Effects of Human-Caused Greenhouse Gas Emissions on U.S. Climate, Health, and Welfare (2025)

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Effects of Human-Caused Greenhouse Gas Emissions on U.S. Climate, Health, and Welfare Committee on Anthropogenic Greenhouse Gases and U.S. Climate: Evidence and Impacts Climate Crossroads Board on Atmospheric Sciences and Climate Board on Health Sciences Policy National Research Council Executive Office Division on Earth and Life Studies Health and Medicine Division Consensus Study Report Prepublication Copy 

NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001 This study was supported by the National Academy of Sciences Arthur L. Day Fund and Ralph J. Cicerone and Carol M. Cicerone Endowment for NAS Missions. Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any individual or organization that provided support for the project. International Standard Book Number-13: 978-0-309-XXXXX-X Digital Object Identifier: https://doi.org/10.17226/29239 This publication is available from the National Academies Press, 500 Fifth Street, NW, Keck 360, Washington, DC 20001; (800) 624-6242; https://nap.nationalacademies.org. The manufacturer’s authorized representative in the European Union for product safety is Authorised Rep Compliance Ltd., Ground Floor, 71 Lower Baggot Street, Dublin D02 P593 Ireland; www.arccompliance.com. Copyright 2025 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and National Academies Press and the graphical logos for each are all trademarks of the National Academy of Sciences. All rights reserved. Printed in the United States of America. Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2025. Effects of Human-Caused Greenhouse Gas Emissions on U.S. Climate, Health, and Welfare. Washington, DC: National Academies Press. https://doi.org/10.17226/29239. Prepublication Copy 

The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, nongovernmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. Tsu-Jae Liu is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org. Prepublication Copy 

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. Proceedings published by the National Academies of Sciences, Engineering, and Medicine chronicle the presentations and discussions at a workshop, symposium, or other event convened by the National Academies. The statements and opinions contained in proceedings are those of the participants and are not endorsed by other participants, the planning committee, or the National Academies. Rapid Expert Consultations published by the National Academies of Sciences, Engineering, and Medicine are authored by subject-matter experts on narrowly focused topics that can be supported by a body of evidence. The discussions contained in rapid expert consultations are considered those of the authors and do not contain policy recommendations. Rapid expert consultations are reviewed by the institution before release. For information about other products and activities of the National Academies, please visit www.nationalacademies.org/about/whatwedo. Prepublication Copy 

COMMITTEE ON ANTHROPOGENIC GREENHOUSE GASES AND U.S. CLIMATE: EVIDENCE AND IMPACTS SHIRLEY M. TILGHMAN (NAS/NAM) (Chair), Princeton University DAVID T. ALLEN (NAE), The University of Texas at Austin SUSAN ANENBERG, The George Washington University MICHELE BARRY (NAM), Stanford University CHARLES T. DRISCOLL, JR. (NAE), Syracuse University SUSAN HANSON (NAS), Clark University (retired) CHRIS T. HENDRICKSON (NAE), Carnegie Mellon University MARIKA HOLLAND, National Science Foundation National Center for Atmospheric Research GEORGE M. HORNBERGER (NAE), Vanderbilt University ARTHUR LEE, Chevron (retired) KARI C. NADEAU (NAM), Harvard University CHARLES W. RICE, Kansas State University DREW T. SHINDELL (NAS), Duke University GRAEME L. STEPHENS (NAE), NASA Jet Propulsion Laboratory DAVID W. TITLEY, Rear Admiral, U.S. Navy (retired) JOHN C. WALL (NAE), Cummins, Inc. (retired) Study Staff AMANDA STAUDT, Senior Director of Climate Crossroads KASEY WHITE, Director of the Board on Atmospheric Sciences and Climate APRIL MELVIN, Associate Director of the Polar Research Board and Senior Program Officer with the Board on Atmospheric Sciences and Climate KATHERINE BOWMAN, Senior Program Officer with the Board on Health Sciences Policy AMANDA PURCELL, Associate Director and Senior Program Officer with Climate Crossroads LINDSAY MOLLER, Research Associate with the Board on Atmospheric Sciences and Climate NANCY HUDDLESTON, Communications Director with the Division on Earth and Life Studies MADDI NICOL, Senior Program Assistant with the Division on Engineering and Physical Sciences NOTE: See Appendix B, Disclosure of Unavoidable Conflicts of Interest. Prepublication Copy v 



