A State of Knowledge Report from theU.S. Global Change Research Program
The full report can be found online at www.globalchange.gov/usimpacts CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi Cambridge University Press 32 Avenue of the Americas, New York, NY 10013-2473, USA www.cambridge.org Information on this title: www.cambridge.org/9780521144070 This report was produced by an advisory committee chartered under the Federal Advisory Committee Act, for the Subcommittee on Global Change Research, and at the request of the U.S. Government. Therefore, the report is in the public domain. Some materials used in the report are copyrighted and permission was granted to the U.S. government for their publication in this report. For subsequent uses that include such copyrighted materials, permission for reproduction must be sought from the copyright holder. In all cases, credit must be given for copyrighted materials. First published 2009 Printed in the United States of America A catalog record for this publication is available from the British Library. ISBN 978-0-521-14407-0 paperback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party Internet Web sites referred to in this publication and does not guarantee that any content on such Web sites is, or will remain, accurate or appropriate. Information regarding prices, travel timetables, and other factual informa- tion given in this work are correct at the time of first printing, but Cambridge Univer- sity Press does not guarantee the accuracy of such information thereafter.Recommended Citation:Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009.The bars at the bottom of the front cover show the global annual average temperature from 1900-2008, see page 17.
Federal Advisory Committee Authors Co-Chairs and Editors-in-Chief Thomas R. Karl, Jerry M. Melillo, Thomas C. Peterson, NOAA National Climatic Data Center Marine Biological Laboratory NOAA National Climatic Data Center Author Team David M. Anderson, Jack A. Kaye, Michael J. Savonis, NOAA World Data Center for National Aeronautics and Space U.S. Department of Transportation Paleoclimatology Administration H. Gerry Schwartz, Jr., Donald F. Boesch, Jay H. Lawrimore, Consultant/Transportation University of Maryland Center for NOAA National Climatic Data Center Eileen L. Shea, Environmental Science James J. McCarthy, NOAA National Climatic Data Center/ Virginia R. Burkett, Harvard University Integrated Data and Environmental U.S. Geological Survey A. David McGuire, Applications Center Lynne M. Carter, U.S. Geological Survey/University of John M.R. Stone, Adaptation Network, Louisiana Alaska Fairbanks Carleton University State University Edward L. Miles, Bradley H. Udall, Stewart J. Cohen, University of Washington University of Colorado/NOAA Earth Environment Canada and University of Evan Mills, System Research Laboratory British Columbia Lawrence Berkeley National Laboratory John E. Walsh, Nancy B. Grimm, Jonathan T. Overpeck, University of Alaska Fairbanks Arizona State University University of Arizona Michael F. Wehner, Jerry L. Hatfield, Jonathan A. Patz, Lawrence Berkeley National Laboratory U.S. Department of Agriculture University of Wisconsin at Madison Thomas J. Wilbanks, Katharine Hayhoe, Roger S. Pulwarty, Oak Ridge National Laboratory Texas Tech University NOAA Climate Program Office and Earth . Donald J. Wuebbles, Anthony C. Janetos, System Research Laboratory University of Illinois Pacific Northwest National Laboratory Benjamin D. Santer, Lawrence Livermore National Laboratory Senior Science Writer and Lead Graphic Designer Senior Science Writer Lead Graphic Designer Susan J. Hassol, Sara W. Veasey, Climate Communication, LLC NOAA National Climatic Data Center Key Support Personnel Jessica Blunden, Editorial Assistant, Jolene McGill, Logistical Support, Susanne Skok, Copy Editor, STG, Inc. NOAA National Climatic Data Center STG, Inc. Marta Darby, Copy Editor, STG, Inc. Deborah J. Misch, Graphics Support, Mara Sprain, Editorial Support, STG, Inc. STG, Inc. David Dokken, CCSP Technical Advisor, USGCRP William Murray, Technical Support, Michael Squires, Cartographic Support, STG, Inc. NOAA National Climatic Data Center Byron Gleason, Data Analysis/Visualization, NOAA National Climatic Data Center Susan Osborne, Copy Editor, Jeff VanDorn, Technical and Graphics Support, STG, Inc. ATMOS Research Glenn M. Hyatt, Graphics Support, NOAA National Climatic Data Center Tim Owen, Logistical Support, David Wuertz, Data Analysis/Visualization, NOAA National Climatic Data Center NOAA National Climatic Data Center Clare Keating, Editorial Support, Texas Tech University Ray Payne, Printing Support Christian Zamarra, Graphics Support, NOAA Nationa Climatic Data Center STG, Inc. Staci Lewis, Technical Advisor, NOAA Deborah Riddle, Graphics Support, NOAA National Climatic Data Center
Federal Executive TeamActing Director, U.S. Global Change Research Program: .........................................................................................Jack A. KayeDirector, U.S. Global Change Research Program Office: ..........................................................................................Peter A. SchultzLead Agency Principal Representative to CCSP (through January 2009),National Oceanic and Atmospheric Administration: .................................................................................................Mary M. GlackinLead Agency Principal Representative to CCSP,Product Lead, National Oceanic and Atmospheric Administration: .........................................................................Thomas R. KarlLead Agency Principal Representative to CCSP,Group Chair Synthesis and Assessment Products, Environmental Protection Agency: ...........................................Michael W. SlimakSynthesis and Assessment Product Coordinator,U.S. Global Change Research Program Office: .........................................................................................................Fabien J.G. LaurierCommunications Advisor/Coordinator/Editor,U.S. Global Change Research Program Office: .........................................................................................................Anne M. WapleSpecial Advisor, National Oceanic and Atmospheric Administration: .....................................................................Chad A. McNuttFederal Advisory Committee Designated Federal Official,National Oceanic and Atmospheric Administration: .................................................................................................Christopher D. Miller Reviewers Blue Ribbon ReviewersRobert W. Corell, Linda O. Mearns, Global Change Program, H. John Heinz III Environmental and Societal Impacts Center for Science, Economics and the Group, National Center for Atmospheric Environment ResearchRobert A. Duce, Gerald A. Meehl, Department of Atmospheric Sciences, Climate and Global Dynamics Division, Texas A&M University National Center for Atmospheric ResearchKristie L. Ebi, John Reilly, Independent consultant, ESS, LLC Sloan School of Management, Alexandria, VA Massachusetts Institute of TechnologyChristopher B. Field, Susan Solomon, Carnegie Institution NOAA, Earth System Research LaboratoryWilliam H. Hooke, Atmospheric Policy Program, American Steven C. Wofsy, Meteorological Society Harvard UniversityMichael C. MacCracken, Climate Institute Communication ReviewersRobert Henson, Jack W. Williams, University Corporation for Atmospheric Research American Meteorological Society This report on Global Climate Change Impacts in the United States was prepared in accordance with Section 515 of the Treasury and General Government Appropriations Act for Fiscal Year 2001 (Public Law 106-554) and the information quality act guidelines issued by the Department of Commerce and NOAA pursuant to Section 515 <http://www.noaanews.noaa.gov/stories/iq.htm>.
