The global transition to mitigate severe climate risks requires a rapid and unprecedented societal transformation. However, national governments haven't implemented actions at a sufficient pace and scale, lacking robust political support. Consequently, various actors—states, cities, businesses, etc.—are taking significant emission reduction measures, challenging conventional understanding of global success models and their potential impact. The value of this "bottom-up" action in achieving 1.5 °C compatible pathways is potentially much greater than previously realized. This principle of diversified, non-state action has been recognized for decades, forming the basis for climate actor coalitions even before the Paris Agreement. The Paris Agreement preamble recognizes the importance of multi-level government and diverse actor engagement. Post-Paris, these activities expanded, highlighting the potential impact of broader engagement. International initiatives focusing on carbon neutrality, science-based targets, and various sectors could reduce emissions significantly beyond national commitments alone. Domestically, numerous non-national governments and actors are committing to increasingly ambitious climate actions. However, our understanding of these impacts on national emissions and enabling policy platforms remains limited. Methodologies are evolving to better understand the potential for rapid emissions reductions, accounting for diverse targets and policies at various scales. Data availability and integration methods into national-level projections remain challenges. This paper focuses on the United States, quantifying the potential scope and implications of current actions by states, cities, and businesses using new aggregation and analysis methods. It evaluates the potential for widespread application of the examples set by the most ambitious actors and demonstrates how expanded action can provide a higher policy ambition baseline for further federal action.

The paper draws upon a substantial body of literature highlighting the importance of subnational and non-state actors in climate action. It references studies emphasizing the insufficient pace of national government action (Rockström et al., 2017; Höhne et al., 2017; Iacobuta et al., 2018; The London School of Economics and Political Science, 2018; UNEP, 2018), the role of the Paris Agreement in fostering non-state engagement (Hale, 2016; Rayner, 2010; United Nations, 2015), and the increasing number and scope of such initiatives (Hsu et al., 2015; Chan et al., 2015; Meckling et al., 2015; Hsu et al., 2018; Kuramochi et al., 2017; Data Driven Yale, NewClimate Institute, PBL, 2018; NewClimate Institute et al., 2019; Hsu et al., 2017). The literature also acknowledges challenges in assessing the aggregate impact of these actions (Hsu et al., 2019), including data heterogeneity and the need for sophisticated modeling approaches. Existing research on US state-level climate policy is cited, showing their potential to drive national emissions reductions (Rabe, 2008; Peterson & Rose, 2006; Martin & Saikawa, 2017), and the role of multi-level governance (Goulder & Stavins, 2011) and the interaction between federal and state level actions (Vandenbergh & Gilligan, 2017). The authors refer to previous assessments of U.S. emissions reduction potential, such as Kuramochi et al. (2019) and the Rhodium Group (Larsen et al., 2017), to contextualize their findings.

The research employed a novel, comprehensive strategy to assess the scope and emissions impact of non-federal climate actions in the United States. This involved integrating data on baselines and sector-specific policies and commitments into an economy-wide estimate of the U.S. emissions trajectory to 2030. The process included data collection, baseline development, aggregation, and modeling, supplemented by stakeholder engagement to inform policy scenarios. The study utilized a modified version of the open-source global integrated assessment model GCAM-USA, renamed GCAM-AP for this study, with a 50-state resolution. Because the resolution of IAMs is insufficient to capture detailed impacts of diverse non-federal actions, a new methodology was developed to aggregate data on current sub-national measures for interaction with GCAM-AP. The aggregation process drew from GCAM inputs, aggregating the effects of diverse policies affecting emissions while avoiding double-counting. The process was calibrated to GCAM-AP baselines, and a structured methodology determined included policies, impacts, and the extent of activity beyond previous policy architectures. Historical emissions data, activity data, and policy/target information from various sources were used in the sector-level analysis and projections. GCAM-AP then used the sectoral outputs from the aggregation to assess the economy-wide emissions impacts. Three scenarios were projected: 1) Current Measures (existing policies), 2) Enhanced Non-federal (scaled-up non-federal actions), and 3) Comprehensive (enhanced non-federal plus ambitious federal action). The enhanced non-federal scenario tiered states based on the likelihood of accelerated action, using proxies like climate organization membership, leadership, targets, and existing policies. The comprehensive scenario built upon the non-federal scenario, adding federal policies that complement non-federal actions. The scenarios were analyzed for emissions reductions considering socioeconomic and technological change, fossil fuel prices, and land sink size. Sensitivity analyses were conducted to assess uncertainties and provide a range of possible emissions associated with different levels of climate action.

The study found that current measures (existing policies) would deliver a 25% reduction in emissions below 2005 levels by 2030 (with a range of 21-29%). Expanding high-impact actions across leading states in the enhanced non-federal scenario could increase reductions to 37% below 2005 levels by 2030. A comprehensive strategy combining enhanced non-federal action with renewed federal engagement could achieve reductions of approximately 49% by 2030. The comprehensive scenario's emission reductions are consistent with a global 1.5°C trajectory. The non-federal scenario alone would put the U.S. on a pathway roughly consistent with a 2°C trajectory. Both higher-ambition scenarios showed an accelerating annual decarbonization rate, surpassing the recent average rate. The power sector, buildings, and transportation showed the biggest near-term reduction possibilities for state and city policy leverage. Reductions consistent with higher-ambition outcomes require additional activity in other sectors. The largest 2030 reductions arise in the power sector, but actions in other sectors support deeper emission reductions beyond 2030. The power sector transition in higher-ambition scenarios would involve rapid changes, surpassing past historical capacity addition rates. The estimates for the impact of existing policies, including state and city actions (current measures scenario), are broadly consistent with coarser estimates made by other researchers (Kuramochi et al., 2019; Rhodium Group, Larsen et al., 2017). The higher-ambition scenarios in this study represent novel findings.

The findings demonstrate the significant potential of subnational actions to contribute to national-level climate goals. The results highlight an opportunity to enhance the long-term effectiveness of climate policy by involving diverse actors, connecting decisions more directly to constituents, and incorporating local or regional benefits. This bottom-up approach addresses political challenges associated with ambitious national climate policies. The study acknowledges potential political challenges and limitations in data availability, particularly concerning business actions and data limitations in other parts of the world. The approach used in the study is transferable, although further development is needed. The authors emphasize the importance of further research on the social and governance mechanisms by which bottom-up action can generate robust long-term results.

The study concludes that integrating subnational and national climate action is crucial for achieving ambitious emission reduction targets. The US case study shows how subnational efforts can significantly reduce emissions and lay the groundwork for more comprehensive national policies. Further research should focus on data collection and refining methodologies for analyzing subnational actions in different contexts globally, and further explore the social and governance mechanisms driving successful bottom-up climate action.

The study acknowledges several limitations. Data limitations on some actions, particularly certain business actions, likely lead to underestimation of their potential impact. Data scarcity is more acute in many parts of the world, highlighting a need for better data collection strategies. The resource-intensive nature of the advanced modeling approach used suggests the need for developing simpler, more accessible methods. The analysis primarily focuses on emissions impacts, without a comprehensive examination of social and governance aspects of subnational action.