Cfc12

FIGURE 14.24 Calculated changes in adjusted radiative forcings from 1979 to 1994 due to changes in 03 (both stratospheric and tropospheric based on SAGE data) and other greenhouse gases (adapted from Hansen et al., 1997b).

counterbalances about half of the total positive forcing due to the other greenhouse gases. It should be noted that this estimated change in forcing due to the change in 03 is quite sensitive both to the change in the vertical profile and to the change in the temperature profile, which, of course, are related (e.g., see Zhong et al., 1996; and Shine et al., 1998). For example, using the changes in 03 derived using a different data set (SAGE/TOMS data) gives a predicted net change in radiative forcing due to 03 of -0.2 W m-2 compared to —0.28 W m~2 shown in Fig. 14.24. In addition, as seen later when all known anthropogenic perturbations are considered together, other estimates show the positive forcing by tropospheric ozone being larger than the negative forcing due to stratospheric ozone.

The chlorinated and brominated compounds, therefore, are expected to exert not only a direct positive radiative forcing (leading to warming) by their interaction with terrestrial infrared radiation in the usual manner of a greenhouse gas but also an indirect negative radiative forcing (leading to cooling) due to changes in stratospheric ozone which changes the radiative flux at the tropopause. The net effect is thus expected to be less than that predicted from the direct radiative forcings in Table 14.5, which only take into account the direct greenhouse gas effect.

Daniel et al. (1995), for example, have estimated the global warming potentials for a variety of chlorine- and bromine-containing gases involved in stratospheric ozone destruction, taking into account the indirect negative forcing as well as the direct positive forcing. Table 14.6 summarizes both the direct GWPs and the net values where the indirect, negative forcing due to ozone destruction has been taken into account. There are a number of simplifying assumptions that have been made in such calculations, but they are illustrative of the importance of considering the indirect as well as

TABLE 14.6 Direct and Net Global Warming Potentials for Some Chlorine- and Bromine-Containing Gases and Halons Relative to COz

Time horizon

TABLE 14.6 Direct and Net Global Warming Potentials for Some Chlorine- and Bromine-Containing Gases and Halons Relative to COz

Time horizon

Gas

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