June 2009Members of Congress:On behalf of the National Science and Technology Council, the U.S. Global Change Research Program is pleased totransmit to the President and the Congress this state of knowledge report: “Global Climate Change Impacts in the UnitedStates.” This report summarizes the science of climate change and the impacts of climate change on the United States,now and in the future.As our nation strives to develop effective policies to respond to climate change, it is critical to have the latest and bestscientific information to inform decision making. More than a year in the making, this report provides that information.It is the first report in almost a decade to provide an extensive evaluation of climate change impacts on the United Statesat the regional level.An expert team of scientists operating under the authority of the Federal Advisory Committee Act, assisted bycommunication specialists, wrote the document. The report was reviewed and revised based on comments from expertsand the public in accordance with the Information Quality Act guidelines issued by the Department of Commerce andthe National Oceanic and Atmospheric Administration.We highly commend the authors and support personnel of both this report and the underlying Synthesis and AssessmentProducts for the outstanding quality of their work in providing sound and thorough science-based information for policyformulation and climate change research priority setting. We intend to use the essential information contained in thisreport as we make policies and decisions about the future, and we recommend others do the same. Sincerely, Dr. John Holdren Dr. Jane Lubchenco Director, Administrator, Office of Science and Technology Policy National Oceanic and Atmospheric Administration
CONTENTS About this Report......................................................................................... 7 Executive Summary.................................................................................... 9 Global Climate Change...................................................................... 13 . National Climate Change.................................................................. 27 . Climate Change Impacts by Sector W . ater Resources........................................................................................ 41 Energy Supply and Use.............................................................................. 53 T . ransportation............................................................................................ 61 . A . griculture................................................................................................... 71 . Ecosystems...................................................................................................79 Human Health.............................................................................................89 . S . ociety...........................................................................................................99
CONTENTS Regional Climate Change Impacts Northeast.................................................................................................107 . Southeast....................................................................................................111 Midwest..................................................................................................... 117 . G reat Plains..............................................................................................123 . Southwest................................................................................................. 129 . Northwest................................................................................................135 . Alaska.........................................................................................................139 Islands.........................................................................................................145 Coasts........................................................................................................149 An Agenda for Climate Impacts Science..............................153 Concluding Thoughts............................................................................. 157 Author Team Biographies........................................................................... 159 Primary Sources of Information.............................................................. 161 Acronyms and Abbreviations....................................................................164 References ..........................................................................................................165 Photo and Figure Credits............................................................................188 .
About this Report About this ReportWhat is this report? What are its sources?This report summarizes the science of climate change The report draws from a large body of scientific in-and the impacts of climate change on the United States, formation. The foundation of this report is a set of 21now and in the future. It is largely based on results of Synthesis and Assessment Products (SAPs), which werethe U.S. Global Change Research Program (USGCRP),a designed to address key policy-relevant issues in climateand integrates those results with related research from science (see page 161); several of these were also sum-around the world. This report discusses climate-related marized in the Scientific Assessment of the Effects ofimpacts for various societal and environmental sec- Climate Change on the United States published in 2008.tors and regions across the nation. It is an authoritative In addition, other peer-reviewed scientific assessmentsscientific report written in plain language, with the goal were used, including those of the Intergovernmentalof better informing public and private decision making Panel on Climate Change, the U.S. National Assessmentat all levels. of the Consequences of Climate Variability and Change, the Arctic Climate Impact Assessment, the National Research Council’s Transportation Research BoardWho called for it, who wrote it, and who report on the Potential Impacts of Climate Change onapproved it? U.S. Transportation, and a variety of regional climate impact assessments. These assessments were augmentedThe USGCRP called for this report. An expert team of with government statistics as necessary (such as popula-scientists operating under the authority of the Federal tion census and energy usage) as well as publicly avail-Advisory Committee Act, assisted by communication able observations and peer-reviewed research publishedspecialists, wrote the document. The report was exten- through the end of 2008. This new work was carefullysively reviewed and revised based on comments from selected by the author team with advice from expert re-experts and the public. The report was approved by its viewers to update key aspects of climate change sciencelead USGCRP Agency, the National Oceanic and Atmo- relevant to this report. The icons on the bottom of thisspheric Administration, the other USGCRP agencies, page represent some of the major sources drawn uponand the Committee on the Environment and Natural Re- for this synthesis report.sources on behalf of the National Science and Technol-ogy Council.b This report meets all Federal requirements On the first page of each major section, the sourcesassociated with the Information Quality Act, including primarily drawn upon for that section are shown usingthose pertaining to public comment and transparency. these icons. Endnotes, indicated by superscript numbers and compiled at the end of the book, are used for specific references throughout the report. See page 161 for descriptions of these sources. a. The U.S. Global Change Research Program (USGCRP), which was established in 1990 by the Global Change Research Act, encompasses the Climate ChangeScience Program (CCSP).b. A description of the National Science and Technology Council (NSTC) can be found at www.ostp.gov/cs/nstc. 7
