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Radiative Forcing of Climate Change

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6 Radiative Forcing of Climate Change 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 Contents Executive Summary 351 6.8.3.1 Carbonaceous aerosols 377 6.8.3.2 Combination of sulphate and 6.1 Radiative Forcing 353 carbonaceous aerosols 378 6.1.1 Definition 353 6.8.3.3 Mineral dust aerosols 378 6.1.2 Evolution of Knowledge on Forcing Agents 353 6.8.3.4 Effect of gas-phase nitric acid 378 6.2 Forcing-Response Relationship 353 6.8.4 Indirect Methods for Estimating the Indirect 6.2.1 Characteristics 353 Aerosol Effect 378 6.2.2 Strengths and Limitations of the Forcing 6.8.4.1 The “missing” climate forcing 378 Concept 355 6.8.4.2 Remote sensing of the indirect effect of aerosols 378 6.3 Well-Mixed Greenhouse Gases 356 6.8.5 Forcing Estimates for This Report 379 6.3.1 Carbon Dioxide 356 6.8.6 Aerosol Indirect Effect on Ice Clouds 379 6.3.2 Methane and Nitrous Oxide 357 6.8.6.1 Contrails and contrail-induced 6.3.3 Halocarbons 357 cloudiness 379 6.3.4 Total Well-Mixed Greenhouse Gas Forcing 6.8.6.2 Impact of anthropogenic aerosols Estimate 358 on cirrus cloud microphysics 379 6.3.5 Simplified Expressions 358 6.9 Stratospheric Aerosols 379 6.4 Stratospheric Ozone 359 6.4.1 Introduction 359 6.10 Land-use Change (Surface Albedo Effect) 380 6.4.2 Forcing Estimates 360 6.11 Solar Forcing of Climate 380 6.5 Radiative Forcing By Tropospheric Ozone 361 6.11.1 Total Solar Irradiance 380 6.5.1 Introduction 361 6.11.1.1 The observational record 380 6.5.2 Estimates of Tropospheric Ozone Radiative 6.11.1.2 Reconstructions of past variations Forcing since Pre-Industrial Times 362 of total solar irradiance 381 6.5.2.1 Ozone radiative forcing: process 6.11.2 Mechanisms for Amplification of Solar studies 362 Forcing 382 6.5.2.2 Model estimates 363 6.11.2.1 Solar ultraviolet variation 382 6.5.3 Future Tropospheric Ozone Forcing 364 6.11.2.2 Cosmic rays and clouds 384 6.6 Indirect Forcings due to Chemistry 365 6.12 Global Warming Potentials 385 6.6.1 Effects of Stratospheric Ozone Changes on 6.12.1 Introduction 385 Radiatively Active Species 365 6.12.2 Direct GWPs 386 6.6.2 Indirect Forcings of Methane, Carbon 6.12.3 Indirect GWPs 387 Monoxide and Non-Methane Hydrocarbons 365 6.12.3.1 Methane 387 6.6.3 Indirect Forcing by NOx Emissions 366 6.12.3.2 Carbon monoxide 387 6.6.4 Stratospheric Water Vapour 366 6.12.3.3 Halocarbons 390 6.12.3.4 NOx and non-methane 6.7 The Direct Radiative Forcing of Tropospheric hydrocarbons 391 Aerosols 367 6.7.1 Summary of IPCC WGI Second Assessment 6.13 Global Mean Radiative Forcings 391 Report and Areas of Development 367 6.13.1 Estimates 391 6.7.2 Sulphate Aerosol 367 6.13.2 Limitations 396 6.7.3 Fossil Fuel Black Carbon Aerosol 369 6.14 The Geographical Distribution of the Radiative 6.7.4 Fossil Fuel Organic Carbon Aerosol 370 Forcings 396 6.7.5 Biomass Burning Aerosol 372 6.14.1 Gaseous Species 397 6.7.6 Mineral Dust Aerosol 372 6.14.2 Aerosol Species 397 6.7.7 Nitrate Aerosol 373 6.14.3 Other Radiative Forcing Mechanisms 399 6.7.8 Discussion of Uncertainties 374 6.15 Time Evolution of Radiative Forcings 400 6.8 The Indirect Radiative Forcing of Tropospheric 6.15.1 Past to Present 400 Aerosols 375 6.15.2 SRES Scenarios 402 6.8.1 Introduction 375 6.15.2.1 Well-mixed greenhouse gases 402 6.8.2 Indirect Radiative Forcing by Sulphate 6.15.2.2 Tropospheric ozone 402 Aerosols 375 6.15.2.3 Aerosol direct effect 402 6.8.2.1 Estimates of the first indirect effect 375 6.15.2.4 Aerosol indirect effect 404 6.8.2.2 Estimates of the second indirect effect and of the combined effect 375 Appendix 6.1 Elements of Radiative Forcing Concept 405 6.8.2.3 Further discussion of uncertainties 377 6.8.3 Indirect Radiative Forcing by Other Species 377 References 406 Radiative Forcing of Climate Change 351 Executive Summary differences, the limited information on pre-industrial O3 distributions, and the limited data that are available to • Radiative forcing continues to be a useful tool to estimate, to a evaluate the model trends for modern (post-1960) first order, the relative climate impacts (viz., relative global conditions. A rank of medium is assigned for the LOSU of mean surface