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Dieses Kapitel untersucht die verschiedenen Ansätze zur Klimaregulierung, die von den USA verfolgt werden. , betont die bedeutende Rolle von Initiativen auf staatlicher Ebene bei der Förderung des Klimaschutzes. Untersucht werden die Gründungsmitglieder der Under2-Koalition, darunter Kalifornien, Oregon, Vermont und Washington, und ihre bahnbrechenden Bemühungen bei der Umsetzung klimapolitischer Maßnahmen. Die Analyse erstreckt sich auf frühe und späte Schreiner wie Connecticut, Massachusetts, Minnesota, New Hampshire, New York, Rhode Island, Colorado, Hawaii, Maine, Maryland, Nevada, New Jersey und Virginia und bietet einen vergleichenden Überblick über ihre Klimapolitik und -instrumente. Das Kapitel diskutiert auch die Führungsrollen dieser Staaten und kategorisiert sie in kognitive, strukturelle, unternehmerische und beispielhafte Führung. Es schließt mit einer vergleichenden Analyse der von diesen Staaten verabschiedeten Strategien, Instrumente und Ziele, die Einblicke in die Effektivität und Auswirkungen ihrer Strategien zur Klimapolitik bietet. Das Kapitel unterstreicht die Bedeutung von Maßnahmen auf staatlicher Ebene zur Förderung der Klimapolitik und die Rolle der Under2-Koalition bei der Förderung subnationaler Klimapolitik.
KI-Generiert
Diese Zusammenfassung des Fachinhalts wurde mit Hilfe von KI generiert.
Abstract
This chapter examines climate policy efforts across 17 US states that joined the Under2 Coalition at different timepoints, analyzing founding members (California, Oregon, Vermont, and Washington), early joiners (Connecticut, Massachusetts, Minnesota, New Hampshire, New York, and Rhode Island), and late joiners (Colorado, Hawaii, Maine, Maryland, Nevada, New Jersey, and Virginia). The analysis systematically compares climate policy adoption, policy instrument diversification, and greenhouse gas emissions reduction targets across these membership categories. California emerges as the dominant climate leader, leveraging its unique Clean Air Act waiver and market power to demonstrate cognitive, structural, entrepreneurial, and exemplary leadership. Other founding members show more variable patterns, with partisan politics creating significant constraints. Early joiners like Massachusetts and New York amplified existing frameworks through the Under2 Coalition membership, while states like New Hampshire represent followership despite early participation. Late joiners generally maintain lower policy effort levels, though some developed ambitious frameworks prior to coalition entry. The findings provide mixed support for the propositions regarding founding member leadership and policy durability.
Introduction
Climate governance in the United States has followed an inconsistent path at the federal level, largely shaped by partisan divisions. The Democratic Party has generally promoted climate action, while the Republican Party has been less ambitious and even dismantled existing climate frameworks (Jotzo et al., 2018; Masur, 2022; Rabe, 2010; Tosun et al., 2023). However, the multileveled nature of US governance arrangements—spanning federal, state, and local levels—has created alternative pathways for policy-based climate action. Despite partisan divides that extend to subnational levels, state governments have emerged as important actors in climate action (Mildenberger, 2021; Trachtman, 2019). This has produced a decentralized approach to climate governance rather than a traditional top-down model, exemplifying what scholars such as Jordan et al. (2018), Jordan and Huitema (2014), Jordan et al. (2015), and Tobin et al. (2024) have identified as a broader shift toward polycentric climate governance (see Chap. 2).
State-level climate initiatives emerged in the early 2000s, with California, Oregon, and Washington launching the West Coast Governors Global Warming Initiative in 2003 to reduce GHG emissions (duVair et al., 2007). This was followed in 2005 by a coalition of northeastern states that signed the RGGI, which became operational in 2009 as the first mandatory carbon cap-and-trade program in the United States. These pioneering efforts introduced renewable energy mandates, economy-wide emissions targets, and market-based carbon pricing mechanisms during a period of federal inaction on climate policy (Karapin, 2018).
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State momentum continued building through the 2010s as international climate negotiations progressed. In May 2015, California partnered with the German state of Baden-Württemberg to convene subnational leaders for the signing of the U2C MoU, strengthening subnational support in the run-up to the Paris Climate Conference.
Even though the Trump administration (2017–2021 and 2025-present) withdrew from the Paris Agreement, the number of U2C members expanded in the United States (see also Chap. 1). This participation makes the United States a laboratory for examining subnational climate governance dynamics. This chapter leverages this rich empirical context.
Founding Members
California
California’s climate leadership story began decades before the inception of U2C and has its roots in an environmental legacy stretching back to the 1960s that would prove instrumental in shaping twenty-first-century climate policy (Allison et al., 2016; Leffel, 2018; Mazmanian & Jurewitz, 2020). This foundation enabled California to pioneer subnational climate action in the 2000s, beginning with groundbreaking vehicle emission standards in 2002 (Meckling & Nahm, 2018) and culminating in the landmark Global Warming Solutions Act (GWSA) in 2006, better known as AB 32, one of the world’s first comprehensive climate framework laws (Bedsworth & Hanak, 2013; Meckling & Nahm, 2022; Sperling & Eggert, 2014).
What transformed California from a regional leader into a global climate innovator was a unique combination of legal authority and market power. Unlike any other state, California holds an exclusive federal waiver1 under the Clean Air Act (CAA) that allows it to set stricter vehicle emission standards, while other states can only choose between federal standards or California’s more stringent requirements. This legal advantage, combined with California’s massive automotive market, creates what scholars term a “super-regulator” effect—the state’s innovations ripple outward to shape both subnational and national policy (Carlson, 2008). The sheer size of California’s car market often compels manufacturers to meet the state’s standards nationwide rather than maintain separate production lines, generating what political economists call the “California effect” (Perkins & Neumayer, 2012; Vogel, 1995).