U.S. Global Change Research Program Global Climate Change Impacts in the United States Does this report deal with options for Rather, in the context of impacts, this report identi- responding to climate change? fies examples of actions currently being pursued in various sectors and regions to address climate While the primary focus of this report is on the change, as well as other environmental problems impacts of climate change in the United States, that could be exacerbated by climate change such as it also deals with some of the actions society is urban air pollution and heat waves. In most cases, already taking or can take to respond to the climate there is currently insufficient peer-reviewed infor- challenge. Responses to climate change fall into two mation to evaluate the practicality, effectiveness, broad categories. The first involves “mitigation” costs, or benefits of these measures, highlighting a measures to reduce climate change by, for example, need for research in this area. Thus, the discussion reducing emissions of heat-trapping gases and par- of various public and private adaptation examples ticles, or increasing removal of heat-trapping gases should not be viewed as an endorsement of any from the atmosphere. The second involves “adapta- particular option, but rather as illustrative examples tion” measures to improve our ability to cope with of approaches being tried. or avoid harmful impacts and take advantage of beneficial ones, now and in the future. Both of these are necessary elements of an effective response How is the likelihood of various strategy. These two types of responses are linked in outcomes expressed given that the that more effective mitigation measures reduce the future is not certain? amount of climate change, and therefore the need for adaptation. When it is considered necessary to express a range of possible outcomes and identify the likelihood This report underscores the importance of mitiga- of particular impacts, this report takes a plain- tion by comparing impacts resulting from higher language approach to expressing the expert judg- versus lower emissions scenarios. The report shows ment of the author team based on the best available that choices made about emissions in the next few evidence. For example, an outcome termed “likely” decades will have far-reaching consequences for has at least a two-thirds chance of occurring; an climate change impacts. Over the long term, lower outcome termed “very likely,” at least a 90 percent emissions will lessen both the magnitude of climate chance.1 In using these terms, the Federal Advisory change impacts and the rate at which they appear. Committee has taken into consideration a wide range of information, including the strength and While the report underscores the importance of consistency of the observed evidence, the range and mitigation as an essential part of the nation’s climate consistency of model projections, the reliability of change strategy, it does not evaluate mitigation particular models as tested by various methods, and technologies or undertake an analysis of the ef- most importantly, the body of work addressed in fectiveness of various approaches. These issues are earlier synthesis and assessment reports. Key sourc- the subject of ongoing studies by the U.S. Govern- es of information used to develop these character- ment’s Climate Change Technology Program and izations of uncertainty are referenced in endnotes. several federal agencies including the Department of Energy, Environmental Protection Agency, National Oceanic and Atmospheric Administration, How does this report address Department of Transportation, and Department of incomplete scientific understanding? Agriculture. The range of mitigation responses be- ing studied includes more efficient production and This assessment identifies areas in which scientific use of energy, increased use of non-carbon-emitting uncertainty limits our ability to estimate future energy sources, and carbon capture and storage. climate change and its impacts. The section on An Agenda for Climate Impacts Science at the end of Adaptation options also have the potential to moder- this report highlights some of these areas. ate harmful impacts of current and future climate variability and change. While this report does ad- dress adaptation, it does not do so comprehensively.8
Executive Summary Executive Summary Observations show that warming of the climate is unequivocal. The global warming observed over the past 50 years is due primarily to human-induced emissions of heat-trapping gases. These emissions come mainly from the burning of fossil fuels (coal, oil, and gas), with important contributions from the clearing of forests, agricultural practices, and other activities. Warming over this century is projected to be considerably greater than over the last century. The global average temperature since 1900 has risen by about 1.5ºF. By 2100, it is projected to rise another 2 to 11.5ºF. The U.S. average temperature has risen by a comparable amount and is very likely to rise more than the global average over this century, with some variation from place to place. Several factors will determine future temperature increases. Increases at the lower end of this range are more likely if global heat-trapping gas emissions are cut substantially. If emissions continue to rise at or near current rates, temperature increases are more likely to be near the upper end of the range. Volcanic eruptions or other natural variationscould temporarily counteract some of the human-induced warming, slowing the rise in globaltemperature, but these effects would only last a few years.Reducing emissions of carbon dioxide would lessen warming over this century and beyond. Siz-able early cuts in emissions would significantly reduce the pace and the overall amount of climatechange. Earlier cuts in emissions would have a greater effect in reducing climate change than com-parable reductions made later. In addition, reducing emissions of some shorter-lived heat-trappinggases, such as methane, and some types of particles, such as soot, would begin to reduce warmingwithin weeks to decades.Climate-related changes have already been observed globally and in the United States. Theseinclude increases in air and water temperatures, reduced frost days, increased frequency and inten-sity of heavy downpours, a rise in sea level, and reduced snow cover, glaciers, permafrost, and seaice. A longer ice-free period on lakes and rivers, lengthening of the growing season, and increasedwater vapor in the atmosphere have also been observed. Over the past 30 years, temperatures haverisen faster in winter than in any other season, with average winter temperatures in the Midwestand northern Great Plains increasing more than 7ºF. Some of the changes have been faster thanprevious assessments had suggested.These climate-related changes are expected to continue while new ones develop. Likely futurechanges for the United States and surrounding coastal waters include more intense hurricanes withrelated increases in wind, rain, and storm surges (but not necessarily an increase in the number ofthese storms that make landfall), as well as drier conditions in the Southwest and Caribbean. Thesechanges will affect human health, water supply, agriculture, coastal areas, and many other aspectsof society and the natural environment.This report synthesizes information from a wide variety of scientific assessments (see page 7) andrecently published research to summarize what is known about the observed and projected conse-quences of climate change on the United States. It combines analysis of impacts on various sectors 9
U.S. Global Change Research Program Global Climate Change Impacts in the United States such as energy, water, and transportation at the Projections of future climate change come from national level with an assessment of key impacts on careful analyses of outputs from global climate specific regions of the United States. For example, models run on the world’s most advanced comput- sea-level rise will increase risks of erosion, storm ers. The model simulations analyzed in this report surge damage, and flooding for coastal communi- used plausible scenarios of human activity that ties, especially in the Southeast and parts of Alaska. generally lead to further increases in heat-trapping Reduced snowpack and earlier snow melt will alter emissions. None of the scenarios used in this report the timing and amount of water supplies, posing assumes adoption of policies explicitly designed to significant challenges for water resource manage- address climate change. However, the level of emis- ment in the West. sions varies among scenarios because of differences in assumptions about population, economic activity, Society and ecosystems can adjust to some climatic choice of energy technologies, and other factors. changes, but this takes time. The projected rapid Scenarios cover a range of emissions of heat-trap- rate and large amount of climate change over this ping gases, and the associated climate projections century will challenge the ability of society and illustrate that lower emissions result in less climate natural systems to adapt. For example, it is difficult change and thus reduced impacts over this century and expensive to alter or replace infrastructure and beyond. Under all scenarios considered in designed to last for decades (such as buildings, this report, however, relatively large and sustained bridges, roads, airports, reservoirs, and ports) in re- changes in many aspects of climate are projected by sponse to continuous and/or abrupt climate change. the middle of this century, with even larger changes by the end of this century, especially under higher Impacts are expected to become increasingly severe emissions scenarios. for more people and places as the amount of warm- ing increases. Rapid rates of warming would lead In projecting future conditions, there is always to particularly large impacts on natural ecosystems some level of uncertainty. For example, there is a and the benefits they provide to humanity. Some of high degree of confidence in projections that future the impacts of climate change will be irreversible, temperature increases will be greatest in the Arctic such as species extinctions and coastal land lost to and in the middle of continents. For precipitation, rising seas. there is high confidence in projections of continued increases in the Arctic and sub-Arctic (including Unanticipated impacts of increasing carbon dioxide Alaska) and decreases in the regions just outside and climate change have already occurred and the tropics, but the precise location of the transition