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 HOUSE MAJORITY LEADER STEVE

SCALISE AND 73 OTHER MEMBERS OF

CONGRESS AS AMICI CURIAE IN SUPPORT OF

PETITIONERS

______________

MICHAEL BUSCHBACHER

JARED M. KELSON

Counsel of Record

LAURA B. RUPPALT

BOYDEN GRAY PLLC

800 Connecticut Ave NW,

Suite 900

Washington, DC 20006

(202) 955-0620

jkelson@boydengray.com

i

QUESTIONS PRESENTED

1. 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.

2. Whether this Court has statutory and Article III

jurisdiction to hear this case.

ii

TABLE OF CONTENTS

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

INTRODUCTION AND SUMMARY OF

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

ARGUMENT .............................................................. 5

I.

THIS CASE SEEKS TO REGULATE GREENHOUSEGAS EMISSIONS AROUND THE WORLD .................. 5

II.

CROSSBORDER EMISSIONS ARE THE EXCLUSIVE

DOMAIN OF FEDERAL LAW .................................... 8

A.

Constitutional Structure and a Century

of Precedent Dictate That Disputes Over

Crossborder Emissions Require Federal

Rules of Decision ......................................... 8

B.

The Clean Air Act Is a Comprehensive

Scheme that Preserves Federal Authority

Over Crossborder Emissions..................... 16

1. The Clean Air Act Confirms Federal

Authority Over Crossborder Emissions.. 17

2. Statutory Interpretation Confirms

Congress Did Not Authorize New State

Power Over Crossborder Emissions ....... 21

3. The Clean Air Act’s Saving Clauses Do

Not Authorize Respondents’ Claims ....... 23

III. APPLYING STATE LAW TO INTERNATIONAL

EMISSIONS INTERFERES WITH THE EXCLUSIVE

AUTHORITY OF THE FEDERAL GOVERNMENT

OVER FOREIGN AFFAIRS...................................... 26

CONCLUSION ......................................................... 32

iii

TABLE OF AUTHORITIES

CASES

Page(s)

