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.;

SUNCOR ENERGY SALES INC.; EXXON MOBIL CORP.,

Petitioners,

v.

COUNTY COMMISSIONERS OF BOULDER COUNTY;

CITY OF BOULDER,

Respondents.

__________

On Writ of Certiorari to the

Supreme Court of Colorado

__________

BRIEF OF THE FRONTIER INSTITUTE,

INDEPENDENCE INSTITUTE, AND

MANHATTAN INSTITUTE

AS AMICI CURIAE

SUPPORTING PETITIONERS

__________

Marcella Burke

Paul B. Simon

Connor Mighell

BURKE LAW GROUP PLLC

1000 Main Street,

Suite 2300

Houston, TX 77002

(832) 987-2214

marcella@burkegroup.law

Ilya Shapiro

Counsel of Record

MANHATTAN INSTITUTE

52 Vanderbilt Ave.

New York, NY 10017

(212) 599-7000

ishapiro@manhattan.

institute

May 19, 2026

Additional counsel listed on signature page

i

QUESTION PRESENTED

The Colorado Supreme Court found that the Clean

Air Act’s framework did not preempt all state and local

regulation of emissions and allowed Boulder County to

bring tort claims under Colorado law against Suncor

Energy and Exxon Mobil Corporation for damages

from carbon emissions.

The question presented is:

Whether federal law precludes state-law claims

seeking relief for injuries allegedly caused by the effects of interstate and international greenhouse-gas

emissions on the global climate.

ii

Table of Contents

QUESTION PRESENTED ........................................... i

TABLE OF AUTHORITIES ....................................... iii

INTEREST OF AMICI CURIAE ................................. 1

INTRODUCTION AND SUMMARY OF

ARGUMENT........................................................... 2

ARGUMENT ................................................................ 4

I. HORIZONTAL FEDERALISM IS

DEEPLY ROOTED IN

CONSTITUTIONAL HISTORY ....................... 4

A. The History of Horizontal Federalism ........ 4

B. The Constitutional Structure of

Horizontal Federalism ................................. 8

II. FOUR ASPECTS OF HORIZONTAL

FEDERALISM ARE RELEVANT TO

THIS CASE ..................................................... 11

A. Overreaching.............................................. 12

B. Exclusions .................................................. 16

C. Favoritism .................................................. 18

D. Externalities .............................................. 21

III. THE COLORADO SUPREME COURT

IGNORED HORIZONTAL FEDERALISM.... 25

CONCLUSION .......................................................... 30

iii

Cases

TABLE OF AUTHORITIES

Am. Elec. Power Co., Inc. v. Connecticut,

564 U.S. 410 (2011) .................................... 22–23, 26

Bigelow v. Virginia, 421 U.S. 809 (1975) ............ 25, 27

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

517 U.S. 559 (1996) .................................... 14, 15, 25

Bordenkircher v. Hayes, 434 U.S. 357 (1978) ........... 14

Canaday v. Anthem Companies, Inc.,

9 F.4th 392 (6th Cir. 2021) ...................................... 2

Carroll v. Lanza, 349 U.S. 408 (1955)......................... 9

Chisholm v. Georgia, 2 U.S. 419 (1793) .................... 11

City of Milwaukee v. Illinois, 451 U.S. 304 (1981) ... 22

City of New York v. Chevron Corp.,

993 F.3d 81 (2d Cir. 2021) ........................... 3, 23–24

Cnty. Comm’rs of Boulder Cnty. v. Suncor Energy

USA, Inc., 586 P.3d 161 (Colo. 2025)............... 26–27

Coyle v. Smith, 221 U.S. 559 (1911) ...................... 8, 21

Durfee v. Duke, 375 U.S. 106 (1963) ......................... 19

Franchise Tax Bd. of Cal. v. Hyatt,

587 U.S. 230 (2019) ...................................... 2, 19–20

Frick v. Pennsylvania, 268 U.S. 473 (1925) .............. 14

Guy v. Baltimore, 100 U.S. 434 (1880) ................ 16–17

Hampton v. McConnell, 3 Wheat. 234 (1818) ........... 19

Hanson v. Denckla, 357 U.S. 235 (1958) ................... 13

Home Ins. Co. v. Dick, 281 U.S. 397 (1930) .............. 12

Illinois v. City of Milwaukee, 406 U.S. 91 (1972) ..... 22

iv

Int’l Paper Co. v. Ouellette,

479 U.S. 481 (1987) .............................. 23, 26, 27, 29

Int’l Shoe v. Washington, 326 U.S. 310 (1945) ......... 12

Kansas v. Colorado, 206 U.S. 46 (1907) ........ 21–22, 29

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

565 U.S. 625 (2012) .......................................... 15, 27

Mayor & City Council of Baltimore v.

