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assumptions are contained in these estimates, and to understand those assumptions and the role that clouds could play, cloud sensitivity calculations have been made to illustrate the range of surface temperature for various assumptions of cloud properties.

In these calculations, the Mitigation Panel used the assumed abundances and optical properties shown in Table Q.1 and a global surface albedo of 15.4 percent. The model has three layers of clouds under global average conditions. It is assumed that clouds, once formed, will have the same effects over their entire lifetimes and that they will have optical properties identical to those of current low-level clouds, which are assumed to be unchanging during the seeding process. Unfortunately, these assumptions contain many uncertainties. These sensitivity calculations show that the effects of clouds depend not only on the fraction of a given cloud type, but also on the surface albedo beneath the clouds. The special role of the low-level cloud and its varying effect as the surface albedo changes add considerable complication because the surface albedo varies from about 4 to 20 percent over some water to as high as 90 percent over pure snow or ice (Hummel and Reck, 1979). This means that once a cloud is formed it may start with a cooling effect and end up in an area where it could produce either greater or lesser cooling, with the slight possibility of even a heating effect.

Albrecht (1989) (see also Twomey and Wojciechowski, 1969) suggests that the average low-cloud reflectivity would increase if the abundance of cloud condensation nuclei (CCN) were to increase through emission of SO2.

TABLE Q.1 Assumed Properties of Average Global Clouds