Attribution of Extreme Weather Events in the Context of Climate Change PREPUBLICATION COPY Committee on Extreme Weather Events and Climate Change Attribution Board on Atmospheric Sciencees and Climate Division on Earth and Life Studies This prepublication version of Attribution of Extreme Weather Events in the Context of Climate Change has been provided to the public to facilitate timely access to the report. Although the substance of the report is final, editorial changes may be made throughout the text and citations will be checked prior to publication. The final report will be available through the National Academies Press in spring 2016. Copyright © National Academy of Sciences. All rights reserved. Attribution of Extreme Weather Events in the Context of Climate Change THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001 This study was supported by the David and Lucile Packard Foundation under contract number 2015- 63077, the Heising-Simons Foundation under contract number 2015-095, the Litterman Family Foundation, the National Aeronautics and Space Administration under contract number NNX15AW55G, the National Oceanic and Atmospheric Administration under contract number EE- 133E-15-SE-1748, and the U.S. Department of Energy under contract number DE-SC0014256, with additional support from the National Academy of Sciences’ Arthur L. Day Fund. 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: International Standard Book Number-10: Digital Object Identifier: 10.17226/21852 Additional copies of this report are available for sale from the National Academies Press, 500 Fifth Street, NW, Keck 360, Washington, DC 20001; (800) 624-6242 or (202) 334-3313; http://www.nap.edu. Copyright 2016 by the National Academy of Sciences. All rights reserved. Printed in the United States of America Cover imagery courtesy of Cameron Beccario, http://earth.nullschool.net/. Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2016. Attribution of Extreme Weather Events in the Context of Climate Change. Washington, DC: The National Academies Press. doi: 10.17226/21852. PREPUBLICATION COPY Copyright © National Academy of Sciences. All rights reserved. Attribution of Extreme Weather Events in the Context of Climate Change 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. Ralph J. Cicerone 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 advisinng the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. C. D. Mote, Jr., 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 Academies also encourage education and research, recognize outstanding contribuutions 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.national-academies.org. PREPUBLICATION COPY Copyright © National Academy of Sciences. All rights reserved. Attribution of Extreme Weather Events in the Context of Climate Change Copyright © National Academy of Sciences. All rights reserved. Attribution of Extreme Weather Events in the Context of Climate Change COMMITTEE ON EXTREME WEATHER EVENTS AND CLIMATE CHANGE ATTRIBUTION DAVID W. TITLEY (Chair), Pennsylvania State University, University Park GABRIELE HEGERL, University of Edinburgh, UK KATHARINE L. JACOBS, University of Arizona, Tucson PHILIP W. MOTE, Oregon State University, Corvallis CHRISTOPHER J. PACIOREK, University of California, Berkeley J. MARSHALL SHEPHERD, University of Georgia, Athens THEODORE G. SHEPHERD, University of Reading, UK ADAM H. SOBEL, Columbia University, New York, NY JOHN WALSH, University of Alaska, Fairbanks FRANCIS W. ZWIERS, University of Victoria, BC, Canada National Academies of Sciences, Engineering, and Medicine Staff KATHERINE THOMAS, Program Officer LAUREN EVERETT, Program Officer AMANDA PURCELL, Associate Program Officer RITA GASKINS, Administrative Coordinator ERIN MARKOVICH, Program Assistant PREPUBLICATION COPY v Copyright © National Academy of Sciences. All rights reserved. Attribution of Extreme Weather Events in the Context of Climate Change BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE A.R. RAVISHANKARA (Chair), Colorado State University, Fort Collins GERALD A. MEEHL (Vice Chair), National Center for Atmospheric Research, Boulder, CO LANCE F. BOSART, University at Albany-SUNY, NY MARK A. CANE, Columbia University, Palisades, NY SHUYI S. CHEN, University of Miami, FL HEIDI CULLEN, Climate Central, Princeton, NJ PAMELA EMCH, Northrop Grumman Aerospace Systems, Redondo Beach, CA ARLENE FIORE, Columbia University, Palisades, NY WILLIAM B. GAIL, Global Weather Corporation, Boulder, CO LISA GODDARD, Columbia University, Palisades, New York MAURA HAGAN, National Center for Atmospheric Research, Boulder, CO TERRI S. HOGUE, Colorado School of Mines, Golden ANTHONY JANETOS, Joint Global Change Research Institute, College Park, MD EVERETTE JOSEPH, University at Albany-SUNY, NY RONALD “NICK” KEENER, JR., Duke Energy Corporation, Charlotte, NC JOHN R. NORDGREN, The Climate Resilience Fund, Bainbridge Island, WA JONATHAN OVERPECK, University of Arizona, Tucson ARISTIDES A.N. PATRINOS, New York University, Brooklyn S.T. RAO, North Carolina State University, Raleigh DAVID A. ROBINSON, Rutgers, The State University of New Jersey, Piscataway CLAUDIA TEBALDI, Climate Central, Princeton, NJ Ocean Studies Board Liaison DAVID HALPERN, Jet Propulsion Laboratory, Pasadena, CA Polar Research Board Liaison JENNIFER FRANCIS, Rutgers, The State University of New Jersey, Marion, MA National Academies of Sciences, Engineering, and Medicine Staff AMANDA STAUDT, Director EDWARD DUNLEA, Senior Program Officer LAURIE GELLER, Program Director KATHERINE THOMAS, Program Officer LAUREN EVERETT, Program Officer ALISON MACALADY, Associate Program Officer AMANDA PURCELL, Associate Program Officer RITA GASKINS, Administrative Coordinator ROB GREENWAY, Program Associate SHELLY FREELAND, Financial Associate MICHAEL HUDSON, Senior Program Assistant ERIN MARKOVICH, Program Assistant PREPUBLICATION COPY vi Copyright © National Academy of Sciences. All rights reserved. Attribution of Extreme Weather Events in the Context of Climate Change Preface Extreme weather has affected human society since the beginning of recorded history and certainly long before then. Humans, along with every other living thing on the Earth, have adapted to a certain range of variability in the weather. Although extreme weather can cause loss of life and significant damage to property, people and virtually every other creature have, at least to some degree, adapted to the infrequent extremes they experience within their normal climatic zone. Humans’ use of fossil fuel since the start of the Industrial Revolution has begun to modify the Earth’s climate in ways that few could have imagined a century ago. The consequences of this change to the climate are seemingly everywhere: average temperatures are rising, precipitation patterns are changing, ice sheets are melting, and sea levels are rising. These changes are affecting the availability and quality of water supplies, how and where food is grown, and even the very fabric of ecosystems on land and in the sea. Despite these profound changes, climate change and its associated risks still may appear distant and remote in both time and space to many people. The natural daily and seasonal variability of the weather can mask the changes in the overall climate. However, when people experience extreme events that they believe may be occurring with different—usually greater—frequency in time or with increased intensity, many ask about the connection between climate change and extreme events. Effective, rigorous, and scientifically defensible analysis of the attribution of extreme weather events to changes in the climate system not only helps satisfy the public’s desire to know, but can also provide valuable information about the future risks of such events to emergency managers, regional planners, and policy-makers at all levels of government. A solid understanding of extreme weather event attribution in the context of a changing climate can help provide insight into and confidence in the many risk calculations that underpin much of society’s building codes; land, water, health and food management; insurance; transportation networks; and many additional aspects of daily life. There are compelling scientific reasons to study extreme weather event