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Introduction and Policy Context
Greenhouse gas (GHG) emissions drive climate change, and transportation is the largest source of GHG emissions in the United States and the fastest-growing emissions source globally.1 In 2020, petroleum products accounted about 90 percent of the U.S. transportation sector energy use, biofuels accounted for about 5 percent, natural gas accounted for about 3 percent, and electricity accounted for less than 1 percent (U.S. Energy Information Administration, 2020). In order to mitigate further effects of climate change, the adoption of low-carbon energy technologies, such as fuels with low GHG emissions, will be of paramount importance (NASEM, 2021a).
TRANSPORTATION EMISSION REDUCTION POLICIES IN THE UNITED STATES
There are a number of federal and state policies that aim to reduce GHG emissions from transportation. This section briefly reviews those policies, with notes on their similarities, differences, or relationship to a low-carbon fuel policy.
Renewable Fuels Standard
The Renewable Fuel Standard (RFS) program,2 which is implemented by the U.S. Environmental Protection Agency (EPA), aims to reduce life-cycle GHG emissions from transportation fuels, expand the U.S. renewable fuels sector, and reduce reliance on imported oil. This standard specifies volumes of renewable fuels to be blended into domestic transportation fuels. Although the RFS addresses only biofuels and does not include other low-carbon fuels, such as electricity, it has provided regulatory experience with life-cycle assessment (LCA)3 of GHG emissions of fuel production pathways.
Much of existing U.S. ethanol production was exempted from meeting GHG reduction targets; new renewable fuel production was required to show reductions in estimated life-cycle GHG reductions relative to a 2005 petroleum baseline specified by EPA. Specifically, biomass-based diesel and advanced biofuels must meet a 50 percent life-cycle GHG reduction; cellulose biofuels must meet a 60 percent reduction; and conventional biofuels, such as ethanol derived from corn starch at new facilities, must meet a 20 percent reduction (EPA, n.d.-b). Life-cycle GHGs are defined under the Clean Air Act (42 U.S.C. § 7545(0)) to include direct and “significant” indirect emissions, such as land use changes related to the full fuel life cycle. Since 2010, more than 200 fuel pathways have been approved using methods of LCA (EPA, n.d.-a).
Corporate Average Fuel Economy Standards
Perhaps the most prominent federal programs regulating transportation fuel consumption in the United States are the Corporate Average Fuel Economy (CAFE) standards, regulated by the National Highway Traffic Safety Administration (Federal Register, 2020), and their companion light-duty fleet GHG emissions standards, regulated by the EPA (Federal Register, 2021). CAFE and the light-duty GHG standard do not set GHG emissions limits for fuels. But CAFE uses a placeholder for the efficiency contribution of alternative fuels, and the light-duty GHG standard treats electricity and hydrogen as zero
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1 Information from a presentation to the committee by V. Reed, J. Fitzgerald, and A. Haq of the Bioenergy Technologies Office of the U.S. Department of Energy, “Life-Cycle Analysis for Biofuels and Bio-Products.”
2 This program was authorized under the Energy Policy Act of 2005 and expanded under the Energy Independence and Security Act of 2007 (EPA, n.d.-b).
3 The terms life-cycle assessment and life-cycle analysis are used interchangeably in this report, and the acronym, LCA, is used for both.
emission fuels (NASEM, 2021b). These assumptions, in effect, incentivize electric vehicle and alternative fuel adoption, and may have several effects on future vehicle GHG emissions (Gan et al., 2021; Jenn et al., 2019).
State-Level Low-Carbon Fuel Standards
At the state level, California and Oregon have adopted low-carbon fuel standards (LCFSs). Both standards are based on life-cycle carbon reduction pathways. Credits are given for fuels based on life-cycle GHG reductions.
California first adopted an LCFS in 2009, with the goal of reducing transportation fuel carbon intensity (CI) by at least 20 percent by 2030 (California Air Resources Board, 2020). LCA is used to calculate a CI score, including both the direct and indirect effects of fuel production, which in turn is used for crediting purposes. Oregon’s Clean Fuels program, started in 2016, also assesses life-cycle GHGs of fuels, and it requires a 10 percent reduction in pollution from transportation fuels used in Oregon below 2015 levels by 2025 (Oregon Department of Environmental Quality, n.d.) This program also uses a credit system and applies to all fuels used in Oregon. Oregon and California’s programs are aligned with regards to system boundary definitions, as well as data availability and transparency.4 In a presentation to this committee, a representative from the Oregon program highlighted the opportunity for shared learning and information exchange regarding LCA.
There is growing interest in a national LCFS. A recent majority report from the congressional staff of the House Select Committee on the Climate Crisis calls for Congress to develop an LCFS that builds on the RFS, sets a benchmark for liquid and non-liquid fuels that is technology- and feedstock-neutral, and is tied to LCA for determining a fuel’s CI (Select Committee on the Climate Crisis, 2020). Additionally, a report on behalf of a bipartisan network of former EPA career employees has called on the agency to evaluate the adoption of a federal LCFS (Environmental Protection Network, 2020).
