Fusing subnational with national climate action is central to decarbonization: the case of the United States

The study addresses how subnational (state, city, business) climate actions can be integrated with national strategies to accelerate U.S. decarbonization. Against insufficient national progress and limited political support for rapid action, non-federal actors have expanded commitments, potentially delivering significant emissions reductions. The paper frames the research question: to what extent can existing and enhanced subnational commitments shape national emissions trajectories and provide a foundation for more ambitious federal policy consistent with 1.5–2 °C pathways? It highlights the Paris Agreement’s recognition of multi-level governance, the rise of coalitions (e.g., We Are Still In, U.S. Climate Alliance), and the need for robust methods to quantify and aggregate diverse, overlapping subnational policies while avoiding double counting.

Prior work has long emphasized the importance of diversified, non-state climate action and coalition-based approaches, embedded in the Paris Agreement preamble. Post-Paris, international cooperative initiatives (e.g., on coal phaseout, science-based targets, HFCs, industrial efficiency) have proliferated, with estimates suggesting up to 15–18 Gt CO2e/yr potential beyond NDCs. Global assessments have begun quantifying non-state action impacts but often lack national detail. In the U.S. context, literature documents states’ critical role in energy and climate policy in a federal system, through RPS/CES, energy efficiency, transportation standards, and land-use authority. Studies have also noted challenges of state–federal policy interactions, potential conflicts, and political dynamics. Earlier projections of U.S. emissions under current policies (e.g., Kuramochi et al.; Rhodium Group) provided national ranges broadly consistent with this paper’s current-measures scenario but did not fully incorporate the recent wave of subnational commitments or detailed aggregation methods presented here.

The authors develop a two-step, integrated analytical framework linking bottom-up aggregation of subnational policies to an economy-wide integrated assessment model (IAM), GCAM-AP (a modified GCAM-USA with 50-state resolution). Steps: (1) Scenario development: three scenarios through 2030—Current Measures (existing enacted federal and non-federal policies only), Enhanced Non-federal (scaled-up state/city/business actions across sectors using a tiered ambition framework), and Comprehensive (federal policies layered on top of enhanced non-federal actions). (2) Data collection and state-level aggregation: survey all 50 states and 285 large cities; identify high-impact actions; gather policy, target, and activity data; estimate reference (no-policy) sectoral baselines; quantify impacts for each actor level; aggregate to state-by-sector metrics while eliminating overlaps and double counting. Actions include RPS/CES, coal retirements, nuclear retention, oil and gas methane controls, RGGI caps, energy efficiency resource standards (EERS), ZEV mandates and procurement, vehicle standards, HFC regulations and programs, and land sink maintenance. (3) Economy-wide modeling: convert state-level sector metrics (e.g., TWh renewables, EV sales, HFC reductions) into GCAM-AP inputs. GCAM-AP represents detailed energy resources, transformation sectors, and technologies (including CCS/CCUS, renewables, nuclear), solving for market equilibrium in 5-year steps with exogenous technology learning. Some actions (coal retirements, land sink) are input directly. Sensitivity analysis explores uncertainty in socioeconomics (population/GDP), fossil fuel prices, renewable costs, coal retirements, and land sink magnitude. Scenario specifics: Current Measures only includes binding/implemented policies (e.g., 28-state RPS raising renewables to 26% generation by 2030; coal to 16%; nuclear at 17%; EERS in 20 states yielding cumulative 1566 TWh electricity and 2360 Bcf gas savings, 541 Mt CO2e avoided 2020–2030; methane regs in 7 states plus federal new-source rules at 75% effectiveness, 995 Mt CO2e cumulative avoided; RGGI, 160 Mt CO2e cumulative avoided; ZEV mandates in 10 states with 13.5 million cumulative EV sales 2020–2030; HFC regs in select states plus federal leakage standards, 160 Mt CO2e cumulative avoided; land sink held at −714 Mt CO2e). Enhanced Non-federal tiers states by ambition and scales policies: clean electricity to 61% (40% renewables, 17% nuclear) by 2030; coal ~7% of generation; gas without CCUS ~32%; oil and gas methane −34%; buildings direct emissions −28% vs. 2005 via electrification and strong EERS; transportation EVs reach 61% of LDV sales; heavy-duty EVs 15% of new sales in Tier 1/2; LDV ICE efficiency improves 4%/yr post-2026; cumulative EV sales ~62 million (2020–2030); industry direct CO2 −5% vs. 2005; HFCs and F-gases −6% vs. 2005; land sink +~80 Mt CO2e vs. today; economy-wide caps in Tier 1 reduce national emissions ~10%. Comprehensive layers federal actions: federal CES and tax incentives drive ~77% clean electricity (49% renewables) by 2030; near-total coal phaseout; conventional gas 23% and gas with CCUS 12% of generation; nationwide methane −60%; all new buildings electrified; appliance electrification from 2030; federal retrofit financing; buildings direct emissions −31% vs. 2005; EVs 62% of LDV sales and 100% of bus sales; ICE efficiency improves 4%/yr 2026–2030; scrappage incentives; cumulative EV sales ~64 million; industry direct CO2 −7.5% vs. 2005; HFCs −37% vs. 2005; land sink +~167 Mt CO2e vs. today; economy-wide caps reduce national emissions ~11%. The methodology aligns with ICAT guidance, Global Covenant of Mayors methodology, and GHG Protocol standards.