B.P. P.L.C., 2026 WL 809501

(Md. Mar. 24, 2026) ................................ 3, 23–24, 28

Nat’l Pork Producers Council v. Ross,

598 U.S. 356 (2023) .......................... 2, 16–17, 18, 30

Native Village of Kivalina v. ExxonMobil Corp.,

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

Nielsen v. Oregon, 212 U.S. 315 (1909) ..................... 14

Northwest Airlines Inc. v. County of Kent,

510 U.S. 355 (1994) ................................................ 16

Pennoyer v. Neff, 95 U.S. 714 (1877) ......................... 12

People of State of Ill. v. City of Milwaukee,

731 F.2d 403 (7th Cir. 1984) .................................. 26

Pike v. Bruce Church, Inc.,

397 U.S. 137 (1970) .......................................... 17–18

Planche v. Fletcher, 99 Eng. Rep. 164 (1779) ............. 5

Presley v. Etowah County Comm’n,

502 U.S. 491 (1992) ................................................ 21

San Diego Building Trades Council v. Garmon,

359 U.S. 236 (1959) ................................................ 15

Shelby County v. Holder, 570 U.S. 529 (2013) .... 20, 21

Somerset v. Stewart, 98 Eng. Rep. 499 (1772) ............ 4

v

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

538 U.S. 408 (2003) ................................................ 25

South Carolina v. Katzenbach,

383 U.S. 301 (1966) ................................................ 20

Toomer v. Witsell, 334 U.S. 385 (1948) ..................... 24

Underwriters Nat. Assur. Co. v. N.C. Life & Acc. &

Health Ins. Guaranty Ass’n,

455 U.S. 691 (1982) .................................... 18, 19, 29

World-Wide Volkswagen Corp. v. Woodson,

444 U.S. 286 (1980) ................................ 2, 12–13, 24

Constitutions, Statutes, and Regulations

42 U.S.C. § 7416......................................................... 23

42 U.S.C. § 7521(a)(1) ............................................ 3, 28

42 U.S.C. § 7604(e) .................................................... 23

An Act to Provide for the Government of the

Territory North-West of the River Ohio,

art. V § 8, 1 Stat. 50 (1789) ................................ 8, 19

Art. of Confed. of 1781, art. IV, § 1 ............................. 6

Colo. Const. art. XIV .................................................. 25

Ordinance of April 23, 1784,

26 J. Cont’l Cong. 275 (1784) ................................... 8

Rescission of the Greenhouse Gas Endangerment

Finding and Motor Vehicle Greenhouse Gas

Emission Standards Under the Clean Air Act,

91 Fed. Reg. 7686 (Feb. 18, 2026)................ 4, 28–29

U.S. Const. art. I, §10 .............................................. 8, 9

U.S. Const. art. IV, §1.................................................. 9

U.S. Const. art. IV, §2.................................................. 9

vi

Other Authorities

2 The Records of the Constitutional Convention

of 1787 (Max Farrand ed., 1937) ......................... 6–7

Allen Erbsen, Horizontal Federalism,

93 Minn. L. Rev. 493 (2008)................. 2, 3, 6, 11, 18

Angus Thuermer, “Hageman Proposes a Boulder,

Colorado, Fossil-Fuel-Free Experiment,”

Cap City News, Aug. 8, 2024 ................................. 25

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 (1987) ..... 4

Emer de Vattel, The Law of Nations at I, § 18

(J. Chitty ed., 1834) (1758) ...................................... 5

Jill Elaine Hasday, Interstate Compacts in a

Democratic Society: The Problem of Permanency,

49 Fla. L. Rev. 1 (1997) ........................................ 8–9

Julian N. Eule, Laying the Dormant Commerce

Clause to Rest, 91 Yale L.J. 425 (1982) ................... 9

Mark D. Rosen, State Extraterritorial Powers

Reconsidered,

85 Notre Dame L. Rev. 1133 (2010) ...................... 10

Peter S. Onuf, New State Equality: The Ambiguous

History of a Constitutional Principle,

18 Publius 53 (1988) ................................................ 8

Samuel Issacharoff & Catherine M. Sharkey,

Backdoor Federalization,

53 UCLA L. Rev. 1353 (2006) ................................ 10

The Federalist No. 44 (Madison) ................................. 9

The Federalist No. 7 (Hamilton) ............................... 10

vii

The Federalist No. 80 (Hamilton) ............................. 10

Willis L.M. Reese, Legislative Jurisdiction,

78 Columbia L. Rev. 1587 (1978)..................... 10–11

1

INTEREST OF AMICI CURIAE 1

The Frontier Institute is an independent research and educational institution with the mission to

keep the spirit of the western frontier alive with sound

public policy and education programs that empower

Montanans to be pioneers, innovators and risk takers.

To those ends, the Frontier Institute is dedicated to

upholding the separation-of-powers requirements of

the United States and Montana Constitutions that foster democratic accountability and sound public policy.

The Independence Institute is a 501(c)(3) public

policy research organization in Denver, founded on the

eternal truths of the Declaration of Independence. The

briefs and scholarship of research director David Kopel

have been cited in seven opinions of this Court and

over 130 lower-court opinions. The Institute’s senior

fellow in constitutional studies, law professor Robert

Natelson, has been cited in 13 opinions of this Court.

The Manhattan Institute (MI) is a nonprofit public policy research foundation whose mission is to develop and disseminate new ideas that foster greater

economic choice and individual responsibility, to advance the flourishing of America’s great cities.

This case interests amici because a proper understanding of federalism holds that federal law precludes

contrary state law. The transaction of actual interstate

commerce, such as energy production—and its attendant pollution—is a quintessentially federal area.

Having one regulatory structure here allows for legal

stability and the efficient allocation of resources.

No counsel for any party authored this brief in any part; nobody

other than amici funded its preparation or submission.

1

2

INTRODUCTION AND

SUMMARY OF ARGUMENT

“Constitutional federalism has two distinct dimensions: the federal government must interact with the

states, and states must interact with each other.” Allen Erbsen, Horizontal Federalism, 93 Minn. L. Rev.

493, 501 (2008). The former interactions are more familiar—and are the focus of constitutional-law classes—but other significant friction can occur from interactions among states or their inhabitants.

The principles governing these interactions have

been called many things: “antidiscrimination,” Nat’l

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

(2023); “comity,” Franchise Tax Bd. of Cal. v. Hyatt,

587 U.S. 230, 245 (2019); “interstate federalism,” see

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

286, 293 (1980); and “horizontal federalism,” Canaday

v. Anthem Companies, Inc., 9 F.4th 392, 410 (6th Cir.

2021) (Donald, J., dissenting) (citation omitted).

We will use the last term, “horizontal federalism.”

This case implicates four recurring “sources of interstate friction” that this Court regularly analyzes,

based on the following principles:

•

Overreaching: A state may not adjudicate,

tax, regulate, or punish conduct that occurs beyond its borders to deter conduct that is lawful

in other states.

•

Exclusions: States (especially commercially

powerful ones) may not leverage their regulations to restrict other states’ policymaking.

•

Favoritism: The federal government may intervene when states regulate in favor of local

3

interests in ways that burden constitutional

rights.

•

Externalities: Federal law governs when a

state pursues a policy that affects another, and

both states have equal rights of action.

See Erbsen, 93 Minn. L. Rev. at 514.

Horizontal federalism emerges from the rich scholarship and experience that informed the framers and

the federal structure they established. The Colorado

Supreme Court neglected that framework here, so its

judgment should be reversed.

Two developments since briefing on the cert. petition was completed have made the case for reversal

stronger. First, the Maryland Supreme Court has now

considered substantially similar climate-tort claims

and reached the opposite result from its Colorado

counterpart. In Mayor & City Council of Baltimore v.

B.P. P.L.C., 2026 WL 809501 (Md. Mar. 24, 2026)

(“Baltimore”), the court affirmed dismissal of local-government state-law tort claims seeking damages for

harms allegedly caused by global greenhouse-gas

emissions. The majority agreed that those claims are

displaced by federal common law, displaced in turn by

the Clean Air Act, and not authorized by the Act’s savings clauses. Id. at *23–25. The court expressly aligned

itself with the dissent below, as well as with the Second Circuit’s decision in City of New York v. Chevron

Corp., 993 F.3d 81 (2d Cir. 2021). Baltimore, 2026 WL

809501, at *20 n.18.

Second, the EPA has rescinded the 2009 endangerment finding for greenhouse gases under section

202(a)(1) of the Clean Air Act, 42 U.S.C. § 7521(a)(1),

and repealed the implementing motor-vehicle

4

greenhouse-gas standards. Rescission of the Greenhouse Gas Endangerment Finding and Motor Vehicle

Greenhouse Gas Emission Standards Under the Clean

Air Act, 91 Fed. Reg. 7686 (Feb. 18, 2026). That action

underscores that greenhouse-gas policy requires national coordination and federal judgment, not tort

rules that project one state’s policy choices into other

states.