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However, California’s true genius lies not just in leveraging its unique automotive advantage, but in creating a replicable governance architecture through AB32. The framework established clear GHG emissions targets while delegating implementation authority to the capable Air Resources Board, which designs and continuously updates programs through successive scoping plans. This approach evolved from sector-specific regulations into a comprehensive, economy-wide system featuring cap-and-trade mechanisms layered onto detailed standards spanning transportation, electricity, buildings, land-use, and beyond—all anchored by long-term carbon neutrality targets.
When California co-founded U2C in 2015, it did not represent a policy shift but rather an amplification of existing momentum. The initiative provided California with an international platform to accelerate policy diffusion (Boyd, 2019), positioning the state as a powerful engine for spreading climate innovation globally.
Oregon
Oregon combines the traits of a leader and a follower, having developed a climate governance that is innovative yet fragmented because its progress has often been hindered by internal political divisions. This tension has defined Oregon’s climate journey from its early recognition of the challenge to its ongoing struggle to translate vision into comprehensive action.
The state’s climate awareness emerged remarkably early, establishing a global warming taskforce in 1988—nearly two decades before most jurisdictions began climate planning (Graves et al., 2020). Yet it took until 2007 for this early recognition to crystallize into legislation, when House Bill 3543 finally established GHG reduction goals and created the Oregon Global Warming Commission to provide strategic guidance and monitor progress (Graves et al., 2020). The Commission has since functioned as Oregon’s climate conscience, serving as an agenda-setter that develops roadmaps and recommendations to drive sectoral action.
Oregon’s followership is evident in its adoption of California’s proven approaches, including the Low-Emissions Vehicle (LEV) standards and ambitious targets for 100% Zero-Emissions Vehicle (ZEV) sales by 2035. However, the state’s leadership ambitions have repeatedly collided with political reality. Oregon failed to meet its 2020 benchmark of reducing emissions 10% below 1990 levels and has been unable to enact the economy-wide cap-and-trade program that would anchor its climate strategy.
The most dramatic illustration of this political dysfunction came during legislative attempts in 2019 and 2020, when ambitious climate bills collapsed amid unprecedented partisan warfare. Despite Democratic majorities in both chambers, Republicans wielded Oregon’s two-thirds quorum requirement as a weapon, repeatedly walking out to deny quorum and effectively paralyzing the legislative process. Democrats lacked the supermajority needed to override these tactics, creating a governance crisis that highlighted the fragility of climate action in polarized political environments (VanderHart, 2025).
Faced with legislative deadlock, Governor Kate Brown demonstrated adaptive governance by pivoting to executive action, directing state agencies to pursue science-based GHG emissions reductions through regulatory channels (Executive Order No. 20-04) (Lorenzen, 2020). The Department of Environmental Quality responded by establishing declining emission caps for fossil fuel suppliers—a workaround that achieved some policy goals despite the absence of comprehensive legislation.
U2C membership offers Oregon external legitimacy and international visibility that helps it sustain climate ambition when internal political divisions threaten to derail progress. When Oregon does lead effectively, it exemplifies cognitive leadership—contributing knowledge, analysis, and innovative thinking that advances the broader climate agenda, even when its own implementation capacity remains constrained by political realities.
Vermont
Despite being a small state with limited market influence, Vermont positioned itself as an early climate innovator, consistently punching above its weight in clean energy and environmental policy. The state’s leadership trajectory began with followership in 1996 when it adopted California’s LEV standards. This early commitment to stringent environmental standards continued in 2001 when Vermont co-signed a regional climate action plan with New England Governors and Eastern Canadian Premiers (NEG-ECP), demonstrating its willingness to engage in cross-border climate cooperation (see Chap. 4). Vermont maintained this momentum by establishing statutory GHG emissions targets in 2006 and by becoming the first state to ban hydraulic fracturing in 2012—a bold move that signaled its commitment to transitioning away from fossil fuels (Scarcioffolo et al., 2021).
By the mid-2010s, Vermont’s clean energy progress was quantifiably impressive, boasting the nation’s least fossil fuel-intensive electricity sector (Sovacool et al., 2014). This achievement provided a strong foundation when Vermont joined U2C, viewing membership as diplomatic positioning within a global network of subnational climate ambition. The U2C platform amplified Vermont’s leadership profile, giving the state an international stage to showcase its achievements and influence broader climate discourse.
However, Vermont’s most significant climate policy evolution came after joining U2C. The 2020 GWSA marked a turning point by establishing legally binding climate obligations and creating a Climate Council responsible for designing consecutive action plans. This comprehensive framework consolidated Vermont’s position as an exemplary leader—a state that demonstrates how ambitious climate action can be achieved regardless of size or market power, inspiring other small jurisdictions to pursue similarly bold approaches.
Washington
Washington began its climate efforts later than other U2C founding members, but it ultimately built one of the United States’ most comprehensive regulatory frameworks through a combination of executive persistence and strategic legislative timing. Washington’s initial climate foray came through the Energy Independence Act in 2006, which required major utilities to source 15% of their electricity from renewables by 2020. The state government quickly expanded its ambition, establishing a GHG emissions reduction framework with reporting requirements and GHG emissions targets by 2008, followed by Executive Order 09-05 in 2009, which directed state agencies to implement cross-sectoral climate strategies. By the time Washington co-founded U2C in 2015, this solid foundation was already in place, positioning the state for the acceleration that would follow.
The post-2015 period revealed both Washington’s climate ambitions and its political constraints. Progress accelerated significantly but relied heavily on executive action, as a Republican-controlled Senate consistently blocked legislative efforts to strengthen GHG emissions targets. The Clean Air Rule in 2016 introduced a cap-and-reduce system for large emitters, demonstrating the administration’s determination to advance climate policy despite legislative gridlock. This executive-driven approach continued with landmark legislation in 2019: the Clean Energy Transformation Act, mandating 100% clean electricity by 2045, and the Clean Buildings Act, establishing performance standards for large commercial buildings.
