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

No. 25-170

In the

Supreme Court of the United States

SUNCOR ENERGY (U.S.A.) INC., et al.,

Petitioners,

v.

COUNTY COMMISSIONERS OF

BOULDER COUNTY, et al.,

Respondents.

On Writ of Certiorari to the

Supreme Court of Colorado

BRIEF OF PREEMPTION SCHOLARS

AS AMICI CURIAE

IN SUPPORT OF RESPONDENTS

Michael D. Hausfeld

Hausfeld LLP

1200 17th Street, N.W.,

Suite 600

Washington, DC 20036

mhausfeld@hausfeldllp.com

Ernest A. Young

Counsel of Record

3208 Fox Terrace Drive

Apex, NC 27502

(919) 360-7718

young@law.duke.edu

A dam J. Levitt

Daniel Rock Flynn

Dicello Levitt LLP

10 North Dearborn Street,

6th Floor

Chicago, IL 60602

alevitt@dicellolevitt.com

dflynn@dicellolevitt.com

Counsel for Amici Curiae

394572

(800) 274-3321 • (800) 359-6859

i

QUESTIONS PRESENTED

1.

Whether the Constitution or the Clean Air Act,

42 U.S.C. § 7401 et seq., categorically preempts

state law nuisance claims for damages litigated

against private parties regarding the marketing

and production of greenhouse gases; and

2.

Whether Congress intended the Clean Air Act’s

scheme for regulating air pollution emissions to

preempt plaintiffs’ state common law claims for

deceptive marketing and upstream production of

fossil fuels.

ii

TABLE OF CONTENTS

Page

QUESTIONS PRESENTED . . . . . . . . . . . . . . . . . . . . . . . i

TABLE OF CONTENTS . . . . . . . . . . . . . . . . . . . . . . . . . ii

TABLE OF CITED AUTHORITIES . . . . . . . . . . . . . . . iv

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

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

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

I.

No broad principle of structural

preemption governs this case . . . . . . . . . . . . . . . . 3

II. Defendants’ examples of str uctu ra l

preempt ion a re either m ist a ken or

inapplicable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

A. This Court’s federal common law cases

on interstate nuisance claims are far

more limited than Defendants recognize,

and in any event have been superseded

by the Clean Air Act . . . . . . . . . . . . . . . . . . 10

B. This Court has explicitly refused

to concoct a broad constitutional

principle barring state regulation with

extraterritorial effects . . . . . . . . . . . . . . . . 17

iii

Table of Contents

Page

C. Foreign affairs preemption is, at best,

a narrow doctrine dependent on

state interferences with particular

national foreign policies embodied in

positive law . . . . . . . . . . . . . . . . . . . . . . . . . . 19

III. The Clean A ir Act does not preempt

Plaintiffs’ claims . . . . . . . . . . . . . . . . . . . . . . . . . . 23

A. The CAA does not regulate the activities that are the focus of Plaintiffs’

lawsuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

B. The CAA does not preempt any field . . . . . 26

CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

iv

TABLE OF CITED AUTHORITIES

Page

CASES

Allstate Ins. Co. v. Hague,

449 U.S. 302 (1981) . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Am. Elec. Power Co. v. Connecticut,

564 U.S. 410 (2011) . . . . . . . . . . . . . . . 17, 23-25, 28, 29

Am. Ins. Assn. v. Garamendi,

539 U.S. 396 (2003) . . . . . . . . . . . . . . . . . . . . . . . . 20, 21

Arizona v. United States,

567 U.S. 387 (2012) . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Arkansas v. Oklahoma,

503 U.S. 91 (1992) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Atherton v. FDIC,

519 U.S. 213 (1997) . . . . . . . . . . . . . . . . . . . . . . . . . 7, 17

Bell v. Cheswick Generating Station,

734 F.3d 188 (3d Cir. 2013) . . . . . . . . . . . . . . . . . . . . . 28

City of Milwaukee v. Illinois & Michigan,

451 U.S. 304 (1981) . . . . . . . . . . . . . . . . . . . . . 13, 15, 16

Cooley v. Bd. of Wardens of Port of Philadelphia,

53 U.S. (12 How.) 299 (1851) . . . . . . . . . . . . . . . . . . . . . 4

Crosby v. Nat’l Foreign Trade Council,

530 U.S. 363 (2000) . . . . . . . . . . . . . . . . . . . . . . . . 20, 23

v

Cited Authorities

Page

F. Hoffmann-La Roche Ltd. v. Empagran S.A.,

542 U.S. 155 (2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Fla. Lime & Avocado Growers, Inc. v. Paul,

373 U.S. 132 (1963) . . . . . . . . . . . . . . . . . . . . . . . . 26-27

Franchise Tax Bd. v. Hyatt,

587 U.S. 230 (2019) . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Georgia v. Tennessee Copper Co.,

206 U.S. 230 (1907) . . . . . . . . . . . . . . . . . . . . . . . . 10, 11

Gibbons v. Ogden,

22 U.S. (9 Wheat.) 1 (1824) . . . . . . . . . . . . . . . . . . . 4, 14

Grupo Mexicano de Desarrollo S.A. v.

