Exploring decarbonization pathways for USA passenger and freight mobility

The transportation sector, historically a significant source of GHG emissions (28% in the US), is undergoing a potential transformation. While advancements in alternative fuels and powertrains, new mobility options, and climate change mitigation efforts offer opportunities for decarbonization, the future of mobility remains uncertain. Past projections often underestimated the sector's decarbonization potential, relying heavily on carbon capture and storage to compensate for limited emission reductions from transportation itself. However, recent progress in electric vehicles (EVs) and other clean fuel technologies, along with shifts in thinking about achieving structural mobility changes, suggest significantly greater potential for affordable and sustainable transportation decarbonization. Key strategies include vehicle electrification (supported by a decarbonized electricity grid), increased public transit and active travel, improved urban planning, low-carbon fuels, and coordinated policies.

Existing literature highlights the challenges of reducing transportation emissions, citing inevitable demand increases, slow turnover of vehicle stock and infrastructure, and the sunk costs associated with the petroleum-dominant transportation system. Studies have shown varied projections for emissions pathways across different models, with transportation often displaying the largest variation and residual emissions. While some optimistic projections suggest aggressive emissions reductions by 2050 through strategies such as vehicle electrification, biofuel use, and mode shifts, skepticism remains regarding the efficacy of some decarbonization strategies. Improved fuel efficiency may negatively impact mode shifting and alternative fuel adoption. Many studies emphasize the necessity of reducing travel demand to achieve significant decarbonization. The effectiveness of shared mobility services is also debated, with some studies suggesting potential congestion and induced trips, while others show emission reductions in specific contexts. The limited availability of high-resolution models that collectively capture the transportation system and incorporate new technologies and mobility solutions further complicates accurate projection of the future.

This study employs the Transportation Energy & Mobility Pathway Options™ (TEMPO) model to simulate the evolution of U.S. passenger and freight mobility emissions to 2050. TEMPO is a novel model that incorporates household-level travel decisions, technology adoption across diverse demographics and geographies, and a disaggregation of freight travel demand by operating segment. Two analyses are conducted: (1) a univariate sensitivity analysis varying one input at a time to rank variables by their isolated impact on 2050 emissions; and (2) a multivariate uncertainty analysis exploring a broad range of scenarios by varying multiple inputs in parallel to identify potential pathways to deep decarbonization. A baseline scenario aligned with the 2019 Annual Energy Outlook (AEO) reference case provides a comparative benchmark. 2173 long-term scenarios were simulated, varying 51 exogenous input variables based on expert elicitation and literature review, representing optimistic and pessimistic bounds. The Sobol sequence algorithm was used to select a quasi-uniform distribution of variable combinations for the multivariate analysis.

Univariate analysis revealed that mandating 100% light-duty ZEV sales by 2030 was the most impactful variable in reducing emissions, followed by reductions in battery costs and increased residential charging access which significantly impacted BEV adoption. Multivariate analysis, incorporating 2000 scenarios, showed a wide range of emission reduction outcomes, with the maximum potential reaching an 89% GHG reduction relative to 2019. Deep decarbonization (80% reduction from the 2050 baseline) scenarios consistently featured a fully decarbonized electricity grid coupled with significant reductions in travel demand and widespread ZEV adoption. The median deep decarbonization scenario projected 1000 TWh of electricity demand for mobility in 2050, but the range spanned 120-3000 TWh, emphasizing the uncertainty around future electricity needs. High ZEV adoption, particularly in the freight sector, led to significant increases in electricity demand, even in scenarios with reduced travel demand. Sustainable biofuel played a crucial role, particularly for aviation, with projected needs ranging from 10-42 billion gallons in 2050 for domestic travel alone. The most consistent driver of emissions reductions across deep decarbonization pathways was reduced travel demand, with 88% of these scenarios showing reductions in passenger or freight travel demand. Deep decarbonization scenarios showed high ZEV adoption (median 76% light-duty and 68% medium-heavy-duty stock share in 2050), although a small number achieved decarbonization even without significant ZEV adoption through substantial biofuel use and travel demand reduction. In the freight sector, scenarios with significant FCEV adoption required high hydrogen refueling infrastructure, low hydrogen prices, and substantial improvements in FCEV costs and performance.

The findings demonstrate that achieving deep decarbonization requires a multifaceted approach involving not only technological advancements (widespread ZEV adoption) but also behavioral changes (reduced travel demand) and supportive policies (decarbonized grid, sustainable biofuel production). The study highlights the critical role of a decarbonized electricity grid, underscoring the interconnectedness between the power and transportation sectors. While some pathways to deep decarbonization are possible without significant travel demand reductions, these pathways necessitate even more rapid and extensive ZEV adoption than previously projected. The substantial uncertainty around future electricity demand underscores the importance of developing strategies for managing travel demand to optimize electricity system planning and investments. The high reliance on biofuels in many scenarios, especially for aviation, necessitates consideration of biomass supply constraints and the need for significant improvements in biofuel life-cycle emissions.

This research presents a comprehensive exploration of decarbonization pathways for U.S. mobility, showing that no single solution exists. Achieving deep decarbonization requires a combination of rapid ZEV adoption, a clean electricity grid, sustainable biofuel production, and potentially reduced travel demand. Future research should focus on endogenous modeling of travel demand, improving the representation of long-distance travel options (including high-speed rail and AVs), examining the role of MaaS, and further investigating the impact of charging infrastructure on ZEV adoption. This work stresses the need for integrated planning of the energy and transportation sectors to effectively address climate change.

The TEMPO model does not currently incorporate endogenous feedbacks between travel demand, pricing mechanisms, or urban form. The analysis does not include all modes of transportation (e.g., autonomous vehicles, micromobility), and the life-cycle emissions of vehicle manufacturing and infrastructure are not explicitly considered. The study assumes exogenous reductions in travel demand, and does not model the potential impact of changes in urbanization patterns and competition for biomass resources. Finally, the model's assumptions regarding biofuel supply and aviation technology are simplified.