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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Appendix A
Conclusions and Recommendations

Conclusions

FUNDAMENTALS OF LIFE-CYCLE ASSESSMENT

Conclusion 2-1: The approach to LCA needs to be guided on the basis of the question the analysis is trying to answer. Different types of LCA are better suited for answering different questions or achieving different objectives, from fine tuning a well-defined supply chain to reduce emissions, to understanding the global, economy-level effect of a technology or policy change.

Conclusion 2-2: Process-based ALCAs entail bottom-up accounting where emissions are assigned to products or processes based on modeling approach of a static world. Process-based ALCA can identify major sources of emissions in well-defined supply chains and identify opportunities to reduce supply chain carbon intensity, especially when case-specific process-data can be used instead of generic data. Economic input-output life cycle assessment (EIO LCA) identifies implications of interactions across broad sectors of the economy. It can capture emissions that may not be immediately apparent if only a well-defined supply chain is evaluated. It also is helpful in flagging emissions sources that are far-removed from the foreground system but are major contributors to total environmental effects. Hybrid Process/EIO ALCA identifies major sources of emissions beyond well-defined supply chains to include economy-wide effects. CLCA assesses the net effect of a decision or action, such as a change in fuel use or a change in policy, on total GHG emissions.

Conclusion 2-3: LCA results can vary depending on which methods are used, which data are used, which assumptions are made, what scope is defined, and what question is asked.

LIFE-CYCLE ASSESSMENT IN A LOW-CARBON FUEL STANDARD POLICY

Conclusion 3-1: The carbon intensities of fuels used in an LCFS are not necessarily equivalent to the full climate consequences of their adoption. Increased use of a fuel with a low carbon intensity, as defined in an LCFS, could potentially decrease or increase carbon emissions relative to the baseline, depending on policy design and other factors. Regulatory impact assessments that use CLCA to project the consequences of policy can help assess the extent to which a given policy design with particular carbon intensity estimates will result in reduced GHG emissions.

Conclusion 3-2: More research is needed to evaluate effective methods to collectively leverage the strengths of CLCA, ALCA, and verification methods in achieving LCFS objectives.

KEY CONSIDERATIONS: DIRECT AND INDIRECT EFFECTS, UNCERTAINTY AND VARIABILITY, AND SCALE OF PRODUCTION

Conclusion 4-1: Dividing emissions into direct and indirect can be used when identifying and classifying sources of emissions, but it can cause confusion, even if carefully defined and transparently presented in an LCA.

Conclusion 4-2: Direct and indirect emissions are concepts distinct from the concepts of attributional and consequential LCA.

Conclusion 4-3: Explicitly considering parametric, scenario, and model uncertainty can help to represent the degree of confidence in model results.

Conclusion 4-4: Up-to-date LCA studies are needed to inform policy.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Conclusion 4-5: LCA studies can produce different estimates depending on regional scope or assumptions

Conclusion 4-6: ALCA studies may produce substantially different results depending on modeling choices about how emissions are assigned to co-products.

Conclusion 4-7: LCA of commercial-scale production for processes that have not been commercialized involve assumptions that can introduce substantial uncertainty, including effects of interactions among multiple uncertain data or parameters, and so may be particularly sensitive to uncertainty.

Conclusion 4-8: Variability in methods and circumstances under which fuels are produced may be associated with differential economic returns. When this is the case, a techno-economic analysis may be helpful to understand the conditions under which market actors will produce the fuel.

Conclusion 4-9: Research is warranted on how the carbon intensity and economics of fuel production may change over time.

Conclusion 4-10: The scale of production can affect life-cycle GHG emissions, and current LCA methods often do not explicitly incorporate changes in production scale into their calculations.

Conclusion 4-11: More research is needed to develop LCA methodologies for incorporating scale dependence.

