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Suggested Citation:"Front Matter." 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.
×

Current Methods for Life-Cycle Analyses
of Low-Carbon Transportation Fuels
in the United States

Committee on Current Methods for Life Cycle Analyses of Low-Carbon
Transportation Fuels in the United States

Board on Environmental Studies and Toxicology

Board on Agriculture and Natural Resources

Division on Earth and Life Studies

Board on Energy and Environmental Systems

Division on Engineering and Physical Sciences

A Consensus Study Report of

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THE NATIONAL ACADEMIES PRESS
Washington, DC
www.nap.edu

Suggested Citation:"Front Matter." 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.
×

NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001

This activity was supported by contracts between the National Academy of Sciences and Breakthrough Energy, contract number 10005236. Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any organization or agency that provided support for the project.

International Standard Book Number-13: 978-0-309-27393-0
International Standard Book Number-10: 0-309-27393-5
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Suggested citation: 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. https://doi.org/10.17226/26402.

Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Consensus Study Reports published by the National Academies of Sciences, Engineering, and Medicine document the evidence-based consensus on the study’s statement of task by an authoring committee of experts. Reports typically include findings, conclusions, and recommendations based on information gathered by the committee and the committee’s deliberations. Each report has been subjected to a rigorous and independent peer-review process and it represents the position of the National Academies on the statement of task.

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Suggested Citation:"Front Matter." 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.
×

COMMITTEE ON CURRENT METHODS FOR LIFE CYCLE ANALYSES OF LOW-CARBON TRANSPORTATION FUELS IN THE UNITED STATES

Members

VALERIE M. THOMAS (Chair), Georgia Institute of Technology

AMOS A. AVIDAN (NAE), Bechtel Corporation (retired)

JENNIFER B. DUNN, Northwestern University

PATRICK L. GURIAN, Drexel University

JASON D. HILL, University of Minnesota, St. Paul

MADHU KHANNA, University of Illinois, Urbana-Champaign

ANNIE LEVASSEUR, École de technologie supérieure, Montreal, Quebec, Canada

JEREMY I. MARTIN, Union of Concerned Scientists, Washington, DC

JEREMY J. MICHALEK, Carnegie Mellon University

STEFFEN MUELLER, University of Illinois, Chicago

NIKITA PAVLENKO, International Council on Clean Transportation, Washington, DC

DONALD W. SCOTT, Scientific Certification Systems, St. Louis, Missouri

CORINNE D. SCOWN, Lawrence Berkeley National Lab

DEV S. SHRESTHA, University of Idaho, Moscow

FARZAD TAHERIPOUR, Purdue University

YUAN YAO, Yale University

Staff

CAMILLA YANDOC ABLES, Study Co-Director

BRENT HEARD, Study Co-Director

CLIFFORD S. DUKE, Board Director

TAMARA DAWSON, Program Coordinator

KYRA HOWE, Research Assistant

Report Editor

EUGENIA GROHMAN

Sponsor

BREAKTHROUGH ENERGY

Suggested Citation:"Front Matter." 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.
×

BOARD ON ENVIRONMENTAL STUDIES AND TOXICOLOGY

Members

FRANK W. DAVIS (Chair), University of California, Santa Barbara

DANA BOYD BARR, Emory University, Atlanta, GA

ANN M. BARTUSKA, U.S. Department of Agriculture (retired), Washington, DC

GERMAINE M. BUCK LOUIS, George Mason University, Fairfax, VA

FRANCESCA DOMINICI, Harvard University, Boston, MA

GEORGE GRAY, The George Washington University, Washington, DC

R. JEFFREY LEWIS, ExxonMobil Biomedical Sciences, Inc., Annandale, NJ

LINSEY C. MARR, Virginia Polytechnic Institute and State University, Blacksburg, VA

MARIE LYNN MIRANDA, Children’s Environmental Health Initiative, Notre Dame, IN

R. CRAIG POSTLEWAITE, U.S. Department of Defense, Burke, VA

REZA J. RASOULPOUR, Corteva Agriscience, Indianapolis, IN

JOSHUA TEWKSBURY, Smithsonian Tropical Research Institute, Panamá

SACOBY M. WILSON, University of Maryland, College Park, MD

TRACEY J. WOODRUFF, University of California, San Francisco, San Francisco, CA

Staff

CLIFFORD S. DUKE, Director

RAYMOND A. WASSEL, Scholar and Director of Environmental Studies

NATALIE ARMSTRONG, Associate Program Officer

KATHRYN GUYTON, Senior Program Officer

KALEY BEINS, Program Officer

LAURA LLANOS, Finance Business Partner

TAMARA DAWSON, Program Coordinator

LESLIE BEAUCHAMP, Senior Program Assistant

THOMASINA LYLES, Senior Program Assistant

Suggested Citation:"Front Matter." 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.
×

