The landscape of international climate governance is undergoing a fundamental transformation as municipal and regional governments increasingly bypass national capitals to forge their own foreign policies. For two decades, the C40 Cities Climate Leadership Group has served as a primary vehicle for this shift, evolving from a small gathering of mayors into a powerhouse of 97 global cities representing one-quarter of the global economy. This burgeoning movement, often referred to as subnational diplomacy, has reached a critical juncture as states like California and coalitions like the U.S. Climate Alliance establish direct ties with foreign sovereigns, creating a parallel track of international relations that persists even when national governments retreat from global environmental commitments.
The recent friction between the State of California and the United States federal government highlights the growing complexity of this trend. Earlier this year, California Governor Gavin Newsom formalized a memorandum of understanding (MoU) with the United Kingdom’s Labour government, a move designed to bypass federal gridlock and accelerate transatlantic cooperation on decarbonization, technical expertise exchange, and economic innovation. The agreement specifically targets resilience measures and research cooperation, positioning California—the world’s fifth-largest economy—as a quasi-independent actor on the global stage. This move has reignited a fierce constitutional and political debate over the limits of state power and the necessity of regional climate action in the face of federal opposition.
A Chronology of Subnational Climate Action
The trajectory of subnational climate diplomacy is marked by several key milestones that have moved regional governments from the periphery to the center of global policy. The C40 was founded in 2005, but its influence surged following the 2015 Paris Agreement, where local leaders argued that because cities consume over two-thirds of the world’s energy and account for more than 70% of global carbon emissions, they must be the primary engines of implementation.
In 2013, a landmark achievement in regional cooperation occurred when California and Quebec officially linked their cap-and-trade markets. This partnership created a cross-border carbon market that allowed businesses in both jurisdictions to trade emissions allowances, effectively establishing a regional carbon price independent of federal mandates in either Washington or Ottawa. Despite legal challenges, this market has remained operational for over 12 years, serving as a blueprint for other subnational entities.
The election of Donald Trump in 2016 triggered a second wave of subnational mobilization. In response to the U.S. withdrawal from the Paris Agreement, the U.S. Climate Alliance was formed—a bipartisan coalition of governors committed to meeting the goals of the accord. Today, this alliance includes more than 20 states representing over half of the U.S. population and a significant portion of its GDP. During this period, the Trump administration actively sought to dismantle these efforts, most notably by filing a lawsuit to terminate the California-Quebec agreement, arguing it violated the Treaty Clause of the U.S. Constitution, which prohibits states from entering into treaties with foreign powers.
The legal landscape shifted again under the Biden administration. In 2021, the Department of Justice dropped the challenge against the California-Quebec link. By 2022, President Biden signed legislation formalizing the role of subnational diplomacy, leading to the creation of the Office of the Special Representative for City and State Diplomacy within the State Department. However, with the return of the Trump administration, these formal federal supports have been rescinded, returning subnational actors to a posture of defensive autonomy.
The California-UK Accord and Federal Friction
The memorandum of understanding signed between California and the United Kingdom represents a sophisticated evolution of state-level diplomacy. Unlike traditional treaties, MoUs are typically non-binding and focus on "technical cooperation" and "information sharing," a distinction that provides states with a degree of legal protection against federal interference. The agreement with Westminster emphasizes the development of clean energy technologies, the strengthening of supply chains, and joint efforts to address climate-driven disasters such as wildfires and flooding.
The reaction from the White House was immediate and characteristic of the broader ideological divide. President Trump publicly criticized the deal, labeling Governor Newsom’s actions as "inappropriate" and suggesting that foreign governments should not be engaging with individual states on matters of national importance. This criticism reflects a broader federal strategy that includes the cancellation of billions of dollars in clean-energy subsidies, the reduction of the Environmental Protection Agency’s (EPA) regulatory authority, and a renewed emphasis on the extraction and export of fossil fuels, particularly coal.
The tension underscores a "bifurcated" American foreign policy. While the federal government prioritizes energy independence through traditional fuels and retreats from international climate forums, states like California, Washington, and New York are aggressively pursuing the energy transition. This creates a confusing landscape for international partners who must navigate a patchwork of regulations and commitments across different levels of American government.
