1. Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties Report from the NRC Committee on Radiative Forcing of Climate commissioned by the Climate Change Science Program released December 2004 Daniel J. Jacob (chair), Roni Avissar, Gerard C. Bond, Stuart Gaffin, Jeffrey T. Kiehl, Judith L. Lean, Ulrike Lohmann, Michael E. Mann, Roger A. Peilke, V. Ramanathan, L.M. Russell

2. Conceptual framework of climate forcing, response, and feedback CLIMATE FORCING AGENTS  Emissions of greenhouse gases and precursors, aerosols and precursors, and biogeochemically active gases  Solar irradiance and insolation changes  Land-cover changes CLIMATE RESPONSE Temperature, precipitation, vegetation, etc. HUMAN ACTIVITIES Fuel, industry, agriculture… NATURAL PROCESSES Sun, orbit, volcanoes Non-radiative Forcing Direct Radiative Forcing Indirect Radiative Forcing Feedback Societal Impacts CHANGE IN CLIMATE SYSTEM COMPONENTS  Atm. lapse rate  Atm. composition  Evapotranspiration

3. Climate change research and policy has relied on global top-of-atmosphere (TOA) radiative forcing TOA forcing from 1750 to present [IPCC, 2001]

4. Strengths and limitations of TOA radiative forcing concept StrengthsLimitations  Linearly related to equilibrium change in global mean surface temperature in GCMs  Simple, physical, robust, easy to compute  Enables comparison of different forcing agents  Enables comparison of different models  Has established use in policy analysis  Directly observable from space  Inferable from observed changes in ocean heat content  Does not account for vertical structure of forcing  Does not characterize regional response  Does not characterize hydrological response  Does not accommodate nonlinear response from large perturbations The TOA radiative forcing concept should be retained and expanded

5. Account for vertical structure of radiative forcing The relationship between TOA radiative forcing and surface temperature is affected by the vertical distribution of forcing within the atmosphere, particularly for absorbing aerosols and for land-use driven changes in evapotranspiration.  Characterize dependence of climate response on the vertical structure of radiative forcing;  Test ability of climate models to reproduce observed vertical structure of forcing;  Report global mean radiative forcing at both the surface and the TOA in climate change assessments; .Develop practical tools for incorporating surface radiative forcing in policy analyses and integrated assessment models. Priority recommendations: Aerosol radiative forcing

6. Determine the importance of regional variation in radiative forcing  Quantify and compare climate responses from regional radiative forcings in different climate models, and report results in climate change assessments;  Use climate records to investigate relationships between regional radiative forcing and climate response. Regional variations in radiative forcing may have important regional and global climatic implications that are not resolved by the concept of global mean radiative forcing. Priority recommendations: JJA forcing by tropospheric ozone

7. Determine the importance of non-radiative forcings  Improve understanding and parameterizations of aerosol-cloud thermodynamic interactions and land-atmosphere interactions in climate models;  Develop improved land-use and land-cover classifications at high resolution for the past and present, as well as scenarios for the future. Several types of forcings—most notably aerosols, land-use and land-cover change, and modifications to biogeochemistry— impact the climate system in non-radiative ways, in particular by modifying the hydrological cycle and vegetation dynamics. 1700 1990 1900 Historical changes in land cover Priority recommendations:

8. Addressing Key Uncertainties 1. Conduct accurate long-term monitoring of radiative forcing variables Spectrally resolved radiances from space, ocean heat content 2. Advance the attribution of decadal to centennial climate change Mine climate forcing and trend records for past 1000 years 3, Reduce uncertainties associated with indirect aerosol radiative forcing Improve parameterizations used in models 4, Better quantify the direct radiative effects of aerosols Improve understanding of aerosol sources, mixing states, sinks 5. Better quantify radiative forcing by ozone Improve understanding of strat-trop exchange, lightning NO x 6, Integrate climate forcing criteria in environmental policy analysis Examine climate impacts of policies directed at air quality and land use, develop GWPs for short-lived agents, determine additivities of forcings