Reviewers 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: MAUREEN CROPPER (NAS), University of Maryland, College Park KRISTIE EBI, University of Washington RONG FU (NAE), University of California, Los Angeles INEZ FUNG (NAS), University of California, Berkeley TIMOTHY GALLAUDET, U.S. Navy (retired) LYNN GOLDMAN (NAM), The George Washington University SARAH HOBBIE (NAS), University of Minnesota DIANA LIVERMAN (NAS), University of Arizona HUSSAM MAHMOUD, Vanderbilt University KATHERINE MCCOMAS, Cornell University RICHARD MESERVE (NAE), Carnegie Institution for Science WILLIAM PRESS (NAS), The University of Texas at Austin ERIC RIGNOT (NAS/NAE), University of California, Irvine ROBERT ROHDE, Berkeley Earth CATHY WOTEKI (NAM), Iowa State University 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 KAI N. LEE, Center for Ocean Solutions, Stanford University, and Owl of Minerva LLC, and CYNTHIA M. BEALL (NAS), Case Western Reserve 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. Prepublication Copy vii 



Contents FOREWORD .................................................................................................................................................... xi PREFACE........................................................................................................................................................ xiii SUMMARY ........................................................................................................................................................ 1 1 INTRODUCTION ............................................................................................................................................ 3 1.1 Study Charge, 3 1.2 Report Structure, 4 1.3 Committee’s Approach, 6 1.4 Geographic Focus of the Report, 7 1.5 Human Health and Public Welfare Impacts Considered, 7 2 NATURAL AND HUMAN-CAUSED INFLUENCES ON EARTH’S ENERGY IMBALANCE ........................................................................................................................................... 9 2.1 Key Messages, 9 2.2 Greenhouse Gases and Their Emissions, 9 2.3 Earth’s Energy Imbalance, 16 2.4 Attribution of Earth’s Warming and Energy Imbalance to Human Activities, 19 3 OBSERVED CLIMATE CHANGES FROM HUMAN-CAUSED GREENHOUSE GAS EMISSIONS ............................................................................................................................................ 22 3.1 Key Messages, 22 3.2 Temperature, 23 3.3 Precipitation and Hydrological System, 25 3.4 Ocean Heat and Chemistry, 29 3.5 Changes in Physical and Biological Systems, 32 3.6 Wildfires, 35 3.7 Whiplash and Compound Events, 36 4 IMPACTS OF GREENHOUSE GAS EMISSIONS ON FUTURE CLIMATE ............................37 4.1 Key Messages, 37 4.2 Climate Models, 37 4.3 Expected Changes in U.S. Climate, 38 4.4 Abrupt Climate Change, 39 5 IMPACTS ON HUMAN HEALTH..................................................................................................... 41 5.1 Key Messages, 41 5.2 Temperature Effects, 42 5.3 Air Quality, 45 5.4 Additional Extreme Weather Events, 51 5.5 Climate-Sensitive Diseases, 52 5.6 Compounding and Cascading Effects, 57 6 IMPACTS ON PUBLIC WELFARE .................................................................................................. 58 6.1 Key Messages, 58 Prepublication Copy ix 

x Contents 6.2 Considering the Evidence, 58 6.3 Drivers of Ecosystem Change, 59 6.4 Food Production and Agriculture, 60 6.5 Forests, 63 6.6 Grasslands, 64 6.7 Coastal Ocean Ecosystems, 65 6.8 Water Resources, 68 6.9 Energy, Infrastructure, and Settlements, 68 REFERENCES ................................................................................................................................................ 72 APPENDIXES A COMMITTEE MEMBER BIOGRAPHIES.................................................................................... 108 B DISCLOSURE OF UNAVOIDABLE CONFLICTS OF INTEREST ......................................... 114 C EXECUTIVE SUMMARY OF THE TECHNICAL SUPPORT DOCUMENT FOR ENDANGERMENT AND CAUSE OR CONTRIBUTE FINDINGS FOR GREENHOUSE GASES UNDER SECTION 202(A) OF THE CLEAN AIR ACT (EPA, 2009) ......................... 116 Prepublication Copy 