Together, these developments confirm what was already true at cert: Colorado tort law is the wrong instrument for resolving injuries allegedly caused by interstate and international emissions.

ARGUMENT

I. HORIZONTAL FEDERALISM IS DEEPLY

ROOTED IN CONSTITUTIONAL HISTORY

Horizontal federalism 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).

A. The History of Horizontal Federalism

A few years before the Revolution, British courts

were articulating the doctrines of extraterritoriality

and comity in jurisdictions that American lawyers like

the Framers followed closely. One jurist, Lord Mansfield, held that a slave brought from America could not

be detained in England because “[t]he state of slavery

. . . is so odious, that nothing can be suffered to support

it, but positive law.” Somerset v. Stewart, 98 Eng. Rep.

499, 510 (1772). So British jurists held that a law of

one jurisdiction was not given effect in another if

5

repugnant to that jurisdiction’s fundamental principles. Likewise, in Planche v. Fletcher, 99 Eng. Rep.

164, 165 (1779), Mansfield ruled that “one nation does

not take notice of the revenue laws of another.” There,

a contract to evade French customs law could not be

enforced in an English court, because English courts

would not enforce French tax law.

European law-of-nations theorists also shaped

early American understandings of comity and equal

sovereignty. One popular treatise stated: “A dwarf is

as much a man as a giant; a small republic is no less a

sovereign state than the most powerful kingdom.”

Emer de Vattel, The Law of Nations at I, § 18 (J. Chitty

ed., 1834) (1758). This idea of sovereign equality posited that no state may subordinate another by force of

law. Mutual recognition and voluntary respect were

ideal, even necessary, when crafting a union of states.

The idea consistently advanced in these writings is

that the authority of a sovereign stops at its borders.

Every court and country applies and fashions its own

law. They may respect the statutes “carried with” the

subjects of other sovereigns as a matter of comity, but

they only adopt the ones they deem persuasive or appropriate and disregard the rest. Any further entanglement would lead to confusion about which law applies and undermine individuals’ reliance interest in

their conduct’s being governed by the law of the location where that conduct takes place.

Post-Revolution, the Constitution’s framers

demonstrated acute awareness that unrestrained

state action leads to interstate friction and conflict.

Our first charter, the Articles of Confederation, viewed

6

each state as its own sovereign, and provided “the free

inhabitants” of each state with

all privileges and immunities of free citizens in

the several States, and the people of each State

shall have free ingress and regress to and from

any other State, and shall enjoy therein all the

privileges of trade and commerce, subject to the

same duties, impositions and restrictions as the

inhabitants thereof respectively . . . .

Art. of Confed. of 1781, art. IV, § 1. Yet under the Articles, states often “pursued conflicting self-interests

at their collective expense,” enacting protectionist

measures and interfering with commerce. Erbsen, 93

Minn. L. Rev. at 511. They coined their own money,

raised their own armies, and erected trade barriers,

“creating systemic friction that left them collectively

worse off.” Id. at 533. The delegates to the Constitutional Convention emerged from a years-long battle

with that problematic interstate construction, producing a system of government with a different approach.

It bears noting from the outset that the Constitution contains no explicit provision requiring blanket

interstate equality. At the Constitutional Convention,

the section that ultimately became Article IV, Section

3 initially included language providing that new states

would be admitted “on the same terms” as current

states. 2 The Records of the Constitutional Convention

of 1787 at 454 (Max Farrand ed., 1937). Gouverneur

Morris moved to strike this language. James Madison

opposed Morris’s motion, arguing that new states “neither would nor ought to submit to a Union which degraded them from an equal rank with the other

States.” Id. George Mason likewise argued that Morris’s suggestion appeared intended to deter western

7

emigration, but that this was impossible. Id. Mason

continued that “the best policy” regarding these new

states would be “to treat them with that equality

which will make them friends not enemies.” Id. Morris

agreed that stopping western emigration was impossible, but “did not wish to throw the power into the[]

hands” of newly admitted states. Id. Roger Sherman

notably stated that he was in favor of “fixing an equality of privileges by the Constitution,” and therefore opposed the motion. John Langdon chimed in to support

the motion because he wondered about circumstances

“which would render it inconvenient” to admit new

states on equal footing with established ones. Id.

North Carolina’s Hugh Williamson also clarified that

while existing states “enjoy an equality now, and for

that reason are admitted to [Congress] in the Senate,”

this reason did not apply to new states. Id. The body

ultimately accepted Morris’s edits by a 9–2 vote. Id.

This debate is highly instructive as to the Framers’

perspective on interstate relations. Morris’s edit provoked vocal debate from Madison, Mason, and Sherman—leading architects of the Constitution’s framework—because it ran counter to their conviction that

existing states should have equality of privileges. But

Morris and his supporting delegates, who won the day,

thought it best to leave to Congress’s judgment the exact admission terms for new states.

The Convention’s view was not that states were unequal or lacked equal sovereignty. It was that the

elected representatives of existing states should decide

what privileges new states had when admitting them.

Although the Convention did not settle this question,

the Continental Congress in New York incorporated

the Northwest Ordinance under the new Constitution,

8

requiring that new states would enter the union “on

equal footing” with existing states. An Act to Provide

for the Government of the Territory North-West of the

River Ohio, art. V § 8, 1 Stat. 50, 53 n.a (1789). This

was based on a 1784 ordinance drafted by Thomas Jefferson proposing that the western states be incorporated on an equal basis with the original thirteen. Ordinance of April 23, 1784, 26 J. Cont’l Cong. 275

(1784). Every state since has been admitted “with an

express declaration of equality” with existing states.

Peter S. Onuf, New State Equality: The Ambiguous

History of a Constitutional Principle, 18 Publius 53, 54

(1988). And this Court later described the equal-footing principle as equality “in all respects whatsoever.”

Coyle v. Smith, 221 U.S. 559, 567, 569 (1911) (cleaned

up) (discussing the admission of several states).

B. The Constitutional Structure of Horizontal

Federalism

Despite disagreement at the drafting stage, the

Constitution established a federal structure that

treats states as equal sovereigns with limited authority to encroach on each other’s respective domains.

States may not, for example, coin money or lay duties

without Congress’s consent. U.S. Const. art. I, §10. In

addition to these pointed restraints on interstate activity, the Constitution includes provisions designed to

foster comity and equality between the several states.

The Interstate Compact Clause, U.S. Const. art. I,

§10, cl.3, allows states to resolve disagreements by

“Agreement or Compact” with congressional approval.