Supporting Data: The Economic Power of Subnational Actors
The significance of subnational diplomacy is best understood through the economic and environmental data of the participants. The U.S. Climate Alliance states, if considered as a single nation, would represent the third-largest economy in the world, trailing only the U.S. as a whole and China.
- Emissions Impact: C40 cities have collectively committed to halving their emissions by 2030. Analysis from the group suggests that if all 97 member cities meet their targets, it would prevent 1.9 gigatonnes of CO2 equivalent from entering the atmosphere—roughly equal to the annual emissions of the European Union.
- Economic Clout: California’s GDP exceeds $3.8 trillion. Its commitment to a 100% clean energy grid by 2045 drives massive investment in battery storage and solar technology, sectors that the current federal administration has signaled it may no longer support through the Inflation Reduction Act (IRA) frameworks.
- Market Linkages: The California-Quebec carbon market covers approximately 80% of California’s greenhouse gas emissions. As of 2024, the market has generated billions in revenue for climate resilience projects, demonstrating that subnational markets can remain solvent and effective even without federal oversight.
The Transatlantic Subnational Resilience Framework
As federal cooperation between the U.S. and the EU faces potential stagnation, scholars and policy experts are proposing new structures to institutionalize regional cooperation. Jakob Wiedekind, a professor of political science and international relations at the University of North Carolina at Chapel Hill, has advocated for the creation of a "Transatlantic Subnational Resilience Framework."
Writing in the Georgetown Journal of International Affairs, Wiedekind argues that the "fluctuating" nature of American federal policy necessitates a more stable, decentralized avenue for cooperation. He points to the European Union’s "REPowerEU" initiative as a model. Launched following the 2022 invasion of Ukraine, REPowerEU utilized a "multi-level governance" approach where the European Commission set broad directives, but individual member states and regional governments shared the technical expertise required to rapidly pivot away from Russian gas toward renewables.
Wiedekind’s proposed framework would focus on:
- Depoliticization: By shifting the focus from high-level climate targets (which often attract political ire) to practical "resilience" topics like wildfire management, flood mitigation, and grid stability, subnational actors can engage across the political aisle.
- Commercialization: Leveraging clean technology as a business opportunity rather than a regulatory burden. This approach is designed to attract interest from Republican-led states that may be skeptical of climate rhetoric but are keen on the economic benefits of manufacturing solar components or wind turbines.
- Decentralized Dialogue: Establishing regular, structured meetings between regional leaders that exist outside the United Nations Framework Convention on Climate Change (UNFCCC) and the annual COP summits, which Wiedekind suggests have become overly politicized and cumbersome.
Broader Impact and Implications
The persistence of subnational diplomacy suggests that the global energy transition has achieved a level of momentum that is difficult for any single national government to fully halt. The "work-arounds" developed by mayors and governors are not merely symbolic; they represent a pragmatic response to the physical and economic realities of climate change.
For the international community, the rise of state-level diplomacy offers a hedge against political volatility in Washington. European and Asian partners are increasingly viewing states like California as reliable long-term partners for technological and academic exchange. However, this trend also carries risks. A fragmented regulatory environment can increase costs for businesses operating across state lines and may lead to prolonged legal battles over the constitutional limits of state authority.
The current clash between the Trump administration and California is a litmus test for the future of the American federalist system. If California’s MoUs and carbon market links continue to survive judicial and executive challenges, it will solidify the role of states as independent diplomatic actors. This would mark a permanent shift in how global policy is conducted, moving toward a "polycentric" model where power is distributed among various levels of government.
Ultimately, the growth of these subnational networks reflects a realization that the impacts of climate change—floods in Valencia, wildfires in the Sierra Nevada, or rising sea levels in New York—are felt locally. As long as these local impacts continue to intensify, the motivation for cities and states to engage in their own diplomacy will likely outweigh the pressures of national political shifts. The "Transatlantic Subnational Resilience Framework" and similar initiatives represent the next phase of this evolution: a world where the dialogue on survival persists through every channel available, regardless of the stance taken by the heads of state.
