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Working Group I: The Scientific Basis |
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6.15.2.2 Tropospheric ozoneThe radiative forcings associated with future tropospheric O3 increases are calculated on the basis of the O3 changes calculated by Chapter 4 and presented in Appendix II, Table II.2.5 for the various SRES scenarios. The mean forcing per DU estimated from the various models and given in Table 6.3 (i.e., 0.042 Wm-2/DU) is used to derive these future forcings. Most scenarios lead to increases in the abundances of tropospheric O3 and consequently to positive radiative forcings in 2050 and 2100. Scenarios A1fi, A2, and A2p provide the maximum tropospheric O3 forcings reaching 0.89 Wm-2 in 2100. Only scenario B1 predicts a decrease in tropospheric O3 and a negative forcing of -0.16 Wm-2 in 2100. 6.15.2.3 Aerosol direct effectThe direct radiative forcing due to anthropogenic sulphate, black carbon (BC), and organic carbon (OC) aerosols are assessed using the scenarios described in Chapter 5, Section 5.5.3 and the column burdens presented in Appendix II, Tables II.2.7, II.2.8 and II.2.9 respectively. Scenarios for mineral dust are not considered here, as there is no �best estimate� available (Sections 6.7.6 and 6.13). For each aerosol species, the ratio of the column burdens for the particular scenario to that of the year 2000 is multiplied by the �best estimate� of the present day radiative forcing. The year 2000 �best estimate� radiative forcing is then subtracted to give the radiative forcing for the period from the year 2000 to the date of the scenario. Estimates of the direct radiative forcing for each of the aerosol species are given in Table 6.15. The uncertainty associated with these estimates of the direct radiative forcing is necessarily higher than the estimates for the year 2000. This is because estimates for the year 2000 are constrained as best as possible to observations. Additionally, the simplified approach applied here does not account for changes in the spatial pattern of the global distribution of the aerosol species that may arise from changes in the geographic distribution of emissions. The uncertainty is therefore estimated as a factor of three for the radiative forcing by sulphate aerosols, and a factor of four for the radiative forcing by BC and OC aerosols. For sulphate aerosols, only scenarios A1fi, A2, and A2p show a negative direct radiative forcing for the period 2000 to 2050, with all scenarios showing positive radiative forcings over the period 2000 to 2100. This contrasts with the IS92a scenario which predicted an increasingly negative radiative forcing due to the higher column burden of sulphate. For BC aerosols the majority of the scenarios show an increasingly positive direct radiative forcing in 2050 and 2100 (except for B1 and B1p). For OC aerosols, the opposite is true, with the majority of scenarios showing an increasingly negative direct radiative forcing in 2050 and 2100 (except for B1 and B1p). The direct aerosol radiative forcing evolution due to sulphate, black and organic carbon aerosols taken together varies in sign for the different scenarios. The magnitudes are much smaller than that for CO2. As with the trace gases, there is considerable variation amongst the different scenarios.
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