The National Academies of Sciences, Engineering, and Medicine (NASEM) announced on October 19, 2022, the release of a report finding that life cycle assessments (LCA) of transportation fuels are valuable tools for measuring environmental impacts, but uncertainties remain in the current models and further research should be conducted to strengthen their reliability. The report recommends ways to improve models, increase reporting and transparency, perform targeted verification of emissions, and other avenues that will better inform policymaking for reducing greenhouse gas (GHG) emissions from transportation fuels. According to NASEM, LCAs have been increasingly applied in the development of transportation fuel policy to estimate and help reduce GHG emissions from fuels such as electricity, biofuels, synthetic fuels, and hydrogen. NASEM states that the report finds that there is no single LCA method capable of answering all questions related to the climate impacts of a transportation fuel, and that both attributional LCA (ALCA) and consequential LCA (CLCA) have important roles to play. CLCA, which considers the consequences of a policy or decision, such as the market effects of production changes, should be used to understand wide-ranging impacts of proposed changes on net GHG emissions. ALCA, which assigns portions of observed environmental impacts from human activities to specific goods and services, can be used to attribute emissions in well-defined supply chains and help identify opportunities to reduce carbon intensity throughout that supply chain. Hybrid methods that use a combination of process-based and economic input-output methodologies can also be useful in some circumstances. In all cases, modelers should provide transparency, justification, and sensitivity or robustness analysis for modeling choices.

NASEM notes that the report contains a number of other findings and recommendations for assessing the emissions of specific transportation fuels and their feedstocks, including the following for biofuels:

• Study of land use changes from biofuels has been the topic of intense study over the last decade. Substantial uncertainties remain on key components of the models used to assess the impacts. More research into this area should be supported; and

• Biofuel production facilities typically produce additional products. The distinction between what qualifies as a co-product, byproduct, or waste can be unclear, creating uncertainty in LCA models.