FIGURE 14.30 Measured (—) and model-predicted (•••) change in monthly mean temperatures (a) at the earth's surface and (b) in the stratosphere (observations at 30 mbar and 10°S, model results for the 10- to 70-mbar layer from 8°S to 16°S) (adapted from Lacis and Mishchenko, 1995).

1997). Additional discussion of the effects of volcanic eruptions on surface temperatures is found in Section D.3.

Finally, while sulfate is a major component of aerosol particles, particularly in the Northern Hemisphere where fossil fuel burning is extensive, it is certainly not the sole particle component. As discussed in detail in Chapter 9, nitrate, organics and elemental carbon, inorganic soil elements, and sea salt components are all found in tropospheric particles to varying extents, depending on the region. Some regional sources such as biomass burning may also have quite widespread and global impacts (e.g., Kaufman et al., 1991; Penner et al., 1992). These other sources will also scatter light and contribute to direct aerosol forcing (e.g., see Andreae et al., 1995). While less attention has been paid to assessing the contribution of these other components (Penner et al., 1994), there are increasing indications that these may prove to be quite important in direct forcing by aerosol particles (e.g., see Andreae et al., 1995; Penner et al., 1998).

For example, Fig. 14.31 shows one set of calculations of direct radiative forcing by sulfate particles as well as those from biomass burning and from fossil fuel combustion in the Northern Hemisphere, Southern Hemisphere, and the global average, respectively (Penner et al., 1998). In this case, the fossil fuel particles were assumed to contain both black carbon, which absorbs radiation and hence has a positive radiative forcing (see following section), and organic carbon, which scatters light (negative radiative forcing). The biomass particles were assumed to take up water as if they contained 30% (NH4)2S04 by mass and to scatter light. Figure 14.31 illustrates that the direct radiative forcing o o

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