The political breakthrough came in 2021 when shifting partisan control finally enabled comprehensive legislative action. The legislature enacted the Climate Commitment Act—a sophisticated cap-and-invest program modeled on California’s pioneering one—alongside the Clean Fuel Standard, which mandates a 20% reduction in the carbon intensity of transportation fuel by 2035 (Roy Chowdhury et al., 2025). These measures transformed Washington’s regulatory architecture, extending state authority across economic sectors while creating market-based instruments to achieve GHG emissions targets.
Washington’s approach exemplifies cognitive leadership in climate governance. Rather than pioneering entirely new approaches, the state systematically studied and adapted proven models from other jurisdictions—particularly California’s cap-and-trade system—while developing innovative solutions tailored to its specific economic and political context.
Early Joiners
Connecticut
Connecticut’s foundational climate policy emerged in 2007 with Public Act 07-242 (an act concerning electricity and energy efficiency), which integrated energy efficiency requirements into state planning and established a renewable portfolio standard. This was quickly followed by the GWSA in 2008, which set GHG emissions limits and created a reporting framework. Then, in 2009, Connecticut became a founding member of the RGGI, positioning it early in this pioneering multi-state cap-and-trade system.
Where Connecticut truly distinguished itself was in pioneering climate finance mechanisms. The creation of the Clean Energy Finance and Investment Authority in 2011 through Public Act No. 11-80 marked a watershed moment, as this institution later evolved into theUnited States' first state green bank. This institutional innovation has since been replicated across multiple US states, representing Connecticut’s most significant contribution to climate policy diffusion across the United States (Nix et al., 2025). The state reinforced its regional approach in 2013 by signing the Multi-State ZEV MoU, a particularly strategic move given that transportation had become Connecticut’s largest GHG emitting sector.
However, Connecticut’s climate trajectory reveals inconsistent commitment. When the state joined U2C in 2015, it aligned with Governor Dan Malloy’s broader diplomatic approach and provided some international visibility, but the core priorities remained RGGI participation and advancing climate finance rather than achieving a comprehensive transformation toward sustained climate action. This uneven commitment has become more apparent in recent years, with Connecticut’s withdrawal from the Transportation & Climate Initiative Program in late 2021 and the decision of its legislative committee in 2023 to stall a proposal to adopt California’s Advanced Clean Cars II standards.
Despite these setbacks, Connecticut can be attributed some degree of climate leadership. The state demonstrated exemplary leadership by creating replicable models—particularly the green bank concept—that other jurisdictions could adopt. Simultaneously, Connecticut showed entrepreneurial leadership by innovating financial mechanisms that addressed market failures in clean energy investment, proving that climate leadership can emerge through institutional creativity rather than just regulatory ambition.
Massachusetts
Massachusetts established its climate policy foundation in 2008 with two landmark pieces of legislation that would define its long-term approach. The GWSA set legally binding targets, including an 80% GHG emissions reduction below 1990 levels by 2050, while empowering the Executive Office of Energy and Environmental Affairs to establish interim GHG emissions reduction targets and enforcement mechanisms. Simultaneously, the Green Communities Act provided the planning frameworks and investment tools needed to support local-level policy implementation. The state reinforced this regulatory foundation by participating in the launch of RGGI that same year, adding market-based carbon pricing to its emerging climate policy portfolio.
What distinguishes Massachusetts is its approach to climate policy development—systematically building on existing foundations rather than pursuing dramatic policy shifts. The Clean Energy and Climate Plans of 2010 and 2015 provided regular progress assessments toward 2020 goals, while the state’s decision to join U2C in 2015 reflected both its commitment to maintaining ambition and its strategic interest in learning from peer jurisdictions (Union of Concerned Scientists, 2016).
The next phase of Massachusetts’ climate policy evolution was characterized by consolidation rather than dramatic expansion. The 2050 Decarbonization Roadmap (2020) provided detailed sectoral pathways, while An Act Creating a Next-Generation Roadmap for Climate Policy (2021) updated the state’s target to net-zero GHG emissions by 2050. Rather than introducing entirely new policy instruments, this period emphasized expanding and reinforcing existing approaches. The Climate Leader Communities initiative, launched in 2025, exemplifies this strategy by broadening municipal engagement through an enhanced version of the earlier Green Communities framework.
This systematic layering approach reflects Massachusetts’ distinctive form of cognitive leadership in climate governance. The state excels at synthesizing research, analyzing climate policy performance, and adapting successful models from other jurisdictions while maintaining long-term strategic coherence. Combined with exemplary leadership—particularly its role in demonstrating how sustained institutional commitment can generate comprehensive climate frameworks—Massachusetts offers a compelling model for states seeking to build durable, evidence-based climate governance systems.
Minnesota
While Minnesota’s climate governance journey illustrates how early momentum can stall in the absence of sustained political commitment, it also demonstrates how renewed leadership can revitalize ambitious climate action.
Minnesota began as an early climate pioneer, launching its Climate Change Action Plan in 2003 to integrate climate concerns into energy and environmental policy frameworks. This early foundation was strengthened by the Next Generation Energy Act in 2007, which established GHG emissions reduction goals, and the comprehensive Climate Change Advisory Group report in 2008, which provided detailed sectoral strategies for legislative action. These initiatives created what appeared to be a solid institutional foundation for sustained climate progress.
However, promise did not translate into performance. Minnesota missed its 2015 GHG emissions reduction target, and the 2016 Climate Solutions report delivered a sobering assessment: the state was not on track to meet its future climate goals. This implementation gap revealed the difference between setting ambitious targets and creating the political and institutional capacity necessary to achieve them.
Minnesota’s decision to sign the U2C MoU in October 2015 reflected this limited ambition, treating the international initiative primarily as an external benchmark rather than as a catalyst for enhanced action (Minnesota Pollution Control Agency, 2017). During this period, Minnesota’s role was fundamentally that of a follower—maintaining basic commitments while other states advanced more aggressive policies.
The turning point came in 2019, when Governor Tim Walz issued an executive order (19-37) establishing a Climate Subcabinet to coordinate climate action across state agencies. This institutional innovation signaled a shift toward more systematic climate governance. Progress accelerated in 2021 with the Energy Conservation and Optimization Act, which expanded efficiency mandates, and the adoption of California’s LEV and ZEV standards—Minnesota’s belated embrace of proven climate policy models.