VERIFICATION

Conclusion 5-1: In verification to evaluate land use change at a national level, specifying the approach used to evaluate the extent, location, and type of agricultural expansion and the degree of uncertainty aids in transparency and clarity.

Conclusion 5-2: Insight into the degree of agricultural expansion domestically into ecologically important, but potentially small, land parcels requires more frequent data with higher spatial resolution and ideally high producer and user accuracy.

Conclusion 5-3: In verification to evaluate electricity load shifts from national electric vehicle policies, specifying the approach used to evaluate the extent, location, and type of load expansion to be verified and the degree of uncertainty will aid transparency and clarity.

Conclusion 5-4: While smart charging has potential to provide information about the carbon intensity of retail electric vehicle load, the assignment of specific generators to specific loads relies on assumptions from either an attributional frame (e.g.: under what conditions renewable generation should be assigned to electric vehicle load or to another load) or from a consequential perspective (e.g.: what emissions would look like in a counterfactual scenario without electric vehicle load).

Conclusion 5-5: Since satellite data allow for monitoring of international land use change, it would be possible to use satellite data to monitor international land use change, support calculations of LUC impacts, and support results from economic models used to estimate international land use change GHG emissions.

Conclusion 5-6: Certification and verification approaches have been implemented in contemporary LCFSs to inform values for many parameters that influence emissions.

Conclusion 5-7: Certification through protocols and methods that are consistent or compatible across regions and countries may mitigate global trade barriers.

Conclusion 5-8: There are a number of issues relating to the choice of certification protocols that use verification, including the cost to fuel providers, the benefits of reciprocity among protocols, and whether

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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protocols act as trade barriers. These should be weighed against the net costs or benefits that verification provides to society including the carbon footprint of the certification process itself.

Conclusion 5-9: Certification protocols that use verification strategies can complement initial fuel pathway modeling with LCA and associated models (e.g., economic models used to estimate land use changes) to lessen the impacts of uncertainty in LCA results and to inform policymakers of the effects of an LCFS as they unfold. This insight can aid in policy adjustments if undesirable effects arise over the course of the policy.

SPECIFIC METHODOLOGICAL ISSUES RELEVANT TO A LOW CARBON FUEL STANDARD

Conclusion 6-1: The carbon intensity of fuels derived from methane that would otherwise be released (e.g., methane from manure or landfill) is strongly influenced by assumptions in the LCA of the alternative fate of methane pollution and is subject to dramatic change if relevant regulations or practices change.

Conclusion 6-2: Different biogenic carbon accounting methods exist and the choice of method affects the carbon intensity of fuels.

Conclusion 6-3: Given the importance of soil organic carbon changes in influencing life-cycle GHG emissions of biofuels, investments are needed to enhance data availability and modeling capability to estimate soil organic carbon change. Capabilities to evaluate permanence of soil organic carbon changes should also be developed.

Conclusion 6-4: Several metrics in addition to global warming potential for 100 years are now available with differing emphases such as short-term, long-term, or cumulative impacts.

Conclusion 6-5: To make a meaningful comparison of the LCA of transportation fuels, the vehicles that use those fuels should be considered.

Conclusion 6-6: If an LCA uses a single point estimate for efficiency of each vehicle type, its conclusions may vary substantially depending on which vehicle design (make-model-trim) is used to represent each fuel type.

Conclusion 6-7: If an LCA uses a single point estimate for efficiency of each vehicle type, its conclusions may vary substantially depending on which use conditions are assumed.

Conclusion 6-8: Specifically formulated high-octane fuels in combination with dedicated fuel engine technologies can provide efficiency improvements in fuel combustion that affect LCA results.

Conclusion 6-9: Ignoring vs. including vehicle production emissions in an LCA could affect its conclusion about which transportation fuels have the lowest carbon emission implications per unit of transportation services delivered.

Conclusion 6-10: A per-vehicle-mile functional unit is, on its own, not fully informative for comparing transportation fuels for weight-constrained or space-constrained applications, such as Class 8 trucks.