Am. Elec. Power Co. v. Connecticut,

564 U.S. 410 (2011) ........................................ 14–16

Am. Ins. Ass’n v. Garamendi,

539 U.S. 396 (2003) .................................. 26, 27, 31

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

517 U.S. 559 (1996) ................................................ 7

Bond v. United States,

572 U.S. 844 (2014) ........................................ 21, 22

Buckman Co. v. Plaintiffs’ Legal Comm.,

531 U.S. 341 (2001) .............................................. 22

Burnet v. Brooks,

288 U.S. 378 (1933) .............................................. 10

Cipollone v. Liggett Grp., Inc.,

505 U.S. 504 (1992) ................................................ 7

City of Milwaukee v. Illinois,

451 U.S. 304 (1981) ........................................ 13, 14

City of New York v. Chevron,

993 F.3d 81 (2d Cir. 2021) ................................... 12

Clearfield Tr. Co. v. United States,

318 U.S. 363 (1943) .............................................. 10

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

615 F.3d 291 (4th Cir. 2010) ................................ 16

iv

Dubin v. United States,

599 U.S. 110 (2023) .............................................. 23

Franchise Tax Bd. of Cal. v. Hyatt,

587 U.S. 230 (2019) .......................................... 9–11

Fuld v. Palestine Liberation Org.,

606 U.S. 1 (2025) .................................................. 16

Georgia v. Tenn. Copper Co.,

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

Gregory v. Ashcroft,

501 U.S. 452 (1991) .............................................. 21

Hinderlider v. La Plata River & Cherry Creek

Ditch Co., 304 U.S. 92 (1938)............................... 14

Hines v. Davidowitz,

312 U.S. 52 (1941) ................................................ 32

Illinois v. City of Milwaukee,

406 U.S. 91 (1972) .................................... 12–14, 16

International Paper Co. v. Ouellette,

479 U.S. 481 (1987) ............................ 13, 14, 25, 26

Kansas v. Colorado,

206 U.S. 46 (1907) .......................................... 14, 15

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

565 U.S. 625 (2012) ................................................ 7

Learning Res., Inc. v. Trump,

146 S. Ct. 628 (2026) ............................................ 21

v

Maryland v. EPA,

958 F.3d 1185 (D.C. Cir. 2020) ............................ 18

Mayor & City Council of Baltimore v. B.P. P.L.C.,

353 A.3d 1142 (Md. 2026) ...................................... 8

Merrick v. Diageo Americas Supply, Inc.,

805 F.3d 685 (6th Cir. 2015) ................................ 25

Missouri v. Illinois,

180 U.S. 208 (1901) ........................................ 10, 11

Missouri v. Illinois,

200 U.S. 496 (1906) .............................................. 11

Montgomery v. Caribe Transp. II, LLC, No. 24-1238,

2026 WL 1336188 (U.S. May 14, 2026) ................. 7

Native Vill. of Kivalina v. ExxonMobil Corp.,

696 F.3d 849 (9th Cir. 2012) ................................ 22

New Hampshire v. Louisiana,

108 U.S. 76 (1883) .................................................. 9

New Jersey v. New York,

283 U.S. 473 (1931) .............................................. 12

New York v. New Jersey,

256 U.S. 296 (1921) .............................................. 12

North Dakota v. Minnesota,

263 U.S. 365 (1923) .............................................. 12

Ohio v. Wyandotte Chems. Corp.,

401 U.S. 493 (1971) .............................................. 13

vi

Parker Drilling Mgmt. Servs., Ltd. v. Newton,

587 U.S. 601 (2019) ........................................ 22, 24

Roberts v. Sea-Land Servs., Inc.,

566 U.S. 93 (2012) ................................................ 20

San Diego Building Trades Council v. Garmon,

359 U.S. 236 (1959) ................................................ 7

Tex. Indus., Inc. v. Radcliff Materials, Inc.,

451 U.S. 630 (1981) .............................................. 10

Texas v. United States,

507 U.S. 529 (1993) .............................................. 21

U.S. Forest Serv. v. Cowpasture River Pres. Ass’n,

590 U.S. 604 (2020) ........................................ 21, 25

United States v. Belmont,

301 U.S. 324 (1937) .............................................. 31

United States v. Locke,

529 U.S. 89 (2000) .......................................... 22, 25

United States v. Pink,

315 U.S. 203 (1942) .............................................. 27

Whitman v. Am. Trucking Ass’ns,

531 U.S. 457 (2001) .............................................. 25

Zschernig v. Miller,

389 U.S. 429 (1968) .............................................. 32

vii

CONSTITUTIONAL PROVISIONS

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

U.S. Const. art. I, § 10 ............................................... 26

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

U.S. Const. art. VI ....................................................... 9

STATUTES

5 U.S.C. § 553 ............................................................ 20

42 U.S.C. §§ 7401–7515 ............................................ 17

42 U.S.C. § 7401 ........................................................ 16

42 U.S.C. § 7402 ........................................................ 19

42 U.S.C. § 7409 ........................................................ 17

42 U.S.C. § 7410 .................................................. 17, 18

42 U.S.C. § 7411 ........................................................ 17

42 U.S.C. § 7415 .................................................. 19, 28

42 U.S.C. § 7416 .................................................. 23, 24

42 U.S.C. § 7426 ........................................................ 18

42 U.S.C. § 7436 ........................................................ 30

42 U.S.C. § 7502 ........................................................ 17

42 U.S.C. § 7506a ................................................ 18, 19

viii

42 U.S.C. § 7507 ........................................................ 20

42 U.S.C. § 7509 ........................................................ 17

42 U.S.C. § 7511c....................................................... 19

42 U.S.C. § 7521 ........................................................ 19

42 U.S.C. § 7543 ........................................................ 20

42 U.S.C. § 7545 ........................................................ 20

42 U.S.C. § 7547 ........................................................ 19

42 U.S.C. § 7571 ........................................................ 19

42 U.S.C. § 7573 ........................................................ 20

42 U.S.C. § 7604 ........................................................ 24

42 U.S.C. § 7607 .................................................. 18, 20

42 U.S.C. §§ 7651–7651o ........................................... 20

42 U.S.C. §§ 7661–7661f ........................................... 17

42 U.S.C. § 7661a ...................................................... 18

42 U.S.C. § 7661d ...................................................... 18

42 U.S.C. §§ 7671–7671q .......................................... 20

Accelerating Deployment of Versatile, Advanced

Nuclear for Clean Energy Act of 2024,

Pub. L. No. 118-67, div. B, 138 Stat. 1447 .......... 30

Act of October 27, 2000,

Pub. L. No. 106-377, 114 Stat. 1441 .................... 30

ix

Energy Act of 2020,

Pub. L. No. 116-260, div. Z, 134 Stat. 1182 ........ 30

Global Climate Protection Act of 1987,

Pub. L. No. 100-204, 101 Stat. 1331 .................... 27

Nuclear Energy Innovation and Modernization Act,

Pub. L. No. 115-439, 132 Stat. 5565 (2019) ........ 30

Strom Thurmond National Defense Authorization

Act for Fiscal Year 1999, Pub. L. No. 105-261,

112 Stat. 1920 (1998) ........................................... 30

Utilizing Significant Emissions with Innovative

Technologies Act, Pub. L. No. 116-260, div. S,

134 Stat. 1182 (2020) ........................................... 30

INTERNATIONAL AGREEMENTS

Convention on Long-Range Transboundary Air

Pollution, T.I.A.S. No. 10,541 (1979) ................... 28

La Paz Agreement, T.I.A.S. No. 10,827 (1983) ........ 28

Vienna Convention for the Protection of the Ozone

Layer, T.I.A.S. No. 11,097 (1985) ........................ 28

Montreal Protocol, T.I.A.S. No. 89-101 (1987) ......... 28

U.S.-Canada Air Quality Agreement,

T.I.A.S. No. 11,783 (1991) .................................... 28

OTHER AUTHORITIES

American Heritage Dictionary of the

English Language (1969) ..................................... 24

x

Colo. Air Pollution Control Div., Dep’t of Pub.

Health & Env’t, Colorado Statewide Inventory of

Greenhouse Gas Emissions and Sinks 2025,

Executive Summary Report (Dec. 2025) ................ 6

Colorado, World Population Review,

https://perma.cc/HJ63-XJNS

(last visited May 21, 2026) ................................... 6

Exec. Order No. 14162, Putting America First in

International Environmental Agreements,

90 Fed. Reg. 8455 (Jan. 30, 2025) ....................... 29

The Federalist No. 42 (James Madison)

(Clinton Rossiter ed., 1961) ................................. 26

The Federalist No. 44 (James Madison)

(Clinton Rossiter ed., 1961) ................................. 27

The Federalist No. 80 (Alexander Hamilton)

(Clinton Rossiter ed., 1961) ................................. 15

H.R. Rep. No. 95-294 (1977)...................................... 21

Historical GHG Emissions, ClimateWatch,

https://perma.cc/2MKP-HFJE

(last visited May 11, 2026) .................................... 6

Memorandum of January 7, 2026, Withdrawing the

United States from International Organizations,

Conventions, and Treaties That Are Contrary to

the Interests of the United States,

91 Fed. Reg. 2281 (Jan. 16, 2026) ....................... 29

xi

Remarks Announcing the Formal Entry of the

United States into the UNFCCC Paris

Agreement in Hangzhou, China, 2 Pub. Papers

of Barack Obama 1122 (Sept. 3, 2016) ................ 29

Richard K. Lattanzio, Cong. Rsch. Serv., R46204,

The United Nations Framework Convention on

Climate Change, the Kyoto Protocol, and the

Paris Agreement: A Summary (Jan. 29, 2020) .... 28

S. Res. 98, 105th Cong.,

143 Cong. Rec. 15808 (1997) .......................... 29, 30

Joseph Story,

Commentaries on the Constitution (1833) ........... 10

St. George Tucker,

Blackstone’s Commentaries (1803) ...................... 10

U.S. Dep’t of State, Press Release, On the U.S.

Withdrawal from the Paris Agreement

(Nov. 4, 2019) ....................................................... 29

U.S. Dep’t of State, Press Release, The United

States Officially Rejoins the Paris Agreement

(Feb. 19, 2021) ...................................................... 29

World Population Review,

https://perma.cc/HJ63-XJNS

(last visited May 20, 2026) .................................... 6

1

INTEREST OF AMICI CURIAE 1

Amici curiae are 74 elected Members of Congress,

many of whom sit on committees with jurisdiction

over energy, natural resources, and environmental

matters. Amici have a strong and direct interest in

preserving the authority vested in the federal

government by the Constitution over crossborder—

both interstate and international—greenhouse-gas

emissions, including the fundamental power of

Congress to regulate interstate and foreign commerce.

The decision below supplants the legislative

prerogative of Congress and would allow a patchwork

of conflicting state laws around the country to govern

matters of uniquely federal concern. Amici file this

brief to defend their role in setting energy,

environmental, and foreign policy for the United

States, including the regulation of crossborder

emissions, and to prevent the use of state law to

undermine the statutory schemes that Congress has

enacted and refined for decades.

The following is the full list of amici:

1 No party’s counsel authored this brief in whole or in part, and

no person or entity other than amici or their counsel made a

monetary contribution intended to fund its preparation or

submission.

2

House of Representatives

Steve Scalise

Brett Guthrie

Bruce Westerman

Mark Alford

Brian Babin

Troy Balderson

Aaron Bean

Andy Biggs

Gus M. Bilirakis

Lauren Boebert

Vern Buchanan

Ken Calvert

Mike Carey

Buddy Carter

Ben Cline

Michael Cloud

Andrew Clyde

James Comer

Jeff Crank

Warren Davidson

Chuck Edwards

Jake Ellzey

Ron Estes

Gabe Evans

Julie Fedorchak

Randy Fine

Vince Fong

Virginia Foxx

Scott Franklin

Craig Goldman

Lance Gooden

H. Morgan Griffith

Harriett Hageman

Mike Haridopolos

Pat Harrigan

Mark Harris

Diana Harshbarger

Clay Higgins

Erin Houchin

Richard Hudson

Wesley Hunt

Jeff Hurd

Ronny L. Jackson

Nick Langworthy

Laurel Lee

Julia Letlow

Michael McCaul

Addison McDowell

John McGuire

Mark Messmer

Cory Mills

Barry Moore

Riley M. Moore

Tim Moore

Dan Meuser

Troy Nehls

Dan Newhouse

Bob Onder

Burgess Owens

Gary Palmer

Scott Perry

Mike Rogers

John Rose

3

Michael Rulli

Derek Schmidt

Adrian Smith

Jason Smith

Pete Stauber

Claudia Tenney

Glenn Thompson

Beth Van

Duyne Ann Wagner

Tim Walberg

Ryan Zinke

4

INTRODUCTION AND SUMMARY OF

ARGUMENT

In recent years, multiple state and local

governments have launched a courtroom war against

the American energy industry. Unsatisfied with the

laws passed by Congress, including the Clean Air Act,

they attempt to wield state law and state court

judgments to pursue crushing penalties against

energy companies for harms allegedly caused by the

effects of global greenhouse-gas emissions on the

global climate. In doing so, they would dictate

national energy, environmental, and foreign policy

themselves. This approach upends the constitutional

balance between federal and state authority and

undermines the federal legislative process.