The most significant breakthrough came in 2023 when the legislature enacted an updated Climate Action Framework, committing Minnesota to net-zero GHG emissions by 2050. This commitment finally aligned Minnesota with peer states that had made similar pledges years earlier, marking the state’s transition from climate laggard to renewed participant in ambitious subnational climate action. Minnesota’s recent trajectory suggests that while early leadership advantages can be squandered, they can also be recovered through focused political commitment and institutional reform.
New Hampshire
New Hampshire’s climate governance exemplifies how early institutional development does not guarantee sustained policy ambition. The state established credible climate planning mechanisms but has struggled to translate this groundwork into the comprehensive climate policy frameworks adopted by peer jurisdictions, resulting in a persistent gap between climate recognition and climate action.
New Hampshire’s climate journey began promisingly with the establishment of the Climate Change Policy Task Force in 2007, which produced the state’s first climate action plan in 2009. This foundational document recommended policies across multiple sectors and established GHG emissions reduction targets, though notably these remained non-binding. The state reinforced its early climate commitment by joining RGGI in 2009, integrating its Clean Power Act into the regional carbon market and demonstrating willingness to participate in innovative multi-state climate mechanisms.
However, this initial momentum gave way to an extended plateau characterized by limited policy development. New Hampshire joined U2C in 2015—an early joiner among US states—and approved the Energy Efficiency Resource Standard in 2016, but these remained isolated actions rather than components of an expanding climate policy framework. Even these modest advances faced political volatility, with the efficiency standard being disrupted in 2021 before being restored and updated in early 2022, illustrating the fragility of climate policy in politically competitive environments.
Today, New Hampshire significantly lags behind other U2C members in both the scope and ambition of its climate policies. The state lacks binding GHG emissions reduction targets, has not adopted electric vehicle (EV) mandates, and offers limited incentive programs for clean energy transitions—policy gaps that are particularly striking given its early engagement with climate planning. This performance suggests that New Hampshire’s role within U2C has been more symbolic than substantive, maintaining membership without embracing the policy innovations that characterize more ambitious coalition members.
Despite these shortcomings, New Hampshire has maintained consistency in certain areas, particularly through its continued participation in RGGI and modest progress on climate framework development and sectoral decarbonization efforts. This pattern reveals a state that recognizes climate challenges and maintains basic institutional capacity but lacks the political commitment or advocacy coalition strength necessary to advance comprehensive climate governance. New Hampshire thus represents a cautionary example of how early climate engagement can stagnate without sustained political leadership and lacking capability or willing to innovate new climate policies.
New York
New York’s climate leadership originated during the 1970s energy crisis, when the state pioneered energy efficiency programs that would become foundational to its later climate strategy. This early experience with energy policy provided institutional knowledge that proved crucial as climate concerns emerged. The state formalized its climate ambitions in 2002 with the State Energy Strategy, which introduced New York’s first GHG emissions reduction goals. By 2009, New York had become a founding member of RGGI, and one year later adopted its first comprehensive Climate Action Plan, successfully embedding market-based carbon pricing into its policy framework.
New York’s decision to join U2C in 2015 aligned naturally with its existing climate strategy, though the coalition’s direct influence on the state’s trajectory is difficult to isolate. What is clear is that the period following U2C membership witnessed decisive policy developments that consolidated New York’s position as a national climate leader. The pivotal moment came in 2019 with the passage of the Climate Leadership and Community Protection Act, landmark legislation that mandated a zero-emissions electricity grid by 2040 and created the Climate Action Council to develop detailed implementation pathways through comprehensive scoping plans.
New York’s approach to the most challenging sectors—transportation and buildings—reveals strategic adaptation of proven models rather than policy innovation from scratch. The state adopted California-style vehicle standards, implementing Advanced Clean Trucks regulations in 2021 and Advanced Clean Cars II standards in 2022. For buildings, New York broke new ground by becoming the first state to mandate all-electric systems for new buildings, commencing in 2023 on a phased schedule.
The scale and sophistication of New York’s climate framework reflect multiple dimensions of subnational leadership (Karapin, 2018). The state demonstrates structural leadership through its economic size and influence: as home to the nation’s largest state public power utility, New York’s policy choices ripple throughout energy markets. Cognitive leadership manifests itself in New York’s systematic development of evidence-based climate policies and institutional frameworks that other states can transfer at. Finally, it also exhibits exemplary leadership by proving that large, economically diverse states can accelerate decarbonization while maintaining economic competitiveness, providing a powerful demonstration effect for other major state economies.
Rhode Island
Rhode Island is another small state with strong climate governance. It entered regional alliances early, co-signing the NEG-ECP’s Climate Change Action Plan in 2001 and, a year later, publishing its own Greenhouse Gas Action Plan (2002). Agenda-setting was complemented by the Least-Cost Procurement statute (2006), which required utilities to pursue cost-effective efficiency. Rhode Island signed the RGGI memorandum in 2007, well before its launch, and by 2009, when it entered into force, the state had already positioned itself within the region’s market-based carbon regime.
In 2014, the Resilient Rhode Island Act set up a cross-sectoral agency and established long-term goals to tackle mitigation, while other states joining U2C supplied external benchmarking. The decisive policy adopted after joining the coalition was the Act on Climate (2021), which made Rhode Island’s climate goals legally binding, including GHG net-zero by 2050 (Kim and Park, 2024). This same act also mandated iterative planning (updates and strategies) while retaining the aspirational nature of previous goals. Rhode Island’s trajectory shows fundamentally cognitive leadership, but there are also instances of exemplary leadership, such as the aforementioned statute for energy efficiency.
