado or elsewhere.” Pet.App.4a. It is “common

sense and basic economics” that raising the “cost of

conduct will make that conduct less common.” City of

New York, 993 F.3d at 93 (cleaned up). Any relief

would be “a de facto regulation on greenhouse gas

emissions.” Id. at 96. Boulder cannot “hid[e] the

obvious”; it seeks “a global remedy for a global issue.”

API, 63 F.4th at 719 (Stras, J., concurring). It is a

“regulatory measure[] designed to benefit in-state

economic interests by burdening” producers in other

States. Dep’t of Rev. of Ky. v. Davis, 553 U.S. 328, 338

(2008). While not every lawsuit against the energy

industry is an attempt to regulate emissions, that’s

what “this case is about.” Pet.App.33a (Samour, J.,

dissenting).

Sometimes claimants will try to disguise their

regulatory efforts by targeting purportedly misleading

disclosures about the energy production (rather than

the energy production activities themselves). But

because Boulder deems “altering the climate” to be a

public nuisance, J.A. 113, the cloak of consumer

protection is not available here. The only way for

energy companies to avoid liability altogether,

according to the complaint, would be to halt the

production, sale, and use of their products everywhere.

And regardless of the label, forcing global energy

companies to cease doing business everywhere is not

among a State’s constitutional powers. Colorado law

can reach “persons and property within the limits of

its own territory.” Hoyt v. Sprague, 103 U.S. 613, 630

(1880). But virtually all the conduct Boulder targets

occurred outside of Colorado, well beyond the proper

regulatory sphere of state law. See City of New York,

993 F.3d at 92; cf. BMW, 517 U.S. at 573 n.20. The

causal chain necessarily passes through interstate

emissions at every step. No historical analogue for

what Boulder is trying to do can be found, and the

court below was wrong to cast this suit as a matter “of

traditional state responsibility.” Pet.App.23a-24a; see

Pet.App.27a (Samour, J., dissenting).

II. If No Federal Rule Applies, The States Will

Suffer.

If the Court embraces Boulder’s view, then Amici

States, their citizens, and our Nation’s energy sector

suffer serious damage. Boulder alone demands

billions of dollars, and every decision like the one

below is a green light for future plaintiffs. At least one

county in Colorado already has a copycat lawsuit.

Pet.App.25a (Samour, J., dissenting). Such suits will

in turn cause job losses, tax losses, and critical

industry harms in States like Alabama and West

Virginia that depend on traditional energy

production. The Court should consider the enormous

costs and uncertainty of litigation in dozens of state

courts around the country when shaping the eventual

rule. See supra n.1. Although some courts appreciate

the grave constitutional problems with these suits,

others do not.

While the grave threat these suits pose to equal

sovereignty and our Nation’s energy infrastructure

are reason enough for this Court to reverse, the

problem is really bigger than that. The theory used

against energy producers here can be expanded to

target any extraterritorial activity that purportedly

“exacerbate[s] climate change.” Pet.App.2a. And

really, the same theory could be extended to justify

action against any activity that produces nationwide

impacts.

It’s already happening. For example, New York

sued “the world’s largest producer of beef products, for

misleading the public about its environmental

impact.”4 The beef producer’s pledge to reach “Net

Zero by 2040” was allegedly misleading because the

company “plans to grow global demand for its

product,” rather than “reduce production of and

demand for” it. Complaint, ¶¶143-44, New York v.

JBS USA Food Co., No. 450682/2024 (N.Y. Super. Ct.

4 Office of N.Y. Att’y Gen., Attorney General James Sues World’s

Largest Beef Producer for Misrepresenting Environmental Impact

of Their Products, Feb. 28, 2024, tinyurl.com/28udz5pa.

filed Feb. 28, 2024). That case was tossed, given the

ease of pleading that deep-pocketed companies have

“exacerbate[d] climate change,” Pet.App.2a, one can

only expect to see more suits like these.

And the problems aren’t limited to lawsuits either.