These agreements have been frequently employed to

govern relations between states since the founding.

See Jill Elaine Hasday, Interstate Compacts in a Democratic Society: The Problem of Permanency, 49 Fla. L.

9

Rev. 1, 3–4 nn.14–18 (1997) (listing compacts). While

such arrangements are allowed, any “Treaty, Alliance,

or Confederation” between states is prohibited. U.S.

Const. art. I, §10, cl.1. This language likely refers to

contracts between states without Congress’s say-so, or

military or external-facing agreements. It was part of

the Articles of Confederation and “for reasons which

need no explanation, is copied into the new Constitution.” The Federalist No. 44 (Madison), at 281 (Clinton

Rossiter ed., 1961). Those self-evident reasons included ensuring that the powers expressly given to the

federal government would not be adulterated by states

working at cross-purposes to the broader Union.

The Constitution also includes another clause similar to a provision in the Articles, providing that the

“Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several

States.” Art. IV, §2, cl.1. While pared down from its

earlier counterpart, the constitutional version has a

wider sweep—preserving the legal and natural rights

of American citizens in all contexts, not merely a commercial one. Based on its lineage and language, this

clause governs state legislative power “in commercial

matters where Congress has not yet acted.” Julian N.

Eule, Laying the Dormant Commerce Clause to Rest,

91 Yale L.J. 425, 448 (1982). And all the above does not

obviate the dormant Commerce Clause, which speaks

to relations between states that specifically concern

commerce—addressed infra at II.B.

Another clause requires states to give “Full Faith

and Credit” to the “public Acts, Records, and judicial

Proceedings of every other State.” U.S. Const. art. IV,

§1; see also Carroll v. Lanza, 349 U.S. 408, 413 (1955)

(clause prevents states from “adopting any policy of

10

hostility” to each other’s acts). This extraterritoriality

doctrine sets the boundaries for how states must treat

the decisions and laws of other states.

Other constitutional provisions help complete the

horizontal federalism framework. Five of the nine provisions in Article III addressing jurisdiction concern

the interaction between states, emphasizing the framers’ concern that courts should referee “bickering and

animosities” between these co-equal sovereigns. The

Federalist No. 80 (Hamilton) at 477. Hamilton specifically worried about this issue, warning that states exercising “distinctions, preferences, and exclusions . . .

would beget discontent,” causing “outrages,” and then

“reprisals and wars.” Federalist No. 7 (Hamilton). Rather than addressing the externalities that activity legal in one state might cause in another, the Constitution sets this problem aside for judicial resolution—

and the courts’ solution has often been explicit federal

control, to avoid the very kinds of problems in interstate relations that the Articles of Confederation failed

adequately to address. See Samuel Issacharoff & Catherine M. Sharkey, Backdoor Federalization, 53 UCLA

L. Rev. 1353, 1368–98 (2006) (discussing federalization of areas traditionally under state control).

At bottom, equality among the states is a due process concern. “[D]ue process primarily protects individuals from being unfairly subject to another state’s

laws.” Mark D. Rosen, State Extraterritorial Powers

Reconsidered, 85 Notre Dame L. Rev. 1133, 1137–38

(2010). Scholarship regarding extraterritoriality has

long noted that horizontal federalism “protect[s] persons against the unfair application of a law” outside

proper borders while also “furthering other interstate

11

. . . values[.]” Willis L.M. Reese, Legislative Jurisdiction, 78 Columbia L. Rev. 1587, 1589 (1978).

This Court recognized that due process connection—and the idea that protecting it would be the job

of the federal courts—four years after the Constitution’s adoption. In Chisholm v. Georgia, 2 U.S. 419

(1793) (holding abrogated by 11th Amendment), in an

opinion by Justice James Wilson, the Court explained

that the Constitution’s goal of domestic tranquility “is

most likely to be disturbed by controversies between

states,” a consequence the Constitution seeks to avoid

“by the establishment and by the exercise of a superintending judicial authority.” Id. at 465.

Together, these provisions provide the framework

of horizontal federalism. The Constitution preserves

each state’s authority to regulate activity occurring

within its bounds while restraining that power beyond

a state’s borders. How that framework constrains state

tort law in cases involving interstate and international

emissions is the subject of Part II.

II. FOUR ASPECTS OF HORIZONTAL FEDERALISM ARE RELEVANT TO THIS CASE

Because horizontal federalism is a structural doctrine inferable from the Constitution, rather than delineated in an explicit clause, how may lower courts

“weave wisps of structure into judicially enforceable

standards”? Erbsen, 93 Minn. L. Rev. at 582. A robust

body of the Court’s case law has given shape to aspects

of horizontal federalism, in a variety of contexts. All of

it has in common the presupposition that state power

is limited by the equal sovereignty of other states and

by a common interest in preventing interstate friction—the “reprisals and wars” of Hamilton’s warning.

12

A. Overreaching

With narrow exceptions, states cannot tax or regulate beyond their borders, or punish conduct that was

lawful where it occurred.

1. Personal jurisdiction

Courts have long recognized the limits this principle imposes in the law of personal jurisdiction.

In the post-Civil War period, this Court found in

Pennoyer v. Neff, 95 U.S. 714, 722 (1877), that “no

State can exercise direct jurisdiction and authority

over persons or property without its territory.” Because “the several States are of equal dignity and authority,” one state extending its reach to conduct beyond its borders is “an encroachment upon the independence” of the state where the affected persons or

property are actually located. Id. at 722–23.

The Court eventually softened Pennoyer’s requirement that state jurisdiction screeches to a stop at its

borders. It allowed jurisdiction where contacts existed

with the forum state, but affirmed that such jurisdiction must not transgress “traditional notions of fair

play and substantial justice.” Int’l Shoe v. Washington,

326 U.S. 310, 316 (1945) (citation omitted). The federal

system does not permit states to hale every outsider

with whom they have contact into court:

Even if the defendant would suffer minimal or no

inconvenience from being forced to litigate before

the tribunals of another State; even if the forum

State has a strong interest in applying its law to

the controversy; even if the forum State is the

most convenient location for litigation, the Due

Process Clause, acting as an instrument of

13

interstate federalism, may sometimes act to divest the State of its power to render a valid judgment.

World-Wide Volkswagen, 444 U.S. at 294.

The strictures of personal jurisdiction are “a consequence of territorial limitations on the power of the respective States” that safeguard individual fairness and

interstate sovereignty. Hanson v. Denckla, 357 U.S.