All. Bond Fund, Inc.,

527 U.S. 308 (1999) . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Hencely v. Fluor Corp.,

146 S. Ct. 1086 (2026) . . . . . . . . . . . . . . . . . . . . . . . . 2, 8

Hinderlider v. La Plata River & Cherry Creek

Ditch Co.,

304 U.S. 92 (1938) . . . . . . . . . . . . . . . . . . . . . . . . . 11, 12

Houston, E. & W.T.R. Co. v. United States,

234 U.S. 342 (1914) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Hughes v. Talen Energy Mktg., LLC,

578 U.S. 150 (2016) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

vi

Cited Authorities

Page

Huron Portland Cement Co. v. City of Detroit,

362 U.S. 440 (1960) . . . . . . . . . . . . . . . . . . . . 3-4, 14, 27

Illinois v. City of Milwaukee,

406 U.S. 91 (1972) . . . . . . . . . . . . . . . . . . . . . . . . . 11, 15

Int’l Paper Co. v. Ouellette,

479 U.S. 481 (1987) . . . . . . . . . . . . 16-18, 24, 25, 28, 29

Kansas v. Garcia,

589 U.S. 191 (2020) . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Leisy v. Hardin,

135 U.S. 100 (1890) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Medellin v. Texas,

552 U.S. 491 (2008) . . . . . . . . . . . . . . . . . . . . . . . . 22, 23

Merrick v. Diageo Americas Supply, Inc.,

805 F.3d 685 (6th Cir. 2015) . . . . . . . . . . . . . . . . . . . . 28

Mintz v. Baldwin,

289 U.S. 346 (1933) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Missouri v. Illinois,

200 U.S. 496 (1906) . . . . . . . . . . . . . . . . . . 10, 11, 15, 16

Nat’l Foreign Trade Council v. Natsios,

181 F.3d 38 (1st Cir. 1999) . . . . . . . . . . . . . . . . . . . . . 23

vii

Cited Authorities

Page

Nat’l Pork Producers Council v. Ross,

598 U.S. 356 (2023) . . . . . . . . . . . . . . . . . . . . . . 6, 18, 19

NLRB v. Jones & Laughlin Steel Corp.,

301 U.S. 1 (1937) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

O’Melveny & Myers v. FDIC,

512 U.S. 79 (1994) . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 17

Ohio v. EPA,

603 U.S. 279 (2024) . . . . . . . . . . . . . . . . . . . . . . 8, 14, 28

ONEOK, Inc. v. Learjet, Inc.,

575 U.S. 373 (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Princess Lida of Thurn & Taxis v. Thompson,

305 U.S. 456 (1939) . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Puerto Rico Dept. of Consumer Affs. v.

Isla Petroleum Corp.,

485 U.S. 495 (1988) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Rice v. Santa Fe Elevator Corp.,

331 U.S. 218 (1947) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Robbins v. Shelby Cty. Taxing Dist.,

120 U.S. 489 (1887) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

S.C. Hwy. Dept. v. Barnwell Bros., Inc.,

303 U.S. 177 (1938) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

viii

Cited Authorities

Page

Thurlow v. Com. of Mass.,

46 U.S. (5 How.) 504 (1847) . . . . . . . . . . . . . . . . . . . . . . 5

Train v. Nat. Res. Def. Council, Inc.,

421 U.S. 60 (1975) . . . . . . . . . . . . . . . . . . . . . . . . . 14, 28

United Haulers Ass’n v. Oneida-Herkimer

Solid Waste Mgt. Auth.,

550 U.S. 330 (2007) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

United States v. Kimbell Foods, Inc.,

440 U.S. 715 (1979) . . . . . . . . . . . . . . . . . . . . . . . . . 7, 17

Virginia Uranium, Inc. v. Warren,

587 U.S. 761 (2019) . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Virginia v. EPA,

108 F.3d 1397 (D.C. Cir. 1997) . . . . . . . . . . . . . . . . . . 14

Wallis v. Pan Am. Petroleum Corp.,

384 U.S. 63 (1966) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Wheeldin v. Wheeler,

373 U.S. 647 (1963) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Wickard v. Filburn,

317 U.S. 111 (1942) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Willson v. Black Bird Creek Marsh Co.,

27 U.S. (2 Pet.) 245 (1829) . . . . . . . . . . . . . . . . . . . . . . . 5

ix

Cited Authorities

Page

Zschernig v. Miller,

389 U.S. 429 (1968) . . . . . . . . . . . . . . . . . . . . . 20, 21, 23

STATUTES

42 U.S.C. § 7401(a)(3) . . . . . . . . . . . . . . . . . . . . . 2, 14-16, 27

42 U.S.C. § 7416 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

42 U.S.C. § 7604(e) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

OTHER AUTHORITIES

William Baude, Jack Goldsmith, John Manning,

James Pfander, & Amanda Tyler, Hart and

Wechsler’s The Federal Cour ts and the

Federal System (8th ed. 2025) . . . . . . . . . . . . . . . . . . 11

Anthony J. Bellia, Jr. & Bradford R. Clark,

General Law in Federal Cour t, 54 Wm.

& Mary L. Rev. 655 (2013) . . . . . . . . . . . . . . . . . . . . . 12

Sarah H. Cleveland, Crosby and the “OneVoice” Myth in U.S. Foreign Relations,

46 Vill. L. Rev. 975 (2001) . . . . . . . . . . . . . . . . . . . . . . 22

Brannon P. Denning, National Pork Producers

Council v. Ross: Extraterritoriality is Dead,

Long Live the Dormant Commerce Clause,

2023 Cato Sup. Ct. Rev. 23 . . . . . . . . . . . . . . . . . . . . . 18

x

Cited Authorities

Page

John P. Dwyer, The Practice of Federalism Under

the Clean Air Act, 54 Md. L. Rev. 1183 (1995) . . 14, 28

Henry J. Friendly, Praise of Erie—And of the

New Federal Common Law, 39 N.Y.U. L.

Rev. 383 (1964) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

St ephen Ga rdbau m, New Deal Co n stitutionalism and the Unshackling of the States,

64 U. Chi. L. Rev. 483 (1997) . . . . . . . . . . . . . . . . . . . . 6

Stephen A. Gardbaum, The Nature of Preemption,

79 Cornell L. Rev. 767 (1994) . . . . . . . . . . . . . . . . . . . . 8

Jack L. Goldsmith, Federal Courts, Foreign

Affairs, and Federalism, 83 Va. L. Rev. 1617

(Nov. 1997) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 21

Jack Goldsmith & Alan Sykes, The Internet

a n d th e Do r m a n t Co m m e r c e C l a u se,

110 Yale L. J. 785 (2001) . . . . . . . . . . . . . . . . . . . . . . . 19

Louis Henkin, Foreign Affairs and the United

States Constitution (2d ed. 1996) . . . . . . . . . . . . . . . 21

Julian Ku, Guber nator ial Foreign Policy,

115 Yale L. J. 2380 (2006) . . . . . . . . . . . . . . . . . . . . . . 22

xi

Cited Authorities

Page

Douglas Laycock, Equal Citizens of Equal

and Territorial States: The Constitutional

Foundations of Choice of Law, 92 Colum. L.

Rev. 249 (1992) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Daniel A. Lyons, Protecting States in the New

World of Energy Federalism, 67 Emory

L. J. 921 (2018) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Thomas W. Merrill, The Disposing Power of the

Legislature, 110 Colum. L. Rev. 452 (2010) . . . . . . . . 8

Caleb Nelson, Preemption, 86 Va. L. Rev. 225

(2000) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 27

Caleb Nelson, The Persistence of General Law,

106 Colum. L. Rev. 503 (2006) . . . . . . . . . . . . . . . 10, 12

Ernest A. Young, Foreign Affairs Federalism

in the United States, The Oxford Handbook

of Comparative Foreig n Relations Law

(Curtis Bradley ed., 2019) . . . . . . . . . . . . . . . . . . . . . 22

Ernest A. Young, Preemption and Federal

Common Law, 83 Notre Dame L. Rev. 1639

(2008) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Ernest A. Young, ‘The Ordinary Diet of the Law’:

The Presumption Against Preemption in the

Roberts Court, 2011 Sup. Ct. Rev. 253 (2012) . . . . . . 6

1

INTEREST OF AMICI

Amici are law professors interested in federal

preemption of state law.1 Our interest arises from teaching

and writing in a variety of related fields, including

administrative law, constitutional law, environmental

law, health law, and torts. Daniel Farber is the Sho Sato

Professor at the University of California, Berkeley School

of Law. Thomas McGarity holds the William Powers, Jr.

and Kim Heilbrun Chair in Administrative Law at the

University of Texas School of Law. Nina Mendelson is

the Joseph Sax Collegiate Professor at the University of

Michigan Law School. David Rubenstein holds the William

Scott Research Chair at the University of Kansas School

of Law. Wendy Wagner holds the Richard Dale Chair at

the University of Texas School of Law. Ernest Young is

the Alston & Bird Distinguished Professor at Duke Law

School.

Each of us believes that legal scholarship should be

concerned with and made useful to the decision of issues

presently before the courts. We join this brief solely in

our individual capacities as lawyers and scholars, and

the views expressed here should not be attributed to our

institutions.

1. Pursuant to Sup. Ct. R. 37.6, counsel for amici represent

that they authored this brief in its entirety and that none of the

parties or their counsel, nor any other person or entity other than

amici or their counsel, made a monetary contribution intended to

fund the preparation or submission of this brief.