Respondents Boulder County Commissioners and

the City of Boulder are among those who have taken

that course. They dress their complaint in the

language of state law, but they cannot escape that

every claim in some way turns on global greenhousegas emissions. The sheer magnitude of the alleged

damages would restructure the American energy

industry—if not bankrupt it altogether—and cause

ripple effects worldwide, especially when multiplied

by the dozens of similar cases across the country.

This lawsuit fails multiple times over. It discards

a century of precedent recognizing that States have no

authority to regulate emissions that originate beyond

their borders—i.e., “crossborder” emissions—a rule

that results from a constitutional structure that

created a union of equally sovereign States and one

federal government. It misreads the Clean Air Act to

discover an expansive new state power that Congress

never conferred. And it conflicts with the longstanding

5

approach of Congress and Presidents of both political

parties to address international emissions through

cooperative, negotiated agreements and voluntary

commitments rather than the unilateral, coercive

liability that Respondents pursue here.

Straightforward application of precedent resolves

this case on any of these grounds. This Court should

reverse.

ARGUMENT

I.

THIS CASE SEEKS TO REGULATE GREENHOUSEGAS EMISSIONS AROUND THE WORLD

Respondents advance an extraordinary theory

that Colorado may reach beyond its borders and apply

state law to global greenhouse-gas emissions. If

accepted, that theory would permit every State to

project its own emissions standards to regulate

conduct everywhere. And it would obstruct activities,

such as the production, refining, marketing, sale, and

use of oil and gas, that are lawful where they occur

and often already subject to extensive federal

regulation under statutes like the Clean Air Act.

No one disputes that States have some authority

over emissions that originate within their borders and

harm local air quality, but that is not what this case

is about. Respondents instead seek relief for the

purported “alteration of the climate” caused by

greenhouse-gas emissions from around the world. Am.

Compl. ¶¶ 1–2, 5–6, 62, 81–82, Bd. of Cnty. Comm’rs

of Boulder Cnty. v. Suncor Energy (U.S.A.), Inc.,

No. 2018CV30349 (Colo. Dist. Ct. June 11, 2018).

They attribute these emissions to the worldwide oil

and gas activities of Petitioners, including operations

6

in “the Canadian tar sands,” id. ¶¶ 384–88, 397;

“Latin America,” id. ¶ 389; and “Angola, Canada,

Qatar, Russia and the United Arab Emirates,” id.

¶ 394.

The crossborder nature of these claims is

inescapable. For perspective, only approximately

0.23% of greenhouse-gas emissions today originate in

Colorado. 2 Roughly 10% originate in the United

States. 3 The rest—including the vast majority of

emissions that Respondents blame for their alleged

harms—originate in foreign countries.

The Colorado Supreme Court reasoned that

because Respondents merely “seek damages from

upstream producers for harms stemming from the

production and sale of fossil fuels,” they “do not seek

to regulate [greenhouse-gas] emissions.” Pet.App.17a,

2 Compare Colo. Air Pollution Control Div., Dep’t of Pub. Health

& Env’t, Colorado Statewide Inventory of Greenhouse Gas

Emissions and Sinks 2025, Executive Summary Report 1 n.1, 3

(Dec. 2025), https://oitco.hylandcloud.com/POP/DocPop/DocPop

.aspx?docid=57931862 (estimating 2023 Colorado emissions of

118 million metric tons of carbon-dioxide equivalent), with

Historical GHG Emissions, ClimateWatch, https://perma.cc/

2MKP-HFJE (last visited May 11, 2026) (estimating 2023 global

emissions of 50.8 gigatons carbon-dioxide equivalent). Although

Colorado is responsible for approximately 0.23% of emissions, it

is home to only about 0.073% of the world population. See World

Population Review, https://perma.cc/W8K8-L8FP (last visited

May 21, 2026) (estimating a world population of 8,298,979,488);

Colorado, World Population Review, https://perma.cc/HJ63XJNS (last visited May 21, 2026) (estimating a Colorado

population of 6,036,620).

3 See Historical GHG Emissions, supra note 2 (estimating 2023

global and U.S. emissions of 50.8 and 5.44 gigatons carbondioxide equivalent, respectively).

7

21a. That is wrong. The claims here necessarily

regulate greenhouse-gas emissions because they

would assign liability based on the alleged effects of

those emissions. Respondents thus seek to impose

their own damages-backed rules on when and to what

extent greenhouse-gas emissions are appropriate,

regardless of whether those emissions result from

conduct that Congress has permitted.

This Court has repeatedly recognized that

“remedies form an ingredient of any integrated

scheme of regulation,” and “[t]he obligation to pay

compensation can be, indeed is designed to be, a

potent method of governing conduct and controlling

policy.” San Diego Building Trades Council v.

Garmon, 359 U.S. 236, 247 (1959). Thus “regulation

can be as effectively exerted through an award of

damages as through some form of preventive relief.”

Id. And even a “salutary effort to redress private

wrongs or grant compensation for past harm” does not

change that conclusion. Id.; see also Montgomery v.

Caribe Transp. II, LLC, No. 24-1238, 2026 WL

1336188, at *4 (U.S. May 14, 2026); Kurns v. R.R.

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

of N. Am., Inc. v. Gore, 517 U.S. 559, 572–73 (1996);

Cipollone v. Liggett Grp., Inc., 505 U.S. 504, 521

(1992).

The sheer magnitude of the damages sought here

confirms that point. Respondents demand payment

for a breathtaking array of alleged past, present, and

future injuries, likely exceeding “billions of dollars.”

See Am. Compl. ¶¶ 450, 532–35, supra (seeking

compensation to manage pest infestations; respond to

wildfires; provide medical treatment for heat-induced

illness; rebuild roads and bridges; design and

construct “alternative” buildings; replace existing

8

flood control; offset reduced agricultural production

and employee productivity; educate the public; and

analyze and monitor impacts, among other things).

Plaintiffs in a similar suit alleged a price tag of “at

least $50 Billion” for a single county. Second Am.

Compl. at 211, County of Multnomah v. Exxon Mobil

Corp., No. 23-CV-25164 (Or. Cir. Ct. Oct. 7, 2024).

Damages of that scale would restructure the

American energy industry, if not bankrupt it

altogether when multiplied across dozens of similar

suits around the country.

Reviewing similar claims, the Supreme Court of

Maryland observed that the “nature and scope of the

damages sought by the local governments … reflect

that their claims seek to regulate conduct outside [the

State] that is causing global warming.” Mayor & City

Council of Baltimore v. B.P. P.L.C., 353 A.3d 1142,

1174 (Md. 2026). The same is true here. “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 1173; see also Pet.App.32a

(Samour, J., joined by Boatright, J., dissenting).

II. CROSSBORDER EMISSIONS ARE THE EXCLUSIVE

DOMAIN OF FEDERAL LAW

A. Constitutional Structure and a Century

of Precedent Dictate That Disputes Over

Crossborder Emissions Require Federal

Rules of Decision

For more than a century, this Court has

consistently held that disputes over crossborder

emissions are governed by federal law. States are

not—and never have been—free to impose their own

9

law and policy preferences on emissions originating

beyond their borders. That settled rule follows directly

from the relationship among the States, and between

the States and the federal government, established by

the Constitution.