FOSSIL AND GASEOUS FUELS FOR ROAD TRANSPORTATION

Conclusion 7-1: Additional data, reporting, and transparency are needed for petroleum sector operations, including improved information on venting and flaring of methane.

Conclusion 7-2: More emissions inventory data from natural gas systems are needed, particularly regarding emissions from storage tanks.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Conclusion 7-3: The share of natural gas extracted from shale as a share of overall domestic consumption in the United States has increased rapidly and additional research and data collection will be necessary to better understand its production process and climate implications.

Conclusion 7-4: Assumptions on co-product handling methods have broad implications on natural gas LCAs. Additional research and data collection on industry practices can assist in the understanding of choice of co-product handling for natural gas production.

Conclusion 7-5: The selection of methane emissions leakage rate within an LCA has profound impacts on the overall estimated climate impact of natural gas production. Additional research and data collection is necessary to identify representative leakage rates for the natural gas industry and is essential to enable comparison of natural gas LCA across studies.

Conclusion 7-6: The life-cycle emissions attributable to green hydrogen are sensitive to assumptions on the upstream source of electricity used for electrolysis, as the difference in emissions between hydrogen produced from renewable electricity and even grid-average electricity is substantial.

AVIATION AND MARITIME FUELS

Conclusion 8-1: Non-CO₂ effects from aviation fuels may be high but remain uncertain. The largest non-CO₂ impact from aviation fuel combustion may be aviation-induced cloudiness, with the remaining contributions being much smaller.

Conclusion 8-2: Reduced aviation fuel aromatic content, whether through processing of fossil fuels or blending of alternative aviation fuels may have beneficial climate effects on non-CO₂ emissions. However, additional research is necessary to more accurately assess the contribution of non-CO₂ effects from the fuel cycle on a consistent basis with existing LCA of alternative aviation fuels. Due to the nonlinearity of these effects, additional testing is necessary to evaluate the effect of alternative aviation fuel blending on non-CO₂ emissions.

Conclusion 8-3: The overall addition of CO₂ and NO_x to the atmosphere from aviation fuel combustion is well-characterized; however, there is substantial uncertainty on the emissions of sulfur, soot, and aviation-induced cloudiness.

Conclusion 8-4: The combustion emissions from aviation fuel are proportional to the quantity of fuel consumed. However, non-CO₂ impacts are non-linear and do not necessarily correspond proportionally to fuel switching. Furthermore, changes in airplane routing, such as location, altitude, and time of day may also influence non-CO₂ impacts of aviation.

Conclusion 8-5: Though there is evidence that fuel blending can mitigate the impact of some non-CO₂ climate forcing; attributing these impacts to fuel switching in policies may result in inaccurate crediting of these fuels.

Conclusion 8-6: The blending of alternative aviation fuels with different energy densities, as well as the introduction of alternative technologies such as electric-drive or hydrogen-powered airframes, may change the operating weight and efficiency of aircraft. In order to compare the emissions of these alternative fuels to conventional jet fuel on a consistent basis, these changes in efficiency must be taken into consideration.

Conclusion 8-7: There are some variations in the life-cycle emissions attributable to alternative aviation fuels at facilities for some fuel pathways, depending on whether they are configured to maximize alternative aviation fuel output or to maximize yields of other co-products, such as middle distillates.

Conclusion 8-8: Estimating the life-cycle GHG emissions of very low sulfur fuel oil will depend upon information about individual refinery choices in meeting marine fuel sulfur requirements.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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BIOFUELS

Conclusion 9-1: Improved data on biofuel feedstock production, including energy consumption, yield, and fertilizer application at fine spatial resolutions may be useful for some applications. Data quality improvements may support improved GHG accounting in biofuel feedstock production, especially should a performance-based LCFS be developed that accounts for spatially-explicit fertilizer and energy consumption, and land management practices like cover crop planting, land clearing, overfertilization, manure application, use of nitrification inhibitors, or noncompliance with long-term soil carbon storage incentives.

