GLOBAL CLIMATE CHANGE
CONTENTS CHAPTER 1 SCIENTIFIC ASSESSMENT OF THE EFFECTS OF GLOBAL CHANGE ON THE UNITED STATES 1 COMMITTEE ON ENVIRONMENT AND NATURAL RESOURCES CHAPTER 2 POTENTIAL IMPACTS OF CLIMATE CHANGE IN THE UNITED STATES 255 CONGRESSIONAL BUDGET OFFICE CHAPTER 1 SCIENTIFIC ASSESSMENT OF THE EFFECTS OF GLOBAL CHANGE ON THE UNITED STATES SECTION I EXECUTIVE SUMMARY I 1 INTRODUCTION THE CLIMATE IS CHANGING AND THESE CHANGES ARE AFFECTING THE WORLD AROUND US IN ORDER TO DEAL WITH THE CHANGES THAT ARE TAKING PLACE NOW AND TO PREPARE FOR THOSE THAT ARE LIKELY TO HAPPEN IN THE FUTURE DECISIONMAKERS NEED INFORMATION ABOUT GLOBAL CHANGE AND ITS EFFECTS ON THE NATION AND THE WORLD WE LIVE IN THIS NATIONAL SCIENTIFIC ASSESSMENT INTEGRATES EVALUATES AND INTERPRETS THE FINDINGS OF THE U S CLIMATE CHANGE SCIENCE PROGRAM CCSP AND DRAWS FROM AND SYNTHESIZES FINDINGS FROM PREVIOUS ASSESSMENTS OF THE SCIENCE INCLUDING REPORTS AND PRODUCTS BY THE INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE IPCC IT ANALYZES CURRENT TRENDS IN GLOBAL CHANGE BOTH NATURAL AND HUMAN INDUCED AND IT PROJECTS MAJOR TRENDS FOR THE FUTURE IT ANALYZES THE EFFECTS OF THESE CHANGES ON THE NATURAL ENVIRONMENT AGRICULTURE WATER RESOURCES SOCIAL SYSTEMS ENERGY PRODUCTION AND USE TRANSPORTATION AND HUMAN HEALTH IT IS INTENDED TO HELP INFORM DISCUSSION OF THE RELEVANT ISSUES BY DECISIONMAKERS STAKEHOLDERS AND THE PUBLIC AS SUCH THIS REPORT ADDRESSES THE REQUIREMENTS FOR ASSESSMENT IN THE GLOBAL CHANGE RESEARCH ACT OF 1990 1 THIS ASSESSMENT ADDRESSES NOT ONLY CLIMATE CHANGE BUT ALSO OTHER CHANGE IN THE GLOBAL ENVIRONMENT INCLUDING WATER RESOURCES OCEANS ATMOSPHERIC CHEMISTRY LAND PRODUCTIVITY AND ECOLOGICAL SYSTEMS THAT MAY ALTER THE CAPACITY OF EARTH TO SUSTAIN LIFE THIS BROADER SET OF CHANGES IS REFERRED TO AS GLOBAL CHANGE AS DEFINED IN THE GLOBAL CHANGE RESEARCH ACT 1 OVER THE PAST SEVERAL YEARS OUR UNDERSTANDING OF CLIMATE VARIABILITY AND CHANGE AND OUR ABILITY TO ESTIMATE THEIR FUTURE EFFECTS HAS IMPROVED SIGNIFICANTLY THE CONCLUSIONS IN THIS ASSESSMENT BUILD ON THE VAST BODY OF OBSERVATIONS MODELING DECISION SUPPORT AND OTHER TYPES OF ACTIVITIES CONDUCTED UNDER THE AUSPICES OF CCSP AND FROM PREVIOUS ASSESSMENTS OF THE SCIENCE INCLUDING REPORTS AND PRODUCTS BY THE IPCC CCSP AND OTHERS THIS ASSESSMENT AND THE UNDERLYING ASSESSMENTS HAVE BEEN SUBJECTED TO AND IMPROVED THROUGH RIGOROUS PEER REVIEWS I 2 CAUSES OF CLIMATE CHANGE OUR UNDERSTANDING OF CLIMATE CHANGE CONTINUES TO GROW ENABLING SCIENTISTS TO DRAW INCREASINGLY CERTAIN CONCLUSIONS ABOUT ITS CAUSES AND IMPACTS FOR EXAMPLE IN THEIR MOST RECENT ASSESSMENT OF CLIMATE CHANGE SCIENCE THE IPCC CONCLUDED THAT IT IS UNEQUIVOCAL THAT THE AVERAGE TEMPERATURE OF EARTH S SURFACE HAS WARMED RECENTLY AND IT IS VERY LIKELY GREATER THAN 90 PROBABILITY 2 THAT MOST OF THIS GLOBAL WARMING IS DUE TO INCREASED CONCENTRATIONS OF HUMAN GENERATED GREENHOUSE GASES SEVERAL LINES OF