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FIGURE 14.26 Estimated direct tropospheric heating ( + ) effects of chlorine- and bromine-containing gases and indirect cooling ( —) effects due to their destruction of stratospheric ozone, assuming a bromine enhancement factor of a = 40 (from Daniel et al, 1995).

illustrates their calculated temporal changes due to these two effects. As stratospheric ozone depletion becomes less severe in the future, the indirect, negative radiative forcing that has been in part counterbalancing the positive forcing will decrease; as a result, the net change in radiative forcing in the coming decades due to halocarbons is expected to be more steep than if their only effect was through direct positive radiative forcing, leading to heating (Solomon and Daniel, 1996; Hansen et al., 1997b).

As discussed in Chapter 13, the short- and long-term replacements for CFCs and halons in use or contemplated for use at the present time are typically compounds containing hydrogen (to shorten their tropospheric lifetimes, hence decreasing the amounts that reach the stratosphere) and/or fluorine (which does not participate in stratospheric ozone destruction to a significant extent). One concern with respect to these substitutes is their potential effect on climate since they are also greenhouse gases. Table f4.7 summarizes direct radiative forcings calculated for some of these compounds, as well as relative GWPs using either CFC-11 or C02 as the reference compound. Clearly, their contributions to radiative forcing may be significant and hence are taken into account in assessing the overall impacts of CFC replacements.

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