Diplomacy Without D.C.: The Legal Case for State-Level Climate Diplomacy

The formation of the UN Framework Convention on Climate Change (UNFCCC) in the 1990s signified the first movement toward international action on climate change. This landmark convention marked a universal recognition of the urgency to address issues ranging from pollution to land degradation, ultimately culminating in international agreements such as the Kyoto Protocol and the Paris Agreement. [1] The Paris Agreement achieved notable success by urging countries such as Japan, China, and those in the EU to establish carbon neutrality goals and adopt net-zero targets, progress indicative of the growing internationalization of climate action. [2] From countries striving to demonstrate technological and ideological independence in renewable energy to China and India actively resisting emission caps, current events highlight the deep entrenchment of climate action in global politics, particularly for industrialized powers and international leaders. As the second-largest manufacturing nation and the second most polluting country in the world, the United States bears a particular burden in establishing broader standards for climate commitments. [3] However, U.S. constitutional doctrine has created ambiguity surrounding the nation’s capacity to implement climate action domestically. The legal instability caused by the executive branch’s ability to join or withdraw from international agreements unilaterally undermines long-term climate accountability in the court of international opinion. Specifically, the U.S. withdrawal from the Paris Agreement under President Trump, followed by reentry under President Biden, and a subsequent withdrawal from the current administration, proves the volatility of the U.S.’s approach to entering the commitment. Despite federal ambivalence, multiple states have pursued campaigns to champion Paris commitments from the ground up. These pursuits are by no means exempt from scrutiny. In response to a U.S.-filed lawsuit in 2020 over California’s cap-and-trade carbon reduction plan, Assistant Attorney General Jeffrey Bossert Clark released the following: "The state of California has veered outside of its proper constitutional lane to enter into an international emissions agreement…The power to enter into such agreements is reserved to the federal government, which must be able to speak with one voice in the area of U.S. foreign policy.” [4] Despite federal actors under the Trump administration vehemently countering efforts by states to develop cross-border climate policies, this comment will argue that non-federal actors in the United States possess the legal autonomy to engage in the Paris Climate Agreement through subnational diplomacy, presenting a viable and often preferable alternative to federal action.

With reference to interpretations of Article 1, Section 10 of the U.S. Constitution, state international agreements have proven both permissible and effective despite challenges from the Executive Branch, foreshadowing a potential for federalism to have a greater influence on international cooperation on climate change. In the face of President Trump’s decision to withdraw from the Paris Climate Agreement, upwards of 24 U.S. states have vowed their alliance to the U.S. Climate Alliance to advance the agreement’s objectives at the state level, which broadly entail “substantially reducing global greenhouse gas emissions to hold global temperature increase to well below 2°C above pre-industrial levels and pursue efforts to limit it to 1.5°C above pre-industrial levels.” [5] As U.S. climate commitments continue to be contested by shifting administrations, recent, unprecedented court rulings have affirmed states’ international negotiating positions, absent congressional approval. In 2013, California and Quebec sought to establish a “cap-and-trade" agreement, which aimed to create a cross-border market for the trading of their exclusive emission allowances. The program imposes regulatory authority on the overall emissions produced by contributing entities. The “cap” is delegated among the sources through allowances and traded on a carbon market. Sources must track and report their emissions to surrender the equivalent number of allowances at the end of a compliance period. [6] California sought to formally harmonize its program with Quebec’s in response to the Trump administration's defiant opposition to the 1992 United Nations Framework Convention on Climate Change and its rejection of the Paris Agreement to allow for joint auctions and provide for the equivalence and interchangeability of compliance instruments. The agreement resembles a traditional treaty, with 20 definitive articles and provisions that accommodate withdrawal, amendment, and enforcement. [7] Additionally, specifications of the agreement's duration and requirements cite: “The Agreement shall enter into full force and effect on the first day of the month following the date of receipt of notification from the last of the Parties informing the other Party that the legally required measures have been completed." [8] The agreement was inherently binding because it imposed consequences for noncompliance and was based on legal enforceability. Assured that this agreement was preempted under the Foreign Affairs Doctrine and presented an apparent violation of state constitutional authority, the Trump administration filed a complaint.