“After independence, the States considered

themselves fully sovereign nations.” Franchise Tax

Bd. of Cal. v. Hyatt, 587 U.S. 230, 237 (2019). That

changed in 1789 when the Constitution transformed

“the States from a loose league of friendship into a

perpetual Union based on the ‘fundamental principle

of equal sovereignty among the States,’” id. at 246

(quoting Shelby County v. Holder, 570 U.S. 529, 544

(2013)), subject to a new “national government,” New

Hampshire v. Louisiana, 108 U.S. 76, 90 (1883).

That adjustment is manifest throughout the

Constitution. For example, Article I denies States “the

traditional diplomatic and military tools that foreign

sovereigns possess,” Hyatt, 587 U.S. at 245, and

Articles I and II vest those powers in Congress and the

President, see, e.g., U.S. Const. art. I, § 8, cl. 3

(Commerce Clause), cl. 4 (Naturalization Clause),

cl. 10 (Define and Punish Clause), cl. 11 (Declare War

Clause); id. art. II, § 2, cl. 2 (Treaty and Appointments

Clauses). “Article IV also imposes duties on the

States” towards one another that were “not required

by international law.” Hyatt, 587 U.S. at 245–46

(discussing U.S. Const. art. IV, § 1 (Full Faith and

Credit Clause), § 2 (Privileges and Immunities

Clause)). And Article VI subordinates state to federal

law, declaring the “Constitution, and the Laws of the

United States … the supreme Law of the Land.” U.S.

Const. art. VI, cl. 2.

10

This structure also “implies certain constitutional

limitations on the sovereignty” of each State. Hyatt,

587 U.S. at 245 (cleaned up); see, e.g., id. at 247

(“There are many other constitutional doctrines that

are not spelled out in the Constitution but are

nevertheless implicit in its structure and supported by

historical practice[.]”); Burnet v. Brooks, 288 U.S. 378,

401 (1933) (“The limits of state power are defined in

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

Federal Union.”). As relevant here, the Constitution

“implicitly forbids” the application of state law when

States have “conflicting rights,” Hyatt, 587 U.S. at 246

(cleaned up), including where “the interstate or

international nature of the controversy makes it

inappropriate for state law to control,” Tex. Indus.,

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

(1981); see also 1 St. George Tucker, Blackstone’s

Commentaries, app. note D, at 152 (1803) (“[T]he

municipal laws of no one state can be resorted to as a

general rule for the rest.”); 3 Joseph Story,

Commentaries on the Constitution 7–8 (1833)

(explaining certain issues, such as bankruptcy, are

“incapable of being redressed by the states” because of

their interstate nature). Those disputes require

federal rules of decision, and “[i]n absence of an

applicable Act of Congress it is for the federal courts

to fashion the governing rule of law according to their

own standards,” i.e., federal common law. Clearfield

Tr. Co. v. United States, 318 U.S. 363, 367 (1943).

This Court has long recognized that disputes over

crossborder air and water emissions—including

pollutants or effluents—are among those requiring

federal rules of decision. In Missouri v. Illinois

(“Missouri I”), 180 U.S. 208 (1901), Missouri filed an

original action to enjoin the discharge of sewage from

11

Chicago into an interstate river system. This Court

held the case could proceed, explaining that “[i]f

Missouri were an independent and sovereign state all

must admit that she could seek a remedy by

negotiation, and, that failing, by force.” Id. at 241. But

“[d]iplomatic powers and the right to make war

having been surrendered to the general government,

it was to be expected that upon the latter”—i.e., the

federal government—“would be devolved the duty of

providing a remedy, and that remedy, we think, is

found in the constitutional provisions” of Article III.

Id.

The Court emphasized the need for federal law

when revisiting the dispute five years later in

Missouri v. Illinois (“Missouri II”), 200 U.S. 496

(1906). The controversy was an issue “of international

importance,” the Court explained, which “must follow

and apply [federal] rules, even if legislation of one or

both of the states seems to stand in the way.” Id. at

518, 520.

The Court extended its reasoning in Missouri I

and Missouri II to air emissions the very next term.

Georgia v. Tennessee Copper Co., 206 U.S. 230 (1907),

involved an original action by Georgia to enjoin copper

companies operating in Tennessee “from discharging

noxious gas” that drifted across state lines. Id. at 236.

The Court explained that “by their union,” States

“made the forcible abatement of outside nuisances

impossible to each.” Id. at 237. The Constitution,

instead, provided for a “suit in this Court,” governed

by federal common law. Id. Indeed, that these early

disputes over interstate air and water emissions were

original actions in this Court underscores the

“conflicting rights” of the States involved, Hyatt, 587

U.S. at 246 (cleaned up), and confirms the need for

12

federal rules of decision, notwithstanding any state

law. See also New York v. New Jersey, 256 U.S. 296

(1921); North Dakota v. Minnesota, 263 U.S. 365

(1923); New Jersey v. New York, 283 U.S. 473 (1931).

Since then, “a mostly unbroken string of cases has

applied federal law to disputes involving interstate air

or water pollution.” City of New York v. Chevron, 993

F.3d 81, 91 (2d Cir. 2021) (collecting cases). This Court

has explained:

Federal common law and not the varying

common law of the individual States is …

entitled and necessary to be recognized as a

basis for dealing in uniform standard with the

environmental rights of a State against

improper impairment by sources outside its

domain.

Illinois v. City of Milwaukee (“Milwaukee I”), 406 U.S.

91, 107 n.9 (1972) (quoting Texas v. Pankey, 441 F.2d

236, 241 (10th Cir. 1971)). That is because “[w]hen we

deal with air and water in their ambient or interstate

aspects,” “there is an overriding federal interest in the

need for a uniform rule of decision” and “the

controversy touches basic interests of federalism.” Id.

at 103, 105 n.6.

This Court was clear that only federal law “can

provide an adequate means for dealing with such

claims,” and federal common law applies “[u]ntil the

field has been made the subject of comprehensive

legislation or authorized administrative standards.”

Id. at 107 n.9 (quoting Pankey, 441 F.2d at 241).

Indeed, federal common law in this area existed

precisely because “state law cannot be used” given the

“significant conflict between … federal policy or

13

interest and the use of state law” in crossborder air

and water disputes. City of Milwaukee v. Illinois

(“Milwaukee II”), 451 U.S. 304, 313 & n.7 (1981)

(cleaned up). 4

International Paper Co. v. Ouellette, 479 U.S. 481

(1987), affirmed these principles in detail. Surveying

its case law, the Court reiterated that “regulation of

interstate water pollution is a matter of federal, not

state, law.” Id. at 488. The “implicit corollary” of its

holding that interstate disputes over water pollution

“should be resolved by reference to federal common

law,” the Court explained, is that “state common law

was preempted.” Id.

The Court continued that “federal common law

governed … interstate water” until Congress passed

the Clean Water Act, which “now occupied the field”

just as federal common law had previously. Id. at 487,

489. In other words, the Clean Water Act created a

“comprehensive” and “all-encompassing program of

water pollution regulation,” and any claims under

state law must be “specifically preserved” in the

saving clauses of the statute. Id. at 492 (cleaned up).