Preface

Emissions of greenhouse gases, especially carbon dioxide from the combustion of fossil fuels, are changing the climate in ways that will have significant and long-term effects globally, including on the economy of the United States and the welfare of its people and environment. Transportation fuels are one of the largest sources of U.S. greenhouse gas emissions. The most widely used transportation fuels—gasoline, diesel, and jet fuel—are made from petroleum and emit carbon dioxide when combusted. The alternatives to petroleum transportation fuels, including electricity, biofuels, synthetic fuels, and hydrogen, also have emissions of carbon dioxide and other greenhouse gases, from the tailpipe, from the production processes, or from wider supply-chain contributions, depending on the fuel. Determining the total net emissions of these alternative fuels requires understanding how fuels are made and how they affect markets. Life-cycle assessment (LCA) is a method to account for the environmental impact of a product throughout its life cycle, including resource extraction, production, and all other supply-chain impacts. Existing LCAs of transportation fuels differ in their methods, such as data, system boundary, and assumptions, and have produced differing results.

Breakthrough Energy provided funding to the National Academies of Sciences, Engineering, and Medicine for a committee to conduct an assessment of the methods for life-cycle analyses of low-carbon transportation fuels in the United States, with the aim of developing a reliable and coherent approach for applying LCA to developing low-carbon fuel standards.

To gather information from experts on various fuel LCA questions, the committee invited presentations from Bo Weidema on consequential LCA, James Hileman on aviation fuels, Michael Wang on LCA of transportation fuels, Amgad Elgowainy on hydrogen fuels, Joule Bergerson on fossil fuels, Richard Plevin on uncertainties associated with LCA for transportation fuels, Tyler Lark and Seth Spawn on land use change, and John Field on soil carbon implications of biofuel production. I thank all of these individuals for sharing their time and expertise with the committee.

I thank the experienced scientists with deep expertise in LCA of fuels who served on this committee for their steadfast dedication to the work of preparing this report. Those efforts include hours of literature reviews, multiple committee meetings, working with and learning from numerous presenters with expertise related to LCA and transportation fuels, and writing working drafts with many edits to make the report readable and of high quality.

I also thank our wonderful team from the National Academies who worked diligently for many months to keep us on track and gave their total support throughout the entire process. On the committee’s behalf, I especially thank our study co-directors, Camilla Yandoc Ables and Brent Heard, for their assistance through every aspect of the development of this report. On behalf of the committee, I also thank project staff members Tamara Dawson, Kyra Howe, Cliff Duke, Robin Schoen, John Holmes, and Ray Wassel.

Special thanks to the representative of Breakthrough Energy, Maria Martinez, and to the representatives of federal and state agencies for the information they provided to the committee. Lastly, I thank the members of the public who contributed to the committee’s knowledge and understanding of issues important to the study and ultimately to the life-cycle evaluation of greenhouse gas emissions from transportation fuels.

Valerie M. Thomas, Chair

Committee on Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States

Page viii Cite
Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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.
×

Acknowledgments

This Consensus Study Report was reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise. The purpose of this independent review is to provide candid and critical comments that will assist the National Academies of Sciences, Engineering, and Medicine in making each published report as sound as possible and to ensure that it meets the institutional standards for quality, objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process.

We thank the following individuals for their review of this report:

STEVEN BARRETT, Massachusetts Institute of Technology

MIGUEL BRANDÃO, KTH Sweden

JOHN FIELD, Oak Ridge National Laboratory

KEN GILLINGHAM, Yale University

DAVID HASSENZAHL, California State University, Chico

DON O’CONNOR, S&T Squared Consultants Inc.

MARK REID (NAS), Harvard & Smithsonian

NAGORE SABIO, ExxonMobil

NORBERT SCHMITZ, International Sustainability and Carbon Certification (ISCC)

TIMOTHY SEARCHINGER, Princeton University

MICHAEL WANG, Argonne National Laboratory

Although the reviewers listed above provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations of this report nor did they see the final draft before its release. The review of this report was overseen by CHRIS HENDRICKSON (NAE), Carnegie Mellon University and JEFFERSON TESTER (NAE), Cornell University. They were responsible for making certain that an independent examination of this report was carried out in accordance with the standards of the National Academies and that all review comments were carefully considered. Responsibility for the final content rests entirely with the authoring committee and the National Academies.

Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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.
×
Suggested Citation:"Front Matter." 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.
×
Page xiii Cite
Suggested Citation:"Front Matter." 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.
×

2-1 Example of System Boundaries in ALCA and CLCA

3-1 Attributional LCA Models Used for Transportation Fuel Policies

FIGURES

2-1 Four phases of life-cycle assessment

2-2 Illustration of how attributional and consequential LCA address different questions

2-3 LCA approaches by research question and relationship to average and marginal emission

2-4 Illustration of the relationships between attributional and consequential emissions and average and marginal emission factors for a single fuel

3-1 Nested compliance categories within the EPA Renewable Fuel Standard (RFS2)

3-2 Illustration of potential consequential outcomes in regulatory impact assessment for a low-carbon fuel standard (LCFS)

4-1 Illustration of different delineations between direct emissions and indirect emissions for various corn ethanol LCA studies

5-1 Hypothetical illustration showing reported average emission rates can be substantially lower than true average emission rates

6-1 GHG emission flows across the life cycle of biofuel

6-2 Soil carbon sequestration rate changes with time

6-3 Range of vehicle energy consumption rates (inverse of efficiency) by fuel type

6-4 Relationship between fuel consumption rate and ambient temperature

6-5 Estimated average energy consumption (watt hour) per mile for a Nissan Leaf in different regions of the United States based on regional ambient temperature over the year

6-6 A comparison between current and beyond Li-ion batteries for electrifying semi trucks

7-1 Petroleum products made from a barrel of crude oil, worldwide, 2020

7-2 Hydrogen produced from different technologies and feedstocks

7-3 Global demand for pure hydrogen, 1975-2018

7-4 U.S. Dry natural gas production, 2000-2050

7-5 Location of shale gas plays in the continental United States

7-6 Steps in the natural gas supply chain from recovery to delivery

7-7 Bottom-up methane (CH4) emissions estimation tool

7-8 Evolution of the Intergovernmental Panel on Climate Change (IPCC) estimate of methane global warming potential from 1990-2021

8-1 The principal emissions from aviation operations and the atmospheric processes that lead to changes in radiative forcing components

8-2 Radiative forcing components in milliwatts per square meter from global aviation as evaluated from preindustrial times until 2018

8-3 Aircraft power profiles and power to energy ratio by flight segment

9-1 Land available for conversion in commonly-used models for assessing land use change in biofuel production

10-1 Conceptual illustration of the difference in approaches for assessing power sector emissions from PEV charging

10-2 Relationship of average and marginal power grid emission factors to attributional and consequential LCA

Suggested Citation:"Front Matter." 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.
×

10-3 Illustration of why the emissions implications of charging a PEV in a region can differ from the emissions of the average grid mix in that region

10-4 Hypothetical dispatch curve

10-5 Example of regression results identifying sources of generation changes in the MRO grid region

10-6 Conceptual illustration of energy balance maintained at every time step of a simulated dispatch model

10-7 Illustration of how the relative life-cycle GHG emissions of a particular PEV compared to with a gasoline vehicle can depend on many factors

TABLES

2-1 Definitions of Attributional and Consequential LCA

2-2 Relationship between System Boundary and LCA Type

2-3 Summary of Major Approaches to LCA

4-1 Definitions of Direct and Indirect Emissions in the Literature

4-2 Relationship between Direct/Indirect Emissions and Attributional/Consequential LCA

4-3 Examples of Factors Often Referred to as “Direct” Or “Indirect” in the Transportation Fuels Literature

5-1 Studies Used in U.S. EPA’s 2018 Triennial Report to Congress

8-1 Summary of Approved and Pending Alternative Aviation Fuel Production Pathways

8-2 Sample of Published LCAs of Aviation Fuels

8-3 Comparison of Physical Properties across Different Energy Carriers Used for Aviation

8-4 Marine Fuels That Could Be Included in an LCFS

10-1 Sample of Published Studies Assessing PEV Emissions in the United States

10-2 Comparison of Approaches to Estimate Grid Emissions from PEV Charging

10-3 Comparison of Marginal AVERT Factors with eGrid, by Selected States and Regions

10-4 Comparison of Estimated Emission Factors for Changing Electricity Load

Suggested Citation:"Front Matter." 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.
×