In 2019, the Trump administration initiated legal proceedings against California in the U.S. District Court for the Eastern District of California in United States v. California, asserting that the state had contravened Article I, Section 10 of the Constitution, which forbids states from forming any “Treaty, Alliance, or Confederation.” [9] The plaintiff's arguments were ultimately rejected on the basis that the agreement did not impede federal foreign affairs and was therefore not subject to dormant preemption, as well as the fact that the agreement failed to constitute the components of a genuine treaty. The agreement did not encroach upon federal diplomatic directive for war, mutual government, the cessation of sovereignty, or general commercial purposes. Furthermore, the court iterated that the agreement contained no enforceable restriction against a party unilaterally modifying or terminating its obligations. “[T]he simple fact that California retains the power to modify its enacting regulations,” said the court, “means unilateral termination of California’s participation in the Agreement is possible.” [10] Regarding the Compact Clause claim, the District Court concluded that the agreement lacked all the defining indicia of a treaty. Unlike treaties, declarations, and alliances, which typically articulate non-binding principles of cooperation, subnational agreement efforts do not create enforceable legal obligations and are hence not subject to the scrutiny of the former. This determination was bolstered by the U.S. Supreme Court’s limitation of the Compact Clause to agreements that jeopardized federal sovereignty in the 1893 decision of Virginia v. Tennessee. [11] Although the Court recognized that the program did extend beyond state responsibility, the U.S. failed to provide evidence that it intruded upon federal power. The linkage between the two programs persists, having produced a surplus of monetary reward for the state of California: nearly $33 billion to fund climate solutions across the state and $15 billion in bill credits returned to utility customers. [12] In Québec, the program generated $2.2 million from 2013 to 2018, and industrial emitters in Québec reduced their emissions by almost 800,000 t CO2e between 2012 and 2014. [13] Beyond the program’s effectiveness, the Court’s determination has considerably reinforced the legal framework supporting climate diplomacy. By clarifying classifications of agreements and affirming the legitimacy of state-led diplomacy, it establishes a pathway for increased cross-border cooperation in climate policy.

Historically, city ordinances, municipal regulations, and state-level policies have functioned as impactful counterweights to federal authority, enabling subnational governments to act independently, particularly in defense of public health and safety. [14] The Cold War–a period of national crisis–exemplifies the profound force subnational diplomacy had in confronting contentious national policies. [15] Communities across the country mobilized to express their discontent with U.S. nuclear armament. Throughout the 1980s, nearly 4,000 communities in seventeen countries, including hundreds in the U.S., declared themselves "nuclear free zones" (NFZs). [16] Across the United States, case law has substantiated the legality of NFZs, further emphasizing the legal plausibility of local attempts to counteract federal action. [17] In Arthur D. Little, Inc. v. Commissioner of Health and Hospitals of Cambridge (1985), the Supreme Judicial Court of Massachusetts upheld a municipal regulation that prohibited the testing, storage, transportation, and disposal within Cambridge city limits of five highly toxic chemical warfare agents. [18] Arthur D. Little contested the city’s ordinances, citing that local restrictions on federal defense contracts are undermined by the federal government’s war and defense power. The Court ultimately ruled in favor of Massachusetts’ state police power and the validity of the Cambridge regulation. The obligation of a city government, as outlined in the case, is "to preserve the health of the inhabitants.” The court was unwilling to invalidate a health regulation unless it "conflicts with federal law or would frustrate the federal scheme" or unless the court determined that "from the totality of the circumstances that Congress sought to occupy the field to the exclusion of the States.” [18] The preservation of state power in upholding a city’s ability to protect the health of its inhabitants has broader implications for federalism, specifically in cases where local policy conflicts with efforts pursued by federal actors (i.e., Department of Defense). This case exemplifies the state’s capacity to enact policies that prioritize the health of its constituents. Given that recent environmental cases against the federal administration have often cited a violation of the fundamental right to life (Juliana v. United States (2015), Lighthiser v. Trump (2025)), it is notable that state autonomy has historically been upheld for the preservation of public health. As the current administration moves to incentivize fossil fuel production, undermine environmental justice efforts, and incapacitate the EPA—actions that collectively represent a significant reversal of not only environmental progress but also public health advancements—states and cities may have the legal authority to intervene and mitigate the risks posed to their residents.