more are possible in the future. For example, it has between these is less certain. At local to regional recently been observed that the increase in atmo- scales and on time frames up to a few years, natural spheric carbon dioxide concentration is causing an climate variations can be relatively large and can increase in ocean acidity. This reduces the ability of temporarily mask the progressive nature of global corals and other sea life to build shells and skeletons climate change. However, the science of making out of calcium carbonate. Additional impacts in the skillful projections at these scales has progressed future might stem from unforeseen changes in the considerably, allowing useful information to be climate system, such as major alterations in oceans, drawn from regional climate studies such as those ice, or storms; and unexpected consequences of highlighted in this report. ecological changes, such as massive dislocations of species or pest outbreaks. Unexpected social or This report focuses on observed and projected economic changes, including major shifts in wealth, climate change and its impacts on the United States. technology, or societal priorities would also affect However, a discussion of these issues would be our ability to respond to climate change. Both incomplete without mentioning some of the actions anticipated and unanticipated impacts become more society can take to respond to the climate chal- challenging with increased warming. lenge. The two major categories are “mitigation” and “adaptation.” Mitigation refers to options for limiting climate change by, for example, reducing10
Executive Summaryheat-trapping emissions such as carbon dioxide, altering its zoning and building codes to place fewermethane, nitrous oxide, and halocarbons, or re- structures in harm’s way and making buildingsmoving some of the heat-trapping gases from the less vulnerable to damage from floods, fires, andatmosphere. Adaptation refers to changes made other extreme events. Some adaptation options thatto better respond to present or future climatic and are currently being pursued in various regions andother environmental conditions, thereby reducing sectors to deal with climate change and/or otherharm or taking advantage of opportunity. Effective environmental issues are identified in this report.mitigation measures reduce the need for adaptation. However, it is clear that there are limits to howMitigation and adaptation are both essential parts of much adaptation can achieve.a comprehensive climate change response strategy. Humans have adapted to changing climatic condi-Carbon dioxide emissions are a primary focus of tions in the past, but in the future, adaptations willmitigation strategies. These include improving be particularly challenging because society won’t beenergy efficiency, using energy sources that do not adapting to a new steady state but rather to a rapidlyproduce carbon dioxide or produce less of it, captur- moving target. Climate will be continually chang-ing and storing carbon dioxide from fossil fuel use, ing, moving at a relatively rapid rate, outside theand so on. Choices made about emissions reductions range to which society has adapted in the past. Thenow and over the next few decades will have far- precise amounts and timing of these changes willreaching consequences for climate-change impacts. not be known with certainty.The importance of mitigation is clear in compari-sons of impacts resulting from higher versus lower In an increasingly interdependent world, U.S.emissions scenarios considered in this report. Over vulnerability to climate change is linked to the fatesthe long term, lower emissions will lessen both the of other nations. For example, conflicts or massmagnitude of climate-change impacts and the rate migrations of people resulting from food scarcityat which they appear. Smaller climate changes that and other resource limits, health impacts, or envi-come more slowly make the adaptation challenge ronmental stresses in other parts of the world couldmore tractable. threaten U.S. national security. It is thus difficult to fully evaluate the impacts of climate change on theHowever, no matter how aggressively heat-trapping United States without considering the consequencesemissions are reduced, some amount of climate of climate change elsewhere. However, such analy-change and resulting impacts will continue due to sis is beyond the scope of this report.the effects of gases that have already been released.This is true for several reasons. First, some of these Finally, this report identifies a number of areas ingases are very long-lived and the levels of atmo- which inadequate information or understandingspheric heat-trapping gases will remain elevated for hampers our ability to estimate future climatehundreds of years or more. Second, the Earth’s vast change and its impacts. For example, our knowl-oceans have absorbed much of the heat added to the edge of changes in tornadoes, hail, and ice stormsclimate system due to the increase in heat-trapping is quite limited, making it difficult to know ifgases, and will retain that heat for many decades. and how such events have changed as climate hasIn addition, the factors that determine emissions, warmed, and how they might change in the future.such as energy-supply systems, cannot be changed Research on ecological responses to climate changeovernight. Consequently, there is also a need is also limited, as is our understanding of socialfor adaptation. responses. The section titled An Agenda for Climate Impacts Science at the end of this report offers someAdaptation can include a wide range of activities. thoughts on the most important ways to improve ourExamples include a farmer switching to growing knowledge. Results from such efforts would informa different crop variety better suited to warmer or future assessments that continue building ourdrier conditions; a company relocating key busi- understanding of humanity’s impacts on climate,ness centers away from coastal areas vulnerable and climate’s impacts on us.to sea-level rise and hurricanes; and a community 11
U.S. Global Change Research Program Global Climate Change Impacts in the United States Key Findings 1. Global warming is unequivocal and primarily human-induced. Global temperature has increased over the past 50 years. This observed increase is due primarily to human- induced emissions of heat-trapping gases. (p. 13) 2. Climate changes are underway in the United States and are projected to grow. Climate-related changes are already observed in the United States and its coastal waters. These include increases in heavy downpours, rising temperature and sea level, rapidly retreating glaciers, thawing permafrost, lengthening growing seasons, lengthening ice-free seasons in the ocean and on lakes and rivers, earlier snowmelt, and alterations in river flows. These changes are projected to grow. (p. 27) 3. Widespread climate-related impacts are occurring now and are expected to increase. Climate changes are already affecting water, energy, transportation, agriculture, ecosystems, and health. These impacts are different from region to region and will grow under projected climate change. (p. 41-106, 107-152) 4. Climate change will stress water resources. Water is an issue in every region, but the nature of the potential impacts varies. Drought, related to reduced precipitation, increased evaporation, and increased water loss from plants, is an important issue in many regions, especially in the West. Floods and water quality problems are likely to be amplified by climate change in most regions. Declines in mountain snowpack are important in the West and Alaska where snowpack provides vital natural water storage. (p. 41, 129, 135, 139) 5. Crop and livestock production will be increasingly challenged. Many crops show positive responses to elevated carbon dioxide and low levels of warming, but higher levels of warming often negatively affect growth and yields. Increased pests, water stress, diseases, and weather extremes will pose adaptation challenges for crop and livestock production. (p. 71) 6. Coastal areas are at increasing risk from sea-level rise and storm surge. Sea-level rise and storm surge place many U.S. coastal areas at increasing risk of erosion and flooding, especially along the Atlantic and Gulf Coasts, Pacific Islands, and parts of Alaska. Energy and transportation infrastructure and other property in coastal areas are very likely to be adversely affected. (p. 111, 139, 145, 149) 7. Risks to human health will increase. Harmful health impacts of climate change are related to increasing heat stress, waterborne diseases, poor air qual- ity, extreme weather events, and diseases transmitted by insects and rodents. Reduced cold stress provides some benefits. Robust public health infrastructure can reduce the potential for negative impacts. (p. 89) 8. Climate change will interact with many social and environmental stresses. Climate change will combine with pollution, population growth, overuse of resources, urbanization, and other social, economic, and environmental stresses to create larger impacts than from any of these factors alone. (p. 99) 9. Thresholds will be crossed, leading to large changes in climate and ecosystems. There are a variety of thresholds in the climate system and ecosystems. These thresholds determine, for example, the presence of sea ice and permafrost, and the survival of species, from fish to insect pests, with implications for society. With further climate change, the crossing of additional thresholds is expected. (p. 76, 82, 115, 137, 142) 10. Future climate change and its impacts depend on choices made today. The amount and rate of future climate change depend primarily on current and future human-caused emissions of heat-trapping gases and airborne particles. Responses involve reducing emissions to limit future warming, and adapting to the changes that are unavoidable. (p. 25, 29)12