The Court then interpreted the Clean Water Act’s

saving clauses to allow only claims brought “pursuant

to the law of the source State,” consistent with

4 In Ohio v. Wyandotte Chemicals Corp., 401 U.S. 493 (1971), this

Court declined to exercise original jurisdiction over a suit by Ohio

against out-of-state chemical makers that it claimed were

dumping mercury into streams that reached Lake Erie,

suggesting the claims “would have to be adjudicated under state

law.” Id. at 498 n.3. That aspect of the decision was unanimously

rejected the following year. See Milwaukee I, 406 U.S. at 102 n.3.

The short-lived nature of that foray reinforces that state law

cannot apply to crossborder emissions.

14

preexisting federal common law. Id. at 497–99. A

contrary decision would have allowed “a number of

different states to have independent and plenary

regulatory authority over a single discharge,” leading

to a “chaotic confrontation between sovereign states”

that Congress could not have intended. Id. at 496

(cleaned up). As a result, the Court concluded, the

Clean Water Act “pre-empts state law to the extent

that the state law is applied to an out-of-state point

source.” Id. at 500; see also Milwaukee II, 451 U.S. at

317–18.

Most recently, in American Electric Power Co.

(“AEP”) v. Connecticut, 564 U.S. 410 (2011), the Court

continued this precedent in the context of a dispute

about the alleged harms of greenhouse-gas emissions.

Explaining that “air and water in their ambient or

interstate aspects” are “meet for federal law

governance,” this Court was clear that “borrowing the

law of a particular State” to govern disputes over

crossborder emissions “would be inappropriate.” Id. at

421–22 (cleaned up).

Throughout these cases, this Court has often

compared disputes over air and water emissions to the

“apportionment of interstate waters” and “questions

of boundaries,” subjects which indisputably require

federal rules of decision (or state compacts approved

by Congress). Milwaukee I, 406 U.S. at 105 (citing

Hinderlider v. La Plata River & Cherry Creek Ditch

Co., 304 U.S. 92, 110 (1938); Kansas v. Colorado, 206

U.S. 46, 98 (1907)). 5 The Court has further explained

See also Hinderlider, 304 U.S. at 110 (“Jurisdiction over

controversies concerning rights in interstate streams is not

5

(continued)

15

what makes air and water cases unique from many

other contexts—the relevant conduct “reaches,

through the agency of natural laws, into the territory

of another state,” which requires a rule of decision

that “will recognize the equal rights of both [States]

and at the same time establish justice between them.”

Kansas, 206 U.S. at 97–98 (emphasis added)

(discussing Missouri I, 180 U.S. 208).

These considerations and the corresponding need

for federal law are even more applicable in cases about

the effect of greenhouse-gas emissions on global

climate. “Greenhouse gases once emitted become well

mixed in the atmosphere,” and “emissions in New

Jersey may contribute no more to flooding in New

York than emissions in China.” AEP, 564 U.S. at 422

(cleaned up). The intermingled nature of these

emissions underscores the need for a federal rule of

decision, rather than allowing a patchwork of

disparate state regimes.

Much of the reasoning in these cases also applies

with equal force to international emissions. If States

lack authority to regulate emissions originating in

other States, it would be absurd to conclude they could

somehow apply their state laws to emissions

originating in other countries. This Court has never

different from those concerning boundaries. These have been

recognized as presenting federal questions.”); Kansas, 206 U.S.

at 97–98 (discussing Missouri I, 180 U.S. 208); cf. The Federalist

No. 80, at 477–78 (Alexander Hamilton) (Clinton Rossiter ed.,

1961) (“But there are many other sources, besides interfering

claims of boundary, for which bickering and animosities may

spring up among the members of the Union…. Whatever

practices may have a tendency to disturb the harmony between

the States, are proper objects of federal superintendence and

control.”).

16

suggested that nonsensical result. To the contrary, in

Milwaukee I, this Court emphasized the “overriding

federal interest in the need for a uniform rule of

decision” by relying on case law about exclusive

federal authority over an international matter. 406

U.S. at 105 n.6 (citing Banco Nacional de Cuba v.

Sabbatino, 376 U.S. 398, 421–27 (1964)). It would be

passing strange if state law somehow had greater

purchase in an area implicating the “exclusive

authority” of the federal government “in international

relations and with respect to foreign intercourse and

trade.” Fuld v. Palestine Liberation Org., 606 U.S. 1,

15 (2025) (cleaned up); see infra Part III.

Crossborder emissions are thus “undoubtedly”

one of the areas “where the basic scheme of the

Constitution … demands” the use of federal law. AEP,

564 U.S. at 421–22.

B. The Clean Air Act Is a Comprehensive

Scheme that Preserves Federal Authority

Over Crossborder Emissions

Against that backdrop, Congress legislated the

Clean Air Act as a detailed scheme “to protect and

enhance the quality of the Nation’s air resources.” 42

U.S.C. § 7401(b)(1). To call the Act “comprehensive

would be an understatement.” North Carolina ex rel.

Cooper v. Tenn. Valley Auth., 615 F.3d 291, 298 (4th

Cir. 2010).

AEP held that, in regulating air emissions,

Congress “displaced” the corresponding federal

common law. 564 U.S. at 423–24. This confirmed the

comprehensive nature of the Act. See Milwaukee I,

406 U.S. at 107 n.9 (explaining federal common law

applies “[u]ntil the field has been made the subject of

17

comprehensive

legislation

or

authorized

administrative standards” (cleaned up)). The Act did

not—expressly or impliedly—authorize a new and

expansive role for States to regulate an area from

which they have long been excluded.

1. The Clean Air Act Confirms Federal

Authority Over Crossborder Emissions

Far from disrupting the traditional allocation of

authority over crossborder emissions recognized by

this Court, the text and structure of the Clean Air Act

reinforce it. The Act retains federal authority over

interstate and international air concerns, while

assigning States a circumscribed role in regulating

emissions that originate within their borders.

The cornerstone of the Act is Title I, 42 U.S.C.

§§ 7401–7515, which directs the U.S. Environmental

Protection Agency (“EPA”) to set national standards

for maximum levels of air pollutants as necessary to

“protect the public health” and “welfare,” id.

§ 7409(b). States, in turn, determine how to meet

those standards within their borders, but must

submit their “implementation plans” to EPA for

approval. Id. § 7410(a), (l). If a State does not attain

or maintain the national air quality standards, EPA

can impose sanctions, id. § 7509; mandate plan

revisions, id. §§ 7410(k)(5), 7502(b)–(d); or impose a

federal plan, id. § 7410(c). Title I further regulates

emissions from new or modified “stationary” sources,

such as power plants, using a similar model. Id.

§ 7411.

Title V, id. §§ 7661–7661f, establishes a

permitting program for large emissions sources that

works together with Title I. EPA sets minimum

18

permitting standards, and States develop and

implement permitting programs for in-state sources,

subject to EPA approval and oversight. Id. § 7661a.

Congress further designed the Act to address

crossborder emissions. The so-called “good neighbor”

provision requires each State’s implementation plan

to prohibit emissions that will “contribute

significantly to nonattainment in, or interfere with

maintenance” of an air quality standard by another

State. Id. § 7410(a)(2)(D)(i). States must also notify

neighboring States if a proposed new source could

affect their air quality. Id. § 7661d(a)(2); see also id.

§ 7426(a)(1) (requiring new sources to notify “all

nearby States” prior to construction).

