UNDERSTANDING CLIMATE CHANGE FEEDBACKS
1 INTRODUCTION 1 2 CLOUD WATER VAPOR AND LAPSE RATE FEEDBACKS 7 3 SEA ICE FEEDBACKS 27 4 OCEAN HEAT UPTAKE AND OCEAN CIRCULATION FEEDBACKS 34 5 TERRESTRIAL HYDROLOGY AND VEGETATION FEEDBACKS 45 6 ATMOSPHERIC CHEMICAL FEEDBACKS 62 7 BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE 74 8 MODES OF VARIABILITY 93 9 RECOMMENDATIONS 96 REFERENCES 104 CONTENTS 1 INTRODUCTION THE CONSENSUS VIEW OF SCIENTISTS IS THAT HUMAN ACTIVITIES ARE CHANGING EARTH S CLIMATE AND THAT THIS COULD HAVE VERY IMPORTANT CONSEQUENCES FOR HUMAN LIFE AND NATURAL ECOSYSTEMS IPCC 2001B NRC 2002 PROJECTIONS1 OF HOW CLIMATE MIGHT CHANGE AS A RESULT OF HUMAN ACTIVITIES REMAIN UNCERTAIN HOWEVER THE INTERNATIONAL PANEL ON CLIMATE CHANGE IPPC THIRD ASSESSMENT REPORT TAR PROJECTS THAT ANNUALLY AND GLOBALLY AVERAGED SURFACE TEMPERATURE WILL INCREASE BY 1 4 C TO 5 8 C DURING THE INTERVAL BETWEEN 1990 AND 2100 CUBASCH ET AL 2001 THE LARGE RANGE OF POSSIBLE WARMING RESULTS IN APPROXIMATELY EQUAL MEASURE FROM TWO SOURCES FIRST THE RATE AT WHICH HUMANS WILL RELEASE GREENHOUSE GASES AND MAKE OTHER CHANGES IN THE NATURAL ENVIRONMENT OF EARTH IN THE FUTURE IS DIFFICULT TO PREDICT THE FUTURE RATES OF HUMAN MODIFICATION OF THE ENVIRONMENT DEPEND ON SOCIAL ECONOMIC AND POLITICAL PROCESSES AS WELL AS TECHNOLOGICAL INNOVATION AND DIFFUSION AND ARE UNKNOWN POLICY MAKERS MAY MAKE DIFFERENT CHOICES IF SCIENTISTS PROVIDE CREDIBLE INFORMATION ABOUT THE MAGNITUDE AND STRUCTURE OF THE CLIMATE RESPONSE TO GREENHOUSE GAS RELEASES THE SECOND SOURCE OF UNCERTAINTY IS HOW THE CLIMATE SYSTEM OF EARTH WILL RESPOND TO HUMAN FORCING INTERACTIONS AMONG PHYSICAL CHEMICAL AND BIOLOGICAL PROCESSES THAT DETERMINE THE RESPONSE OF THE CLIMATE SYSTEM TO HUMAN ACTIVITIES ARE NOT FULLY UNDERSTOOD IF THE CARBON DIOXIDE CONCENTRATION IN THE ATMOSPHERE WERE DOUBLED AND THE CLIMATE WERE ALLOWED SUFFICIENT TIME TO COME INTO A NEW EQUILIBRIUM THE PROJECTED UNCERTAINTY IN THE WARMING OF THE GLOBAL MEAN SURFACE TEMPERATURE WOULD STILL BE LARGE 1 IN THIS DOCUMENT WE HAVE GENERALLY TRIED TO FOLLOW THE IPCC PRACTICE OF USING THE WORD PROJECTION WHEN REFERRING TO ESTIMATES OF FUTURE CLIMATES THAT ARE HYPOTHETICAL IN THE SENSE THAT THEY DEPEND ON AN ASSUMPTION OF A PARTICULAR SCENARIO FOR EMISSIONS AND HENCE RADIATIVE FORCING WE USE THE WORD PREDICTION WHEN THE ANSWER IS NOT CONTINGENT ON A CLIMATE FORCING SCENARIO OR THE CLIMATE FORCING SCENARIO IS CONSIDERED FIXED SUCH AS IN THE PROBLEM OF CALCULATING THE EQUILIBRIUM RESPONSE TO DOUBLED CO2 1 1 5 C TO 4 5 C ACCORDING TO THE INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE IPCC 2001A OUR INABILITY TO RELIABLY DETERMINE