Global Climate Change CLIMATE IMPACTS NATIONAL LEVEL Global Climate Change Key Messages: • Human activities have led to large increases in heat-trapping gases over the past century. • Global average temperature and sea level have increased, and precipitation patterns have changed. • The global warming of the past 50 years is due primarily to human-induced increases in heat-trapping gases. Human “fingerprints” also have been identified in many other aspects of the climate system, including changes in ocean heat content, precipitation, atmospheric moisture, and Arctic sea ice. • Global temperatures are projected to continue to rise over this century; by how much and for how long depends on a number of factors, including the amount of heat-trapping gas emissions and how sensitive the climate is to those emissions.Key SourcesThis introduction to global climatechange explains very briefly what has 800,000 Year Record of Carbon Dioxide Concentrationbeen happening to the world’s climateand why, and what is projected tohappen in the future. While this reportfocuses on climate change impacts inthe United States, understanding thesechanges and their impacts requiresan understanding of the globalclimate system.Many changes have been observed inglobal climate over the past century.The nature and causes of these changeshave been comprehensively chronicledin a variety of recent reports, such asthose by the Intergovernmental Panel Lüthi et al.; Tans; IIASA 2on Climate Change (IPCC) and theU.S. Climate Change Science Program Analysis of air bubbles trapped in an Antarctic ice core extending back 800,000 years documents the Earth’s changing carbon dioxide concentration. Over this long period,(CCSP). This section does not intend to natural factors have caused the atmospheric carbon dioxide concentration to varyduplicate these comprehensive efforts, within a range of about 170 to 300 parts per million (ppm). Temperature-related databut rather to provide a brief synthesis, make clear that these variations have played a central role in determining the globaland to integrate more recent work with climate. As a result of human activities, the present carbon dioxide concentration of about 385 ppm is about 30 percent above its highest level over at least the last 800,000the assessments of the IPCC, CCSP, years. In the absence of strong control measures, emissions projected for this centuryand others. would result in the carbon dioxide concentration increasing to a level that is roughly 2 to 3 times the highest level occurring over the glacial-interglacial era that spans the last 800,000 or more years. 13
U.S. Global Change Research Program Global Climate Change Impacts in the United States Human activities have led to large These emissions are thickening the blanket of increases in heat-trapping gases over heat-trapping gases in Earth’s atmosphere, causing the past century. surface temperatures to rise. The Earth’s climate depends on the functioning of a Heat-trapping gases natural “greenhouse effect.” This effect is the result Carbon dioxide concentration has increased due of heat-trapping gases (also known as greenhouse to the use of fossil fuels in electricity generation, gases) like water vapor, carbon dioxide, ozone, transportation, and industrial and household uses. methane, and nitrous oxide, which absorb heat radi- It is also produced as a by-product during the ated from the Earth’s surface and lower atmosphere manufacturing of cement. Deforestation provides a and then radiate much of the energy back toward source of carbon dioxide and reduces its uptake by the surface. Without this natural greenhouse effect, trees and other plants. Globally, over the past sev- the average surface temperature of the Earth would eral decades, about 80 percent of human-induced be about 60°F colder. However, human activities carbon dioxide emissions came from the burning have been releasing additional heat-trapping gases, of fossil fuels, while about 20 percent resulted from intensifying the natural greenhouse effect, thereby deforestation and associated agricultural practices. changing the Earth’s climate. The concentration of carbon dioxide in the atmo- sphere has increased by roughly 35 percent since Climate is influenced by a variety of factors, both the start of the industrial revolution.3 human-induced and natural. The increase in the carbon dioxide concentration has been the principal Methane concentration has increased mainly as factor causing warming over the past 50 years. Its a result of agriculture; raising livestock (which concentration has been building up in the Earth’s produce methane in their digestive tracts); mining, atmosphere since the beginning of the industrial transportation, and use of certain fossil fuels; sew- era in the mid-1700s, primarily due to the burn- age; and decomposing garbage in landfills. About ing of fossil fuels (coal, oil, and natural gas) and 70 percent of the emissions of atmospheric methane the clearing of forests. Human activities have also are now related to human activities.4 increased the emissions of other greenhouse gases, such as methane, nitrous oxide, and halocarbons.3 Nitrous oxide concentration is increasing as a re- sult of fertilizer use and fossil fuel burning. 2,000 Years of Greenhouse Gas Concentrations Halocarbon emissions come from the release of certain manufactured chemi- cals to the atmosphere. Examples include chlorofluorocarbons (CFCs), which were used extensively in refrigeration and for other industrial processes before their pres- ence in the atmosphere was found to cause stratospheric ozone depletion. The abun- dance of these gases in the atmosphere is now decreasing as a result of international regulations designed to protect the ozone layer. Continued decreases in ozone-deplet- ing halocarbon emissions are expected to reduce their relative influence on climate change in the future.3,5 Many halocarbon Forster et al. 3;Blasing7 replacements, however, are potent green- Increases in concentrations of these gases since 1750 are due to human activities house gases, and their concentrations in the industrial era. Concentration units are parts per million (ppm) or parts per are increasing.6 billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air.14
Global Climate ChangeOzone is a greenhouse gas, and is continually pro- Another type of aerosol, often referred to as sootduced and destroyed in the atmosphere by chemical or black carbon, absorbs incoming sunlight andreactions. In the troposphere, the lowest 5 to 10 traps heat in the atmosphere. Thus, depending onmiles of the atmosphere near the surface, human their type, aerosols can either mask or increase theactivities have increased the ozone concentration warming caused by increased levels of greenhousethrough the release of gases such as carbon mon- gases.13 On a globally averaged basis, the sum ofoxide, hydrocarbons, and nitrogen oxides. These these aerosol effects offsets some of the warminggases undergo chemical reactions to produce ozone caused by heat-trapping gases.10in the presence of sunlight. In addition to trappingheat, excess ozone in the troposphere causes respi- The effects of various greenhouse gases andratory illnesses and other human health problems. aerosol particles on Earth’s climate depend in part on how long these gases and particles remain inIn the stratosphere, the layer above the troposphere, the atmosphere. After emission, the atmosphericozone exists naturally and protects life on Earth concentration of carbon dioxide remains elevatedfrom exposure to excessive ultraviolet radiation for thousands of years, and that of methane forfrom the Sun. As mentioned previously, halocar- decades, while the elevated concentrations of aero-bons released by human activities destroy ozone sols only persist for days to weeks.11,12 The climatein the stratosphere and have caused the ozone hole effects of reductions in emissions of carbon dioxideover Antarctica.8 Changes in the stratospheric and other long-lived gases do not become apparentozone layer have contributed to changes in wind for at least several decades. In contrast, reductionspatterns and regional climates in Antarctica.9 in emissions of short-lived compounds can have a rapid, but complex effect since the geographic pat-Water vapor is the most important and abundant terns of their climatic influence and the resultinggreenhouse gas in the atmosphere. Human activi- surface temperature responses are quite different.ties produce only a very small increase in water One modeling study found that while the greatestvapor through irrigation and combustion process- emissions of short-lived pollutants in summertimees.3 However, the surface warming caused by hu- by late this century are projected