A State that believes emissions from sources in a

neighboring State violate the good neighbor provision

can petition EPA. Id. § 7426(b). If EPA agrees, it can

require the neighboring State to amend its

implementation plan, id. § 7410(k)(5), or adopt a

federal plan, id. § 7410(c). If a State is unsatisfied

with EPA’s resolution, it can sue in a federal court of

appeals. Id. § 7607(b)(1); see, e.g., Maryland v. EPA,

958 F.3d 1185 (D.C. Cir. 2020) (challenging EPA’s

denial of petitions).

Relatedly, EPA can establish—on its own or by

petition from a State—an “interstate … transport

region” when there is “reason to believe that the

interstate transport of air pollutants from one or more

States” impedes another State from meeting the

national air quality standards. 42 U.S.C. § 7506a(a).

Each interstate transport region has a “commission,”

comprising representatives from EPA and each State

within the region, to assess and recommend

“strategies for mitigating the interstate pollution.” Id.

19

§ 7506a(b). A commission can ask EPA to find that an

implementation plan for a State within the region

does not meet the good-neighbor requirements and

must be revised. Id. § 7506a(c). 6 In addition, Congress

provided for States to cooperatively address air

concerns by entering into agreements, with certain

restrictions, contingent on approval from Congress.

Id. § 7402(c).

The Act also addresses international emissions.

When the Secretary of State or an international

agency informs EPA that emissions originating from

a State affect a foreign country, the agency must

notify the State and can require the State to revise its

implementation plan to mitigate the emissions, with

the affected foreign country invited to participate in

related hearings. Id. § 7415(a)–(b). This section only

applies if the foreign country gives the United States

essentially the same rights in relation to emissions

originating in that country. Id. § 7415(c).

The Act’s other substantive titles address issues

that inherently cross state borders and so reflect the

same preeminent federal authority. Title II directs

EPA to set nationwide standards for emissions from

mobile sources—like cars, trucks, trains, and

aircraft—that regularly transit state lines. Id.

§§ 7521(a)(1), 7547, 7571. With one exception, States

are expressly prohibited from adopting or attempting

to enforce their own laws or regulations related to

6 Congress included additional instructions for interstate

transport regions for ozone—a reactive gas with regional

effects—and established a “single transport region for ozone”

comprising the District of Columbia and eleven States in the

northeast. 42 U.S.C. § 7511c(a).

20

emissions from these sources. Id. §§ 7543, 7573. 7 And

Titles IV and VI direct EPA to oversee federal

programs to reduce regional emissions that result in

acid rain and to phase out substances that contribute

to the global depletion of stratospheric ozone,

respectively. See id. §§ 7651–7651o (acid rain),

§§ 7671–7671q (ozone depletion). A State dissatisfied

with EPA’s actions in these programs can petition for

rulemaking, 5 U.S.C. § 553(e); participate in EPA’s

administrative process, id. § 553(c); 42 U.S.C.

§ 7607(d); and, ultimately, sue in federal court, 42

U.S.C. § 7607(b)(1).

Taken together, the Act’s provisions operate as

“an harmonious whole” to preserve federal authority

over matters that implicate multiple States or

demand a national solution, while allowing States to

regulate emissions originating within their own

borders. Roberts v. Sea-Land Servs., Inc., 566 U.S. 93,

100 (2012). Nothing in the Act, whether read alone or

in combination, extends state law to crossborder

emissions.

7 In limited circumstances, California can set its own emissions

standards for some in-state vehicles and engines with EPA

approval, 42 U.S.C. § 7543(b), (e)(2)(A), and other States can opt

in to those standards, id. §§ 7507, 7543(e)(2)(B).

Title II also permits EPA to regulate fuels and fuel additives that

may affect mobile source emissions. Id. § 7545(a). States are

generally prohibited from issuing their own such regulations

without EPA approval. Id. § 7545(c)(4)(A), (C). California, again,

has a special exception. Id. § 7545(c)(4)(B).

21

2. Statutory Interpretation Confirms Congress

Did Not Authorize New State Power Over

Crossborder Emissions

Principles of statutory interpretation confirm the

Clean Air Act did not authorize new state power over

crossborder emissions. See Bond v. United States, 572

U.S. 844, 857 (2014) (“Congress legislates against the

backdrop of certain unexpressed presumptions.”

(cleaned up)).

First, unless “a statutory purpose to the contrary

is evident,” this Court presumes that Congress does

not lightly discard “long-established and familiar

principles.” Texas v. United States, 507 U.S. 529, 534

(1993) (cleaned up). Congress legislated the Act

against the backdrop of the basic structure of the

Constitution and longstanding precedent that

disputes over crossborder emissions demand a

uniform federal rule. See, e.g., H.R. Rep. No. 95-294,

at 329 (1977) (discussing Tenn. Copper Co., 206 U.S.

230). Nothing in the Act indicates that Congress

departed from that baseline. See Learning Res., Inc. v.

Trump, 146 S. Ct. 628, 672 (2026) (Barrett, J.,

concurring) (“background legal conventions and

constitutional structure inform the most natural

reading of a statute”).

Second, to “significantly alter the balance

between federal and state power,” Congress must use

“exceedingly clear language.” U.S. Forest Serv. v.

Cowpasture River Pres. Ass’n, 590 U.S. 604, 621–22

(2020); see also Gregory v. Ashcroft, 501 U.S. 452, 460

(1991) (“If Congress intends to alter the usual

constitutional balance between the States and the

Federal Government, it must make its intention to do

so unmistakably clear in the language of the statute.”

22

(cleaned up) (emphasis added)). This “requirement of

clear statement assures that the legislature has in

fact faced, and intended to bring into issue” the

asserted change in “the federal balance.” Bond, 572

U.S. at 858 (quoting United States v. Bass, 404 U.S.

336, 349 (1971)). If Congress intended the Act to

fundamentally change the relationship between the

federal government and the States by giving States

authority to regulate emissions throughout the

country and around the world, then it needed to use

exceedingly clear language. It did not.

The Colorado Supreme Court wrongly invoked a

different interpretive principle—the “presumption

against preemption”—to conclude that state law could

apply in this case. Pet.App.11a–12a. But this Court’s

traditional framework for statutory preemption “is

applicable only where the overlapping, dual

jurisdiction of the Federal and State Governments

makes it necessary to decide which law takes

precedence.” Parker Drilling Mgmt. Servs., Ltd. v.

Newton, 587 U.S. 601, 610 (2019). Because “state law

has never applied of its own force” to crossborder

emissions, this case “does not present the ordinary

question in pre-emption cases.” Id. Each of

Respondents’ claims in some way turns on crossborder

emissions, no matter how packaged. See supra Part I.

Moreover, “when the State regulates in an area

where there has been a history of significant federal

presence,” the “‘assumption’ of nonpre-emption is not

triggered.” United States v. Locke, 529 U.S. 89, 108

(2000); see also Buckman Co. v. Plaintiffs’ Legal

Comm., 531 U.S. 341, 347–48 (2001). The field of

crossborder emissions claims has always been

governed by federal law—first federal common law,

and now the Clean Air Act. See Native Vill. of Kivalina

23

v. ExxonMobil Corp., 696 F.3d 849, 857 (9th Cir. 2012)

(“If the federal common law cause of action has been

displaced by legislation, that means that ‘the field has

been made the subject of comprehensive legislation’

by Congress.” (quoting Milwaukee II, 451 U.S. at

314)). That leaves no room for claims under state law

like those Respondents bring here.

3. The Clean Air Act’s Saving Clauses Do Not

Authorize Respondents’ Claims

The Colorado Supreme Court invoked the Clean

Air Act’s “saving” clauses, 42 U.S.C. §§ 7416, 7604(e),

to illustrate potential room for state law. Pet.App.14a.

