RADIATIVE FORCING OF CLIMATE CHANGE: CONCEPTS AND UNCERTAINTIES

By ANDREW MARTINEZ

CONTENTS EXECUTIVE SUMMARY INTRODUCTION EARTH’S CLIMATE SYSTEM, RADIATIVE FORCING: PERTURBATION TO EARTH’S ENERGY BUDGET, THEORETICAL DEVELOPMENT OF THE RADIATIVE FORCING CONCEPT, THE RADIATIVE FORCING CONCEPT AND CLIMATE POLICY, STATE OF SCIENTIFIC UNDERSTANDING WELL-MIXED GREENHOUSE GASES, TROPOSPHERIC AND STRATOSPHERIC OZONE, DIRECT EFFECT OF AEROSOLS, INDIRECT EFFECT OF AEROSOLS, LAND-COVER AND LAND-USE CHANGES, SOLAR FORCING, VOLCANIC ERUPTIONS, OCEAN COLOR, TELECONNECTIONS AND RADIATIVE FORCING, RADIATIVE FORCING OVER EARTH’S HISTORY GEOLOGICAL TIMESCALES, GLACIAL-INTERGLACIAL TIMESCALES, HOLOCENE, LAST 1000 YEARS, LAST 25 YEARS, CONTENTS RETHINKING THE GLOBAL RADIATIVE FORCING CONCEPT THE CURRENT CONCEPT, GLOBAL MEAN RADIATIVE FORCING WITH ADJUSTED TROPOSPHERIC TEMPERATURES, GLOBAL MEAN RADIATIVE FORCING AT THE SURFACE, REGIONAL RADIATIVE FORCING, REGIONAL NONRADIATIVE FORCING, OCEAN HEAT CONTENT, UNCERTAINTIES ASSOCIATED WITH FUTURE CLIMATE FORCINGS FUTURE EMISSIONS OF GREENHOUSE GASES AND AEROSOLS, SOLAR VARIABILITY, LAND-USE AND LAND-COVER CHANGE, ABRUPT CLIMATE CHANGE, RESEARCH APPROACHES TO FURTHERING UNDERSTANDING OBSERVATIONS OF RADIATIVE FORCING AND RESPONSE, LABORATORY AND PROCESS STUDIES FOR AEROSOLS, ATMOSPHERIC REANALYSIS AND DATA ASSIMILATION, RELATING CONCENTRATIONS OF GREENHOUSE GASES AND AEROSOLS TO SOURCES, CLIMATE FORCING AND RESPONSE OVER EARTH’S HISTORY, CLIMATE MODELS, RECOMMENDATIONS EXPANDING THE RADIATIVE FORCING CONCEPT, ADDRESSING KEY UNCERTAINTIES, IMPROVING THE OBSERVATIONAL RECORD, ADDRESSING POLICY NEEDS, REFERENCES EXECUTIVE SUMMARY T HE EARTH RECEIVES A CONTINUOUS INFLUX OF ENERGY FROM THE SUN SOME OF THIS ENERGY IS ABSORBED AT THE EARTH’S SURFACE OR BY THE ATMO- SPHERE, WHILE SOME IS REFLECTED BACK TO SPACE AT THE SAME TIME, THE EARTH AND ITS ATMOSPHERE EMIT ENERGY TO SPACE, RESULTING IN AN APPROXIMATE BALANCE BETWEEN ENERGY RECEIVED AND ENERGY LOST KNOWLEDGE OF THE NATURAL AND ANTHROPOGENIC PROCESSES THAT AFFECT THIS ENERGY BALANCE IS CRITICAL FOR UNDERSTANDING HOW EARTH’S CLIMATE HAS CHANGED IN THE PAST AND WILL CHANGE IN THE FUTURE IN ORDER TO ADVANCE UNDERSTANDING OF THIS ISSUE, THE U S CLIMATE CHANGE SCIENCE PROGRAM ASKED THE NATIONAL ACADEMIES TO EXAMINE THE CURRENT STATE OF KNOWLEDGE OF HOW THE ENERGY BALANCE REGULATING EARTH’S CLIMATE IS MODIFIED BY “FORCINGS” INCLUDING GASES AND AEROSOLS, LAND USE, AND SOLAR VARIABILITY AND TO IDENTIFY RELEVANT RESEARCH NEEDS (SEE APPENDIX B FOR THE FULL STATEMENT OF TASK) THIS REPORT PROVIDES THE CONSENSUS VIEW OF THE COMMITTEE THAT WAS FORMED TO UNDERTAKE THE STUDY IN THIS REPORT, THE COMMITTEE PRESENTS SPECIFIC RECOMMENDATIONS FOR EXPANDING CURRENT RADIA- TIVE FORCING CONCEPTS AND METRICS AND OUTLINES RESEARCH PRIORITIES FOR EX- PLOITING THESE CONCEPTS AND METRICS AS TOOLS FOR CLIMATE CHANGE RESEARCH AND POLICY WHAT IS RADIATIVE FORCING? FACTORS THAT DRIVE CLIMATE CHANGE ARE USEFULLY SEPARATED INTO FORCINGS AND FEEDBACKS (FIGURE ES-1) A CLIMATE FORCING IS AN ENERGY IMBALANCE IMPOSED ON THE CLIMATE SYSTEM EITHER EXTERNALLY OR BY HUMAN ACTIVITIES