to come fromman-produced increases in other greenhouse gases Asia, the strongest climate response is projected toleads to an increase in atmospheric water vapor, be over the central United States.13since a warmer climate increases evaporation andallows the atmosphere to hold more moisture. This Human activities have also changed the land sur-creates an amplifying “feedback loop,” leading to face in ways that alter how much heat is reflectedmore warming. or absorbed by the surface. Such changes include the cutting and burning of forests, the replacementOther human influences of other areas of natural vegetation with agricul-In addition to the global-scale climate effects of ture and cities, and large-scale irrigation. Theseheat-trapping gases, human activities also produce transformations of the land surface can cause localadditional local and regional effects. Some of these (and even regional) warming or cooling. Globally,activities partially offset the warming caused by the net effect of these changes has probably been agreenhouse gases, while others increase the warm- slight cooling of the Earth’s surface over the pasting. One such influence on climate is caused by 100 years.14,15tiny particles called “aerosols” (not to be confusedwith aerosol spray cans). For example, the burning Natural influencesof coal produces emissions of sulfur-containing Two important natural factors also influence cli-compounds. These compounds form “sulfate aero- mate: the Sun and volcanic eruptions. Over the pastsol” particles, which reflect some of the incoming three decades, human influences on climate havesunlight away from the Earth, causing a cooling become increasingly obvious, and global tempera-influence at the surface. Sulfate aerosols also tend tures have risen sharply. During the same period,to make clouds more efficient at reflecting sun- the Sun’s energy output (as measured by satelliteslight, causing an additional indirect cooling effect. since 1979) has followed its historical 11-year cycle 15
U.S. Global Change Research Program Global Climate Change Impacts in the United States of small ups and downs, but with no net increase influences, there are also fluctuations in climate (see figure page 20).16 The two major volcanic erup- that occur even in the absence of changes in human tions of the past 30 years have had short-term cool- activities, the Sun, or volcanoes. One example is ing effects on climate, lasting 2 to 3 years.17 Thus, the El Niño phenomenon, which has important these natural factors cannot explain the warming of influences on many aspects of regional and global recent decades; in fact, their net effect on climate climate. Many other modes of variability have been has probably been a slight cooling influence over identified by climate scientists and their effects this period. Slow changes in Earth’s orbit around on climate occur at the same time as the effects of the Sun and its tilt toward or away from the Sun human activities, the Sun, and volcanoes. are also a purely natural influence on climate, but are only important on timescales from thousands to Carbon release and uptake many tens of thousands of years. Once carbon dioxide is emitted to the atmosphere, some of it is absorbed by the oceans and taken up The climate changes that have occurred over the by vegetation, although this storage may be tempo- last century are not solely caused by the human and rary. About 45 percent of the carbon dioxide emit- natural factors described above. In addition to these ted by human activities in the last 50 years is now stored in the oceans and vegetation. The Major Warming and Cooling Influences on Climate rest has remained in the air, increasing 1750-2005 the atmospheric concentration.2,3,18 It is thus important to understand not only how much carbon dioxide is emitted, but also how much is taken up, over what time scales, and how these sources and “sinks” of carbon dioxide might change as climate continues to warm. For example, it is known from long records of Earth’s climate history that under warmer conditions, carbon tends to be released, for instance, from thaw- ing permafrost, initiating a feedback loop in which more carbon release leads to more warming which leads to further release, and so on.19,20 Global emissions of carbon dioxide have been accelerating. The growth rate increased from 1.3 percent per year in the 1990s to 3.3 percent per year be- tween 2000 and 2006.21 The increasing emissions of carbon dioxide are the pri- mary cause of the increased concentra- Forster et al.3 tion of carbon dioxide observed in the The figure above shows the amount of warming influence (red bars) or cooling influence atmosphere. There is also evidence that (blue bars) that different factors have had on Earth’s climate over the industrial age a smaller fraction of the annual human- (from about 1750 to the present). Results are in watts per square meter. The longer the induced emissions is now being taken bar, the greater the influence on climate. The top part of the box includes all the major human-induced factors, while the second part of the box includes the Sun, the only up than in the past, leading to a greater major natural factor with a long-term effect on climate. The cooling effect of individual fraction remaining in the atmosphere volcanoes is also natural, but is relatively short-lived (2 to 3 years), thus their influence and an accelerating rate of increase in is not included in this figure. The bottom part of the box shows that the total net effect the carbon dioxide concentration.21 (warming influences minus cooling influences) of human activities is a strong warming influence. The thin lines on each bar provide an estimate of the range of uncertainty.16
Global Climate ChangeOcean acidification Global Temperature and Carbon DioxideAs the ocean absorbs carbon dioxide from theatmosphere, seawater is becoming less alkaline(its pH is decreasing) through a process gener-ally referred to as ocean acidification. The pH ofseawater has decreased significantly since 1750,22,23and is projected to drop much more dramatically bythe end of the century if carbon dioxide concentra-tions continue to increase.24 Such ocean acidifica-tion is essentially irreversible over a time scale ofcenturies. As discussed in the Ecosystems sectorand Coasts region, ocean acidification affects theprocess of calcification by which living things cre-ate shells and skeletons, with substantial negative NOAA/NCDC32consequences for coral reefs, mollusks, and some Global annual average temperature (as measured over both landplankton species important to ocean food chains.25 and oceans). Red bars indicate temperatures above and blue bars indicate temperatures below the average temperature for the period 1901-2000. The black line shows atmospheric carbon dioxide (CO2) concentration in parts per million (ppm). While there is a clear long-Global average temperature and sea term global warming trend, each individual year does not show alevel have increased, and precipitation temperature increase relative to the previous year, and some yearspatterns have changed. show greater changes than others.33 These year-to-year fluctuations in temperature are due to natural processes, such as the effects of El Niños, La Niñas, and the eruption of large volcanoes.Temperatures are risingGlobal average surface air temperature has in-creased substantially since 1970.26 The estimated reflect the Sun’s heat, this melting causes more heatchange in the average temperature of Earth’s to be absorbed, which causes more melting, result-surface is based on measurements from thousands ing in another feedback loop.20of weather stations, ships, and buoys around theworld, as well as from satellites. These measure- Additionally, temperature measurements above thements are independently compiled, analyzed, and surface have been made by weather balloons sinceprocessed by different research groups. There are a the late 1940s, and from satellites since 1979. Thesenumber of important steps in the data processing. measurements show warming of the troposphere,These include identifying and adjusting for the ef- consistent with the surface warming.30,31 They alsofects of changes in the instruments used to measure reveal cooling in the stratosphere.30 This patterntemperature, the measurement times and loca- of tropospheric warming and stratospheric coolingtions, the local environment around the measuring agrees with our understanding of how atmosphericsite, and such factors as satellite orbital drift. For temperature would be expected to change in re-instance, the growth of cities can cause localized sponse to increasing greenhouse gas concentrations“urban heat island” effects. and the observed depletion of stratospheric ozone.14A number of research groups around the world Precipitation patterns are changinghave produced estimates of global-scale changes Precipitation is not distributed evenly over thein surface temperature. The warming trend that is globe. Its average distribution is governed primarilyapparent in all of these temperature records is con- by atmospheric circulation patterns, the availabilityfirmed by other independent observations, such as of moisture, and surface terrain effects. The firstthe melting of Arctic sea ice, the retreat of moun- two of these factors are influenced by temperature.tain glaciers on every continent,27 reductions in the Thus, human-caused changes in temperature areextent of snow cover, earlier blooming of plants expected to alter precipitation patterns.in spring, and increased melting of the Greenlandand Antarctic ice sheets.28,29 Because snow and ice 17