THE INFLUENCE OF VARIOUS FEEDBACK PROCESSES IS ONE OF THE MOST IMPORTANT REASONS WHY PROJECTIONS OF POSSIBLE FUTURE CLIMATE CHANGE SHOW SUCH WIDE VARIATIONS SCIENTIFIC RESEARCH CAN PROVIDE KNOWLEDGE THAT WILL HELP REFINE AND FOCUS THESE PROJECTIONS SO THAT THEY BECOME MORE ACCURATE OVER TIME CLIMATE SCIENTISTS OFTEN SEPARATE INFLUENCES ON CLIMATE CHANGE INTO FORCINGS AND FEEDBACKS CLIMATE FORCINGS ARE CHANGES THAT INITIATE OUTSIDE OF THE NATURALLY EVOLVING CLIMATE SYSTEM AND CAN BE EITHER NATURAL OR HUMAN CAUSED SEE TABLE 1 1 PROCESSES IN THE CLIMATE SYSTEM THAT CAN EITHER AMPLIFY OR DAMP THE SYSTEM S RESPONSE TO CHANGED FORCINGS ARE KNOWN AS FEEDBACKS FEEDBACKS ARE INTERACTIONS IN THE CLIMATE SYSTEM BETWEEN THE VARIABLES DEFINING THE STATE OF THE ATMOSPHERE OCEAN AND LAND SURFACE THE RANGE OF POSSIBLE OUTCOMES IN CLIMATE CHANGE PROJECTIONS THAT RESULTS FROM THE INTERNAL DYNAMICS OF THE CLIMATE SYSTEM IS THE RESULT OF FEEDBACK PROCESSES AND OUR INABILITY TO CAPTURE THESE ADEQUATELY IN MODELS A FEEDBACK PROCESS IS A PROCESS WHEREBY A CHANGE IN ONE VARIABLE SUCH AS CARBON DIOXIDE CONCENTRATION CAUSES A CHANGE IN TEMPERATURE WHICH CAUSES A CHANGE IN A THIRD VARIABLE SUCH AS WATER VAPOR WHICH IN TURN CAUSES A FURTHER CHANGE IN TEMPERATURE CLIMATE MODELS SUGGEST THAT THE TEMPERATURE CHANGE ENHANCEMENT ASSOCIATED WITH FEEDBACK PROCESSES IS GREATER THAN THE TEMPERATURE CHANGE RESULTING FROM THE DIRECT EFFECT OF THE CARBON DIOXIDE DOUBLING WITHOUT FEEDBACKS IPCC 2001A STOTT AND KETTLEBOROUGH 2002 FIND THAT THE MAGNITUDE OF GLOBAL WARMING OVER THE NEXT 40 YEARS IS INSENSITIVE TO THE RATE OF GREENHOUSE GAS RELEASES IN THEIR STUDY THE RANGE OF POSSIBLE WARMINGS IS DETERMINED BY THE RANGE OF ESTIMATES OF THE STRENGTH OF CLIMATE FEEDBACKS AND NOT BY THE RANGE OF ESTIMATES OF CLIMATE FORCING THEREFORE STUDY OF CLIMATE FEEDBACKS AND CLIMATE SENSITIVITY IS VERY IMPORTANT FOR PROJECTING CLIMATE CHANGES OVER THE NEXT 40 YEARS EVEN IN A SIMPLE LINEAR ANALYSIS THE TEMPERATURE RESPONSE IS NOT LINEAR IN THE STRENGTHS OF THE FEEDBACKS BECAUSE ALL THE OTHER FEEDBACK PROCESSES MODIFY THE TEMPERATURE CHANGE ASSOCIATED WITH ONE FEEDBACK PROCESS HANSEN ET AL 1984 IN A SYSTEM WITH A STRONG POSITIVE FEEDBACK SUCH AS WATER VAPOR FEEDBACK IN THE CLIMATE SYSTEM THE STRONG POSITIVE FEEDBACK PROCESS AMPLIFIES THE CHANGES ASSOCIATED WITH WEAKER FEEDBACK PROCESSES SEE BOX 1 1 THE INTEGRATED EFFECT OF CLIMATE FEEDBACK PROCESSES ON CLIMATE SENSITIVITY CAN BE ESTIMATED BY USING THE OBSERVED RECORD OF GLOBAL MEAN TEMPERATURE OVER THE PAST 120 YEARS IPCC 2001A THIS METHOD REQUIRES ESTIMATES OF THE CLIMATE FORCING CLIMATE SENSITIVITY AND THE UPTAKE OF HEAT UNDERSTANDING