Some States are pursuing legislative avenues to

impose extraterritorial liability, even though “[t]he

limits on a State’s power to enact substantive

legislation are similar to the limits on the jurisdiction

of state courts.” Edgar v. MITE Corp., 457 U.S. 624,

643 (1982) (plurality op.). One needn’t even leave New

York to look for examples. That state (alongside

Vermont) has passed a law that seeks to impose

billions of dollars of retroactive, strict liability on outof-state energy producers. See N.Y. Env’t Conser. Law

§§ 76-0101 to -0103; see also Vt. Stat. tit. 10, §§ 596599c. Other states are considering similar laws. See

Nat’l Caucus of Env’tl Legislators, 2025 Superfund

Legislation (Climate) (2025), tinyurl.com/3enh4k2u.

Just like Boulder, they blame out-of-state entities for

alleged climate change. While the liability under those

laws will come by way of an administrative

assessment—rather than a nuisance judgment—the

pain will be just the same. All these jurisdictions

appear to be trying to “leverage the expense, risks,

and burden to [the energy companies] of defending

[themselves] in multiple jurisdictions” to secure a

much-desired cash stream. Chevron Corp. v. Donziger,

974 F. Supp. 2d 362, 475 (S.D.N.Y. 2014).

The States, upon entering the Union, gave up the

right to use their laws to wage this sort of interstate

conflict. They gave up the right to impose their policy

choices on other States and activities entirely outside

the home State’s jurisdiction. The Court should

recognize as much and embrace a rule that generates

the uniformity and equal respect that the Founders

intended in matters like these.

CONCLUSION

The Court should reverse.

Respectfully submitted,

Steve Marshall

Attorney General

John B. McCuskey

Attorney General

A. Barrett Bowdre

Solicitor General

Counsel of Record

Michael R. Williams

Solicitor General

Robert M. Overing

Principal Deputy

Solicitor General

OFFICE OF THE ALABAMA

ATTORNEY GENERAL

501 Washington Avenue

Montgomery, AL 36130

(334) 242-7300

Barrett.Bowdre@

AlabamaAG.gov

Counsel for Alabama

OFFICE OF THE WEST

VIRGINIA

ATTORNEY GENERAL

State Capitol Complex

Building 1, Room E-26

Charleston, WV 25306

(304) 558-2021

mwilliams@wvago.gov

Counsel for West Virginia

LEGAL REPRESENTATIVES OF THE STATES

CORI MILLS

Acting Attorney General

State of Alaska

LIZ MURRILL

Attorney General

State of Louisiana

TIM GRIFFIN

Attorney General

State of Arkansas

LYNN FITCH

Attorney General

State of Mississippi

JAMES UTHMEIER

Attorney General

State of Florida

CATHERINE L. HANAWAY

Attorney General

State of Missouri

CHRISTOPHER M. CARR

Attorney General

State of Georgia

AUSTIN KNUDSEN

Attorney General

State of Montana

RAÚL R. LABRADOR

Attorney General

State of Idaho

MICHAEL T. HILGERS

Attorney General

State of Nebraska

THEODORE E. ROKITA

Attorney General

State of Indiana

JOHN M. FORMELLA

Attorney General

State of New Hampshire

BRENNA BIRD

Attorney General

State of Iowa

DREW WRIGLEY

Attorney General

State of North Dakota

KRIS W. KOBACH

Attorney General

State of Kansas

DAVE YOST

Attorney General

State of Ohio

RUSSELL M. COLEMAN

Attorney General

Commonwealth of

Kentucky

GENTNER DRUMMOND

Attorney General

State of Oklahoma

DAVID W. SUNDAY, JR.

Attorney General

Commonwealth of

Pennsylvania

ALAN WILSON

Attorney General

State of South Carolina

MARTY JACKLEY

Attorney General

State of South Dakota

KEN PAXTON

Attorney General

State of Texas

DEREK BROWN

Attorney General

State of Utah

KEITH G. KAUTZ

Attorney General

State of Wyoming

This is a copy of a public record, reproduced as it was published. It is not legal advice, and it may not be the version a court would rely on. Check the official source before you cite it.