U.S. Global Change Research Program Global Climate Change Impacts in the United States Observations show that such shifts are occur- well below freezing, and where increased precipi- ring. Changes have been observed in the amount, tation has outpaced melting). The total volume of intensity, frequency, and type of precipitation. glaciers on Earth is declining sharply. The progres- Pronounced increases in precipitation over the past sive disappearance of glaciers has implications not 100 years have been observed in eastern North only for the rise in global sea level, but also for America, southern South America, and northern water supplies in certain densely populated regions Europe. Decreases have been seen in the Mediter- of Asia and South America. ranean, most of Africa, and southern Asia. Changes in the geographical distribution of droughts and The Earth has major ice sheets on Greenland and flooding have been complex. In some regions, there Antarctica. These ice sheets are currently losing have been increases in the occurrences of both ice volume by increased melting and calving of droughts and floods.28 As the world warms, north- icebergs, contributing to sea-level rise. The Green- ern regions and mountainous areas are experienc- land Ice Sheet has also been experiencing record ing more precipitation falling as rain rather than amounts of surface melting, and a large increase in snow.34 Widespread increases in heavy precipitation the rate of mass loss in the past decade.41 If the en- events have occurred, even in places where total tire Greenland Ice Sheet melted, it would raise sea rain amounts have decreased. These changes are level by about 20 feet. The Antarctic Ice Sheet con- associated with the fact that warmer air holds more sists of two portions, the West Antarctic Ice Sheet water vapor evaporating from the world’s oceans and the East Antarctic Ice Sheet. The West Antarc- and land surface.31 This increase in atmospheric tic Ice Sheet, the more vulnerable to melting of the water vapor has been observed from satellites, and two, contains enough water to raise global sea lev- is primarily due to human influences.35,36 els by about 16 to 20 feet.29 If the East Antarctic Ice Sheet melted entirely, it would raise global sea level Sea level is rising by about 200 feet. Complete melting of these ice After at least 2,000 years of little change, sea level sheets over this century or the next is thought to be rose by roughly 8 inches over the past century. virtually impossible, although past climate records Satellite data available over the past 15 years show provide precedent for very significant decreases in sea level rising at a rate roughly double the rate ice volume, and therefore increases in sea level.42,43 observed over the past century.37 There are two principal ways in which global warming causes sea level to Cumulative Decrease in Global Glacier Ice rise. First, ocean water expands as it warms, and therefore takes up more space. Warming has been observed in each of the world’s major ocean basins, and has been directly linked to human influences.38,39 Second, warming leads to the melting of glaciers and ice sheets, which raises sea level by adding water to the oceans. Glaciers have been retreating worldwide for at least the last century, and the rate of retreat has increased in the past decade.29,40 Only a few glaciers are actu- ally advancing (in locations that were Meier et al.27 As temperatures have risen, glaciers around the world have shrunk. The graph shows the cumulative decline in glacier ice worldwide.18
Global Climate ChangeThe global warming of the past 50 years The third line of evidence is based on the broad,is due primarily to human-induced qualitative consistency between observed changesincreases in heat-trapping gases. Human in climate and the computer model simulations“fingerprints” also have been identified of how climate would be expected to change inin many other aspects of the climate response to human activities. For example, whensystem, including changes in ocean heat climate models are run with historical increasescontent, precipitation, atmospheric in greenhouse gases, they show gradual warmingmoisture, and Arctic sea ice. of the Earth and ocean surface, increases in ocean heat content and the temperature of the lower atmo-In 1996, the IPCC Second Assessment Report44 sphere, a rise in global sea level, retreat ofcautiously concluded that “the balance of evi- sea ice and snow cover, cooling of the stratosphere,dence suggests a discernible human influence on an increase in the amount of atmospheric waterglobal climate.” Since then, a number of national vapor, and changes in large-scale precipitation andand international assessments have come to much pressure patterns. These and other aspectsstronger conclusions about the reality of human of modeled climate change are in agreementeffects on climate. Recent scientific assessments with observations.14,49find that most of the warming of the Earth’s surfaceover the past 50 years has been caused by human Finally, there is extensive statistical evidenceactivities.45,46 from so-called “fingerprint” studies. Each fac- tor that affects climate produces a unique patternThis conclusion rests on multiple lines of evi- of climate response, much as each person has adence. Like the warming “signal” that has gradu- unique fingerprint. Fingerprint studies exploit theseally emerged from the “noise” of natural climate unique signatures, and allow detailed comparisonsvariability, the scientific evidence for a human of modeled and observed climate change patterns.44influence on global climate has accumulated over Scientists rely on such studies to attribute observedthe past several decades, from many hundreds of changes in climate to a particular cause or set ofstudies. No single study is a “smoking gun.” Nor causes. In the real world, the climate changes thathas any single study or combination of studies have occurred since the start of the Industrial Revo-undermined the large body of evidence supporting lution are due to a complex mixture of human andthe conclusion that human activity is the primary natural causes. The importance of each individualdriver of recent warming. influence in this mixture changes over time. Of course, there are not multiple Earths, which wouldThe first line of evidence is our basic physical allow an experimenter to change one factor at aunderstanding of how greenhouse gases trap heat, time on each Earth, thus helping to isolate differenthow the climate system responds to increases in fingerprints. Therefore, climate models are usedgreenhouse gases, and how other human and natu- to study how individual factors affect climate. Forral factors influence climate. The second line of ev- example, a single factor (like greenhouse gases) oridence is from indirect estimates of climate changes a set of factors can be varied, and the response ofover the last 1,000 to 2,000 years. These records are the modeled climate system to these individual orobtained from living things and their remains (like combined changes can thus be studied.50tree rings and corals) and from physical quantities(like the ratio between lighter and heavier isotopes For example, when climate model simulations ofof oxygen in ice cores) which change in measurable the last century include all of the major influencesways as climate changes. The lesson from these on climate, both human-induced and natural, theydata is that global surface temperatures over the can reproduce many important features of observedlast several decades are clearly unusual, in that they climate change patterns. When human influenceswere higher than at any time during at least the are removed from the model experiments, resultspast 400 years.47 For the Northern Hemisphere, the suggest that the surface of the Earth would actu-recent temperature rise is clearly unusual in at least ally have cooled slightly over the last 50 years. Thethe last 1,000 years.47,48 clear message from fingerprint studies is that the 19