Late Joiners
Colorado
Colorado’s state-level climate policy started with renewable energy standards, which evolved into comprehensive legislation. Amendment 37 (2004) required large utilities to source 10% of their electricity from renewables by 2015 and created a renewable energy credit market. Colorado then established GHG emissions inventories and preliminary non-binding GHG emissions targets through executive action (2008). Exemplary leadership emerged in 2014 when Colorado became the first US state to regulate methane leakage from oil and gas operations, later becoming a model for federal policy (Dai et al., 2020).
When the United States retreated from the Paris Agreement in 2017, Colorado responded with an executive order adopting Paris-aligned goals and announcing entry to the US Climate Alliance the same day. In 2019, with legislative support, the state enacted the Climate Action Plan to Reduce Pollution with binding reduction targets. Complementary measures included adopting California’s ZEV standards and the Clean Heat Plan Act (2021), requiring utilities to cut emissions from gas use in buildings. In 2023, the GHG emissions targets were updated to achieve net-zero emissions by 2050 with stronger regulatory authority for implementation through the GHG Reduction Measures bill.
Colorado joined the U2C in 2024, later than most US states and after consolidating its climate framework toward full carbon neutrality. The state was a late joiner that did not rely on the coalition for direction but used it to amplify its international credibility. Given its pioneering methane rules, Colorado’s leadership is mainly exemplary and cognitive.
Hawaii
Hawaii’s legal framework began with the GWSA (2007), which is modeled on California’s AB32 and set one of the nation’s first binding GHG emissions reduction targets, attempting to return to 1990 levels by 2020 (University of Hawaii Economic Research Organization, 2008). The following year, the state pursued clean energy through the Hawaii Clean Energy Initiative (2008), partnering at the federal level with the US Department of Energy to combine achieving GHG emissions reductions with strengthening energy security.
Ambition rose with Act 97 (2015), making Hawaii the first state to set a 100% renewable electricity goal to be achieved by 2045. Act 15 (2018) then established a statewide net-zero GHG emissions target for the same year, to be attained through a GHG Sequestration Task Force charged with aligning land-use, forests, soils, and coasts with mitigation. Hawaii joined U2C in 2018 for an external validation of its policy-based climate action and to align its statutory trajectory with global subnational entities. Nevertheless, the state's climate policy revolves more around local energy challenges and regional cooperation than U2C itself.
Subsequent legislation, including Act 74 (2021) and Act 238 (2022), filled gaps in interim milestones regarding vehicle emissions and GHG emissions reduction targets, respectively. This was reinforced through fostering sectoral transitions, such as by implementing energy efficiency portfolio standards in buildings and participation in the Multi-State ZEV Memorandum of Understanding for medium and heavy-duty vehicles. Overall, the binding GHG emissions reduction targets and the cross-sector framework from Act 238 suggest that Hawaii demonstrates both exemplary and cognitive leadership.
Maine
Maine’s climate policy began with the Climate Action Law (2003), mandating the first GHG emissions reduction targets and action plan (2004) with biennial reporting and sectoral recommendations. In 2005, the state co-founded the RGGI, which was launched in 2009 to embed carbon pricing into electricity generation, and later established the Efficiency Maine Trust (2009) to channel revenues into building efficiency.
Progress stalled under Republican Governor Paul LePage (2011–2018), who opposed climate policy and limited new initiatives (Portland Press Herald, 2018). The only notable measure was a biomass procurement program, criticized as economically rather than environmentally driven (McKibben, 2019). However, the institutional architecture remained intact, and momentum returned with Governor Janet Mills of the Democratic Party. The state joined the US Climate Alliance (2019) and created the Climate Council through Public Law 2019, Chapter 476, institutionalizing multi-sectoral working groups and setting statutory GHG emissions reduction targets of 45% below 1990 levels by 2030 and 80% by 2050. This law also reformed the Renewable Portfolio Standard and thereby established heat pump deployment initiatives to accelerate decarbonization.
The “Maine Won’t Wait” Action Plan (2020) translated targets into concrete measures with deadlines and accountability metrics. Maine joined U2C following this resurgence in climate policy in 2021. The revised MoU of the U2C established a net-zero-by-2050 baseline that Maine and other members have committed to, though this did not alter the state’s trajectory but rather positioned Maine among subnational leaders pursuing more ambitious climate action.
Regarding sectoral policies, Maine joined the Multi-State ZEV MoU (2020) and adopted California’s ZEV Program (2021), committing itself to selling 30% of its new medium- and heavy-duty vehicles as zero emission by 2030. Maine considered adopting California’s Advanced Clean Trucks and Advanced Clean Cars II regulations, but the Board of Environmental Protection voted against it in 2024. Despite this setback, Maine demonstrates cognitive leadership through strategic guidance, as exemplified by the “Maine Won’t Wait” Action Plan.
Maryland
The Healthy Air Act (2006), which served as a springboard for the state’s entry into RGGI by limiting power sector emissions, marks the beginning of state-level climate policy in Maryland (Ruth, 2008). The state government then adopted California’s clean-car program (2007), developed a Climate Action Plan (2008), and established its first GHG emissions targets through the GHG Reductions Act (2009). EmPOWER Maryland (2008) subsequently created a platform for incentivizing energy efficiency through energy conservation measures in low-income households.
A stagnation period followed, without the adoption of dedicated climate policies, until the Clean Energy Jobs Act (2019) expanded offshore wind energy. The main turning point came with the Climate Solutions Now Act (2022), which introduced binding net-zero GHG emissions reduction targets for 2045 and required state agencies to develop statewide plans, as articulated in the 2023 Climate Pollution Reduction Plan. Maryland also adopted California’s Advanced Clean Cars II regulations (2023), mandating 100% ZEV sales by 2035.
Entering U2C in 2023 provided Maryland with credibility for its climate policy approach through joining a subnational platform. Maryland has not been a source of horizontal climate policy diffusion to other states, marking it as a follower rather than a leader. Although EmPOWER Maryland represents an innovative measure, the fact that is was not adopted by the other US states indicates that Maryland lacks cognitive leadership.
