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time scales are somewhat smaller (Lean et al., 1992a and Figure 2.3) than the 0.4 to 1.5 percent needed to explain the paleoclimate record. If the climate sensitivity is greater (one inference from Milankovitch GCM studies; Rind et al., 1989; Phillipps and Held, 1994; discussed below) or the global temperature change smaller than indicated, the required solar variability would be reduced. Furthermore, although GCM climate simulations estimate a mean global temperature reduction of 0.46° C for a solar irradiance reduction of 0.25 percent (Rind and Overpeck, 1993), some regions of the Earth's surface may cool and others warm by as much as 1° C as a result of advective changes caused by differential heating of the land and oceans.

The problem of assessing direct solar radiative forcing of climate change is additionally complicated because the extent to which total solar irradiance variability arises from radiative changes at ultraviolet rather than at visible wavelengths (Lean, 1989) determines the altitude of its direct impact on the global system. If this impact shifts to altitudes mostly above the troposphere, total solar irradiance forcing of surface temperature would be reduced. On the other hand, the amplitude of irradiance variations in the visible and infrared portions of the solar spectrum that directly heat the surface, though thought to be small (e.g., Figure 1.1), is not currently known.

While solar radiative changes are probably not the sole driving force of the historical climate record, they nevertheless will need to be understood and quantified in order to unravel the contribution of solar forcing. Indeed, circumstantial evidence points to a solar forcing contribution to the temperature changes observed over the past century (Kelly and Wigley, 1992; Schlesinger and Ramankutty, 1992) that decreases the predicted temperature change associated with a doubling of atmospheric CO2 by nearly half (Lacis and Carlson, 1992).

From the perspective of the U.S. Global Change Research Program, it is important to know how solar irradiance variations can be expected to vary in the future and, in particular, the likelihood that events such as another Little Ice Age, will occur in the coming century. Were the only variations in solar radiative output an 11-year cycle with peak-to-peak amplitude of about 0.1 percent, solar forcing could be expected to modulate the net anthropogenic climate forcing as shown in Figure 2.2. But another scenario is that additional solar forcing might arise from