Those clauses do not permit anything like what

Respondents attempt here.

Section 7416 provides that “nothing” in the Act—

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[.]

42 U.S.C. § 7416. That text does not extend new

powers to States. It is premised on existing “right[s].”

Id. When Congress passed the Act, States had no right

to enforce their own “standard,” “limitation,” or

“requirement” for crossborder emissions, and § 7416

does not change that. See supra Part II.A.

If there were any doubt, the title of the provision

removes it. Dubin v. United States, 599 U.S. 110, 120–

21 (2023) (“[T]he title of a statute and the heading of

a section are tools available for the resolution of a

24

doubt about the meaning of a statute.” (cleaned up)).

Section 7416 is titled “Retention of State Authority.”

42 U.S.C. § 7416 (emphasis added). To “retain”

something is to “keep” what is already in “one’s

possession.” American Heritage Dictionary of the

English Language 1109 (1969). A State cannot retain

a power it never possessed.

Relying on § 7604(e) fails for similar reasons. As

part of the Act’s section permitting citizen suits, it

provides—

Nothing in this section shall restrict any right

which any person (or class of persons) may

have under any statute or common law to seek

enforcement of any emission standard or

limitation or to seek any other relief

(including relief against the Administrator or

a State agency).

42 U.S.C. § 7604(e).

This text again only preserves “right[s]” that

otherwise exist. Id. State law did not apply to

crossborder emissions before the Act, and § 7604(e)

does not change that. Indeed, § 7604(e) is even more

limited than § 7416 because it is specifically limited to

the effect of the citizen suit provision, not the Act as a

whole.

Reading the saving clauses to impliedly extend

state authority into a traditionally federal domain is

also illogical “in their context and with a view to their

place in the overall statutory scheme.” Parker

Drilling, 587 U.S. at 608 (cleaned up). Throughout the

Act, Congress carefully identified the authorities

allocated to States, limited States to regulating

25

emissions originating within their borders, and

subjected state regulation to federal oversight. To

conclude that two “ancillary” saving clauses tacitly

authorize “in vague terms” an expansive new role for

States to regulate crossborder emissions—and outside

federal supervision—would be, at the very least, to

“hide elephants in mouseholes.” Whitman v. Am.

Trucking Ass’ns, 531 U.S. 457, 468 (2001); see also,

e.g., Cowpasture River Pres. Ass’n, 590 U.S. at 621–22.

As this Court has observed, it is “quite unlikely

that Congress would use a means so indirect as the

saving clauses … to upset the settled division of

authority by allowing States to impose additional

unique substantive regulation” in an area otherwise

committed to federal governance. Locke, 529 U.S. at

106. This Court should thus “decline to give broad

effect to [the] saving clauses” because “doing so would

upset the careful regulatory scheme established by

federal law.” Id.; see also id. (“Limiting the saving

clauses as we have determined respects the

established federal-state balance[.]”).

This Court’s analysis of the Clean Water Act’s

parallel saving clauses in Ouellette applies with full

force here. 479 U.S. at 485, 492–96 (interpreting 33

U.S.C. §§ 1365(e), 1370); see Merrick v. Diageo

Americas Supply, Inc., 805 F.3d 685, 695 (6th Cir.

2015) (“What was true for the Clean Water Act holds

true for the Clean Air Act.”). Reading § 7416 and

§ 7604(e) to extend Colorado state law to crossborder

emissions would “undermine” the “carefully drawn”

Clean Air Act by creating “an irrational system of

regulation” in which a single emissions source or

operation could face “a variety” of “‘vague’ and

‘indeterminate’” state common-law standards.

Ouellette, 479 U.S. at 494, 496 (quoting Milwaukee II,

26

451 U.S. at 317). And it would allow States to “do

indirectly what they could not do directly”—regulate

emissions that originate beyond their borders—and

set up a “chaotic confrontation between sovereign

states.” Id. at 495–96 (cleaned up).

The Clean Air Act’s saving clauses do not

authorize the use of state law that Respondents

attempt here.

III. APPLYING STATE LAW TO INTERNATIONAL

EMISSIONS INTERFERES WITH THE EXCLUSIVE

AUTHORITY OF THE FEDERAL GOVERNMENT

OVER FOREIGN AFFAIRS

Respondents do not stop at applying Colorado law

to greenhouse-gas emissions originating within their

State, or even within the United States. They insist

that Colorado state law may govern emissions around

the world. This intrudes on the exclusive authority of

the federal government over foreign affairs.

The Constitution vests all authority over foreign

affairs in the political branches of the federal

government. See supra Part II.A. It then goes further

and denies any such power to the States. U.S. Const.

art. I, § 10, cl. 1 (prohibiting states from entering any

treaty or alliance). These provisions ensure the

Nation speaks “with ‘one voice’” to the world. Am. Ins.

Ass’n v. Garamendi, 539 U.S. 396, 414 (2003).

The Founders recognized the vital need for

“uniformity in this country’s dealings with foreign

nations.” Id. at 413 (cleaned up). As James Madison

explained, “[i]f we are to be one nation in any respect,

it clearly ought to be in respect to other nations.” The

Federalist No. 42, at 264. Indeed, he thought the

27

reasons behind denying States such powers were so

obvious they “need no explanation.” The Federalist

No. 44, at 281.

This Court has been clear that “[n]o State can

rewrite our foreign policy to conform to its own

domestic policies,” because “[p]ower over external

affairs is not shared by the States; it is vested in the

national government exclusively.” United States v.

Pink, 315 U.S. 203, 233 (1942). It is thus beyond

question that where “there is evidence of clear conflict

between the policies adopted by” the federal

government and a State in foreign affairs, “state law

must give way.” Garamendi, 539 U.S. at 421.

Congress and the President have long recognized

that the “global nature” of the effects of greenhousegas emissions requires “international cooperation”

and “a coordinated national policy.” Global Climate

Protection Act of 1987, Pub. L. No. 100-204, § 1102(5)–

(6), 101 Stat. 1331, 1408 (codified at 15 U.S.C. § 2901

note (Global Climate Protection)). And they have

agreed that “United States policy should seek to …

foster cooperation among nations” and “work toward

multilateral agreements.” Id. § 1103(a)(2), (4), 101

Stat. at 1408.

Congress accordingly directed the “President,

through the Environmental Protection Agency,” to “be

responsible for developing and proposing to Congress

a coordinated national policy on global climate

change,” and to work through the Secretary of State

“to coordinate those aspects of United States policy

requiring action through the channels of multilateral

diplomacy.” Id. § 1103(b), (c), 101 Stat. at 1408–09.

Although there is disagreement about the appropriate

role for the President in setting such policy, this

28

framework—like the Clean Air Act’s provisions for

international emissions, 42 U.S.C. § 7415—clearly

leaves no role for the States.

The political branches have thus resolved issues

related

to

international

emissions

through

multilateral, nation-to-nation agreements. 8 They

have approached greenhouse gases the same way,

opting for negotiated agreements and voluntary

commitments over coercive, unilateral action. These

agreements retain flexibility that allows countries—

including the United States—to balance emissions

policy with other priorities, such as the economy,

energy independence, and national security.

For example, in 1992, President George H.W.

Bush signed, and the Senate ratified, the United

Nations Framework Convention on Climate Change

(“UNFCCC”), which established an international

forum for cooperative action on greenhouse-gas

emissions. “[T]his international treaty provides the

structure for collaboration and evolution of efforts

over decades,” but “does not … include quantitative

and enforceable objectives and commitments for any

Party.” 9 Later Presidents have similarly entered and

exited cooperative international agreements on

See, e.g., Convention on Long-Range Transboundary Air

Pollution, T.I.A.S. No. 10,541 (1979); La Paz Agreement, T.I.A.S.