Conclusion 9-2: Estimates of the GHG emissions associated with biofuels from woody biomass depend on the source of wood, forest management practices, and the carbon accounting method.

Conclusion 9-3: The impact of biorefinery co-products, particularly biochar, compost, digestate, or other products meant to be applied to soils, is highly dependent on how these materials are produced and handled and on what land they are applied. Assigning any gGHG offset credit to a biorefinery for producing and exporting these materials requires extensive verification to ensure they deliver the intended benefits. Nutrient-rich materials such as compost only offer fertilizer offset benefits if they are applied in a manner that results in lowered net GHG emissions.

ELECTRICITY AS A VEHICLE FUEL

Conclusion 10-1: ALCA is sometimes used to estimate emissions from electricity consumption because it is easy or because the modeler is interested in an attributional, rather than consequential, question. However, using average emission factors does not answer the question of how emissions will change if PEVs or a PEV policy is adopted. CLCA aims to answer how PEV or PEV policy adoption would change emissions from the power sector.

Conclusion 10-2: For CLCA, regression-based approaches are useful for grounding in data, but simulation-based approaches are needed to project consequential effects of large changes in PEV charging or PEV charging on future grids.

Conclusion 10-3: CLCA for future PEV loads is inherently uncertain, as is any term related to the future, given unknown future conditions that affect consequential emissions, including feedstock prices, regulations, non-vehicle load, and other factors.

Conclusion 10-5: Transportation fuel policies can have co-benefits and tradeoffs in terms of near-term human health effects, climate impacts and other factors.

Recommendations

FUNDAMENTALS OF LIFE-CYCLE ASSESSMENT

Recommendation 2-1: When emissions are to be assigned to products or processes based on modeling choices including functional unit, method of allocating emissions among co-products, and system boundary, ALCA is appropriate. Modelers should provide transparency, justification, and sensitivity or robustness analysis for modeling choices.

Recommendation 2-2: When a decision-maker wishes to understand the consequences of a proposed decision or action on net GHG emissions, CLCA is appropriate. Modelers should provide transparency, justification, and sensitivity/robustness analysis for modeling choices for the scenarios modeled with and without the proposed decision or action.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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LIFE-CYCLE ASSESSMENT IN A LOW-CARBON FUEL STANDARD POLICY

Recommendation 3-1: When some emissions consequences of fuel use are excluded from carbon intensity values in an LCFS, the rationale, justification, and implications for these exclusions should be documented.

Recommendation 3-2: Public policy design based on LCA should ensure through regulatory impact assessment that, at a minimum, the consequential life-cycle impact of the proposed policy is likely to reduce net GHG emissions and increase net benefits to society. Regulatory impact assessments should consider changes in production and use of multiple fuel types (e.g., gasoline, electricity, biofuels, hydrogen).

Recommendation 3-3: LCA practitioners who choose to combine attributional and consequential LCA estimates should transparently document these choices and clearly identify the implications of combining these different types of estimates for the given application, scope and research question.

Recommendation 3-4: Research programs should be created to advance key theoretical, computational, and modeling needs in LCA, especially as it pertains to the evaluation of transportation fuels. Research needs include:

Further development of robust methods to evaluate the GHG emissions from development and adoption of low-carbon transportation fuels, and development or integration of process-based, economic input-output, hybrid, and CLCA methodologies
Products could include the following:
- development of national, open-source, transparent CLCA models for use in LCFS development and assessment
- continued development of national, open-source ALCA models from new or existing models
- evaluation of different approaches to creating, using, or combining ALCA, CLCA, and verification for evaluation of policy outcomes
- quantification of variation between marginal and average GHG emissions for various feedstock-to-fuel pathways; and
- quantification and characterization of the implications of approximations and proxies in LCA, such as comparisons of marginal and average emissions.