EVIDENCE INCLUDING THOSE OUTLINED IN THE FOLLOWING SECTIONS POINT TO A STRONG HUMAN INFLUENCE ON CLIMATE ALTHOUGH THESE INDIVIDUAL LINES OF EVIDENCE VARY IN THEIR DEGREES OF CERTAINTY WHEN CONSIDERED TOGETHER THEY PROVIDE A COMPELLING AND SCIENTIFICALLY SOUND EXPLANATION OF THE CHANGES TO EARTH S CLIMATE INCLUDING CHANGES IN SURFACE TEMPERATURE ICE EXTENT AND SEA LEVEL RISE OBSERVED AT GLOBAL AND CONTINENTAL SCALES OVER THE PAST FEW DECADES IV 1 AND IV 2 3 SEVERAL DIFFERENT TYPES OF GASES IN THE ATMOSPHERE WARM THE PLANET BY TRAPPING ENERGY THAT WOULD OTHERWISE BE EMITTED TO SPACE THESE GREENHOUSE GASES INCLUDE WATER VAPOR CARBON DIOXIDE METHANE OZONE NITROUS OXIDE AND SEVERAL FLUORINE AND CHLORINE CONTAINING GASES OF THE GREENHOUSE GASES THAT ARE INCREASING IN ATMOSPHERIC CONCENTRATION AS A DIRECT RESULT OF HUMAN ACTIVITIES CARBON DIOXIDE IS CONTRIBUTING MOST TO THE RECENT WARMING THE GLOBALLY AVERAGED CONCENTRATION OF CARBON DIOXIDE IN THE ATMOSPHERE HAS INCREASED FROM ABOUT 280 PARTS PER MILLION PPM IN THE 18TH CENTURY TO 383 PPM IN 2007 EMISSIONS OF CARBON DIOXIDE FROM FOSSIL FUEL USE AND FROM THE EFFECTS OF LAND USE CHANGE ARE THE PRIMARY SOURCES OF THIS INCREASE THE CURRENT ATMOSPHERIC CONCENTRATION OF CARBON DIOXIDE GREATLY EXCEEDS THE NATURAL RANGE OF THE LAST 650 000 YEARS 180 TO 300 PPM AS DETERMINED FROM ICE CORES INDEED THE ATMOSPHERIC LEVELS OF ALL MAJOR GREENHOUSE GASES HAVE ALSO INCREASED SIGNIFICANTLY IN THE PAST CENTURY IV 1 A AN INCREASING BODY OF SCIENTIFIC RESEARCH SUPPORTS THE CONCLUSION THAT WHILE GREENHOUSE GASES ARE BUT ONE OF MANY FACTORS THAT AFFECT CLIMATE THEY ARE VERY LIKELY THE SINGLE LARGEST CAUSE OF THE RECENT WARMING OTHER FACTORS THAT AFFECT CLIMATE MAY CONTRIBUTE EITHER A WARMING OR COOLING INFLUENCE FOR EXAMPLE SOME TYPES OF TINY PARTICLES IN THE AIR INTRODUCE A COOLING INFLUENCE THESE PARTICLES KNOWN AS AEROSOLS INCLUDE SULFATE ORGANIC CARBON NITRATE AND MINERAL DUST BLACK CARBON AEROSOLS INTRODUCE A WARMING INFLUENCE DEPOSITION OF BLACK CARBON ON SNOW AND ICE ALSO CONTRIBUTES A WARMING INFLUENCE ON THE CLIMATE BY DECREASING SURFACE REFLECTIVITY THAT WOULD OTHERWISE DEFLECT MORE SOLAR ENERGY BACK INTO SPACE AEROSOLS HAVE BEEN OBSERVED TO INCREASE CLOUDINESS AND CLOUD REFLECTIVITY BOTH OF WHICH HAVE A COOLING INFLUENCE OTHER CHANGES TO SURFACE REFLECTIVITY AS WELL AS VARIATIONS IN SOLAR IRRADIANCE CAN HAVE EITHER WARMING OR COOLING INFLUENCES IV 1 A GLOBAL CLIMATE CHANGE IMPACTS IN THE UNITED STATES 2 STUDIES THAT RIGOROUSLY QUANTIFY THE EFFECT OF DIFFERENT EXTERNAL INFLUENCES ON OBSERVED CHANGES ATTRIBUTION STUDIES CONCLUDE THAT MOST OF THE RECENT GLOBAL WARMING IS VERY LIKELY DUE TO HUMAN GENERATED INCREASES IN GREENHOUSE GAS CONCENTRATIONS A LARGE NUMBER OF CLIMATE MODEL SIMULATIONS SHOW THAT NATURAL FACTORS ALONE CANNOT EXPLAIN THE OBSERVED WARMING IN THE SECOND HALF OF THE 20TH CENTURY OF EARTH S LAND MASSES AND OCEANS OR THAT OF THE NORTH AMERICAN