Modeling Heterogeneous Subnational Climate Action through a Sectoral Tiering Framework: Implications for U.S. Decarbonization Pathways

Kiara Ordonez-Olazabal, Alicia Zhao, Kowan T.V. O'Keefe, Matthew Binsted, Claire V. Squire, Stephanie Vo, Adriana Bryant, Ryna C, Steven J Smith, Kathleen M. Kennedy, Nate Hultman

State governments play a critical role in reducing greenhouse gas emissions to meet the United States’ Paris Agreement targets. However, states vary widely in their capacities and inclinations for enacting climate mitigation policies. Here, we present a sectoral tiering framework to assess state-level mitigation performance across sectors. We implement the tiering system in an integrated assessment model with 50-state resolution of the United States (GCAM-USA) to quantify how heterogeneous ambition shapes national outcomes. We find that in most sectors, states cluster in the lowest-performing tier, which accounts for the majority of sectoral emissions. Enhanced state ambition differentiated by tier can reduce 2035 emissions by 42–51% below 2005 levels, compared with 35% under existing measures. Modest increases in policy adoption and stringency in low-performing states yield larger reductions than comparable increases in high-performing states. These results provide a transparent approach for integrating subnational variation into decarbonization modeling.