KEY CONSIDERATIONS: DIRECT AND INDIRECT EFFECTS, UNCERTAINTY AND VARIABILITY, AND SCALE OF PRODUCTION

Recommendation 4-1: Because the terms “direct” and “indirect” are used differently in different contexts, these terms should be carefully defined and transparently presented when used in LCA studies or policy. Another option is to avoid using the terms “direct” and “indirect” altogether, as they are not considered necessary elements of LCA and may lead to greater confusion.

Recommendation 4-2: Current and future LCFS policies should strive to reduce model uncertainties and compare results across multiple economic modeling approaches and transparently communicate uncertainties.

Recommendation 4-3: LCA studies used to inform policy should explicitly consider parameter uncertainty, scenario uncertainty, and model uncertainty.

Recommendation 4-4: When LCA results are used in policy design or policy analysis, the implications of parameter uncertainty, scenario uncertainty, and model uncertainty for policy outcomes should be explicitly considered, including an assessment of the degree of confidence that a proposed policy will result in reduced GHG emissions and increased social welfare.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Recommendation 4-5: Regulatory agencies should formulate a strategy to keep LCAs up to date, which may involve periodic reviews of key inputs to assess whether sufficient changes have taken place to warrant a re-analysis, and agencies should be aware that substantial changes to LCAs on timescales of less than a decade can occur.

Recommendation 4-6: LCA studies used to inform transportation fuel policy should be explicit about the feedstock and regions to which the study applies and to the extent possible should explicitly report the sensitivity of the results to variation in these assumptions.

Recommendation 4-7: ALCA studies used to inform fuel policy should justify the approach used to handle co-products, and as necessary report sensitivity of results to variation in approaches to assigning emissions to co-products.

Recommendation 4-8: LCA studies used to inform transportation policy regarding processes that do not yet exist at scale should explicitly report sensitivity of findings to uncertainty, in order to produce bounding estimates.

Recommendation 4-9: Modelers should conduct sensitivity analysis to understand implications of variation.

Recommendation 4-10: To effectively inform policymaking, LCA studies should document results for a range of input values.

Recommendation 4-11: Researchers and regulatory agencies should identify additional information to assess impacts of large changes in fuel systems.

Recommendation 4-12: Because LCA-based carbon intensities in current LCFS policy are often not structured to capture nonlinear and non-life cycle implications of large changes in fuel and fuel pathway production volume, policymakers should consider potential complementary policy mechanisms.

VERIFICATION

Recommendation 5-1: Estimates of historical land use change—which may be used to inform economic models that evaluate market-mediated land use change—based on survey or remote sensing data should rely on more than one data source and should include estimates of uncertainty. Higher resolution, higher accuracy, and more frequently collected data sources should be made accessible to the public.

Recommendation 5-2: The research and policy communities should develop frameworks and methodologies for use of satellite data to characterize national and international land use change that may be in part attributable to an LCFS. Examples of framing questions include:

Should an LCFS include measures to mitigate undesirable international land use change, or is it sufficient to monitor international land use change that may be due to the LCFS and these GHG emissions to the associated fuel?
What are the guardrails (e.g., amount and type of land converted to agriculture in a certain region) that a monitoring approach would put in place and, if approached or exceeded, what action would be undertaken as a result?
How can satellite data and economic modeling be most effectively used synergistically to limit GHG emissions from international land use change?
What public data sources will be used to track land use change?
How should uncertainty in land use change estimates be reported?

Recommendation 5-3: If applied, verification requirements should be used consistently and comparably across pathways to encourage technology development and deployment.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Recommendation 5-4: Baselines, if used, should consider (1) the state of technology, (2) inputs from multiple stakeholders, (3) implications for cost of implementation, and (4) incentives that the baselines create for innovation to reduce emissions and for data collection to demonstrate emissions reductions.