Nevada
Nevada started early in developing state-level climate policy with the adoption of the Renewable Portfolio Standard (1997) but stagnated until 2017, when the state government moved rapidly to catch up with the other US states (Selin & Vandeveer, 2021). Nevada first established a Clean Energy Fund, a green bank focused on financing clean growth for underserved communities. However, the inflection point was Senate Bill 254, which set both GHG emissions reduction and net-zero targets. Notably, Nevada’s GHG emissions reduction targets aimed for 45% below 2005 levels by 2030. These targets were complemented by the Climate Strategy (2020), which created a basis for cross-sectoral climate action.
In 2021, Nevada adopted California’s LEV III and ZEV requirements through its Clean Cars Nevada regulations, starting with model year 2025, and joined U2C—mostly for external validation, but also as part of broader efforts to improve climate ambition after years without progress. Subsequently, Nevada developed the Priority Climate Action Plan (2024) to reinforce near-term measures and reach its 2030 goal. The state’s trajectory has been that of a follower due to its stagnation period, but there are signs of emerging cognitive leadership .
New Jersey
New Jersey developed its climate policy through a sequence of laws and regulations, starting with the Global Warming Response Act (2007), which mandated early GHG emissions reduction targets and obliged the Department of Environmental Protection to enable monitoring and reporting systems for emissions. The state later enacted the Offshore Wind Economic Development Act (2010), creating wind renewable energy certificates and authorizing tax incentives for this sector.
Little progress was made besides the Solar Act (2012) while Chris Christie, a Republican governor, was in office (2010–2018), but momentum returned in 2018 when the state re-entered the RGGI and increased renewable energy portfolio and energy efficiency targets through the Clean Energy Act. The EV Act (2020) established EV sales targets and charging infrastructure goals, plus a fund to incentivize EV purchases. These measures were complemented by Executive Order 274 (2021), which filled gaps through an interim goal of 50% emission reductions by 2030.
Despite the stagnation in climate policy developed between 2010 and 2018, New Jersey had developed a robust state-level climate policy by the time it joined U2C (2024), elevating its international profile. Given strategic offshore wind development and multi-sector climate strategies, the state demonstrated cognitive leadership under Democratic governments.
Virginia
Virginia’s early approach to climate policy was limited to non-binding planning documents providing broad guidance, such as the Virginia Energy Plans (2007), but climate action gained momentum after joining U2C in 2017. Despite being a late addition to U2C, Virginia joined early for a southern state.
No immediate action followed its entry into U2C, but in 2020 the state enacted the Virginia Clean Economy Act, establishing binding clean electricity requirements and a Renewable Portfolio Standard. The act mandates retiring carbon-emitting electric generating units by 2045, with earlier deadlines for coal and large oil units (2024), and sets a non-binding goal of net-zero GHG emissions by 2045.
Virginia joined RGGI in 2020—late nationally but as the first southern state to do so, though an attempted withdrawal in 2023 has created legal uncertainty over whether Virginia is still part of the initiative. Momentum for climate policy continued in 2021 with the Commonwealth Clean Energy Policy, extending non-binding net-zero requirements to all major sectors. The transport sector advanced by adopting California’s Advanced Clean Cars I (2021) and Clean Cars II (2022) regulations, requiring 100% ZEV sales by 2035, but in 2024 Governor Glenn Youngkin of the Republicans announced Virginia would stop following Clean Cars II starting in 2025 (Governor of Virginia, 2024). Virginia had also adopted California’s LEV standards (2021), and building codes were updated for better energy efficiency (2023).
Rapid acceleration followed U2C membership, with Virginia beginning to show exemplary and cognitive leadership until 2022. However, under the current Republican governor, Virginia is no longer a leader or follower of leading US states, as the state's climate policy has regressed.
Comparative Analysis
Policies
Figure 3.1 reveals that California began implementing climate policies earlier and accumulated more policies than other founding members. Rather than relying on the U2C framework, the state functioned as its own source of climate policy innovation (Carlson, 2008). Washington entered later with moderate climate policy effort, then accelerated policy development in the late 2010s and early 2020s following the enactment of the Clean Energy Transformation Act (2019) and the establishment of a cap-and-invest program (2021)—changes that occurred once partisan veto points were removed.
Fig. 3.1
Adoption of climate laws, plans, programs, and regulations by founding signatories
Oregon’s policy curve shows a late start with minimal growth until the early 2020s, when the state pivoted toward programs and plans as alternatives for overcoming legislative gridlock. Vermont began policy development earlier than Oregon and Washington, albeit on a smaller scale, before making significant advances in the early 2020s through comprehensive planning initiatives (Sovacool et al., 2014).
While U2C membership corresponds with post-2015 policy adoption across all states, this alignment is less pronounced in California, which had already established its policy leadership before this period.
Figure 3.2 reveals distinct policy development patterns among early joiners. Massachusetts and New York emerge as the most prolific states in climate policy development, both accelerating after 2008/2009—likely driven by RGGI implementation and initial policy frameworks—followed by a second wave from 2019–2023 when Massachusetts enacted several pieces of new legislation.
Fig. 3.2
Adoption of climate laws, plans, programs, and regulations by early-joining signatories
Connecticut demonstrates a more gradual approach, beginning with laws before progressively adding programs, plans, and regulations that reflect initiatives in green banking and transportation. Minnesota experienced an extended plateau from 2008–2015, after which new climate policies emerged consistently.
New Hampshire presents a contrasting case, with minimal new climate policy expansion resulting in low overall totals by 2024. The state’s climate policy landscape remains dominated by non-legislative measures, reinforcing its follower or laggard status. Rhode Island began meaningful climate policy development in 2014, focusing primarily on plans with some legislation before constructing a comprehensive climate framework.
Examining the post-2015 U2C influence, New Hampshire shows limited policy growth during this period, while other early joiners demonstrate substantial increases in their climate policies following U2C participation.
Figure 3.3 illustrates that late joiners generally maintain a lower overall number of climate policies, though with notable variation in timing and approach. Colorado showed modest progress through the 2000s before laws and regulations gained prominence from 2019 onward. Hawaii experienced early advancement (2007–2009), followed by limited activity in the mid-2010s, then sustained climate policy growth.