CLIMATE CHANGE FEEDBACKS 2 TABLE 1 1 CLIMATE FORCING VARIABLES COMPARED TO CLIMATE STATE VARIABLESA CLIMATE FORCING VARIABLES CLIMATE SYSTEM STATE VARIABLES SOLAR IRRADIANCE TEMPERATURE OF AIR LAND AND WATER VOLCANIC ERUPTIONS PRECIPITATION AND SNOW COVER GREENHOUSE GAS PRODUCTION BY HUMANS HUMIDITY CLOUDS AND WINDS AEROSOL PRODUCTION BY HUMANS OCEAN CURRENTS SALINITY AND ICE COVER REACTIVE GAS PRODUCTION BY HUMANS SOIL MOISTURE AND VEGETATION PROPERTIES LAND SURFACE MODIFICATION BY HUMANS AEROSOL DISTRIBUTION ATMOSPHERIC TRACE GAS CONCENTRATION ATHE VARIABLES IN THE LEFT COLUMN ARE NATURAL AND HUMAN CAUSED CLIMATE FORCINGS THAT ARE DEFINED TO BE OUTSIDE THE CLIMATE SYSTEM FOR THE PURPOSES OF THIS REPORT THE PROCESSES THAT COUPLE THE CLIMATE SYSTEM VARIABLES IN THE RIGHT COLUMN CAN RESULT IN CLIMATE FEEDBACK THAT WILL DETERMINE THE RESPONSE OF CLIMATE TO FORCING BY THE CLIMATE SYSTEM AND EACH OF THESE FACTORS IS UNCERTAIN CONSEQUENTLY THE RANGE OF PROBABLE FUTURE CLIMATES IS ONLY LOOSELY CONSTRAINED BY MODELS FITTED TO THE INSTRUMENTAL RECORD OF GLOBAL MEAN TEMPERATURE ANDRONOVA AND SCHLESINGER 2001 USED A MONTE CARLO SIMULATION WITH A SIMPLE CLIMATE SYSTEM MODEL TO ESTIMATE A PROBABILITY DISTRIBUTION FUNCTION FOR CLIMATE SENSITIVITY CLIMATE SENSITIVITY IS HERE DEFINED TO BE THE EQUILIBRIUM RESPONSE OF GLOBAL MEAN SURFACE TEMPERATURE TO DOUBLING CARBON DIOXIDE THEY CONCLUDED THAT THERE IS A 54 PERCENT LIKELIHOOD THAT THE ACTUAL CLIMATE SENSITIVITY LIES OUTSIDE THE RANGE OF 1 5 4 5 C AND THAT THE 90 PERCENT CONFIDENCE INTERVAL FOR CLIMATE SENSITIVITY IS 1 0 9 3 C KNUTTI ET AL 2002 FOUND A 40 PERCENT PROBABILITY THAT THE WARMING WILL EXCEED THE IPCC ESTIMATES BUT ONLY A 5 PERCENT PROBABILITY THAT THE WARMING WILL BE LESS THAN THE IPCC LOWER LIMIT FOREST ET AL 2002 FOUND SIMILARLY THAT THE 5 PERCENT AND 95 PERCENT CONFIDENCE LIMITS ON THE CLIMATE SENSITIVITY ARE 1 4 K TO 7 7 K COMPARED TO THE 1 5 4 5 K RANGE STATED BY IPCC USE OF THE INSTRUMENTAL RECORD OF GLOBAL MEAN TEMPERATURE CANNOT CONSTRAIN CLIMATE SENSITIVITY TO A NARROW RANGE BECAUSE THE CLIMATE FORCING MAGNITUDE AMOUNT OF HEAT STORAGE AND EVEN THE TEMPERATURE RECORD ITSELF ARE NOT KNOWN WITH SUFFICIENT PRECISION AN ENHANCED EFFORT TO UNDERSTAND AND MODEL THE MOST IMPORTANT CLIMATE FEEDBACK PROCESSES IS NEEDED TO IMPROVE OUR FUNDAMENTAL KNOWLEDGE AND WILL LEAD TO BETTER CHARACTERIZATIONS OF THE CLIMATE SYSTEM POTENTIALLY REDUCING THE WIDE RANGES NOW SEEN IN CLIMATE CHANGE PROJECTIONS IMPROVED UNDERSTANDING COMBINED WITH MORE RIGOROUS COMPARISON OF OBSERVED AND MODELED FEEDBACK PROCESSES SHOULD LEAD TO MORE CONFIDENCE IN CLIMATE UNDERSTANDING CLIMATE CHANGE FEEDBACKS 3