235, 251 (1958). Every state’s judicial power ends

where another’s authority begins, and no state can

punish or regulate conduct lawful in another state if

that conduct lacks a meaningful connection to the forum. See Home Ins. Co. v. Dick, 281 U.S. 397, 407–08

(1930) (holding that Texas law “may not validly affect

contracts which are neither made nor are to be performed in Texas”); World-Wide Volkswagen, 444 U.S.

at 292 (explaining that the personal-jurisdiction requirement of minimum contacts “acts to ensure that

States, through their courts, do not reach out beyond

the limits imposed on them by their status as coequal

sovereigns in a federal system.”); id. at 293 (“The sovereignty of each State, in turn, implied a limitation on

the sovereignty of all of its sister States—a limitation

express or implicit in both the original scheme of the

Constitution and the Fourteenth Amendment.”).

This Court’s jurisprudence thus highlights an important doctrinal facet: a state may regulate activity

beyond its physical borders only if that activity has a

meaningful, substantial nexus with it.

2. Due process

But the relevant principle is about more than personal jurisdiction and the requirement of minimum

contacts. Due process constrains states from taxing or

14

regulating in a way that “infring[es] on the policy

choices of other States.” BMW of N. Am., Inc. v. Gore,

517 U.S. 559, 572 (1996); see also Frick v. Pennsylvania, 268 U.S. 473, 477–79 (1925) (holding that the Due

Process Clause bars Pennsylvania from imposing

transfer tax on art in New York owned by Pennsylvania decedent); Nielsen v. Oregon, 212 U.S. 315, 321

(1909) (reversing criminal conviction in Oregon of

Washington resident who fished in Washington using

gear lawful in Washington).

A state may not “impose sanctions . . . in order to

deter conduct that is lawful in other jurisdictions.”

BMW, 517 U.S. at 573. This extraterritorial limitation

applies regardless of whether a state acts through

statute, regulation, or a damages award entered under

a common-law tort. “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.” Id. at 572 n.17; id. at 572

(“[I]t follows from principles of state sovereignty and

comity 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.”).

Because this limitation sounds in due process, the

fact that Boulder County is trying to regulate conduct

via an ex post facto lawsuit—not even an ordinance or

regulation—makes its action even more damning and

worthy of more stringent review by this Court. Such a

lawsuit regulates out-of-state conduct by punishing

parties retroactively and without notice for conduct

that was permissible at the time both where it occurred and in Colorado—meaning that the parties

could not have reasonably foreseen this liability or adjusted their conduct. Boulder’s action violates a fundamental limitation on state power protecting an

15

individual’s right to rely on the law governing his conduct at the time and place that he acts. See BMW, 517

U.S. at 572 (“To punish a person because he has done

what the law plainly allows him to do is a due process

violation of the most basic sort.” (quoting Bordenkircher v. Hayes, 434 U.S. 357, 363 (1978))). Allowing

a state to penalize such conduct ex post facto ignores

the coequal sovereignty of the state where the conduct

took place, in violation of due process.

This constitutional problem does not disappear

merely because Boulder seeks compensatory damages

rather than an injunction. “Regulation can be as effectively exerted through an award of damages as

through some form of preventive relief,” while “[t]he

obligation to pay compensation can be, indeed is designed to be, a potent method of governing conduct and

controlling policy.” San Diego Bldg. Trades Council v.

Garmon, 359 U.S. 236, 247 (1959); accord Kurns v.

R.R. Friction Prods. Corp., 565 U.S. 625, 637 (2012)

(quoting that passage of Garmon). Damages and injunctions are functional equivalents in their effect on

regulating and altering out-of-state conduct. A state

cannot avoid territorial strictures merely by labeling

the cause of action “tort” and the remedy “damages.”

Boulder County’s claims squarely implicate the

overreaching prohibition. The conduct on which Boulder seeks damages—the production, refining, sale, and

promotion of fossil fuels over decades—occurred overwhelmingly outside Colorado, in other states and in

foreign nations whose laws permit—and in many cases

affirmatively encourage—that conduct. A Coloradolaw damages judgment would impose substantial economic consequences on lawful out-of-state conduct

with the practical effect of changing that conduct.

16

The structural-federalism objection here is not a

technicality of preemption doctrine but the very constitutional premise on which Pennoyer, BMW, Garmon, and Kurns all rest.

B. Exclusions

Despite the general prohibition against overreach,

as a practical matter, a large state’s in-state bans on

certain activity may lead to de facto nationwide rules,

especially in the commercial realm. But even these

rules are limited: states may not leverage their restrictive regulations to prevent more permissive policymaking elsewhere.

The dormant Commerce Clause prohibits “economic protectionism—that is, regulatory measures designed to benefit in-state economic interests by burdening out-of-state competitors.” Ross, 598 U.S. at 369

(cleaned up). Protectionist state action amounts to

“discrimination against interstate commerce” in this

telling. Northwest Airlines Inc. v. County of Kent, 510

U.S. 355, 373 n.18 (1994). But while states may not

“build up” commerce by burdening industry and “business of other states,” so long as its goals are not protectionism, “a State may exclude from its territory, or

prohibit the sale therein of any articles which, in its

judgment, fairly exercised, are prejudicial to” its own

citizenry. Guy v. Baltimore, 100 U.S. 434, 443 (1880).

In Ross, petitioners argued that the dormant Commerce Clause operates as a near-blanket prohibition of

“state laws that have the practical effect of controlling

commerce outside the State,” even unintentionally.

598 U.S. at 371 (quotation omitted). The Court disagreed, noting that its prior decisions prohibited only

state statutes that “prevented out-of-state firms from

17

undertaking competitive pricing or deprived businesses and consumers in other States of whatever competitive advantages they may possess.” Id. at 374

(cleaned up). Inferring more would “invite endless litigation and inconsistent results” when any state made

a law that influenced commerce outside its borders. Id.

at 375. The Court noted that antidiscrimination under

the dormant Commerce Clause “may well represent

one more effort to mediate competing claims of sovereign authority under our horizontal separation of powers,” but it does not allow the Court to strike down all

extraterritorial exercises of state power. Id. at 376.

The Ross petitioners advanced another argument,

premised on Pike v. Bruce Church, Inc., 397 U.S. 137,

142 (1970), which held that state statutes regulating

to “effectuate a local public interest” with incidental

interstate commerce effects will be upheld “unless the

burden imposed on such commerce is clearly excessive

in relation to the putative local benefits.” The Court

fractured over how to handle this claim. The plurality

held that the petitioners’ claim that California’s pork

regulations flunk Pike failed because courts cannot

weigh a law’s economic and non-economic effects, and

that such policy choices “belong to the people and their

elected representatives.” Ross, 598 U.S. at 382. Congress may thus step in if a state law disrupts an industry given “its power to adopt federal legislation that

may preempt conflicting state laws.” Id. at 382–83.

But Chief Justice Roberts would have vacated and

remanded. Id. at 395. He noted that most of the Court

agreed that “it is possible to balance benefits and burdens,” even of various kinds, under Pike. Id. at 397.