Fig. 3.3
Adoption of climate laws, plans, programs, and regulations by later-joining signatories
Maine maintained steady policy growth until 2018, then increased climate policy development significantly in 2019–2020 before reaching a high plateau. Maryland’s climate policy development occurred in two distinct phases: initial activity from 2007–2012, followed by minimal change until a substantial push in 2022–2024 that aligned with the adoption of net-zero GHG emissions reduction targets and transportation regulations designed to accelerate progress toward decarbonization, as laid out in its 2023 Climate Pollution Reduction Plan.
Nevada presents a unique case, showing no policy activity until 2019, when it experienced rapid growth driven by comprehensive executive measures. New Jersey followed a similar pattern, with early activity from 2007–2012 followed by stagnation until 2018, when regulations increased notably through 2023.
Virginia had minimal climate policy before 2017—primarily plans—but demonstrated strong growth from 2018 to 2024 that included the enactment of legislative measures. However, the state now appears to be experiencing climate policy reversal.
Across all late joiners, the increase in the number of climate policies corresponds with the Paris Agreement timeframe. Regarding U2C influence, Colorado, New Jersey, and Maryland joined too recently to allow us for assessing potential network effects. Maine joined in 2021 but has not adopted new climate policies since. Nevada exhibits increased climate policy activity since its entry into U2C in 2021, while Hawaii (2018) and Virginia (2017) have long enough been members of U2C to suggest climate policy growth following membership.
Policy Instruments
Figure 3.4 reveals that California maintains the earliest, most extensive, and most diversified climate policy instrument portfolio, utilizing all instrument types except voluntary agreements. Notably, subsidies, standards, and certifications were implemented early on, while taxes, cap-and-trade, and public procurement emerged later. Bans, information instruments, and trading mechanisms began in 2015 and continue operating today.
Fig. 3.4
Policy instruments to reduce regional GHG emissions by founding signatories
Vermont also demonstrates early policy instrument diversification, implementing six instrument types before 2015 and adding two thereafter. Significantly, Vermont pioneered carbon cap-and-trade implementation even before RGGI’s launch in 2009.
Among late joiners, Oregon began the development of its climate policy with standards and subsidies before adding bans, public procurement, and taxes after 2015. Washington initially employed standards, information and trading instruments, and taxes, before incorporating cap-and-trade and bans post-2015.
Figure 3.5 demonstrates that standards represent the most prevalent and earliest policy instrument among early joiners, implemented by every state except New Hampshire. Carbon cap-and-trade systems follow as the second-most common policy instrument, primarily driven by RGGI participation. Minnesota presents a notable exception, lacking RGGI membership and utilizing only standards and subsidies.
Fig. 3.5
Policy instruments to reduce regional GHG emissions by early-joining signatories
Public procurement emerged later as a policy tool, appearing in New York, Connecticut, and Massachusetts. Certification and labeling programs were implemented exclusively in New York and Massachusetts from 2009 onward, while bans spread across multiple states—New York, Rhode Island, Massachusetts, and New Hampshire—during the mid-2010s through early 2020s.
In terms of policy instrument diversification, Massachusetts leads with eight different policy types, followed closely by New York with seven. Minnesota demonstrates the least diversification, employing only two instrument types.
The post-2015 period reveals varying levels of climate policy innovation: New York, Massachusetts, and New Hampshire each added three new instruments and Connecticut incorporated two, while Rhode Island and Minnesota showed no new instrument diversification during this timeframe.
Figure 3.6 reveals that standards typically emerged first among the late joiners, though overall instrument diversification remains more limited compared to earlier joiner groups. Nevada began earliest but subsequently only added subsidies. Information and training instruments appeared early across several states: Colorado (2008), Hawaii (2007), and Maine (2004).
Fig. 3.6
Policy instruments to reduce regional GHG emissions by later-joining signatories
Voluntary agreements show varied adoption patterns, starting in Virginia in 2000, arriving in Hawaii in 2009, and reaching Maine in 2020. Cap-and-trade systems primarily emerged through RGGI participation, beginning in Maine and Maryland in 2007. New Jersey followed a similar trajectory, initially joining RGGI before pausing after 2011 and rejoining in 2018. Virginia implemented cap-and-trade from 2020–2023, but renewal is uncertain due to the state’s RGGI withdrawal. Hawaii represents a unique case, initiating its own statutory cap-and-trade program in 2014.
Subsidies experienced distinct surges: Maryland, Virginia, and Nevada expanded these instruments around 2010, while Maine and New Jersey saw growth in the 2020s. Public procurement appeared in Maine from 2015–2018 through biomass contracts, with Hawaii and Maryland adopting similar instruments in 2021. Bans emerged more recently, featuring in Maryland since 2017 and Virginia since 2020. Notably, taxes remain absent across all late-joiner US states.
Regarding post-2015 instrument adoption, Hawaii (1), Maine (3), Maryland (2), and New Jersey (3) introduced new climate policy instruments during this period. Following U2C membership, only Hawaii added further instruments, though the timeframe remains limited for assessment. Consequently, most policy instruments predate U2C participation among late joiners.
Targets
Figure 3.7 elucidates that all U2C founding members established long-term GHG emissions reduction goals early, beginning in the mid-2000s. Medium-term goals followed a similar pattern, with Oregon as the notable exception, neglecting these until 2020. Short-term goals show more variation: only California implemented early short-term targets, while Vermont added these in 2020.
Fig. 3.7
Greenhouse gas emissions reduction targets by founding signatories
California has progressed beyond its initial benchmarks and currently maintains a 40% GHG emissions reduction goal by 2030 and carbon neutrality by 2045, both stemming from its foundational AB32 framework. Oregon’s medium-term goal adoption in 2020 followed its failed 2019 cap-and-trade legislative attempt, emerging as part of subsequent executive action.
The timing of goal-setting appears unrelated to U2C membership or the post-2015 period. However, the original U2C MoU proposed ambitious 80–95% GHG emissions reduction targets based on climate science.The 2021 MoU revision established the collective goal of achievingnet-zero GHG emissions by 2050. These changes demanded target strengthening by members who had not previously aligned with these standards.