No. 10,827 (1983); Vienna Convention for the Protection of the

Ozone Layer, T.I.A.S. No. 11,097 (1985); Montreal Protocol,

T.I.A.S. No. 89-101 (1987); U.S.-Canada Air Quality Agreement,

T.I.A.S. No. 11,783 (1991).

8

9 Richard K. Lattanzio, Cong. Rsch. Serv., R46204, The United

Nations Framework Convention on Climate Change, the Kyoto

Protocol, and the Paris Agreement: A Summary 2 (Jan. 29, 2020),

https://www.congress.gov/crs-product/R46204.

29

greenhouse gases, including the UNFCCC, as the

priorities and policies of the United States have

evolved. 10

At the same time, Congress has rejected

international efforts to impose mandatory obligations

for greenhouse-gas emissions. In 1997, the Senate

unanimously repudiated the Kyoto Protocol, which

would have imposed “legally binding” emissions

targets on the United States and similarly situated

nations. S. Res. 98, 105th Cong., 143 Cong. Rec. 15808

(1997). Among other things, the Senate expressed

concern over emissions requirements that could

“result in serious harm to the United States economy,

including significant job loss, trade disadvantages,

increased energy and consumer costs, or any

10 In 2016, President Obama signed an executive agreement for

the United States to undertake nonbinding emissions reductions

targets under the UNFCCC Paris Agreement. Remarks

Announcing the Formal Entry of the United States into the

UNFCCC Paris Agreement in Hangzhou, China, 2 Pub. Papers

of Barak Obama 1122, 1122–24 (Sept. 3, 2016), https://perma.cc/

9K57-4YX5. The Trump Administration provided the United

Nations with notice of withdrawal from the Agreement in 2019.

U.S. Dep’t of State, Press Release, On the U.S. Withdrawal from

the Paris Agreement (Nov. 4, 2019), https://perma.cc/JB2G4NWX. The Biden Administration rejoined in 2021. U.S. Dep’t of

State, Press Release, The United States Officially Rejoins the

Paris Agreement (Feb. 19, 2021), https://perma.cc/XJ66-ZP6F.

President Trump withdrew again in 2025. Exec. Order No.

14162, Putting America First in International Environmental

Agreements, 90 Fed. Reg. 8455 (Jan. 30, 2025). In January 2026,

President Trump announced that the United States would also

withdraw from the UNFCCC. Memorandum of January 7, 2026,

Withdrawing

the

United

States

from

International

Organizations, Conventions, and Treaties That Are Contrary to

the Interests of the United States, 91 Fed. Reg. 2281 (Jan. 16,

2026).

30

combination thereof.” Id. Congress then barred the

EPA from implementing or funding the Protocol, Act

of October 27, 2000, Pub. L. No. 106-377, app. A, 114

Stat. 1441, 1441A-41; id., app. B, § 604, 114 Stat. at

1441A-85, and clarified the Protocol could not

interfere with other national interests, such as

military preparedness, see Strom Thurmond National

Defense Authorization Act for Fiscal Year 1999, Pub.

L. No. 105-261, § 1232, 112 Stat. 1920, 2155 (1998).

Congress made clear its position that the United

States should not pursue emissions reductions at all

costs, but must balance emissions policy with other

national interests. 11

Respondents would take a radically “different

tack,” however, and “use an iron fist where the

[federal government] has consistently chosen”

Federal domestic policy has been similar, with Congress

consistently opting for incentives over mandates, while

accommodating other national priorities. In just the last few

years, bipartisan majorities of Congress have passed dozens of

provisions aimed at reducing greenhouse-gas emissions while

advancing economic growth, energy independence, and national

security. This includes funding research into energy storage;

securing supply chains for critical minerals; and accelerating the

commercialization of affordable, safe, and clean nuclear energy.

See, e.g., Accelerating Deployment of Versatile, Advanced

Nuclear for Clean Energy Act of 2024, Pub. L. No. 118-67, div. B,

138 Stat. 1447, 1448–82; Energy Act of 2020, Pub. L. No. 116260, div. Z, 134 Stat. 1182, 2418–2615; Utilizing Significant

Emissions with Innovative Technologies Act, Pub. L. No. 116260, div. S, §§ 101–102, 134 Stat. at 2243–55 (2020); Nuclear

Energy Innovation and Modernization Act, Pub. L. No. 115-439,

132 Stat. 5565 (2019). And when Congress has opted for coercive

regulation, it has been measured, targeted, and ultimately

delayed. See, e.g., 42 U.S.C. § 7436(c)–(h) (imposing a fee on

emissions of methane—a greenhouse gas—above a specified

threshold from a subset of oil and gas facilities).

11

31

another approach. Garamendi, 539 U.S. at 423, 427.

Rather than embracing international cooperation and

voluntary commitments that balance competing

national priorities, Respondents seek to impose “a

different, state system of economic pressure” that

unilaterally compels global emissions reductions by

penalizing the worldwide production, marketing, and

sale of oil and gas. Id. at 423 (quoting Crosby v. Nat’l

Foreign Trade Council, 530 U.S. 363, 376 (2000)).

They substitute their own judgment for that of

Congress and the President, and attempt to dictate

emissions policy not just for the United States, but for

countries around the world.

The claims in this lawsuit, and others like it, have

far “more than incidental effect in conflict with

express foreign policy of the National Government” on

crossborder emissions. Id. at 420. They undermine the

international cooperation and national selfdetermination that has been the hallmark of federal

emissions policy for decades. “[D]oing so undercuts …

diplomatic discretion and the choice” made by

Congress and the President to address international

emissions in that manner, rather than through

unilateral coercive action against energy producers.

Id. at 423–24. The result is a “clear conflict” in

means—if

not

goals—for

addressing

global

greenhouse-gas emissions. Id. at 421.

The Constitution does not permit Respondents to

engage in this significant, purposeful foray into

foreign affairs. “Governmental power over external

affairs is not distributed, but is vested exclusively in

the national government.” United States v. Belmont,

301 U.S. 324, 330 (1937). Therefore, “[o]ur system of

government … imperatively requires that federal

power in the field affecting foreign relations be left

32

entirely free from local interference.” Hines v.

Davidowitz, 312 U.S. 52, 63 (1941). A state law that

“has a direct impact upon foreign relations and may

well adversely affect the power of the central

government to deal with those problems” is foreclosed.

Zschernig v. Miller, 389 U.S. 429, 441 (1968).

Respondents would sweep aside decades of

delicate nation-to-nation negotiations that have

balanced emissions reductions against the economy,

energy policy, and national security, and would

override Congress’s rejection of unilateral mandatory

liability schemes. And if they are allowed to proceed,

more suits under other state laws would inevitably

follow, each imposing its own standards. The world

would hear not one national voice, but a cacophony of

competing state commands. The Court should reject

this attempt by Respondents to establish their “own

foreign policy” from a Boulder courthouse. Id.

CONCLUSION

This Court should reverse the judgment below.

Respectfully submitted,

MAY 21, 2026

MICHAEL BUSCHBACHER

JARED M. KELSON

Counsel of Record

LAURA B. RUPPALT

BOYDEN GRAY PLLC

800 Connecticut Ave NW,

Suite 900

Washington, DC 20006

(202) 955-0620

jkelson@boydengray.com