Accordingly, Chief Justice Roberts—joined by three

others—found that the Court’s precedents distinguish

18

“the costs of complying with a given state regulation

from other economic harms to the interstate market.”

Id. Certain regulations may not impose a cost immediately, or that cost may be “difficult to quantify,” but it

is not “noneconomic” cost. Id. at 399. The chief justice

distinguished his approach from a per se prohibition on

extraterritorial state action by finding that regulations imposing “broad impact requiring . . . compliance

even by producers who do not wish to sell in the regulated market” may fail under Pike. Id. at 402.

It cannot be that the Constitution, without exceptions, prohibits a state from regulating activity wholly

within its bounds in a way that affects commerce beyond them. But when one state makes a law that burdens commercial activity that occurs in another state,

such laws are suspect under horizontal federalism.

C. Favoritism

States also “have an incentive to favor local interests,” but if they do so in a way that burdens the Privileges and Immunities or Full Faith and Credit

Clauses, federal courts must step in to referee the conflict. See Erbsen, 93 Minn. L. Rev. at 521.

Where a state has jurisdiction to rule on a case’s

merits, the judgment of its courts is entitled to full

faith and credit. See Underwriters Nat. Assur. Co. v.

N.C. Life & Acc. & Health Ins. Guaranty Ass’n, 455

U.S. 691, 705–06 (1982). Writing for the Court, Justice

Blackmun emphasized that “the concept of full faith

and credit is central to our system of jurisprudence[,]”

as we are “a union of states,” each with its own courts

that sit in judgment over cases and controversies

properly before them. Id. at 703–04. If two states could

exercise jurisdiction over the same activity,

19

“uncertainty, confusion, and delay” would ensue. Id. at

704. The final merits judgments of state courts thus

have effect “in every other court of the United States,

which it had in the State where it was pronounced.”

Id. (quoting Hampton v. McConnell, 3 Wheat. 234, 235

(1818) (Marshall, C.J.)).

Yet, as noted above, these judgments only have authority “if the court in the first State had power to pass

on the merits—had jurisdiction, that is, to render the

judgment.” Durfee v. Duke, 375 U.S. 106, 110 (1963).

Although the united nature of America’s several states

implies full faith and credit, “the structure of our Nation as a union of States, each possessing equal sovereign powers,” limits the faith and credit any state must

provide to another. Underwriters, 455 U.S. at 704.

This Court has also emphasized the importance of

the “equal sovereign powers” of states in other cases.

As evinced by the Northwest Ordinance, see supra at

I.A, Congress established early in the Nation’s history

that new states joined the Union on equal footing with

existing ones. “Equality of constitutional right and

power is the condition of all the states of the Union, old

and new.” Id. at 575.

The Court recognized the federal system’s mediating role again in Franchise Tax Bd. of Cal. v. Hyatt.

There, a citizen of Nevada sued a California state

agency for alleged torts committed in the course of a

tax investigation. 587 U.S. at 234. The Court held that

Nevada lacked jurisdiction, as a state could not “be

sued by a private party without its consent in the

courts of a different State.” Id. at 233. The Constitution both “assumes that the States retain their sovereign immunity except as otherwise provided” and “fundamentally adjusts the States’ relationship with each

20

other and curtails their ability, as sovereigns, to decline to recognize each other’s immunity.” Id. at 237.

The Court has recognized an outer limit to the federal policing responsibility regarding favoritism, however. When addressing the Voting Rights Act’s reliance on out-of-date data for its preclearance coverage

formula, Chief Justice Roberts grounded his reasoning

that the law as applied was unconstitutional in the

“fundamental principle of equal sovereignty” of the

states. Shelby County v. Holder, 570 U.S. 529, 544

(2013) (citation omitted). Because the VRA held some

states to be “more equal than others,” requiring nine

less-equal states to “beseech the Federal Government

for permission to implement laws that they would otherwise have the right to enact and execute on their

own,” its preclearance regime violated the Constitution. Id. Even though the VRA was adopted when

these states were “geographic areas where immediate

action seemed necessary” to correct race-based voting

discrimination, it was meant to expire after five years.

Id. at 546 (quoting South Carolina v. Katzenbach, 383

U.S. 301, 328 (1966)). Instead it was reauthorized and

extended even after voter turnout equalized, removing

any need for its “unprecedented authority” over an

area reserved to the states under the Tenth Amendment. Id. at 546. As umpire over the situation, the

Court held that this VRA provision, as it then stood,

impermissibly favored some states over others.

The Court will not, therefore, sanction federal intervention that violates the equal power of states to

govern their own affairs if there is no constitutional

need to do so—no violation of privileges or immunities,

full faith and credit, or due process. Anything else

would represent an “extraordinary departure” from

21

federalism. Id. at 557 (quoting Presley v. Etowah

County Comm’n, 502 U.S. 491, 500–01 (1992)).

That equal-sovereignty principle is both older and

broader than the modern voting-rights cases. It is the

same principle the Court has applied to questions of

admission and statehood: “the constitutional equality

of the States is essential to the harmonious operation

of the scheme upon which the Republic was organized.” Coyle, 221 U.S. at 580. Each state enters and

remains in the Union “on an equal footing with the

original States in all respects whatsoever.” Id. at 567.

Shelby County recognized the equal-sovereignty

principle in the vertical dimension—as a limit on unjustified federal differential treatment of the states.

570 U.S. at 544. But the same structural premise operates horizontally—as a constraint on what one state

may impose on another. Kansas v. Colorado stated the

horizontal rule directly:

One cardinal rule, underlying all the relations of

the states to each other, is that of equality of

right. Each state stands on the same level with

all the rest. It can impose its own legislation on

no one of the others, and is bound to yield its own

views to none.

206 U.S. 46, 97 (1907). Shelby County and Kansas thus

reflect the same constitutional commitment, and both

illuminate why Colorado may not project its law into

the territory of coequal sovereigns.

D. Externalities

Finally, when states with different regimes of law

have an equal right of action regarding use of a common resource, the proper solution is a federal one. In

the aforementioned Kansas v. Colorado, Kansas filed

22

an original action to enjoin Colorado’s diversion of water from the Arkansas River, arguing that Colorado’s

upstream irrigation was harming Kansans. 206 U.S.

46, 47–48 (1907). Although the Court noted from the

outset that the suit “involves no question of boundary

or of the limits of territorial jurisdiction[,]” id. at 80, it

held that the Supreme Court was still the proper forum for such a question, as the Court “must be held to

embrace all controversies of a justiciable nature arising within the territorial limits of the nation.” Id. at

83. In settling the matter, the Court held that no state

may “legislate for, or impose its own policy upon the

other” in a matter of interstate concern. Id. at 95. Kansas followed the common-law riparian doctrine while

Colorado embraced the doctrine of public ownership.