Washington exemplifies this alignment process, with the Department of Ecology updating targets in 2020 while pursuing carbon neutrality ahead of the U2C revision, bringing the state into compliance with Paris Agreement commitments (Shields, 2023).
Figure 3.8 shows that the early joiners’ medium- and long-term GHG emissions reduction targets align perfectly, originating from identical policies. Connecticut, Massachusetts, and Minnesota established these targets around 2010, while New York implemented them in the early 2000s. Rhode Island delayed adoption until 2021, and New Hampshire remains without any targets. Among early joiners, Massachusetts alone has adopted short-term targets, doing so in 2021.
Fig. 3.8
Greenhouse gas emissions reduction targets by early-joining signatories
A notable target-setting wave occurred around RGGI’s launch, with subsequent updates across all participating states except New Hampshire and Rhode Island. While not visible in Fig. 3.8, these updates occurred between 2019 and 2023, establishing net-zero commitments by 2050 or similarly ambitious targets.
This timing corresponds with both the Paris Agreement framework and the period following the states’ entry into U2C, suggesting alignment between state-level goal-setting and broader international and coalition commitments.
Figure 3.9 shows that late joiners have not yet established short-term targets. Target adoption patterns vary significantly across states and timeframes. Virginia uniquely focused on long-term targets, implementing these in 2020 through the Clean Economy Act.
Fig. 3.9
Greenhouse gas emissions reduction targets by later-joining signatories
For medium-term targets, Colorado, Hawaii, Maryland, and New Jersey adopted these around 2010—a full decade before Virginia’s target-setting began. Maine and Nevada followed later, establishing medium-term goals in 2019.
Long-term target adoption shows two distinct waves: Colorado (non-binding), Maine, and New Jersey implemented long-term targets before 2010, while the remaining states adopted these during the post-2015 period. Regarding U2C influence, only Virginia and Hawaii established long-term targets after joining the coalition. Hawaii’s targets emerged through Act 15 (2018), passed by the state legislature.
Leadership
Our analysis shows a varied pattern of leadership roles by the US states that joined U2C. California has been cast as a super regulator and diplomatic entrepreneur using its unique economic weight and CAA waiver to innovate climate policies that have diffused across the US states and beyond (Boyd, 2017; Leffel, 2018; Mazmanian & Jurewitz, 2020). The state also shared its regulatory templates and long-term targets, exemplifying all four leadership types outlined by Wurzel et al. (2019): cognitive (agenda-setting), structural (legal leverage and market power), entrepreneurial (coalition-building), and exemplary (policy innovation).
Other founding members, like Oregon and Washington, oscillated between leadership and followership as internal partisan veto points limited structural leadership and forced those states to rely on executive action and U2C membership to reinforce the legitimacy of climate action (Meckling & Trachtman, 2024).
Early joiners, like Massachusetts and New York, show how U2C can amplify existing climate policy trajectories. Both states already had climate targets in place when joining U2C through RGGI and state policies, but U2C membership provided a platform to show cognitive and exemplary leadership with measures such as the Community Protection Act (2019) adopted by the government of New York. On the other hand, Minnesota and New Hampshire represent cases of followership, adopting fewer and less ambitious climate policy instruments before and after joining U2C, which for them was more of an external benchmark than a driver of policy innovation (Busby & Urpelainen, 2020). The differences suggest that besides U2C membership, leadership hinges on several factors such as pre-existing state capacity, political will, and regional collaborations. Even international agreements such as the 2015 Paris Agreement form part of the conditions faciliating climate leadership.
Late joiners, such as Colorado, Hawaii, and Maine, already had domestic climate policies in place and, in this case, U2C seems to have been strategically useful for increasing the visibility of their state-level climate action among other US states. Hawaii pioneered a renewable electricity target (2015) and Colorado was the first to implement methane leakage regulations (2014); although both measures predated membership, they became better known through U2C affiliation (Martin & Bergh, 2019). Maryland, Nevada, and Virginia highlight not just the conditions for exerting leadership but also for effective followership. They embraced ambitious statutory GHG emissions reduction targets in the 2020s, but reversals or partisan disputes have constrained the credibility of these steps.
Conclusion
The US states that have joined U2C at different points in time offer numerous instructive insights, which help us to assess the propositions formulated in Chap. 2.
Regarding Proposition 1 (see Chap. 2), U2C founding members have demonstrated the most sustained and diversified climate policy approach. California leveraged its legal waiver and environmental policy legacy to set the pace for policy-based climate action in the United States. The empirical picture for Washington, Oregon, and Vermont also suggests that reputational pressure affected their climate policy effort, though it was not as consistent over time compared to California. Early U2C joiners exhibited similarly uneven empirical patterns in terms of policy-based climate action. Massachusetts and New York expanded existing climate policy frameworks with statutory targets, while Connecticut, Rhode Island, and Minnesota pursued mixed approaches that integrated regional mechanisms like RGGI and finance innovation without always consolidating comprehensive frameworks. New Hampshire represents an early membership example failing to elevate ambition—despite possible reputational damage—due to weak domestic political will. This outcome might be expected from a late joiner, but returning to Proposition 2 (see Chap. 2), reputational costs seem to have proved insufficient to overcome internal political divisions and maintain high climate policy ambition.
Later entrants joined too recently for definitive confirmation, but broadly speaking, their climate policy effort has remained the lowest—though this trend preceded U2C membership. However, Colorado and Hawaii managed to build ambitious climate policy frameworks before joining the coalition. Maine, Maryland, and Nevada show weaker policy-based climate action but did align with U2C targets after rebooting their climate policy frameworks. The timing suggests that reputational incentives remained secondary to internal political cycles and shifts in federal politics.
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The GWSA is a collective term for laws in various US states that aim to reduce GHG emissions. To distinguish California’s act from those subsequently adopted by other US states, we use the term “AB32” for California’s version.
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