Through the U.S. Climate Alliance, a bipartisan coalition of two dozen governors representing nearly 60 percent of the U.S. economy and 55 percent of the U.S. population, [19] several states are reaffirming their commitment to the Paris Climate Agreement, prompting a critical examination of the legal implications associated with the ability of these subnational actors to pursue such action. In assessing the constitutionality of cities and states vowing to enter the agreement, it is necessary to examine the historical circumstances that have invoked Article 1, Section 10, and how the provision may limit emerging climate policies. In the 1840 Supreme Court case Holmes v. Jennison, four Justices recognized a key distinction between relationships categorically barred by Clause 1 and those requiring approval under the Compact Clause. [21] The case rested on the informality of an agreement between Canadian authorities and the Governor of Vermont, and whether extradition applied in the absence of a congressionally approved treaty. However, the Jennison Court lacked a majority opinion, and no clear judicial or scholarly consensus exists on how to distinguish among the treaties, alliances, confederations, and compacts referenced in Article I, Section 10. Legal research indicates that most state agreements with foreign nations—such as past declarations and MOUs on climate change—are not submitted to Congress for approval and rarely contested on a federal level. [21] In fact, the aforementioned lawsuit against the linkage between California and Quebec’s cap-and-trade program signaled an unprecedented shift in the federal government’s acquiescence. According to American legal scholar Duncan Hollis, “U.S. states simply do not submit their arrangements with foreign powers to Congress for approval. Thus, Congress frequently has no knowledge, let alone views on, what the states might be doing abroad. But make no mistake - U.S. states have very active international agendas.” [22] For instance, in October 2007, Congress remained silent when ten U.S. states teamed up with ten European countries, the European Commission, two Canadian provinces, and New Zealand to establish the “International Carbon Action Partnership" (ICAP), aiming to advance cap-and-trade carbon markets to address global warming. [23] This pattern is expected to continue with specific post-Paris Agreement state actions. For instance, the recent California-China Memorandum of Understanding to enhance cooperation on Green Development explicitly states that it is not legally binding, making it unlikely to implicate the restrictions of Article I, Section 10. [24] 

Other state climate policies, however, are facing greater scrutiny. In 2017, Hawaii Governor David Ige passed legislation, including Hawaii Senate Bill 559, which expanded efforts to reduce greenhouse gas emissions, and Hawaii House Bill 1578, which created a “carbon farming” task force. [25] Although many states had enacted similar legislation prior to the Paris Agreement, Ige’s expressed statement of purpose, “supporting the goals” of the Paris Agreement, regardless of federal action, differentiated it from the former. Beyond merely reinforcing the agreement, Hawaii incorporated relevant sections in the crafting of its legislation. In its 2012 decision, Movsesian v. Versicherung AG, the Ninth Circuit determined that a state law may be deemed unconstitutional even in the absence of a conflict with federal foreign policy if it (1) lacks a legitimate connection to a traditional state responsibility and (2) encroaches on the federal government’s foreign affairs power. [26] The ruling does not specify the framework used to determine “traditional state responsibility." Instead, it relies on an adverse inference whereby any domain not explicitly assigned to the federal government is presumed to belong to the states. In the case of Hawaii, the state invoked an international, legally binding treaty requiring ratification by the federal government as the legal foundation for its legislation. This approach introduces problematic ambiguity regarding whether the legislation exceeds the scope of the state’s “traditional responsibility” by enacting policies derived from an international treaty. Although few state climate laws have been challenged under federal foreign affairs preemption, the 2012 Movsesian inquiry into state legislative intent could subject state laws primarily aimed at opposing the Trump Administration’s withdrawal from the Paris Agreement, such as Hawaii’s, to heightened judicial scrutiny.

As climate change and political polarization intensify, it becomes increasingly imperative to develop pragmatic, collective, and policy-oriented solutions. The evidence presented in this commentary sufficiently validates the legality of such mobilization, which is grounded in international actions and agreements undertaken by states. Nevertheless, subnational leaders are already actively engaging in these initiatives. Following President Trump's cessation from the Paris Agreement in 2017, a consortium of U.S. governors and additional American subnational authorities established the U.S. Climate Alliance, an organization committed to upholding the commitments of the Agreement from a bottom-up perspective. To reinforce these commitments, members of the alliance also founded America's Pledge, an initiative designed to enable local and regional authorities to quantify the impact and capacity of their efforts in emission reduction. These endeavors exemplify the increasing coordination among state leaders in articulating the policy aspirations of American society and underscore the promising potential of climate action rooted in subnational diplomacy. 

Edited by Alicia Lopez-Guerra

[1] Council on Foreign Relations, “History of Climate Action Timeline,” CFR Education, 2025, https://education.cfr.org/learn/timeline/history-climate-action.