Neither state could impose its controlling regime on

the other unilaterally. Id. It fell to this Court to settle

the dispute “in such a way as will recognize the equal

rights of both, and at the same time establish justice

between them.” Id. at 98.

That structural premise—that disputes over transboundary environmental harms are governed by federal law—runs through this Court’s interstate-pollution jurisprudence. “When we deal with air and water

in their ambient or interstate aspects, there is a federal common law.” Am. Elec. Power Co., Inc. v. Connecticut, 564 U.S. 410, 421 (2011) (“AEP”) (quoting Illinois v. City of Milwaukee, 406 U.S. 91, 103 (1972)).

When Congress comprehensively legislates in an area

in which federal common law has operated, federal

common law is displaced. City of Milwaukee v. Illinois,

451 U.S. 304, 316–17 (1981). And once federal common

law is displaced, “the availability vel non of a state

lawsuit depends, inter alia, on the preemptive effect

of” the displacing federal statute. AEP, 564 U.S. at

23

429. AEP applied that framework to greenhouse-gas

emissions specifically, holding that the Clean Air Act

displaces federal common-law abatement claims for

carbon-dioxide emissions. Id. at 424. The Ninth Circuit then applied AEP’s displacement rule to damages

claims. Native Vill. of Kivalina v. ExxonMobil Corp.,

696 F.3d 849, 856–58 (9th Cir. 2012).

Ouellette completes the chain. There, the Court

held that the Clean Water Act’s savings clauses did not

authorize affected-state tort suits seeking to apply the

affected state’s own law to out-of-state sources. Int’l

Paper Co. v. Ouellette, 479 U.S. 481, 497 (1987). Otherwise, a regulated source could be subjected to “a variety of common-law rules established by the different

States,” making it “virtually impossible to predict the

standard for a lawful discharge.” Id. at 496–97. The

Clean Air Act’s savings clauses, 42 U.S.C. §§ 7416 and

7604(e), are materially parallel to the Clean Water Act

savings clauses at issue in Ouellette. See City of New

York v. Chevron Corp., 993 F.3d 81, 99–100 (2d Cir.

2021). Like those provisions, they preserve state authority within the Act’s cooperative federal framework

and do not authorize affected states or localities to impose their own tort law on out-of-state sources for interstate and international emissions. See Mayor &

City Council of Baltimore v. B.P. P.L.C., 2026 WL

809501, at *24–25 (Md. Mar. 24, 2026).

The Second Circuit applied that framework to climate-change tort claims in City of New York. There,

the city sought damages for climate-related harms allegedly caused by the defendants’ global production,

promotion, and sale of fossil fuels. The court rejected

the idea that damages claims avoid federal-law limits

merely because they do not expressly seek emissions

24

standards or injunctive relief. Such a lawsuit, the

court explained, “would regulate cross-border emissions in an indirect and roundabout manner, [but] it

would regulate them nonetheless.” 993 F.3d at 93. And

because the Clean Air Act did not affirmatively authorize that form of affected-state regulation, the

state-law claims could not proceed. Id. at 99. Maryland

has now adopted the same logic. Baltimore, 2026 WL

809501, at *18–25.

The doctrinal line from Kansas to Milwaukee I, Milwaukee II, AEP, Ouellette, Kivalina, City of New York,

and Baltimore points in one direction: affected-state

climate-tort claims seeking relief for global emissions

are precluded by federal law.

* * *

In sum, horizontal federalism is the doctrine that

State A generally may not directly regulate activity

that occurs in State B, unless that activity has a substantial connection with State A. Even in such an instance, the Constitution requires State A to give full

faith and credit to State B’s decisions about the activity, and to regulate in a manner that does not prevent

State B from regulating the activity as it sees fit. A

federal solution may be required to settle secondary effects of differing state policies. To “fuse into one Nation” coequal states, Toomer v. Witsell, 334 U.S. 385,

395 (1948), capacity-and-constraint analysis should

guide the result when state priorities clash.

Horizontal federalism thus acts as a structural “instrument of interstate federalism” that cabins a state’s

ability to reach across borders. World-Wide Volkswagen, 444 U.S. at 294. As the Court has repeatedly

held, a state lacks the constitutional power to punish

a defendant for conduct that was “lawful where it

25

occurred.” State Farm Mutual Auto. Ins. Co. v. Campbell, 538 U.S. 408, 421 (2003) (citing BMW, 517 U.S. at

572, and Bigelow v. Virginia, 421 U.S. 809, 824 (1975)).

Boulder’s claims seek to impose liability on out-ofstate actors for activities that were (and are) expressly

permitted by their home sovereigns and the federal

government—and in Colorado when the conduct at issue occurred. These claims violate the basic principle

that no state may “impose its own policy choice on

neighboring States” as well as the due-process reliance

interests of the citizens of those States and should be

dismissed with prejudice. BMW, 517 U.S. at 571.

III. THE COLORADO SUPREME COURT IGNORED HORIZONTAL FEDERALISM

The court below ignored horizontal federalism principles. Boulder County, a political subdivision of Colorado, see Colo. Const. art. XIV §1, sued out-of-state oil

companies—affiliates of Suncor Energy, headquartered in Canada, and ExxonMobil Corporation, headquartered in Texas—for alleged violations of state tort

law based on the companies’ production and promotion

of fossil fuels throughout the world. Boulder County

alleged that these companies’ activities led to the emission of greenhouse gases that contributed to climate

change that caused harms within Colorado—increased

wildfires, floods, heat, and the like—based on attribution modeling. 2

2 Ironically, when Rep. Harriet Hageman (R-Wyo.) challenged

Boulder to abandon its use of fossil-fuel energy sources given its

professed fear of their alleged effects, city leaders cursorily rejected the suggestion because of the city’s admitted reliance on

them. See Angus Thuermer, “Hageman Proposes a Boulder, Colorado, Fossil-Fuel-Free Experiment,” Cap City News, Aug. 8,

2024, https://tinyurl.com/nd95cu6b.

26

The court below found that these state-law claims

could proceed and were not preempted by the Clean

Air Act because of a savings clause in the federal statute. It stated that federal common-law claims for pollution abatement only applied to “suits brought by one

State to abate pollution emanating from another

state,” Cnty. Comm’rs of Boulder County v. Suncor Energy USA, Inc., 586 P.3d 161, 171 (Colo. 2025) (paraphrasing AEP, 564 U.S. at 419) (emphasis in original),

and that Boulder’s claims were different because they

sought damages from upstream producers rather than

abatement from emitters. That analysis treats the displacement of federal common law as though it revives

affected-state law.

The Court addressed a similar fact pattern in the

Clean Water Act context in Ouellette, a case replete

with horizontal federalism principles. Recognizing an

externality problem, the Court held that “it is not necessary for a federal statute to provide explicitly that

particular state laws are pre-empted.” 479 U.S. at 491.

