Executive Summary

6.1 Radiative Forcing

6.1.1 Definition
6.1.2 Evolution of Knowledge on Forcing Agents

6.2 Forcing-Response Relationship

6.2.1 Characteristics
6.2.2 Strengths and Limitations of the Forcing Concept

6.3 Well-Mixed Greenhouse Gases

6.3.1 Carbon Dioxide
6.3.2 Methane and Nitrous Oxide
6.3.3 Halocarbons
6.3.4 Total Well-Mixed Greenhouse Gas Forcing Estimate
6.3.5 Simplified Expressions

6.4 Stratospheric Ozone

6.4.1 Introduction
6.4.2 Forcing Estimates

6.5 Radiative Forcing By Tropospheric Ozone

6.5.1 Introduction
6.5.2 Estimates of Tropospheric Ozone Radiative Forcing since Pre-Industrial Times
6.5.2.1 Ozone radiative forcing: process studies
6.5.2.2 Model estimates
6.5.3 Future Tropospheric Ozone Forcing

6.6 Indirect Forcings due to Chemistry

6.6.1 Effects of Stratospheric Ozone Changes on Radiatively Active Species
6.6.2 Indirect Forcings of Methane, Carbon Monoxide and Non-Methane Hydrocarbons
6.6.3 Indirect Forcing by NO_x Emissions
6.6.4 Stratospheric Water Vapour

6.7 The Direct Radiative Forcing of Tropospheric Aerosols

6.7.1 Summary of IPCC WGI Second Assessment Report and Areas of Development
6.7.2 Sulphate Aerosol
6.7.3 Fossil Fuel Black Carbon Aerosol
6.7.4 Fossil Fuel Organic Carbon Aerosol
6.7.5 Biomass Burning Aerosol
6.7.6 Mineral Dust Aerosol
6.7.7 Nitrate Aerosol
6.7.8 Discussion of Uncertainties

6.8 The Indirect Radiative Forcing of Tropospheric Aerosols

6.8.1 Introduction
6.8.2 Indirect Radiative Forcing by Sulphate Aerosols
6.8.2.1 Estimates of the first indirect effect
6.8.2.2 Estimates of the second indirect effect and of the combined effect
6.8.2.3 Further discussion of uncertainties
6.8.3 Indirect Radiative Forcing by Other Species
6.8.3.1 Carbonaceous aerosols
6.8.3.2 Combination of sulphate and carbonaceous aerosols
6.8.3.3 Mineral dust aerosols
6.8.3.4 Effect of gas-phase nitric acid
6.8.4 Indirect Methods for Estimating the Indirect Aerosol Effect
6.8.4.1 The “missing” climate forcing
6.8.4.2 Remote sensing of the indirect effect of aerosols
6.8.5 Forcing Estimates for This Report
6.8.6 Aerosol Indirect Effect on Ice Clouds
6.8.6.1 Contrails and contrail-induced cloudiness
6.8.6.2 Impact of anthropogenic aerosols on cirrus cloud microphysics

6.9 Stratospheric Aerosols

6.10 Land-use Change (Surface Albedo Effect)

6.11 Solar Forcing of Climate

6.11.1 Total Solar Irradiance
6.11.1.1 The observational record
6.11.1.2 Reconstructions of past variations of total solar irradiance
6.11.2 Mechanisms for Amplification of Solar Forcing
6.11.2.1 Solar ultraviolet variation
6.11.2.2 Cosmic rays and clouds

6.12 Global Warming Potentials

6.12.1 Introduction
6.12.2 Direct GWPs
6.12.3 Indirect GWPs
6.12.3.1 Methane
6.12.3.2 Carbon monoxide
6.12.3.3 Halocarbons
6.12.3.4 NO_x and non-methane hydrocarbons

6.13 Global Mean Radiative Forcings

6.13.1 Estimates
6.13.2 Limitations

6.14 The Geographical Distribution of the Radiative Forcings

6.14.1 Gaseous Species
6.14.2 Aerosol Species
6.14.3 Other Radiative Forcing Mechanisms

6.15 Time Evolution of Radiative Forcings

6.15.1 Past to Present
6.15.2 SRES Scenarios
6.15.2.1 Well-mixed greenhouse gases
6.15.2.2 Tropospheric ozone
6.15.2.3 Aerosol direct effect
6.15.2.4 Aerosol indirect effect

Appendix 6.1 Elements of Radiative Forcing Concept

References

Co-ordinating Lead Author
V. Ramaswamy

Lead Authors
O. Boucher, J. Haigh, D. Hauglustaine, J. Haywood, G. Myhre, T. Nakajima, G.Y. Shi, S. Solomon

Contributing Authors
R. Betts, R. Charlson, C. Chuang, J.S. Daniel, A. Del Genio, R. van Dorland, J. Feichter, J. Fuglestvedt, P.M. de F. Forster, S.J. Ghan, A. Jones, J.T. Kiehl, D. Koch, C. Land, J. Lean, U. Lohmann, K. Minschwaner, J.E. Penner, D.L. Roberts, H. Rodhe, G.J. Roelofs, L.D. Rotstayn, T.L. Schneider, U. Schumann, S.E. Schwartz, M.D. Schwarzkopf, K.P. Shine, S. Smith, D.S. Stevenson, F. Stordal, I. Tegen, Y. Zhang

Review Editors
F. Joos, J. Srinivasan

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