CONTINENT ON THE OTHER HAND SIMULATIONS THAT INCLUDE HUMAN FACTORS ARE ABLE TO REPRODUCE IMPORTANT LARGE SCALE FEATURES OF THE RECENT CHANGES SEVERAL STUDIES INDICATE THAT NATURAL FACTORS PLAYED AN IMPORTANT ROLE IN THE WARMING OF THE FIRST HALF OF THE 20TH CENTURY ATTRIBUTION STUDIES SHOW THAT IT IS LIKELY GREATER THAN 66 PROBABILITY THAT THERE HAS BEEN A SUBSTANTIAL HUMAN CONTRIBUTION TO THE SURFACE TEMPERATURE INCREASE IN NORTH AMERICA ACCORDING TO THE IPCC MODEL SIMULATIONS OF REGIONAL SUB CONTINENTAL SCALE CLIMATE HAVE IMPROVED AND A LIMITED NUMBER OF REGIONAL CLIMATE FEATURES ARE CONSISTENTLY CAPTURED IN MULTIPLE CLIMATE MODEL PROJECTIONS REGIONAL CHANGES IN THE UNITED STATES THAT ARE CONSISTENTLY PROJECTED IN CLIMATE MODELS ARE DISCUSSED THROUGHOUT THE REPORT AND INCLUDE SOME SIGNIFICANT REGIONAL CHANGES IN TEMPERATURE AND PRECIPITATION THERE ARE LIMITS TO HOW EFFECTIVELY CLIMATE CHANGE AND ITS IMPACTS CAN BE PROJECTED ON SMALL SCALES SUCH AS AT THE SCALE OF A SINGLE CITY OR COUNTY A SINGLE STATE OR EVEN A GROUP OF STATES ESPECIALLY OVER THE NEXT DECADE OR TWO CLIMATE CHANGES ON SMALL SCALES ARE DIFFICULT TO DISCERN IN PART BECAUSE CLIMATE VARIABILITY AVERAGED OVER SMALL REGIONS IS GREATER THAN THE VARIABILITY AVERAGED OVER LARGE REGIONS FOR THESE AND OTHER REASONS ATTRIBUTION OF THE DRIVERS OF LONG TERM TEMPERATURE CHANGES ON TIME SCALES OF LESS THAN 50 YEARS AND AT REGIONAL SCALES E G COUNTY STATE OR MULTIPLE STATES AS OPPOSED TO CONTINENTAL WITH LIMITED EXCEPTIONS HAS NOT YET BEEN ESTABLISHED IV 2 IN ADDITION TO AVERAGE TEMPERATURES RECENT WORK SHOWS THAT HUMAN ACTIVITIES HAVE ALSO LIKELY INFLUENCED EXTREMES IN TEMPERATURE MANY INDICATORS OF CLIMATE EXTREMES INCLUDING THE ANNUAL NUMBERS OF FROST DAYS WARM AND COLD DAYS AND WARM AND COLD NIGHTS SHOW CHANGES THAT ARE CONSISTENT WITH WARMING FOR EXAMPLE THERE IS EVIDENCE THAT HUMAN INDUCED WARMING MAY4 HAVE SUBSTANTIALLY INCREASED THE RISK OF EXTREMELY WARM SUMMER CONDITIONS IN SOME REGIONS DISCERNIBLE HUMAN INFLUENCES EXTEND TO ADDITIONAL ASPECTS OF CLIMATE INCLUDING THE RECENT DECREASES IN ARCTIC SEA ICE EXTENT PATTERNS OF SEA LEVEL PRESSURE AND WINDS AND THE GLOBAL SCALE PATTERN OF LAND PRECIPITATION IV 2 A ACCORDING TO CCSP SYNTHESIS AND ASSESSMENT PRODUCT SAP 3 3 IT IS VERY LIKELY THAT THE HUMAN INDUCED INCREASE IN GREENHOUSE GASES HAS CONTRIBUTED TO THE INCREASE IN SEA SURFACE TEMPERATURES IN THE HURRICANE FORMATION REGIONS THERE IS A STRONG STATISTICAL CONNECTION BETWEEN TROPICAL ATLANTIC SEA SURFACE TEMPERATURES AND ATLANTIC HURRICANE ACTIVITY AS MEASURED BY AN INDEX THAT ACCOUNTS FOR STORM INTENSITY FREQUENCY AND DURATION ON DECADAL TIMESCALES OVER THE PAST 50 YEARS THIS EVIDENCE SUGGESTS A SUBSTANTIAL HUMAN CONTRIBUTION TO RECENT HURRICANE ACTIVITY HOWEVER A CONFIDENT ASSESSMENT OF HUMAN INFLUENCE ON HURRICANES WILL REQUIRE FURTHER STUDIES USING MODELS AND OBSERVATIONS IV 2 A GLOBAL CLIMATE CHANGE IMPACTS IN THE UNITED STATES 3