temperature responses) due to radiatively induced this forcing. perturbations. The practical appeal of the radiative forcing concept is due, in the main, to the assumption that there exists • The changes in tropospheric O3 are mainly driven by a general relationship between the global mean forcing and the increased emissions of CH4, carbon monoxide (CO), non- global mean equilibrium surface temperature response (i.e., the methane hydrocarbons (NMHCs) and nitrogen oxides (NOx), global mean climate sensitivity parameter, λ) which is similar but the specific contributions of each are not yet well quanti- for all the different types of forcings. Model investigations of fied. Tropospheric and stratospheric photochemical processes responses to many of the relevant forcings indicate an approx- lead to other indirect radiative forcings through, for instance, imate near invariance of λ (to about 25%). There is some changes in the hydroxyl radical (OH) distribution and evidence from model studies, however, that λ can be substan- increase in stratospheric water vapour concentrations. tially different for certain forcing types. Reiterating the IPCC WGI Second Assessment Report (IPCC, 1996a) (hereafter • Models have been used to estimate the direct radiative forcing for SAR), the global mean forcing estimates are not necessarily five distinct aerosol species of anthropogenic origin. The global, indicators of the detailed aspects of the potential climate annual mean radiative forcing is estimated as −0.4 Wm−2 (–0.2 responses (e.g., regional climate change). to –0.8 Wm−2) for sulphate aerosols; −0.2 Wm−2 (–0.07 to –0.6 Wm−2) for biomass burning aerosols; −0.10 Wm−2 (–0.03 to • The simple formulae used by the IPCC to calculate the –0.30 Wm−2) for fossil fuel organic carbon aerosols; +0.2 Wm−2 radiative forcing due to well-mixed greenhouse gases have (+0.1 to +0.4 Wm−2) for fossil fuel black carbon aerosols; and been improved, leading to a slight change in the forcing in the range −0.6 to +0.4 Wm−2 for mineral dust aerosols. The estimates. Compared to the use of the earlier expressions, the LOSU for sulphate aerosols is low while for biomass burning, improved formulae, for fixed changes in gas concentrations, fossil fuel organic carbon, fossil fuel black carbon, and mineral decrease the carbon dioxide (CO2) and nitrous oxide (N2O) dust aerosols the LOSU is very low. radiative forcing by 15%, increase the CFC-11 and CFC-12 radiative forcing by 10 to 15%, while yielding no change in the • Models have been used to estimate the “first” indirect effect of case of methane (CH4). Using the new expressions, the anthropogenic sulphate and carbonaceous aerosols (namely, a radiative forcing due to the increases in the well-mixed reduction in the cloud droplet size at constant liquid water greenhouse gases from the pre-industrial (1750) to present time content) as applicable in the context of liquid clouds, yielding + −2 −2 (1998) is now estimated to be 2.43 Wm (comprising CO2 global mean radiative forcings ranging from −0.3 to −1.8 Wm . −2 −2 −2 (1.46 Wm ), CH4 (0.48 Wm ), N2O (0.15 Wm ) and halocar- Because of the large uncertainties in aerosol and cloud bons (halogen-containing compounds) (0.34 Wm−2)), with an processes and their parametrizations in general circulation uncertainty1 of 10% and a high level of scientific understanding models (GCMs), the potentially incomplete knowledge of the (LOSU). radiative effect of black carbon in clouds, and the possibility that the forcings for individual aerosol types may not be −2 • The forcing due to the loss of stratospheric ozone (O3) additive, a range of radiative forcing from 0 to −2 Wm is between 1979 and 1997 is estimated to be −0.15 Wm−2 adopted considering all aerosol types, with no best estimate. (range: −0.05 to −0.25 Wm−2). The magnitude is slightly The LOSU for this forcing is very low. larger than in the SAR owing to the longer period now consid- ered. Incomplete knowledge of the O3 losses near the • The “second” indirect effect of aerosols (a decrease in the tropopause continues to be the main source of uncertainty. precipitation efficiency, increase in cloud water content and The LOSU of this forcing is assigned a medium rank.
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