Put another way, a savings clause does not mean that

federal law fails to preempt state regulation. See id. at

493. States may not impose their own regulations

against out-of-state water pollution sources. See id. at

495. Subjecting a company to potentially 50 different

state nuisance standards for a single course of conduct

would make it “virtually impossible to predict the

standard for a lawful discharge into an interstate body

of water.” Id. at 497 (quoting People of State of Ill. v.

City of Milwaukee, 731 F.2d 403, 414 (7th Cir. 1984)).

The majority below distinguished Ouellette on the

ground that Boulder County was not trying to regulate

emissions but seeking compensation for local harms.

In its telling, the Court in Ouellette was just

27

performing “the very type of preemption analysis that

we have conducted above” to determine whether a suit

under state law could proceed. Boulder County, 586

P.3d at 172. But there is no meaningful difference, in

this instance, between direct regulation via state law

and indirect regulation via state tort. Requiring damages is “a potent method of governing conduct and controlling policy.” Kurns v. R.R. Friction Prods. Corp.,

565 U.S. 625, 637 (2012) (quotation omitted). In fact,

in Ouellette this Court recognized that it ought not

“draw a line” between different types of relief when

evaluating preemption, because then a state might try

to control out-of-state activity via another form of punishment. 479 U.S. at 498 n.19. Again, per this Court’s

horizontal federalism jurisprudence, externalities suggest a potential opportunity for federal preemption.

Respondents’ attempt to reframe this litigation as

a “deception” or “failure-to-warn” case is a transparent

effort to plead around horizontal federalism. This

Court’s decision in Bigelow v. Virginia, 421 U.S. 809

(1975), makes clear that a state may not, “under the

guise of exercising internal police powers, bar a citizen

of another State from disseminating information about

an activity that is legal in that State.” Id. at 824–25.

Because the production, sale, and promotion of fossil

fuels are lawful activities in the jurisdictions where

Petitioners operate, Colorado cannot use its consumerprotection or tort laws to effectively “veto” out-of-state

speech concerning those activities. If a state cannot directly regulate out-of-state emissions or the interstate

commerce of energy, it cannot do so indirectly by penalizing the promotion of that commerce in other

states. Allowing such a theory would enable any locality to bypass the displacement of federal common law

28

simply by alleging that out-of-state producers misled

consumers about their products.

A recent Maryland Supreme Court decision confirms the point. Addressing substantially similar

claims by local Maryland governments—public and

private nuisance, trespass, and failure-to-warn theories pleaded as deception-driven harm to local property—the court affirmed dismissal of the claims. The

majority agreed that the claims are displaced by federal common law, that any federal common law is in

turn displaced by the Clean Air Act under AEP, and

that the Clean Air Act’s savings clauses do not authorize affected-locality suits against out-of-state sources

for global emissions under the Ouellette framework.

Baltimore, 2026 WL 809501, at *18–25. The Maryland

court expressly disagreed with its Colorado counterpart; it aligned instead with the dissent below and the

Second Circuit’s decision in City of New York. Id. at

*20 n.18; see also id. at *1 (determining that “the local

governments, through their various state law claims,

are seeking to regulate air emissions beyond their jurisdictional boundaries”). It also rejected the precise

argument on which the Colorado court relied: “No

amount of creative pleading can masquerade the fact

that the local governments are attempting to utilize

state law to regulate global conduct that is purportedly

causing global harm.” Id. at *20.

But the problem with the judgment below is

broader than statutory preemption alone. 3 To allow

3 As mentioned above, the EPA recently rescinded the 2009 endangerment finding for greenhouse gases under section 202(a)(1)

of the Clean Air Act, 42 U.S.C. § 7521(a)(1), and repealed the implementing motor-vehicle greenhouse-gas standards. Rescission

of the Greenhouse Gas Endangerment Finding and Motor Vehicle

29

the long-arm application of state torts in a manner

that effectively prohibits not only legal activity occurring in other American states, but legal activity

throughout the world, violates horizontal federalism’s

chief tenet regarding overreaching: One state may not,

via its own law, penalize conduct that is legal in another state and occurs within that state’s boundaries.

If Colorado’s tort law were used to judge conduct

that other states permit, regulate, or encourage, Colorado could override the policy judgments of those jurisdictions. That is the horizontal-federalism problem

amici have identified throughout this brief: the export

of one state’s standards in a manner that governs conduct beyond its borders. That is the kind of interstate

tyranny horizontal federalism exists to prevent. The

court below failed to account for that structural injury,

and its failure to do so is a form of willful blindness to

the constitutional consequences of its rule.

Every state may redress action that occurs within

or has sufficient contacts with its territory. See, e.g.,

Underwriters, 455 U.S. at 705–06. But a state may not

apply its own law to settle an injury caused by action

in another state. See Kansas, 206 U.S. at 95. Such controversies fall to the federal courts—and federal law—

to settle. See Ouellette, 479 U.S. at 495. And a state (or

Greenhouse Gas Emission Standards Under the Clean Air Act, 91

Fed. Reg. 7686 (Feb. 18, 2026). Whatever the ultimate fate of that

rulemaking—petitions challenging it have been filed in the D.C.

Circuit—the analysis above does not depend on it. The horizontalfederalism objection to one state’s applying its tort law to interstate and international emissions does not turn on the precise

scope of federal regulation at any given moment, but on the constitutional principle that regulation of activity occurring across

multiple states and abroad must proceed through the federal

framework Congress enacted, not through state tort verdicts.

30

locality) certainly may not apply its law in a manner

that conflicts with another state’s sovereignty. Ross,

598 U.S. at 388.

If this case is allowed to proceed, Boulder County

residents will be able to affect national policymaking,

but nobody outside the county will have any role in

electing the officials pushing here to drive national energy policy through tort suits at the local courthouse.

In cases like these, horizontal federalism’s delicate

balance can only be preserved by a federal umpire.

CONCLUSION

The judgment below should be reversed because

horizontal federalism bars the application of one

state’s tort law to restrict activity in another.

Respectfully submitted,

Marcella Burke

Paul B. Simon

Connor Mighell

BURKE LAW GROUP PLLC

1000 Main Street,

Suite 2300

Houston, TX 77002

(832) 987-2214

marcella@burkegroup.law

May 19, 2026

Ilya Shapiro

Counsel of Record

MANHATTAN INSTITUTE

52 Vanderbilt Ave.

New York, NY 10017

(212) 599-7000

ishapiro@manhattan.

institute

David B. Kopel

INDEPENDENCE

INSTITUTE

727 East 16th Ave.

Denver, CO 80203

(303) 279-6536

david@i2i.org