LIFE-CYCLE ANALYSIS TO ASSESS GREENHOUSE GAS EMISSIONS
To ensure reductions in GHG emissions, metrics and accurate measurements are needed. LCA is the tool used to measure and account for the full environmental impacts of a transportation fuel, including impacts associated with feedstock production or extraction, transportation and manufacturing, and use in vehicles. LCA aims to include emissions that may be considered indirect as well as direct emissions of GHGs. A notable example is land use change stemming from increased demand for biofuels. Published LCA studies have differed in their implementation, with methodological differences affecting choices of system boundaries, quantification of market-induced effects, and allocation of emissions among coproducts. Additionally, there have been questions regarding the quantity, quality, and availability of data used in LCA. If low-carbon fuel policies are to rely on LCA, the methodologies and assumptions need to be assessed, with approaches defined for how to navigate results determined by uncertain parameters, models, and assumptions.
THE COMMITTEE’S CHARGE AND APPROACH
In May 2021 Breakthrough Energy5 requested the National Academies of Sciences, Engineering, and Medicine to appoint an ad hoc committee that would assess current methods for estimating GHG
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4 Information from a presentation to the committee by C. McConnaha, C-A. Wind, and K. Winans, K., “Oregon Department of Environmental Quality Clean Fuels Program Presentation to the National Academy of Sciences, Engineering, and Medicine Committee, “Current Methods for Life Cycle Analyses of Low-Carbon Transportation Fuels in the United States.”
5 Breakthrough Energy, founded by Bill Gates in 2015, is a network of entities supporting investments intended to help attain net-zero greenhouse gas emissions. For information about the organization and its mission, see https://www.breakthroughenergy.org/our-story/our-story.
emissions associated with transportation fuels (liquid and non-liquid) for potential use in a national low-carbon fuels program. The committee’s statement of task is presented in Box 1-1.
Individuals appointed to the committee were chosen for their individual expertise and the relevance of their experience and knowledge to the task, not their affiliation with any institution. All committee members volunteer their time to participate in a National Academies consensus study. Areas of expertise represented on the committee include LCA, fuel production and use (including fossil fuels, biofuels, and electricity), economics, GHG emission modeling, uncertainty analysis, environmental policy decision-making, and biofuel impacts and fuel policy. For biographical sketches of the committee members, see Appendix B.
The committee organized its work by focusing on the methods of LCA and the capabilities needed for potential use in a national low-carbon fuels program. The committee examined general methodological approaches of LCA, key issues for evaluating GHG emissions, issues that arise for transportation fuels, and methodological issues that arise for characteristic types of transportation fuel.
The committee decided not to review or emphasize comparison of the numerical results of different LCAs of transportation fuels, but rather to keep the focus on the methods of GHG emission LCA for fuels. That is, the committee did not include tables compiling or comparing results from different studies, different methods, different years, or different fuels. The committee does not endorse the numerical result of any particular LCA or method. Instead, the committee focused, and the report emphasizes, what methods and approaches could be considered in order to develop reliable quantitative estimates of GHG emissions. Moreover, the committee focused on developing conclusions and recommendations for the use of LCA of transportation fuels that could be used to support an LCFS policy. That is, the committee considered that policymakers and the public would want the LCA to be able to reliably estimate the effect of a low-carbon fuel policy on reducing emissions of GHGs. To that end, the committee and the report emphasize methods to evaluate the consequences of a potential U.S. LCFS policy. The committee also, consistent with its task, identified needs for additional data, methods for data collection, standardized inputs for LCA, and model improvements that could provide a basis for strengthening the reliability and consistency of how LCA is applied for LCFSs.
The committee deliberated and gathered information from June 2021 to February 2022, holding 10 virtual meetings. Five of these meetings included an open session at which the committee members had the opportunity to hear from and have Q&A sessions with a representative of Breakthrough Energy, the study sponsor; federal and state agency representatives; and invited speakers. The invited speakers were requested to submit recorded presentations on topics relevant to the study prior to the open sessions. The agendas for the open session are provided in Appendix C. Video recordings of the speaker presentations and speakers’ slides are available on the study website.
Throughout the study, the committee also received input from interested stakeholders and the public through the study website, public comments periods in the open meetings, or by e‐mail. All submitted comments and documents were added to the study’s public access file, which is available on request from the National Academies’ Public Access Records Office.
ORGANIZATION OF THE REPORT
The report’s 10 chapters are divided in three parts. In addition to this chapter’s general background for the study, Part I covers the phases and types of LCA (Chapter 2) and a discussion of LCA in an LCFS policy (Chapter 3). Part II addresses the general and specific considerations for LCA: direct and indirect effects, uncertainty and variability, and scale of production are discussed in Chapter 4; verification is discussed in Chapter 5; and specific issues and methods for LCA are discussed in Chapter 6. Part III addresses specific fuel issues for LCA: issues related to fossil and gaseous fuels for road transportation are discussed in Chapter 7; issues pertaining to aviation and maritime fuels are discussed in Chapter 8; issues related to biofuels are discussed in Chapter 9; and issues related to electricity as transportation fuel are discussed in Chapter 10.
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