- Coalition scale: By 2019, U.S. subnational actors with climate commitments represented ~68% of GDP, ~65% of population, and ~51% of emissions—comparable to the world’s second-largest economy. - National emissions outcomes: Current Measures deliver ~19% below 2005 by 2025 and ~25% by 2030 (range 21–29% in 2030; 17–21% in 2025). Enhanced Non-federal can achieve up to ~37% below 2005 by 2030 without new federal policies. Comprehensive (non-federal plus ambitious federal action) reaches ~49% below 2005 by 2030 (sensitivity range ~46–52%), broadly consistent with a 1.5 °C-aligned pathway for the U.S. - Decarbonization rates: Historical 2005–2017 average reductions ~1%/yr; after 2025, Enhanced Non-federal ~2.8%/yr and Comprehensive ~5.2%/yr, compared to ~4.4%/yr average needed to reach mid-century net zero. - Sectoral contributions (2030): Largest near-term reductions arise in the power sector via renewables expansion and rapid coal decline; buildings and transportation contribute through electrification, efficiency, and EV adoption; industry, methane, and HFC measures are essential to reach higher-ambition outcomes; land sink enhancement adds 80–167 Mt CO2e/yr relative to today depending on scenario. - Power system transformation: Clean electricity share reaches ~61% (Enhanced Non-federal) and ~77% (Comprehensive) by 2030; coal generation falls to ~7% (Enhanced Non-federal) and near-zero (Comprehensive). New clean capacity additions exceed 75 GW/year by 2030 in high-ambition cases, surpassing historical peaks. - Transportation: Under Enhanced Non-federal, EVs reach ~61% of LDV sales; cumulative 2020–2030 EV sales ~62 million; transportation liquid fuel demand drops ~21% from 2005. Under Comprehensive, EVs reach ~62% LDV sales and 100% bus sales; cumulative EVs ~64 million; liquid fuels down ~22% from 2005. - Methane and HFCs: State actions reduce oil and gas methane ~34% (Enhanced Non-federal) and ~60% nationwide with federal rules (Comprehensive). HFCs and F-gases fall ~6% vs. 2005 (Enhanced Non-federal) and ~37% (Comprehensive). - Validation against prior studies: Current Measures estimates align with prior national projections (e.g., ~15–26% by 2030), while the paper uniquely quantifies higher-ambition subnational and integrated scenarios.

The findings demonstrate that bottom-up, multi-level climate action can materially shift national emissions trajectories and serve as a foundation for more ambitious federal policy. Aggregated subnational policies in power, buildings, transportation, industry, methane, HFCs, and land use can deliver substantial near-term reductions, moving the U.S. toward pathways consistent with global temperature goals. The Enhanced Non-federal scenario shows that, even absent federal policy changes, expanding leading state and city actions can approach a 2 °C-consistent pathway and set the stage for deeper cuts. The Comprehensive scenario illustrates how federal policies can efficiently stitch together and amplify subnational efforts—e.g., by national clean electricity standards, strengthened methane rules, vehicle standards, and support for electrification and industrial decarbonization—achieving about 49% reductions by 2030, broadly aligned with a 1.5 °C trajectory. This bottom-up-first approach can enhance political feasibility and policy durability by tying decisions to local constituencies and co-benefits (health, jobs, cost savings), while informing national strategy design with proven subnational tools. However, multi-level governance is subject to political contestation and uneven adoption, underscoring the need to understand enabling conditions and to address coordination challenges.

This paper develops and applies a coupled aggregation–IAM framework to quantify how subnational commitments can be fused with federal policy to accelerate U.S. decarbonization. It shows that existing policies can reach ~25% below 2005 emissions by 2030, enhanced non-federal actions up to ~37%, and comprehensive national strategies that build on subnational action about ~49% by 2030. Sectoral roadmaps indicate rapid power decarbonization, transportation electrification, building electrification and efficiency, methane and HFC mitigation, industrial improvements, and land-based carbon enhancement as central levers. The approach provides a transferable template for other countries to assess bottom-up contributions and to design nationally determined contributions grounded in local action. Future research should expand high-quality data coverage (especially for businesses and additional subnational actors), develop more accessible versions of the methodology for broader use, and investigate social and governance mechanisms that enable durable scaling of bottom-up actions.

- Data constraints: Incomplete or heterogeneous data for subnational actions—especially corporate initiatives—likely render estimates conservative. - Aggregation complexity: Avoiding double counting across overlapping city, state, and corporate policies requires assumptions that introduce uncertainty. - Modeling uncertainties: Results depend on assumptions about technology costs, fossil fuel prices, economic and population growth, coal retirements, and land sink magnitude; sensitivity ranges partially capture but do not exhaust uncertainties. - Scope and transferability: The U.S. federal system affords substantial subnational authority; results may differ in countries with different governance structures. - Political uncertainty: Implementation and durability of policies are subject to political dynamics and potential legal challenges. - Resource intensity: The approach relies on advanced modeling and significant research effort; simplified methods are needed for broader application.