Acronyms

AAF alternative aviation fuel
AEZ-EF Agro-ecological Zone Emission Factor (model)
ALCA attributional life-cycle assessment; attributional life-cycle analysis
ASTM American Society for Testing and Materials
AVERT AVoided Emissions and geneRation Tool
BCM billion cubic meters
BEPAM Biofuel and Environmental Policy Analysis Model
BEV battery electric vehicle
CA-LCFS California Low Carbon Fuel Standard
CAFE corporate average fuel economy
CA-GREET modified Greenhouse Gases, Regulated Emissions, and Energy Use in Technologies (GREET) model
CARB California Air Resources Board
CARD Center for Agricultural and Rural Development (Iowa State University)
CCLUB Carbon Calculator for Land Use Change from Biofuels Production
CCUS carbon capture utilization and storage
CEDM Center for Climate and Energy Decision Making
CGE computable general equilibrium (model)
CI carbon intensity
CLCA consequential life-cycle assessment; consequential life-cycle analysis
CO2e carbon dioxide equivalent
CORSIA Carbon Offsetting and Reduction Scheme for International Aviation
EEIO environmentally extended input output (LCA)
eGRID Emissions & Generation Resource Integrated Database (of the US Environmental Agency)
EIO economic input output (applied to LCA)
EISA Energy Independence and Security Act
EPA U.S. Environmental Protection Agency
EPA GHGI EPA Greenhouse Gas Emissions Inventory
EPPA Emissions Prediction and Policy Analysis (model)
EU European Union
EV electric vehicle
FAPRI-CARD Food and Agricultural Policy Research Institute (model developed at Center for Agricultural and Rural Development of Iowa State University)
FASOM Forest and Agricultural Sector Optimization Model
GHG greenhouse gas
GLOBIOM Global Biosphere Management model
GREET Greenhouse gases, Regulated Emissions, and Energy use in Technologies model (developed by Argonne National Laboratory)
GTAP Global Trade Analysis Project (model developed in 1994)
GTAP-BIO Global Trade Analysis Project-BIO (model first modified in 2007 with subsequent changes over time)
GTP Global temperature change potential
Suggested Citation:"Front Matter." 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.
×
GWP global warming potential
HEFA hydroprocessed esters and fatty acids
ICAO International Civil Aviation Organization
IEA International Energy Agency
ILUC indirect land use change
ISCC International Sustainability and Carbon Certification
IPCC Intergovernmental Panel on Climate Change
IRENA International Renewable Energy Agency
ISO International Organization for Standardization
ISOs Independent System Operators
kWh kilowatt-hour
LCA life-cycle assessment; life-cycle analysis
LCFS low-carbon fuel standard
LCI life-cycle inventory
LNG liquefied natural gas
LUC land use change
MIRAGE Modeling International Relationships in Applied General Equilibrium (model)
MISO Midcontinent Independent System Operator
MJ megajoule
MMb/d million barrels per day
mW/m2 milliWatts per square meter
OPGEE Oil Production Greenhouse gas Emissions Estimator (Stanford)
PE partial equilibrium (model)
PEV plug-in electric vehicle
PHEV plug-in hybrid electric vehicle
PRELIM Petroleum Life Cycle Inventory Model
RED Renewable Energy Directive (2009)
RED II Renewable Energy Directive (updated 2018)
RFS U.S. Renewable Fuel Standard
RFS1 Renewable Fuel Standard (established in 2005)
RFS2 Renewable Fuel Standard (established in 2007)
RINs Renewable Identification Numbers
RTOs Regional Transmission Operators
SETAC Society of Environmental Toxicology and Chemistry
SOC soil organic carbon
t*km ton-kilometer
UNEP United Nations Environment Programme
USDA U.S. Department of Agriculture
USGS U.S. Geological Survey
Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Page xiii Cite
Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Suggested Citation:"Front Matter." 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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Transportation is the largest source of greenhouse gas emissions in the United States, with petroleum accounting for 90 percent of transportation fuels. Policymakers encounter a range of questions as they consider low-carbon fuel standards to reduce emissions, including total emissions released from production to use of a fuel or the potential consequences of a policy. Life-cycle assessment is an essential tool for addressing these questions. This report provides researchers and practitioners with a toolkit for applying life-cycle assessment to estimate greenhouse gas emissions, including identification of the best approach to use for a stated policy goal, how to reduce uncertainty and variability through verification and certification, and the core assumptions that can be applied to various fuel types. Policymakers should still use a tailored approach for each fuel type, given that petroleum-based ground, air, and marine transportation fuels necessitate different considerations than alternative fuels including biofuels, hydrogen, and electricity. Ultimately, life-cycle assessments should clearly document what assumptions and methods are used to ensure transparency.

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