2

SUMMARY OF ARGUMENT

This Court recently emphasized that “[t]here is no

federal pre-emption in vacuo, without a constitutional

text or a federal statute to assert it.’” Hencely v. Fluor

Corp., 146 S. Ct. 1086, 1093 (2026) (quoting Puerto Rico

Dept. of Consumer Affs. v. Isla Petroleum Corp., 485

U.S. 495, 503 (1988)). That principle decides this case.

Defendants have asserted an unusually aggressive array

of preemption arguments, but each boils down to broad

structural preemption—that is, that Plaintiffs’ claims are

categorically preempted either by the general structure

of constitutional federalism, or by Congress’s wholly

implicit intent in the Clean Air Act (CAA) to preempt the

entire field of interstate air conditions. The constitutional

argument lacks any textual foundation, and the theory

upon which it rests has been rejected explicitly since

the New Deal and in practice since the Marshall Court.

Likewise, Defendants’ implied field preemption argument

is just another form of “pre-emption in vacuo,” and it

founders on Congress’s express provision in the CAA that

“air pollution prevention . . . and air pollution control at

its source is the primary responsibility of States and local

governments.” 42 U.S.C. § 7401(a)(3).

The plaintiffs in this case face multiple significant

challenges on the merits. They must link their theories—

which focus not on emissions but rather on deceptive

marketing and upstream production of fossil fuels—to

climate impacts suffered in Boulder. They carry a weighty

burden to prove causation, and Defendants have other

federal defenses that remain for consideration on remand.

But they should be allowed to prove their case; federal

preemption is not a proxy to screen out suits that may

3

not ultimately prevail on the merits. Preemption under

the Supremacy Clause is the critical line demarcating

the boundary between national and state authority.

Defendants’ approach here, if accepted by this Court,

would utterly transform that boundary with broad and

unpredictable consequences. It would not only reshape

all manner of tort litigation, but it would narrow state

regulatory authority across the board and likely disrupt

cooperative federalism regimes under a wide array of

federal statutes.

ARGUMENT

I.

No broad principle of structural preemption

governs this case.

Defendants’ view of structural preemption is

breathtakingly broad. They argue that the Constitution

preempts state regulation of “ambient” phenomena—by

which they seem to mean phenomena like air pollution

that is often interconnected across state boundaries.

Petr. Br. at 27. 2 That would apply not only to greenhouse

2. Defendants use “ambient”—which typically means

“existing or present on all sides,” see Ambient, Merriam-Webster

Dictionary, https://www.merriam-webster.com/dictionary/ambient

(last visited July 21, 2026)—in an idiosyncratic way to suggest

inherently interstate phenomena. See Petr. Br. at 27 (“Air and

water are ‘ambient’ . . . in the sense that . . . pollution from a single

source can readily flow from one state to another.”). The word

applies more broadly, however—for instance, to the “ambient noise

level in the law library” or the “ambient temperature in Fresno.”

“Ambient” gases in the atmosphere, while not respecting state

lines any more than sound or temperature do, are subject to state

regulation. See Huron Portland Cement Co. v. City of Detroit, 362

4

gases and air or water pollution, but also to crime, market

conditions, communicable diseases, or culture. To say that

only federal law may regulate these subjects would be to

revert to a model of constitutional jurisdiction that this

Court abandoned long ago.

In the nineteenth and early twentieth centuries, this

Court interpreted the Commerce Clause as an exclusive

grant of power to the national government, thus barring

the states from regulating anything that would fall within

that grant. The Constitution thus barred Congress from

regulating intrastate commerce that did not involve “more

states than one,” Gibbons v. Ogden, 22 U.S. (9 Wheat.) 1,

194 (1824), while symmetrically barring the states from

regulating interstate commerce, Leisy v. Hardin, 135

U.S. 100 (1890). At least in theory, this regime foreclosed

all state regulation of cross-border problems. 3 It was

precisely the regime that Defendants describe in their

opening brief.

Even during the “dual federalism” period, however, the

jurisdictional lines were more permeable than Defendants

acknowledge.4 This Court recognized exceptions on both

U.S. 440 (1960). Nothing in the word signifies exclusive federal

regulatory jurisdiction.

3. See, e.g., Robbins v. Shelby Cty. Taxing Dist., 120 U.S. 489,

494 (1887) (“[I]n the matter of interstate commerce the United

States are but one country, and must be subject to one system of

regulations, and not to a multitude of systems.”).

4. See, e.g., Cooley v. Bd. of Wardens of Port of Philadelphia,

53 U.S. (12 How.) 299, 319 (1851) (stating that the national commerce

power created a regime of “selective exclusivity”). By the 1840s,

Chief Justice Taney stated that “the mere grant of power to the

5

sides of the line: states regulated interstate activity for

purposes within the states’ police power, Willson v. Black

Bird Creek Marsh Co., 27 U.S. (2 Pet.) 245, 251-52 (1829),

and national authorities could reach intrastate activity

where necessary to facilitate regulation of interstate

activity, Houston, E. & W.T.R. Co. v. United States, 234

U.S. 342, 351 (1914). Claims seeking damages to deter

fraudulent advertising or defray the cost of mitigating

environmental damage would surely have fallen within the

“police power” exception, even in the heyday of strict limits

on state regulatory authority. Defendants’ categorical

theory of constitutional preemption has never existed in

real life.

It certainly does not exist now. In 1937, this Court

changed course and rejected the dual federalist model.

Landmark decisions like NLRB v. Jones & Laughlin

Steel Corp., 301 U.S. 1 (1937), and Wickard v. Filburn,

317 U.S. 111 (1942), recognized the national economy’s

basic interconnectedness, upholding national power to

regulate even small activities, like producing minute

quantities of wheat exceeding a regulatory quota. At the

same time, this Court significantly narrowed its dormant

Commerce Clause doctrine, generally permitting state

regulation so long as states did not discriminate against

interstate commerce. 5 The antidiscrimination doctrine

general government cannot . . . be construed to be an absolute

prohibition to the exercise of any power over the same subject by

the States. . . . [State] regulations [of commerce] are valid unless

they come in conflict with a law of Congress.” Thurlow v. Com.

of Mass., 46 U.S. (5 How.) 504, 579 (1847) (opinion of Taney, C.J.).

5. See, e.g., S.C. Hwy. Dept. v. Barnwell Bros., Inc., 303

U.S. 177, 189 (1938) (observing that “so long as the state action

6

does not purport to exclude state regulation altogether

from any substantive area. And this Court has “expressly

cautioned against judges using the dormant Commerce

Clause as ‘a roving license for federal courts to decide what

activities are appropriate for state and local governments

to undertake.’” Nat’l Pork Producers Council v. Ross, 598

U.S. 356, 380 (2023) (plurality opinion) (quoting United

Haulers Ass’n v. Oneida-Herkimer Solid Waste Mgt.

Auth., 550 U.S. 330, 343 (2007)). Defendants relabel that

“roving license” as structural preemption, but it amounts

to the same thing.

Since the New Deal, state and national regulatory

jurisdiction have been largely concurrent; the boundary

between them is set by the Supremacy Clause, with

state regulation foreclosed only when it conflicts with the

terms of particular federal statutory regimes.6 Hence

this Court’s cautious approach to federal common law

and preemption. As the post-New Deal Court retreated

from broad notions of constitutional exclusion under the

dormant Commerce Clause, state law could apply in a

does not discriminate, the burden [on interstate commerce] is one

which the Constitution permits”); see Stephen Gardbaum, New

Deal Constitutionalism and the Unshackling of the States, 64 U.