Recommendation 5-5: Combinations of newly developed sensor (including satellite) and supply chain technologies (e.g., database systems, blockchain) could be considered to improve land use change assessments. Policies need to be consistent with verification technology and set realistic expectations for verified LCA values. Data should be made publicly available for external verification. The GHG footprint of verification technologies should be included in the LCA as well.

Recommendation 5-6: An LCFS should consider inclusion of a certification protocol with verification. The protocol and its implementation should be overseen by an agency or group of agencies with the complementary expertise sets needed for success. These expertise sets include insights into multiple energy systems and new technologies, economics, environmental effects of fuels and their production routes, agriculture, fossil fuel production, and electricity generation.

Recommendation 5-7: Certification protocols should be revisited periodically to adapt to the emergence of new verification technology, national and global trends in the energy, transportation, and agriculture sectors, and to update baselines as needed based on evolving common practice.

Recommendation 5-8: Economic modeling and verification processes are complementary to each other and should both be used. Verification processes to assess international- and national-level land use change should use state-of-the art remote sensing technologies, when appropriate, which are evolving toward increased frequency and spatial resolution.

SPECIFIC METHODOLOGICAL ISSUES RELEVANT TO A LOW CARBON FUEL STANDARD

Recommendation 6-1: LCA for LCFS policies should provide as much transparency as possible on the different carbon removal elements of fuel life cycles allowed under the policy, as well as insight into how these may change over time, to inform policymakers and stakeholders. Specifically, LCA pathway analyses used to determine carbon intensity scores should separately indicate the contributions from negative elements (if any) and the counterfactual scenarios, such as avoided CO₂ emissions, avoided methane emissions, carbon capture and sequestration in geologic reservoirs or soil, and use in enhanced oil recovery.

Recommendation 6-2: All biogenic carbon emissions and carbon sequestration generated during the lifecycle of a low-carbon fuel should be accounted for in LCA estimates.

Recommendation 6-3: Research should be conducted to improve the methods for accounting and reporting biogenic carbon emissions.

Recommendation 6-4: Research should be conducted to collect existing soil organic carbon data from public and private partners in an open source database, standardize methods of data reporting, and identify highest priority areas for soil organic carbon monitoring. These efforts could align with the recommendations made in the 2019 National Academies report on negative emissions technologies t to study soil carbon dynamics at depth, to develop a national on-farm monitoring system, to develop a model-data platform for soil organic carbon modeling, and to develop an agricultural systems field experiment network. These efforts should also be extended internationally.

Recommendation 6-5: Research should be conducted to explore remote-sensing and in situ sensor-based methods of measuring soil carbon that can generate more data quickly.

Recommendation 6-6: Use of more than one climate change metric should be considered in the analysis of low-carbon fuel policies.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Recommendation 6-7: Further research should be conducted to better understand the suitability of different GHG metrics for LCA.

Recommendation 6-8: Further research should be conducted to develop a framework to include albedo effects from land cover change, and near-term climate forcers, in LCA of low-carbon fuels.

Recommendation 6-9: Further research should be conducted to better understand the climate implications of increased GHG emissions on the short-term (carbon debt) to support the selection of an appropriate approach to account for the timing of GHG emissions and uptakes in LCA.

Recommendation 6-10: LCA of transportation fuels may include analysis using functional units based on the transportation service provided, such as passenger-mile or ton-mile, or otherwise be based on comparison of comparable transportation services. This may be reported in addition to an energy-based functional unit. LCAs should clearly describe their assumptions for the energy- and service-based functional units, such as through vehicle efficiency, market share, or other factors.

Recommendation 6-11: When comparing life-cycle emissions of different transportation fuels, LCA studies that assess or inform policy should consider the range of vehicle efficiencies within each fuel type to ensure that the comparisons are made on comparable transportation services, such as passenger capacity, payload capacity, and performance.

