The Paris Agreement necessitates ambitious near-term climate targets to limit global warming. The United States, as a major emitter, plays a crucial role in achieving these goals. Its current NDC pledges a 50-52% emissions reduction by 2030 and net-zero by 2050. Recent legislation, such as the Inflation Reduction Act (IRA) and the Bipartisan Infrastructure Law (BIL), represent significant investments in clean energy and emissions reduction. However, achieving more ambitious targets requires a comprehensive assessment of potential pathways. This study addresses this need by modeling existing and enhanced policies across all sectors and greenhouse gases to determine the potential for significantly deeper emissions cuts by 2035. The research question is: What level of greenhouse gas emissions reduction is achievable in the US by 2035 through a high-ambition climate action plan encompassing all sectors and gases, building upon existing policies and incorporating additional measures?

The existing literature highlights the inadequacy of current 2030 NDCs in meeting long-term Paris Agreement goals, emphasizing the need for increased near-term ambition. Studies have examined the potential of various policies, including federal initiatives like the IRA and BIL, and subnational actions by states and cities in driving emissions reductions. This research builds upon these studies by incorporating a more comprehensive and integrated assessment of policies across all sectors, including transportation, electricity, buildings, industry, and methane emissions, to create a detailed pathway analysis for a 2035 target.

The study employs the GCAM-USA-CGS integrated assessment model, which provides state-level detail across all sectors and gases. The model incorporates existing policies from federal and non-federal sources, including the IRA, BIL, and key state-level policies like renewable portfolio standards (RPS), electric vehicle (EV) sales targets, and zero-emission appliance standards. Two scenarios are modeled: 'Current Policies,' representing existing policies, and 'Enhanced Ambition,' which expands upon the Current Policies scenario by incorporating additional, more ambitious policies. The Enhanced Ambition scenario includes actions such as a full phaseout of coal-fired electricity by 2030, accelerated ZEV adoption, zero-emission appliance standards, a comprehensive methane fee, and extensions of IRA tax credits. States are categorized into tiers based on their climate ambition, with Tier 1 states adopting the most ambitious policies, Tier 2 states showing increased ambition, and Tier 3 states making minimal changes. The model assesses GHG emissions reductions relative to 2005 and 2020 levels, providing context on the magnitude of change needed. The model also incorporates sensitivity analyses, varying assumptions on GDP, population growth, energy prices, technology costs, and land-use change to provide a range of potential emission outcomes.

The 'Current Policies' scenario projects a 44% (range: 37-52%) reduction in net GHG emissions by 2035 compared to 2005 levels. The 'Enhanced Ambition' scenario, however, projects a significantly larger reduction of 65% (range: 59-71%). The electricity sector shows the largest emissions reductions in both scenarios, with the Enhanced Ambition scenario projecting a substantial increase in renewable energy sources and phasing out unabated coal-fired power generation. The transportation sector exhibits considerable emissions reductions driven by accelerated EV adoption under the Enhanced Ambition scenario. EV sales targets for both light-duty vehicles and freight trucks are significantly higher in the Enhanced Ambition scenario than under Current Policies. Furthermore, the Enhanced Ambition scenario incorporates vehicle miles traveled (VMT) reduction policies to further decrease transportation emissions. In the buildings sector, the Enhanced Ambition scenario shows substantial emissions reductions due to extended IRA electrification incentives, state-level zero-emission appliance standards, and strengthened efficiency standards. The industrial sector experiences a shift towards electrification and carbon capture, utilization, and storage (CCUS) technologies under the Enhanced Ambition scenario, resulting in greater emissions reductions compared to the Current Policies scenario. Methane mitigation plays a significant role in achieving the Enhanced Ambition targets, with policies implemented to reduce emissions from fossil fuels and agriculture. Finally, negative emissions from land-use, land-use change, and forestry (LULUCF) and carbon dioxide removal (CDR) technologies (direct air capture and BECCS) contribute to the overall emissions reductions in the Enhanced Ambition scenario. Key metrics, such as renewable electricity share, clean electricity share, EV penetration, and space and water heating electrification, are significantly higher under the Enhanced Ambition scenario.

The findings demonstrate that a significantly deeper emissions reduction pathway is achievable in the US by 2035 through a high-ambition approach that goes beyond existing policies. The success of this pathway hinges on the rapid and widespread implementation of policies across various sectors. The transportation sector's potential for rapid decarbonization is highlighted, although challenges remain in scaling up EV charging infrastructure, addressing critical mineral supply chains, and reducing travel demand. The buildings and industrial sectors present challenges due to the high capital costs of electrification and the technical difficulties in decarbonizing high-heat industrial processes. Expanding low-carbon electricity generation to support the higher electrification rates in the Enhanced Ambition scenario will require addressing interconnection queue backlogs, streamlining permitting processes, and potentially implementing utility reform. The role of emerging technologies like direct air capture and BECCS in achieving net-zero goals is also emphasized, acknowledging the need for cost reductions and technological advancements. The study highlights the importance of multi-level governance, involving federal, state, and local governments, to ensure policy durability and to account for potential political and social uncertainties.

This study shows that a 65% reduction in US GHG emissions by 2035 is achievable through a high-ambition scenario incorporating enhanced policies across all sectors. The success depends on rapid policy implementation and technological advancements. Future research could focus on detailed analyses of specific policy instruments and the development of robust strategies to address the challenges of policy implementation, technological deployment, and ensuring broad public and political support for climate action.

The model does not explicitly capture all political, social, and behavioral factors that could influence policy implementation and the uptake of new technologies. Uncertainties exist regarding the availability and cost of critical minerals for EV production, the economic feasibility and public acceptance of certain technologies like direct air capture, and the potential for future policy rollbacks. The model's assumptions regarding future economic growth, population change, and energy prices also introduce uncertainties.