Chi. L. Rev. 483, 489 (1997) (“[T]he dormant Commerce Clause

. . . had been wielded extremely aggressively against the states

during the Lochner era but was drastically scaled back by the

Court starting in 1938.”).

6. See Ernest A. Young, ‘The Ordinary Diet of the Law’:

The Presumption Against Preemption in the Roberts Court, 2011

Sup. Ct. Rev. 253, 257-59, 268-69 (2012); see also Caleb Nelson,

Preemption, 86 Va. L. Rev. 225, 231 (2000) (“Under the Supremacy

Clause, preemption occurs if and only if state law contradicts a

valid rule established by federal law . . . .”).

7

broader range of situations, thereby reducing the need for

federal common law.7 State law can apply even in areas like

federal contracting—a heartland of Judge Friendly’s “new

federal common law.” 8 Under current law, judge-made

federal rules of decision are unavailable unless state law

conflicts with overriding federal interests.9 As Professor

Thomas Merrill has observed, this Court “increasingly

avoids invoking federal common law . . . preferring to

cast its rulings in terms of interpretation of authoritative

federal texts or preemption. Although it would be rash to

declare that federal common law is dead, a heavy aura of

7. See, e.g., Wheeldin v. Wheeler, 373 U.S. 647, 651-52 (1963)

(noting that post-Erie, “[t]he instances where we have created

federal common law are few and restricted,” and refusing to create

a federal common law right of action for abuse of process because

“federal officials are usually governed by local law” in that area).

8. See Henry J. Friendly, In Praise of Erie—And of the

New Federal Common Law, 39 N.Y.U. L. Rev. 383, 408-11 (1964).

Defendants suggest Judge Friendly thought federal common

lawmaking authority necessarily excluded state law, Petr. Br.

at 26, but both Friendly and this Court rejected that view. See

Friendly, supra, at 410; United States v. Kimbell Foods, Inc.,

440 U.S. 715, 728-30 (1979) (stating that even in federal contracts

cases, courts must “determine whether application of state law

would frustrate specific objectives of the federal programs,” and

rejecting “generalized pleas for uniformity”).

9. See Atherton v. FDIC, 519 U.S. 213, 218-19 (1997);

O’Melveny & Myers v. FDIC, 512 U.S. 79, 87 (1994) (stating that

the power to make federal common law depends upon a “‘significant

conflict between some federal policy or interest and the use of state

law’”) (quoting Wallis v. Pan Am. Petroleum Corp., 384 U.S. 63, 68

(1966)); see generally Ernest A. Young, Preemption and Federal

Common Law, 83 Notre Dame L. Rev. 1639 (2008).

8

illegitimacy hangs over any claim for newly minted federal

common law rules.”10

10

Likewise, this Court’s recognition of broad national

legislative authority prompted a shift in preemption

doctrine, requiring clear evidence that Congress intended

to displace state law.11 And Congress’s post-New Deal

enactments generally reflect a legislative commitment

to concurrent state and federal authority. Federal

environmental law accords states a large discretionary

role in the federal scheme,12 often explicitly saves state

law and remedies from preemption, and leaves untouched

many related issues like deceptive marketing of fossil

fuels. Even where Congress has sought to allocate

specific functions exclusively to federal regulators, as

10. Thomas W. Merrill, The Disposing Power of the

Legislature, 110 Colum. L. Rev. 452, 463 (2010).

11. See, e.g., Rice v. Santa Fe Elevator Corp., 331 U.S. 218

(1947); Mintz v. Baldwin, 289 U.S. 346 (1933); see generally

Stephen A. Gardbaum, The Nature of Preemption, 79 Cornell

L. Rev. 767, 806 (1994) (observing that the Court “replaced a

strict division of powers version of federalism with a new version

embodying the presumption that state powers . . . survive unless

clearly ended by Congress. Preemption doctrine was thus modified

to reflect this new presumption, and thereafter, Congress was

affirmatively required to manifest an intent to preempt”).

Although judges occasionally debate the scope or strength of the

Rice presumption, all contemporary preemption cases focus on

Congress’s particularized intent to preempt state law manifested

in the terms of particular statutes—not broad categories of

structural preemption. See Hencely, 146 S. Ct. at 1093.

12. See, e.g., Ohio v. EPA, 603 U.S. 279, 283 (2024) (“The

Clean Air Act envisions States and the federal government

working together to improve air quality.”).

9

in energy law, it has done so in a far more fine-grained

way than Defendants’ categorical rule of structural

preemption would permit, and this Court has rejected

broad preemption arguments in situations implicating

both state and federal responsibilities.13

Whether Plaintiffs’ state law claims are foreclosed

must be a function of any preemptive effect of the statutes

that Congress has enacted—in particular, the Clean Air

Act—not of broad and categorical structural principles

concocted by Defendants and defined by judges.

II. Defendants’ examples of structural preemption are

either mistaken or inapplicable.

Defendants attempt to support their broad and

categorical rule that the Constitution preempts all state

efforts to regulate greenhouse gases by cobbling together

three more specific theories. None is plausible.

13. See ONEOK, Inc. v. Learjet, Inc., 575 U.S. 373, 376 (2015)

(observing that the Natural Gas Act assigns wholesale gas sales to

federal regulation while reserving retail sales to the states, and

holding state antitrust claims against pipelines’ behavior affecting

both were not preempted); Hughes v. Talen Energy Mktg., LLC,

578 U.S. 150, 168 (2016) (Sotomayor, J., concurring) (observing

that, even in finding preemption, the Court had focused on the

specific provisions of the Federal Power Act “rather than resting

on generic pre-emption frameworks unrelated to” that statute).

See Daniel A. Lyons, Protecting States in the New World of Energy

Federalism, 67 Emory L. J. 921 (2018) (describing how Congress

originally sought to divide the energy field between state and

federal regulators by statute but over time the statutory scheme

recognized important areas of concurrent jurisdiction).

10

A.

This Court’s federal common law cases on

interstate nuisance claims are far more limited

than Defendants recognize, and in any event

have been superseded by the Clean Air Act.

Defendants rely most prominently upon a line of

cases in which American states have sued other states, or

sometimes other actors, to abate an interstate nuisance.

These cases are complex, generally focused on issues other

than preemption, and engage substantive matters far

afield from the consumer protection concerns of Plaintiffs’

suits. The decisions tend to offer little explanation and

must be approached with care. They certainly do not

support the categorical rule of structural preemption that

Defendants would derive from them.

Defendants begin with Missouri v. Illinois, 200 U.S.

496 (1906), and Georgia v. Tennessee Copper Co., 206 U.S.

230 (1907), which they read to stand for the proposition

that federal common law governs interstate pollution

disputes. See Petr. Br. at 24-25. That would be surprising,

because “federal common law” in the sense we speak of it

today is largely (outside admiralty) a post-Erie concept.

Neither Missouri nor Tennessee Copper saw any need to

pinpoint the source of nuisance law applied in those cases;

they do not speak in terms of “federal” substantive law.14

Ordinarily, a pre-Erie federal court in such a case would

apply “general” law, and indeed the literature on general

law often cites interstate disputes as an area where

federal courts have resorted to general law.15 Subsequent

14. Defendants interpolate the word “federal” into a quotation

from Missouri, Petr. Br. at 25, but it is not in the original opinion.

