Tracing U.S. Fuel Life-Cycle Greenhouse Gas Emissions in a Multisector Dynamics Model Using LC-GCAM

Model-based analysis of fuel pathways is essential for informing energy and environmental policies. Two major model types are typically used: multisector dynamics models, which capture the broader energy economy, such as GCAM (Global Change Analysis Model), and life-cycle assessment models, such as GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation). Each has distinct strengths and limitations, and recent studies have increasingly adopted hybrid approaches to harness the advantages of both. However, such integration is often time-consuming and complicated by inconsistencies in the system boundaries and technology definitions. We present LC-GCAM, a new tool that enables estimation of life-cycle greenhouse gas emissions and primary energy use for any fuel pathway represented in GCAM. We apply LC-GCAM to 300 scenarios designed to explore key uncertainties affecting the life-cycle performance of future fuel options in the U.S. freight sector. To evaluate LC-GCAM, we compared its results with those from GREET for nine fuel types in a 2030 reference scenario. When input assumptions are modestly aligned, LC-GCAM and GREET estimates typically agree within 13% (aggregate absolute-sum error), although discrepancies can be larger for pathways involving large amounts of land use change emissions. LC-GCAM offers a flexible and efficient approach to generating life-cycle metrics within an integrated modeling framework, supporting robust policy analysis across a wide range of interacting energy system uncertainties.