U.S. Global Change Research Program Global Climate Change Impacts in the United States Separating Human and tern of atmospheric temperature changes, with its Natural Influences on Climate pronounced cooling in the stratosphere, is therefore inconsistent with the hypothesis that changes in the Sun can explain the warming of recent decades. Moreover, direct satellite measurements of solar output show slight decreases during the recent period of warming. The earliest fingerprint work51 focused on changes in surface and atmospheric temperature. Scientists then applied fingerprint methods to a whole range of climate variables,50,52 identifying human-caused climate signals in the heat content of the oceans,38,39 Hegerl et al.49 the height of the tropopause53 (the boundary be- The blue band shows how global average temperatures would tween the troposphere and stratosphere, which has have changed due to natural forces only, as simulated by climate shifted upward by hundreds of feet in recent de- models. The red band shows model projections of the effects cades), the geographical patterns of precipitation,54 of human and natural forces combined. The black line shows drought,55 surface pressure,56 and the runoff from actual observed global average temperatures. As the blue band indicates, without human influences, temperature over the major river basins.57 past century would actually have first warmed and then cooled slightly over recent decades. 58 Studies published after the appearance of the IPCC Fourth Assessment Report in 2007 have also observed warming over the last half-century can- found human fingerprints in the increased levels of not be explained by natural factors, and is instead atmospheric moisture35,36 (both close to the surface caused primarily by human factors.14,50 and over the full extent of the atmosphere), in the Another fingerprint of human effects on Measurements of Surface Temperature climate has been identified by looking at a and Sun’s Energy slice through the layers of the atmosphere, and studying the pattern of temperature changes from the surface up through the stratosphere. In all climate models, increases in carbon di- oxide cause warming at the surface and in the troposphere, but lead to cooling of the strato- sphere. For straightforward physical reasons, models also calculate that the human-caused depletion of stratospheric ozone has had a strong cooling effect in the stratosphere. There is a good match between the model fingerprint in response to combined carbon dioxide and ozone changes and the observed pattern of tro- pospheric warming and stratospheric cooling (see figure on next page).14 In contrast, if most of the observed tempera- ture change had been due to an increase in NOAA/NCDC; Frölich and Lean; Willson and Mordvinov; Dewitte et al. 59 solar output rather than an increase in green- The Sun’s energy received at the top of Earth’s atmosphere has house gases, Earth’s atmosphere would have been measured by satellites since 1978. It has followed its natural warmed throughout its full vertical extent, 11-year cycle of small ups and downs, but with no net increase including the stratosphere.9 The observed pat- (bottom). Over the same period, global temperature has risen markedly (top).6020
Global Climate Changedecline of Arctic sea ice extent,61 and in the In the tropics, all models predicted that with a rise inpatterns of changes in Arctic and Antarctic greenhouse gases, the troposphere would be expectedsurface temperatures.62 to warm more rapidly than the surface. Observa- tions from weather balloons, satellites, and surfaceThe message from this entire body of work is that thermometers seemed to show the opposite behaviorthe climate system is telling a consistent story (more rapid warming of the surface than the tropo-of increasingly dominant human influence – the sphere). This issue was a stumbling block in our un-changes in temperature, ice extent, moisture, and derstanding of the causes of climate change. It is nowcirculation patterns fit together in a physically con- largely resolved.71 Research showed that there weresistent way, like pieces in a complex puzzle. large uncertainties in the satellite and weather balloon data. When uncertainties in models and observationsIncreasingly, this type of fingerprint work is shift- are properly accounted for, newer observational dataing its emphasis. As noted, clear and compelling sets (with better treatment of known problems) are inscientific evidence supports the case for a pro- agreement with climate model results.31,72-75nounced human influenceon global climate. Much Patterns of Temperature Changeof the recent attention is Produced by Various Atmospheric Factors, 1958-1999now on climate changes atcontinental and regionalscales,64,65 and on variablesthat can have large impactson societies. For example,scientists have establishedcausal links between humanactivities and the changes insnowpack, maximum andminimum temperature, andthe seasonal timing of runoffover mountainous regions ofthe western United States.34Human activity is likelyto have made a substantialcontribution to ocean surfacetemperature changes in hur-ricane formation regions.66-68Researchers are also lookingbeyond the physical climatesystem, and are begin-ning to tie changes in thedistribution and seasonalbehavior of plant and animalspecies to human-causedchanges in temperature andprecipitation.69,70For over a decade, one aspect Modified from CCSP SAP 1.163of the climate change story Climate simulations of the vertical profile of temperature change due to various factors, and the effectseemed to show a signifi- due to all factors taken together. The panels above represent a cross-section of the atmosphere fromcant difference between the north pole to the south pole, and from the surface up into the stratosphere. The black lines show the location of the tropopause, the boundary between the lower atmosphere (troposphere) and themodels and observations.14 stratosphere. 21
U.S. Global Change Research Program Global Climate Change Impacts in the United States This does not mean, however, that all remain- mate changes (such as the warming of the surface ing differences between models and observations and troposphere, and the increase in the amount have been resolved. The observed changes in some of moisture in the atmosphere) are driven by very climate variables, such as Arctic sea ice,61,76 some basic physics, which is well-represented in mod- aspects of precipitation,54,77 and patterns of surface els.35 Fourth, climate models can be used to predict pressure,56 appear to be proceeding much more changes in climate that can be verified in the real rapidly than models have projected. The reasons for world. Examples include the short-term global these differences are not well understood. Never- cooling subsequent to the eruption of Mount Pi- theless, the bottom-line conclusion from climate natubo and the stratospheric cooling with increas- fingerprinting is that most of the observed changes ing carbon dioxide. Finally, models are the only studied to date are consistent with each other, and tools that exist for trying to understand the climate are also consistent with our scientific understand- changes likely to be experienced over the course of ing of how the climate system would be expected this century. No period in Earth’s geological history to respond to the increase in heat-trapping gases provides an exact analogue for the climate condi- resulting from human activities.14,49 tions that will unfold in the coming decades.20 Scientists are sometimes asked whether extreme weather events can be linked to human activities.24 Global temperatures are projected to Scientific research has concluded that human influ- continue to rise over this century; by ences on climate are indeed changing the likelihood how much and for how long depends of certain types of extreme events. For example, on a number of factors, including the an analysis of the European summer heat wave of amount of heat-trapping gas emissions 2003 found that the risk of such a heat wave is now and how sensitive the climate is to roughly four times greater than it would those emissions. have been in the absence of human-induced climate change.68,78 Some continued warming of the planet is projected over the next few decades due to past emissions. Like fingerprint work, such analyses of human- Choices made now will influence the amount of fu- caused changes in the risks of extreme events rely ture warming. Lower levels of heat-trapping emis- on information from climate models, and on our sions will yield less future warming, while higher understanding of the physics of the climate system. levels will result in more warming, and more severe All of the models used in this work have imperfec- impacts on society and the natural world. tions in their representation of the complexities of the “real world” climate system.79,80 These are due Emissions scenarios to both limits in our understanding of the climate The IPCC developed a set of scenarios in a Special system, and in our ability to represent its com- Report on Emissions Scenarios (SRES).81 These plex behavior with available computer resources. have been extensively used to explore the potential Despite this, models are extremely useful, for a for future climate change. None of these scenarios, number of reasons. not even the one called “lower”, includes imple- mentation of policies to limit climate change or First, despite remaining imperfections, the current to stabilize atmospheric concentrations of heat- generation of climate models accurately portrays trapping gases. Rather, differences among these many important aspects of today’s weather pat- scenarios are due to different assumptions about terns and climate.79,80 Models are constantly being changes in population, rate of adoption of new improved, and are routinely tested against many technologies, economic growth, and other factors. observations of Earth’s climate system. Second, the fingerprint work shows that models capture not The IPCC emission scenarios also do not encom- only our present-day climate, but also key features pass the full range of possible futures: emissions of the observed climate changes over the past cen- can change less than those scenarios imply, or they tury.47 Third, many of the large-scale observed cli- can change more. Recent carbon dioxide emissions22