15. See, e.g., Caleb Nelson, The Persistence of General Law,

106 Colum. L. Rev. 503, 508-09 (2006).

11

decisions interpreting these cases as predicated on a

federal law of interstate nuisance, see Illinois v. City of

Milwaukee (“Milwaukee I”), 406 U.S. 91, 105 n.6, (1972),

have read that into the decisions.

Any federal common lawmaking in these cases seems

to have rested on far narrower grounds than Defendants

assert. Tennessee Copper did suggest that the national

Constitution entitled Georgia to relief under principles

of federal equity if they could prove a case for nuisance

on the merits. See 206 U.S. at 237-38. But this Court has

recognized a basis for federal equity dating back to the

Founding, see Grupo Mexicano de Desarrollo S.A. v. All.

Bond Fund, Inc., 527 U.S. 308, 318-19 (1999),16 without

extending that authority to formulate federal common law

rules. And federal equity can be no authority for displacing

Plaintiffs’ damages claims here.17

As Defendants acknowledge, Petr. Br. at 24-25, both

this Court and scholars have grounded decisions like

Missouri and Tennessee Copper in a substantive principle

of sovereign equality among states. That principle has no

application to the Plaintiffs’ claims against private actors

here. In Hinderlider v. La Plata River & Cherry Creek

Ditch Co., 304 U.S. 92, 110 (1938), Justice Brandeis wrote

that “whether the water of an interstate stream must be

apportioned between the two States is a question . . . upon

which neither the statutes nor the decisions of either State

16. See also William Baude, Jack Goldsmith, John Manning,

James Pfander, & Amanda Tyler, Hart and Wechsler’s The Federal

Courts and the Federal System 783-88 (8th ed. 2025).

17. See Tennessee Copper, 206 U.S. at 238 (suggesting that

the Court could likely not have provided a remedy in a suit between

private parties or an action at law).

12

can be conclusive.”18 This is an important principle, but

extending it to foreclose any state-law claims involving

cross-border pollution or other “ambient” interstate

activity stretches it beyond all recognition.19 All of the

interstate dispute cases of which we are aware involve

sovereign equality concerns, which arguably demand

a neutral rule of decision to prevent self-dealing. And

substantively, there is a considerable difference between

zero-sum boundary and resource allocation cases, which

often require exclusive judicial jurisdiction, 20 and in

18. Although Hinderlider involved a private party’s suit,

the issue in dispute concerned the allocation of water rights

between Colorado and New Mexico. See 304 U.S. at 95-96. See

also Anthony J. Bellia, Jr., & Bradford R. Clark, General Law in

Federal Court, 54 Wm. & Mary L. Rev. 655, 713 (2013) (“Disputes

between states are beyond the authority of either state to resolve,

and the Constitution itself requires federal courts to apply general

law rules designed to uphold the constitutional equality of the

states.”); Nelson, General Law, supra, at 508 (agreeing that “if a

state disagrees with neighboring states about its boundaries or

its right to the water in interstate rivers, it cannot authoritatively

resolve the dispute simply by passing a statute arrogating all of

the disputed territory or water to itself”).

19. When cases like Franchise Tax Bd. v. Hyatt, 587 U.S.

230 (2019), speak of “the interstate . . . nature of the controversy”

making it “inappropriate for state law to control,” id. at 246, they

are referring to the clash among sovereign equals, not simply

activity that crosses state lines. See id. at 246-27.

20. See Hyatt, 246-47 (emphasizing that “a dispute over

borders or water rights . . . involves a direct conflict between

sovereigns”); Princess Lida of Thurn & Taxis v. Thompson, 305

U.S. 456, 466-67 (1939) (acknowledging the principle that a single

court must have jurisdiction over cases administering a corpus).

13

personam claims for damages based on tortious activity

that crosses a border. The sovereign equality of states

may impose some constraint on choice of law rules, 21 but

the Constitution has never been held to require that all

cases implicating the interests of multiple states must be

governed by federal common law. 22

In any event, the federal common law of interstate

nuisance was itself preempted by the 1972 Amendments

to the Clean Water Act. In City of Milwaukee v. Illinois

& Michigan, 451 U.S. 304, 314 (1981) (“Milwaukee II”),

this Court recognized that “when Congress addresses

a question previously governed by a decision rested on

federal common law the need for such an unusual exercise

of lawmaking by federal courts disappears.” Defendants

nonetheless argue that the preemptive force of the old

interstate nuisance jurisprudence remains, even though

its substance does not. Petr. Br. at 29-30. This argument

depends entirely on the underlying contention that the

Constitution itself forecloses states from regulating

interstate pollution. See id. at 30. For reasons discussed

in Part I, that argument is simply incorrect.

21. See, e.g., Douglas Laycock, Equal Citizens of Equal and

Territorial States: The Constitutional Foundations of Choice of

Law, 92 Colum. L. Rev. 249 (1992) (arguing that the equality of

states ought to require that states may not systematically prefer

their own law over the law of other states).

22. See Allstate Ins. Co. v. Hague, 449 U.S. 302, 312-13 (1981)

(holding that the Constitution requires only that a “State must

have a significant contact or significant aggregation of contacts,

creating state interests, such that choice of its law is neither

arbitrary nor fundamentally unfair”).

14

This Court necessarily rejected any such notion in

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

440 (1960). That case held that Detroit could apply its

Smoke Abatement Code to ships operating in interstate

commerce on Lake Huron. That was true even though the

case fell within the maritime jurisdiction, traditionally an

enclave of extensive federal common lawmaking. Citing

cases all the way back to Gibbons v. Ogden, 22 U.S. (9

Wheat.) 1 (1824), the Court explained:

Legislation designed to free from pollution the

very air that people breathe clearly falls within

the exercise of even the most traditional concept

of . . . the police power. In the exercise of that

power, the states and their instrumentalities

may act, in many areas of interstate commerce

and maritime activities, concurrently with the

federal government. 362 U.S. at 442.

Defendants’ claims of broad constitutional preemption

of state law would also wreak havoc with the CAA’s

structure. That statute delegates broad discretion to state

governments in formulating state implementation plans

to meet federal air quality standards. 23 And as we discuss

in Part III, the CAA incorporates two broad savings

23. See generally Ohio v. EPA, 603 U.S. 279, 283-84 (2024)

(“Because the States bear ‘primary responsibility’ for developing

compliance plans, EPA has ‘no authority to question the wisdom

of a State’s choices of emission limitations.’”) (quoting 42 U.S.C. §

7401(a)(3) and Train v. Nat. Res. Def. Council, Inc., 421 U.S. 60, 79

(1975)); Virginia v. EPA, 108 F.3d 1397, 1406-09 (D.C. Cir. 1997);

John P. Dwyer, The Practice of Federalism Under the Clean Air

Act, 54 Md. L. Rev. 1183 (1995).

15

clauses preserving state regulations and remedies. If the

Constitution prohibits state regulation in this area, then it

is unclear how Congress could nonetheless authorize it. 24

Defendants’ argument diverges dramatically from

how federal common law generally works. The best

understanding of the interstate conflicts cases from

Missouri through Milwaukee I is that, in order to respect

the equality of sovereign litigants, the Court applied

general law principles to cases where neither state could

dictate its own law. And, as Milwaukee II made clear, the

Court did this in the absence of congressional action, but

with the clear understanding that Congress might step

in at any point and weigh the competing considerations

differently. See 451 U.S. at 317 (“[W]e start with the

assumption that it is for Congress, not federal courts,

to articulate the appropriate standards to be applied

as a matter of federal law.”) (internal quotation marks

omitted). Congress having done just that, the measure

of preemption becomes the CAA and any other relevant

federal statutes.