Foreword The U.S. Environmental Protection Agency (EPA) concluded that “six greenhouse gases taken in combination endanger both the public health and the public welfare of current and future generations” in its 2009 “Endangerment Finding.” In a Federal Register Notice published on August 1, 2025, EPA stated that the agency “unreasonably analyzed the scientific record” in making the 2009 Endangerment Finding and that subsequent “developments cast significant doubt on the reliability of the findings.” Such significant claims about the scientific record deserve careful review. The Federal Register Notice proposed repealing the Endangerment Finding and invited public comments. In response to EPA’s request for public input, the National Academies of Sciences, Engineering, and Medicine undertook this independent assessment of the science underpinning the Endangerment Finding. In the Clean Air Act, the U.S. Congress instructed EPA to draw on findings, recommendations, and comments from the Clean Air Science Advisory Committee (CASAC) and the National Academy of Sciences (NAS) (42 U.S. Code § 7607(D)(3)(c)). Advice from CASAC was not available to EPA during the window when it was considering this proposed rulemaking because CASAC was disbanded in January 2025 and EPA was in the process of appointing new members (FR Doc. 2025-07538 (90 FR 18658)). Supporting evidence-informed decision-making by the federal government is a core mission of the National Academies. This study was produced to meet the timeline of the Federal Register Notice (August 1-September 22, 2025) and followed the standard National Academies’ processes for managing conflicts of interest, inviting public comment on the committee members, and thorough peer review of the draft report. This study was supported by the NAS, using funding from two of its endowments. The Arthur L. Day Fund was created for studies on physics of the Earth; its namesake was an expert in geophysics and volcanology who served as vice president of the NAS from 1933-1941. The Ralph J. Cicerone and Carol M. Cicerone Fund was created to honor the service of Dr. Ralph Cicerone, president of the NAS from 2005-2016. I am deeply grateful to Dr. Shirley Tilghman, who ably chaired this committee, and to all of the members and staff who worked tirelessly to complete this report in a timely manner. Marcia McNutt President National Academy of Sciences Prepublication Copy xi 



Preface As the committee undertook this project, it was hard not to think about recent climate-related disasters: the heavy rainfall of Hurricane Helene that destroyed homes and roads in the mountains of North Carolina, the fast-moving wildfires that displaced thousands in Los Angeles and affected air quality for miles around, and the rapid flooding of the Guadalupe River in central Texas that led to at least 135 fatalities. The U.S. Environmental Protection Agency (EPA) concluded in 2009, based on scientific understanding at the time, that emitting greenhouse gases (GHGs) to the atmosphere increased the risk of harms to human health and welfare from changes to the climate, including the risks associated with hurricanes, wildfires, and heavy rainfall, among many others. On the strength of this “Endangerment Finding,” EPA and many state governments instituted their own regulations governing GHG emissions in the intervening years. In August 2025 EPA issued a notice of proposed rulemaking to rescind the Endangerment Finding. This 2009 finding by the EPA Administrator was informed by a companion Technical Support Document (EPA, 2009) that laid out the scientific evidence that emissions of six GHGs posed a threat to human health and welfare. With the aim of informing EPA as it considers the status of the Endangerment Finding, the National Academies undertook this study to evaluate the current state of scientific evidence regarding the impact of human-caused GHGs on climate, with a particular focus on the evidence in the peer-reviewed primary literature that has accumulated since 2009. Specifically, the committee asked whether new evidence since 2009 strengthened or weakened the primary conclusions in EPA (2009) and addressed uncertainties that remain in our understanding of the science of climate change. In addition, the committee identified new issues that were not evident or addressed in EPA (2009). Although climate change in response to GHG emissions is a global issue, the committee concentrated on the effects on human health and welfare in the United States in order to address the statutory concern of EPA. The committee’s charge was to produce a succinct and balanced evaluation of the state of the science, not to make recommendations or advocate for a specific policy. We hope that the report will serve as a critical resource in informing U.S. federal agency decision-making regarding future GHG regulations. The importance of getting the science right weighed heavily on the committee’s deliberations, given the potential significant implications of a changing climate and of the actions proposed to address it. Unlike earthquakes and volcanoes, over which we have no control, responding to the potential harm to human health and welfare from changes in the climate is actionable now. While the short timeline of the study did not lend itself to holding open discussion sessions in person, the committee is grateful to the more than 200 individuals and organizations who responded to a Request for Information. These inputs helped the committee survey the breadth of the literature that has been published since 2009, pointing it to more than 600 peer-reviewed articles. Many of those contributions have been incorporated into the report and influenced its conclusions. I am deeply grateful to the 15 distinguished scientists, engineers, and physicians on the committee, who so generously gave their time and expertise to produce the report. They would not have succeeded without the logistical, managerial, and editorial support of the National Academies’ staff—Katherine Bowman, Nancy Huddleston, April Melvin, Lindsay Moller, Maddi Nicol, Amanda Purcell, and Kasey White, ably led by Amanda Staudt. Everyone, including those who participated in the report review process, was inspired by the importance of the task at hand, and contributed significantly to the final report. Shirley M. Tilghman, Chair Committee on Anthropogenic Greenhouse Gases and U.S. Climate: Evidence and Impacts September 2025 Prepublication Copy xiii