Recommendation 6-12: When comparing life-cycle emissions of different transportation fuels, LCA studies should avoid relying on a single point estimate for efficiency of each vehicle fuel type and instead consider the range of vehicle efficiencies within each fuel type across vehicles and common or likely operating conditions.

Recommendation 6-13: LCAs of high-octane fuels should consider the impact of fuel octane on vehicle efficiency, but for the purpose of broad policy assessment LCA should be based on the actual and anticipated vehicle fleet, and following common practice for fuel vehicle assessments include only combinations that reflect reality.

Recommendation 6-14: For regulatory impact assessment, LCA of transportation fuels and transportation fuel policy should consider a range of estimates for possible changes in the emissions of vehicle production required to convert transportation fuels into transportation services, and the resulting changes in vehicle fleet composition.

Recommendation 6-15: LCA comparing transportation fuels for weight-constrained applications should present a per-ton-mile functional unit and/or explicitly model the logistical implications of payload effects by fuel type.

FOSSIL AND GASEOUS FUELS FOR ROAD TRANSPORTATION

Recommendation 7-1: Policymakers may consider recognizing the variation in GHG emissions across different petroleum fuel pathways, and include mechanisms to reduce these emissions in fuel policies.

Recommendation 7-2: Further research should be done on the key parameters used to assess the climate impacts of natural gas production, such as methane leakage rates. These parameters will evolve as technology advances, data availability increases, and statistical methods may be used to translate the additional data into improved emissions estimates.

Recommendation 7-3: Further research on the climate impacts of natural gas production should draw upon real world activity data in part supplied by the natural gas industry and in part from independent studies using satellite and remote sensing technology to improve methane emissions rate estimates; these should be revisited frequently— at least every five years.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Recommendation 7-4: To ensure renewable electricity is supplied via the grid to produce green hydrogen in the context of an LCFS, certification is necessary to ensure that the source of the electricity and its additionality.

Recommendation 7-5: In the context of an LCFS, LCAs of hydrogen should be well documented with choices of key parameters supported with facility-measured data or well-supported citations from the literature. These key parameters include the choice of energy source for steam-methane reforming or authothermal reforming, the carbon capture level from the waste gaseous stream, source of upstream electricity, and the rate of methane or CO₂ leakage. Where relevant, the approach to quantifying emissions of upstream natural gas production should align with those used elsewhere in an LCFS for other fuels produced from natural gas.

AVIATION AND MARITIME FUELS

Recommendation 8-1: Because the non-CO₂ effects from aviation fuels remain uncertain, research should be done to clarify the magnitude and direction of these effects.

Recommendation 8-2: Alternative fuels and airframe combinations, particularly those with large density differences such as battery electric technology and hydrogen, may impact airplane efficiency and thus influence overall emissions. The comparative LCA of these technologies should use functional units based on the transportation service provided or otherwise be based on comparison of consistent transportation services.

Recommendation 8-3: Alternative aviation fuel LCA estimates developed for fuel policy should reflect existing practices at facilities or the expected behavior in response to future policies.

Recommendation 8-4: LCA of oil-derived marine fuels should use new data as available for the feedstock conversion life-cycle stage. A body such as the International Maritime Organization should strive to collect data that will enable reliable marine fuel LCAs.

Recommendation 8-5: The baseline life-cycle GHG emissions for marine fuels should reflect current industry trends stemming from MARPOL Annex VI and potentially be updated after several years’ time once the industry adjusts more fully to the new regulations through, for example, deployment of more liquefied natural gas-fueled vessels.

Recommendation 8-6: Marine fuel pathways should be evaluated with methods that are consistent with on-road and aviation fuels while considering unique factors in the oil refining and use phase aspects of a marine fuel’s life cycle.

BIOFUELS

Recommendation 9-1: Additional research should be done to assess key parameters and assumptions in forest management practices induced by increased woody biomass demand, including: changes in residue removal rates, stand management and forest productivity, and changes in tree species selection during replanting.