As discussed in Part III, Congress decided to leave

“primary responsibility” for “air pollution prevention

. . . and air pollution control” with “States and local

governments.” 42 U.S.C. § 7401(a)(3). There is no

requirement that Congress expressly “authorize” claims

24. Moreover, this Court has upheld EPA decisions requiring

pollution sources to comply with a downstream affected state’s

water quality standards. See Arkansas v. Oklahoma, 503 U.S.

91, 105 (1992). That would be impermissible if all state regulation

were constitutionally forbidden.

16

under state law. See Petr. Br. at 30. 25 Milwaukee II

acknowledged the presumption against preemption of

state law in this context, while favoring displacement

of federal common law by a new federal statute. See 451

U.S. at 316-17. Defendants’ contrary assertion that some

broad principle of structural preemption survived the

CAA is flatly inconsistent with this Court’s later reading

of Milwaukee II; in Int’l Paper Co. v. Ouellette, 479

U.S. 481 (1987), this Court interpreted Milwaukee II as

holding “that federal legislation now occupied the field,

pre-empting all federal common law. The Court left open

the question of whether injured parties still had a cause of

action under state law.” Id. at 489 (emphasis in original). 26

And Ouellette explicitly held that state law actions could

be brought against polluters under the law of the source

state. 27

25. If there is such a requirement, amici submit that the

savings clause for state remedies “under any statute or common

law,” 42 U.S.C. § 7401(a)(3), should satisfy it.

26. Defendants pick out language in Ouellette stating that

“the regulation of interstate water pollution is a matter of federal,

not state law,” Petr. Br. at 26 (quoting Ouellette, 479 U.S. at 488),

but that language simply described the Court’s earlier holding in

Milwaukee I—not the law after the CWA included an extensive

role for States.

27. See 479 U.S. at 497 (“The saving clause specifically

preserves other state actions, and therefore nothing in the Act bars

aggrieved individuals from bringing a nuisance claim pursuant

to the law of the source State.”) (emphasis in original); see also

id. (“By its terms the CWA allows States . . . to impose higher

standards on their own point sources, and in Milwaukee II we

recognized that this authority may include the right to impose

higher common-law as well as higher statutory restrictions.”)

(citing 451 U.S. at 328). Likewise, Defendants’ quotations to

17

Both Ouellette and AEP considered the scope of

preemption of particular sorts of state law claims under

the specific provisions of the CWA and CAA. That careful

analysis is inconsistent with categorical principles of

structural preemption. The law of interstate nuisance was

never categorical in the way that Defendants suggest, but

in any event that way of thinking surely perished after

the Clean Air Act Amendments.

B. This Court has explicitly refused to concoct

a broad constitutional principle barring state

regulation with extraterritorial effects.

Three years ago, this Court unanimously refused to

recognize an “extraterritoriality doctrine” that would

forbid enforcement of state law merely because it has

the “‘practical effect of controlling commerce outside the

worries about “in irrational system of regulation” or “chaotic

confrontation” between states, Petr. Br. at 26, are in the context

of allowing suits under both source states’ and affected states’

laws. See 479 U.S. at 496. The Court did not see such problems as

intrinsic to state regulation under an appropriate choice of law

regime.

Defendants likewise invoke Am. Elec. Power Co. v. Connecticut,

564 U.S. 410 (2011) (“AEP”), for the categorical proposition that

“federal law applies to disputes involving interstate pollution.”

Petr. Br. at 26. But the language on which they rely is out of

context. It occurs in the course of rejecting the possibility that

federal common law claims might nonetheless adopt a state rule

of decision—a possibility identified in Kimbell Foods and other

cases. See 440 U.S. at 728-29. More recent federal common law

cases have emphasized that, absent a specific conflict with federal

interests or policy, state law applies of its own force, see Atherton,

519 U.S. at 218-19; O’Melveny & Myers, 512 U.S. at 87-88, and

AEP did not foreclose this.

18

State.’” Ross, 598 U.S. at 371. It is venturesome, then,

for Defendants to argue that imposing liability on them

for impacts of climate change in Colorado will shape

their behavior in ways the Constitution forbids. Petr.

Br. at 22. Their alleged change in business practices

relating to marketing and production of fossil fuels is

indistinguishable from the arguments raised and rejected

in Ross that California’s law shaped pork production

practices outside the state. See Ross, 598 U.S. at 371. In

an otherwise divided decision, Defendants’ proposition

failed to garner a single vote. 28

T h i s Cou r t d id a dopt a much more mode st

extraterritoriality rule in Ouellette. By restricting state

law claims to the law of the source state, the Court ensured

that only one state’s rule could govern that source’s

emissions. See 479 U.S. at 493-94. But this rule sprung

not from the Constitution, but from the CWA’s emissions

permitting system, which prioritized the source state. See

id. at 490. Plaintiffs’ claims do not target emissions at all

and thus implicate no similar permitting regime under

the CAA. Ouellette illustrates, however, that Congress

could impose a similar choice of law rule if it desired—

and it highlights that the trial court in this case has yet

to address any choice of law arguments concerning what

state’s law applies to Plaintiff’s common law claims.

In any event, standard extraterritoriality principles

generally permit any given jurisdiction to impose liability

28. Brannon P. Denning, National Pork Producers Council

v. Ross: Extraterritoriality is Dead, Long Live the Dormant

Commerce Clause, 2023 Cato Sup. Ct. Rev. 23, 29 (“[I]f there was

still some life left in extraterritoriality, National Pork Producers

delivered a unanimous coup de grace.”).

19

for harms occurring inside the jurisdiction as a result

of conduct outside its borders. 29 That is all Colorado

purports to do here (and at least some of Defendants’

conduct occurred inside Colorado). As Ross recognized,

“many (maybe most) state laws have the ‘practical effect

of controlling extraterritorial behavior” including “all

manner of ‘libel laws, securities requirements, charitable

registration requirements, franchise laws, tort laws,’

and plenty else besides.” 598 U.S. at 374 (quoting Jack

Goldsmith & Alan Sykes, The Internet and the Dormant

Commerce Clause, 110 Yale L. J. 785, 804 (2001)). These

are, as the Court concluded, “valid exercises of the

States’ constitutionally reserved powers.” Id. at 375. The

constitutional principle Defendants invoke does not exist

in anything like the form they assert.

C.

Foreign affairs preemption is, at best, a narrow

doctrine dependent on state interferences with

particular national foreign policies embodied

in positive law.

Defendants’ foreign affairs preemption argument is a

variant of their constitutional preemption argument, this

time predicated on climate change’s status as a subject of

international cooperation and negotiation among nations.

Ordinarily, successful arguments that state law must

give way to national diplomacy have been grounded in

29. See F. Hoffmann-La Roche Ltd. v. Empagran S.A., 542

U.S. 155, 165 (2004) (“[O]ur courts have long held that application of

our antitrust laws to foreign anticompetitive conduct is nonetheless

reasonable, and hence consistent with principles of prescriptive

comity, insofar as they reflect a legislative effort to redress

domestic antitrust injury that foreign anticompetitive conduct

has caused.”).