[2] Curtis Bradley, “International Agreements by U.S. States: Federalism, Foreign Affairs, and Constitutional Change,” Chicago Unbound (2023).

[3] Caroline Garrett, “Most Polluted Countries in the World: 2021 Ranking.” Selectra, May 28, 2021, https://climate.selectra.com/en/carbon-footprint/most-polluting-countries.

[4] “United States Files Lawsuit against State of California for Unlawful Cap and Trade Agreement with the Canadian Province of Quebec,” Justice.gov, October 23, 2019, https://www.justice.gov/archives/opa/pr/united-states-files-lawsuit-against-state-california-unlawful-cap-and-trade-agreement.

‌[5]  “The Paris Agreement.” United Nations, 2015, https://www.un.org/en/climatechange/paris-agreement.

[6] “The California Cap-And-Trade Program: Overview and Considerations for Congress.” Congress.gov, 2025, www.congress.gov/crs-product/R48314.

[7] “California’s Cap-And-Trade Agreement with Quebec: Surviving Constitutional Scrutiny – Environmental and Energy Law Program,” Harvard Law School, November 4, 2019, eelp.law.harvard.edu/californias-cap-and-trade-agreement-with-quebec-surviving-constitutional-scrutiny/.

[8] Agreement between the California Air Resources Board and the Government of Du Québec Concerning the Harmonization and Integration of Cap-and-Trade Programs for Reducing Greenhouse Gas Emissions, 2013, https://ww2.arb.ca.gov/sites/default/files/cap-and-trade/linkage/ca_quebec_linking_agreement_english.pdf. 

[9] United States v. State, 444 F.Supp.3d 1181 (E.D. Cal. 2020).

[10] Bradley, “International Agreements.”

[11] Virginia v. Tennessee, 148 U.S. 503 (1893).

‌[12] “California Dedicates Nearly $33 Billion to Climate Solutions, Using Money from the Cap-And-Trade Program,” Ca.gov, May 7, 2025, https://ww2.arb.ca.gov/news/california-dedicates-nearly-33-billion-climate-solutions-using-money-cap-and-trade-program. 

[13] Julien Hanoteau, and David Talbot, “Impacts of the Québec Carbon Emissions Trading Scheme on Plant-Level Performance and Employment,” Carbon Management 10, no. 3 (2019): 287–98.

[14] James May, “Subnational Climate Rights in America,” Journal of Constitutional Law 26, no. 3 (2024).

[15] Benjamin Leffel, “Subnational Diplomacy, Climate Governance & Californian Global Leadership.” USC Center on Public Diplomacy,  https://uscpublicdiplomacy.org/publication/subnational-diplomacy-climate-governance-californian-global-leadership 

[16] Luis Li, “State Sovereignty and Nuclear Free Zones.” California Law Review 79, no. 4 (1991): 1169–1204. 

[17] Lori A. Martin, “The Legality of Nuclear Free Zones.” The University of Chicago Law Review 55, no. 3 (1988): 965–1009.

[18] Arthur D. Little, Inc. v. Commissioner of Health and Hospitals of Cambridge, 481 N.E.2d 441 (1985).

[19] “U.S. Climate Alliance to the International Community: ‘We Will Continue America’s Work to Achieve the Goals of the Paris Agreement,’” U.S. Climate Alliance, January 20, 2025, https://usclimatealliance.org/press-releases/alliance-paris-withdrawal/.

‌[20] Holmes v. Jennison, 39 U.S. 540 (1840).

[21] “Constitutional Limits on States’ Power over Foreign Affairs,” Congress.gov, 2022, https://www.congress.gov/crs_external_products/LSB/HTML/LSB10808.web.html.

[22] Duncan Hollis, “The Elusive Foreign Compact,” Missouri Law Review 73, no. 4 (2008).

[23] Rafael Leal-Arcas et al., “The World Trade Organization and Carbon Market Clubs,” The Georgetown Journal of International Law 52 (2021).

[24] “Memorandum of Understanding to Enhance Cooperation on Green Development between the Government of Guangdong Province and the Government of the State of California,” 2023, https://www.gov.ca.gov/wp-content/uploads/2023/10/CA-Guangdong-MOU-Oct-2023.pdf.

[25] Hawaii Act 32 (2017); Hawaii Act 33 (2017).

[26] Movsesian v. Verisherung AG, No. 07-56722 (9th Cir. 2012).