Recommendation 9-2: Research and data collection efforts should be carried out for improved data and modeling related to forest feedstock production and storage, including energy use, yield, inputs, fugitive emissions, and changes in forest carbon stock should be supported.

Recommendation 9-3: Policymakers should exercise caution in crediting biorefineries for GHG emissions sequestration as a result of exporting co-products such as biochar, digestate, and compost, as it risks over-crediting producers for downstream behavior that is not necessarily occurring. The committee recommends that any credits generated from these activities must be contingent on verification that these activities are being practiced.

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Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Recommendation 9-4: Applying credits for carbon sequestration to soil or reduced use of fertilizer should require robust measurement and verification to prove the co-products are applied in a manner that yields net climate benefits.

Recommendation 9-5: Additional review and research is recommended on the key factors affecting induced land use change.

Recommendation 9-6: Beyond research on induced land use change and rebound effects, research should be done to identify and quantify the impacts of other indirect effects of biofuel production, including but not limited to market-mediated effects on livestock markets, land management practices, and dietary change of food type, quantity, and nutritional content.

Recommendation 9-7: Though the study of induced land use changes from biofuels has been the topic of intense study over the last decade, substantial uncertainties remain on many key components of economic models used to assess these impacts. Further work is warranted to update these estimates of market-mediated land use change and the models so as to inform the development and implementation of an LCFS.

Recommendation 9-8: Assessment of the consequential effects from a future proposed policy, such as induced land use change, should be further developed in order to assess the risk of market-mediated effects and emissions attributable to the policy. Consequential assessment can inform the implementation of safeguards within policies such as limits on high-risk feedstocks, can inform the development of supplementary policies, identify hotspots, and reduce the likelihood of unintended consequences.

Recommendation 9-9: To improve understanding of market-mediated effects of biofuels, research should be supported on different modeling approaches, including their treatment of baselines and opportunity costs, and to investigate key parameters used in national and international modeling based on measured data, including various elasticity parameters, soil carbon sequestration, land cover, and emission factors and others.

Recommendation 9-10: Because other market-mediated effects of biofuel production, such as livestock market impacts, land management practices, and changes in diets and food availability may be linked to land use and biofuel demand assessed using induced land use change models, additional research should be done and model improvements undertaken to include these effects.

Recommendation 9-11: Current and future low-carbon fuel policies should strive for transparency in their modeling efforts.

ELECTRICITY AS A VEHICLE FUEL

Recommendation 10-1: Regulatory impact assessment or other analyses estimating the emissions implications of a change in PEV charging load should use a CLCA approach to estimate the implications of power grid emissions and clearly characterize uncertainty of estimates due to assumptions, especially for future scenarios.

Recommendation 10-2: Research should be conducted to estimate how upstream emissions in the power sector change in response to changes in generation.

Recommendation 10-3: Analyses that estimate the emissions implications of changing PEV adoption or PEV policy should provide a transparent assessment of how sensitive or robust the results of the analyses are to reasonable variations in modeling assumptions and future scenarios.

Recommendation 10-4: Analyses estimating the emissions implications of PEV adoption in future power grid scenarios should consider changes in power grid emissions caused by PEV charging in each power grid scenario.

Page 214 Cite

Suggested Citation:"Appendix A: Conclusions and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2022. Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States. Washington, DC: The National Academies Press. doi: 10.17226/26402.

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Recommendation 10-5: LCA to estimate the change in GHG emissions induced by a policy or a change in technology adoption should consider how interaction with existing policies may affect outcomes. For cars and trucks, national fleet standards are key to understanding the net GHG outcomes of technology or policy actions.

Recommendation 10-6: Methods for LCAs of low-carbon transportation fuels can evaluate co-benefits and tradeoffs of transportation policies in terms of climate impact, human health, and other factors.

Recommendation 10-7: Continuing and improved data are needed to support evaluation of the GHG emissions of electricity used as a transportation fuel.