20

positive law—that is, a statute imposing sanctions on

a foreign government that implicitly limits additional

state sanctions, 30 or an executive agreement with another

country that state law may undermine. 31 Defendants have

pointed to no such federal positive law in this case.

Defendants must therefore rely on Zschernig v. Miller,

389 U.S. 429 (1968). That decision struck down an Oregon

statute denying inheritance to heirs from countries—

usually East-bloc communist nations—that were likely to

confiscate the property passed through an Oregon will.

The Court was troubled by the law’s encouragement of

state judges to criticize communist nations at the height

of the Cold War. It held the law preempted because it

“may disturb foreign relations,” id. at 441, even though

no national treaty, statute, or executive agreement was

involved.

Without federal positive law for guidance, courts

applying Zschernig necessarily arrogate the foreign

affairs power to themselves. As Professor Jack Goldsmith

has pointed out, “[t]he content . . . of the judge-made

federal law [in Zschernig] was based on a judicial analysis

of the requirements of U.S. foreign relations.” 32 The

constitutional design, however, “left the determination

of when the national foreign relations interest would be

best served by the exclusion of state power largely to the

30. See Crosby v. Nat’l Foreign Trade Council, 530 U.S.

363 (2000).

31. See Am. Ins. Assn. v. Garamendi, 539 U.S. 396 (2003).

32. Jack L. Goldsmith, Federal Courts, Foreign Affairs, and

Federalism, 83 Va. L. Rev. 1617, 1630 (Nov. 1997).

21

discretion of the political branches.”33 The exercise of that

discretion requires “satisfaction of procedural hurdles

that make it costly and difficult for the political branches

to make foreign relations law” 34 —that is, production of

positive law, rather than, as here, Presidential statements

of opposition to a lawsuit. Zschernig was perceived as a

radical departure from these norms when it was decided,35

and it has remained an isolated outlier since.

To the extent that Zschernig has force at all, it

“resonates most audibly when a state action ‘reflect[s] a

state policy critical of foreign governments and involve[s]

sitting in judgment on them.” Garamendi, 539 U.S. at 439

(Ginsburg, J., dissenting) (quoting Louis Henkin, Foreign

Affairs and the United States Constitution 164 (2d ed.

1996)). Defendants argue for a broader rule preempting

state law based on any “‘likelihood’ that ‘state [action]

will produce something more than incidental effect

in conflict with express foreign policy of the National

Government.’” Petr. Br. at 40 (citing Garamendi, 539

U.S. at 420). That is unsupported in the Constitution’s

text or this Court’s caselaw. 36 Defendants’ categorical

33. Id. at 1645.

34. Id. (noting that this design was meant “to preserve state

influence and protect state interests”).

35. Id. at 1649.

36. Garamendi applied no such rule. It refused to sort out

the meaning of Zschernig, see 539 U.S. at 419-20, and focused

entirely on state law’s interference with “the national position”

on settlement of insurance claims arising from the Holocaust,

“expressed unmistakably in the executive agreements signed by

the President with Germany and Austria,” id. at 421.

22

version of the “one voice” aspiration in foreign affairs

is inconsistent with both the division of foreign affairs

powers among the three national branches but also those

branches’ practice of “tolerat[ing], deferr[ing] to or even

encourag[ing] state and local measures impacting on

foreign affairs.” 37 Because states have strong interests

in international commerce and collective solutions to

cross-border problems, moreover, states necessarily and

generally successfully engage in activities that touch on

foreign relations all the time. 38 And because international

law (and international opinion) are frequently concerned

with nations’ treatment of their own nationals within

their own territory, almost any subject of state activity

or regulation may implicate foreign relations. 39

This Court has rejected claims that state law must

yield absent conflict with a national positive law, even when

it puts the United States in violation of an international

court’s judgment on an issue which has often complicated

American foreign relations. See Medellin v. Texas, 552

U.S. 491 (2008) (upholding state administration of the

37. Sarah H. Cleveland, Crosby and the “One-Voice” Myth

in U.S. Foreign Relations, 46 Vill. L. Rev. 975, 975 (2001).

38. See, e.g., Julian Ku, Gubernatorial Foreign Policy, 115

Yale L. J. 2380 (2006) (collecting examples); Ernest A. Young,

Foreign Affairs Federalism in the United States, The Oxford

Handbook of Comparative Foreign Relations Law 259-75 (Curtis

Bradley ed., 2019) (noting state diplomatic efforts concerning

climate change).

39. See Goldsmith, supra, at 1673 (“[T]he changing nature of

international regulation and concern means that even domestic law

that applies to domestic persons for domestic acts can implicate

foreign relations.”).

23

death penalty in defiance of an International Court of

Justice order). As Justice Scalia noted, Medellin shows

that “[e]ven in its international relations, the Federal

Government must live with the inconvenient fact that

it is a Union of independent States, who have their own

sovereign powers. This is not the first time it has found

that a nuisance and a bother in the conduct of foreign

policy.” Arizona v. United States, 567 U.S. 387, 423-24

(2012) (Scalia, J., concurring in part and dissenting in

part).

Zschernig still stands alone as this Court’s only

case applying “dormant foreign affairs preemption” in

the absence of some federal positive law. This Court has

routinely avoided relying on Zschernig. In the Crosby

case, for example, the Court held a Massachusetts law

imposing sanctions on countries doing business in Burma

to be preempted by a federal statute imposing narrower

sanctions. See 530 U.S. at 373-74. But it did not follow the

First Circuit’s opinion, which had relied on Zschernig;40

rather, the Court found preemption based on its analysis of

the federal Burma sanctions statute enacted by Congress.

See 530 U.S. at 373-74. This case must likewise stand or

fall on statutory preemption.

III. The Clean Air Act does not preempt Plaintiffs’

claims.

This Court has generally focused on statutory

text rather than broad arguments about structural

preemption. The AEP Court said as much, noting that

40. See Nat’l Foreign Trade Council v. Natsios, 181 F.3d

38, 52-53 (1st Cir. 1999).

24

“[i]n light of our holding that the Clean Air Act displaces

federal common law, the availability vel non of a state

lawsuit depends, inter alia, on the preemptive effect of

the federal Act.” 564 U.S. at 429. But the Clean Air Act is

an unlikely source of broad preemption for two reasons.

First, it regulates emissions of pollutants, and plaintiffs’

lawsuits do not seek limits on emissions. Second, the

CAA explicitly sets out a major role for state regulation

and accords the states considerable discretion. That is

inconsistent with any notion that Congress occupied the

field of climate regulation in the CAA.

A.

The CAA does not regulate the activities that

are the focus of Plaintiffs’ lawsuit.

Plaintiffs’ causes of action seek damages for concealing

the real impacts of climate change and for producing fossil

fuels that other actors go on to burn, causing emission of

greenhouse gases. The Clean Air Act, on the other hand,

says nothing about marketing or upstream production;

it “speaks directly to emissions of carbon dioxide” from

power plants and other sources. AEP, 564 U.S. at 424.

Whatever the CAA’s preemptive effect on state regulation

of the same subjects, it does not reach state-law claims

predicated on entirely different activity.

This Court’s cases construing the preemptive impact

of the CAA and the CWA concern the availability of

common law claims that nonetheless target the same

activity as the federal statutes themselves; they are

all cases about emissions. In AEP, plaintiffs brought

“federal common-law public nuisance claims against

carbon-dioxide emitters” and sought “a decree setting

carbon-dioxide emissions for each defendant at an initial

cap.” 564 U.S. at 415. Likewise, in Ouellette, the defendant

25

discharged effluents into Lake Champlain, and plaintiffs

sought both damages relief from those discharges and an

injunction requiring the defendant to restructure its water

treatment system to reduce discharges. See 479 U.S. at

483-84. In each case, then, plaintiffs both targeted the

same activity as the federal statute and sought the same

sorts of remedies that the statute itself sought to impose.

Even so, Ouellette held that state law nuisance claims

were not preempted so long as they rested on the law of

the source state, and AEP reserved the preemption of

state-law nuisance claims as an open question.

The case for preemption is much weaker here, because

Plaintiffs’ claims target activity that the CAA does not

regulate. Defendants will argue that Plaintiffs’ focus

on production and marketing is just artful pleading and

that imposing damages for pre-emissions stage activity

is just an attempt to get at emissions indirectly. This

argument has little purchase with regard to Plaintiffs’

deceptive marketing claims, which fall wholly outside

the CAA’s purview. Even with respect to the upstream

production claims, this case is meaningfully different

from an emissions case. On the merits, Plaintiffs will

bear the potentially difficult burden of tying Defendants’

production to emissions that caused climate change

damages in Boulder. But to say that these additional

steps make no difference for preemption purposes is

necessarily to argue that federal law preempts regulation

of all activities that potentially influence greenhouse

gas emissions. That would include, for example, land

use decisions about siting data centers, promulgation of

“green” building codes, and even educational programs

designed to increase awareness of the dangers of climate

change. Federal statutes that regulate one aspect of a

problem do not ordinarily restrict every other policy

26

measure that might have an impact on the federally

regulated activity.41

Even if the CAA were an instance of field preemption,

it would still be necessary to define the field. An

obviously strong causal connection between one activity

and another will not necessarily broaden the scope of

federal field preemption. In Virginia Uranium, Inc. v.

Warren, 587 U.S. 761 (2019), federal law preempted the

field of regulating the operation and safety of nuclear

power plants. Rather than barring nuclear power plants,

Virginia banned uranium mining in the state. The mining

company argued that the mining ban would have the effect

of discouraging nuclear power plants, noting Virginia’s

admission that it banned mining to protect the public

from the same nuclear safety hazards that federal law

addressed. Id. at 776. Nonetheless, this Court held mining

regulation to be outside the preempted field. Again, the

present case presents a weaker argument for preemption

than the one rejected in Virginia Uranium.

B. The CAA does not preempt any field.

This Court has held that “federal regulation of a

field of commerce should not be deemed preemptive of

state regulatory power in the absence of persuasive

reasons—either that the nature of the regulated subject

matter permits no other conclusion, or that the Congress

has unmistakably so ordained.” Fla. Lime & Avocado

41. See, e.g., Kansas v. Garcia, 589 U.S. 191 (2020) (holding

the federal immigration law restricting prosecution of immigrants

for working without authorization did not prohibit state identity

theft and false information prosecutions immigrants seeking

employment).

27

Growers, Inc. v. Paul, 373 U.S. 132, 142 (1963).42 Not

only have Defendants pointed to no language in the CAA

expressly preempting any field, but the statute’s text

expressly eschews any sort of field preemption.

The CAA begins by declaring that “air pollution

prevention . . . and air pollution control at its source is the

primary responsibility of States and local governments.”

42 U.S.C. § 7401(a)(3). The CAA then adds not one but two

express savings clauses. The first states that, with certain

exceptions, “nothing in this chapter shall preclude or deny

the right of any State or political subdivision thereof to

adopt or enforce (1) any standard or limitation respecting

emissions of air pollutants or (2) any requirement

respecting control or abatement of air pollution,” so

long as it is not “less stringent” than existing standards

adopted under the statute. 42 U.S.C. § 7416. The second

saves state remedies “under any statute or common law”

from preemption by the Clean Air Act’s remedial scheme.

42 U.S.C. § 7604(e). None of these provisions is consistent

with field preemption.

The regulatory structure that the CAA creates

likewise forecloses any such finding that Congress has

preempted the field of emissions regulation. The CAA

is a cooperative federalism arrangement; it assigns the

national EPA responsibility to determine safe levels of

42. The Court cited Huron Portland Cement, which rejected

the notion that federal law governing air pollution or maritime

commerce foreclosed applying local air quality regulations to

vessels engaged in interstate commerce. See 373 U.S. at 142.

See also Nelson, Preemption, supra, at 227 (observing that “[t]

he Court has grown increasingly hesitant to read implicit fieldpreemption clauses into federal statutes”).

28

pollutants but delegates to states the authority to devise

state implementation plans to reach those levels.43 The

states’ ability to formulate and enforce implementation

plans is far too discretionary to be consistent with any

notion of field preemption; rather, the statutory scheme

depends on the states devising their own approaches

to emissions regulation that promise to achieve the air

quality standards that EPA sets.44 If states could not

regulate emissions, they could not fulfill the role in which

Congress has cast them.

Nor does the CAA preempt unilateral state action

outside the CAA framework. By saving state remedies

“under any statute or common law,” the statute makes

clear that states are not limited to enforcing their

implementation plans within the CAA process. Under

the analogous CWA scheme, Ouellette held that state

law nuisance claims are not preempted so long as they

are brought under the law of the source state. 479 U.S.

at 497-98. Likewise, AEP reserved the question whether

the CAA preempts state law nuisance claims. See 564 U.S.

at 429. The majority of lower courts to consider the issue

have held—following Ouellette—that the CAA does not

preempt state nuisance claims brought under the law of

the state in which the emitter is located.45

43. See Ohio, 603 U.S. at 283-84; Dwyer, supra, at 1190-99.

44. See Train, 421 U.S. at 79 (holding that EPA has “no

authority to question the wisdom of a State’s choices of emission

limitations”).

45. See, e.g., Merrick v. Diageo Americas Supply, Inc., 805

F.3d 685 (6th Cir. 2015); Bell v. Cheswick Generating Station, 734

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

29

The nuanced approach of Ouellette and AEP is

fundamentally inconsistent with any sort of broad field

preemption. And Defendants have hardly attempted

to make any narrower claim of conflict or impossibility

preemption. It may be that narrower preemption claims

may prove more persuasive in particular circumstances,

but the exceptionally aggressive preemption arguments

that Defendants have advanced here are completely at

odds with the statute Congress enacted.

CONCLUSION

The Supreme Court of Colorado’s rejection of

Defendants’ preemption defense should be affirmed.

Respectfully submitted,

Michael D. Hausfeld

Hausfeld LLP

1200 17th Street, N.W.,

Suite 600

Washington, DC 20036

mhausfeld@hausfeldllp.com

Ernest A. Young

Counsel of Record

3208 Fox Terrace Drive

Apex, NC 27502

(919) 360-7718

young@law.duke.edu

A dam J. Levitt

Daniel Rock Flynn

Dicello Levitt LLP

10 North Dearborn Street,

6th Floor

Chicago, IL 60602

alevitt@dicellolevitt.com

dflynn@dicellolevitt.com

Counsel for Amici Curiae

August 3, 2026