surfaces and the oceans. The atmospheric composition and the distribution of oceans and land masses combine with the solar energy input to determine the radiative balance, and hence the climate, of the Earth's biosphere. A change, DS W/m2, in solar radiation received by the Earth causes climate forcing of 0.7DS/4 W/m2 which perturbs directly the equilibrium global temperature by an amount DT=l-1(0.7DS/4)°C where the climate sensitivity, l-1, is currently estimated to be in the range 0.3 to 1.0°C/(W/m2) (e.g., Wigley and Raper, 1990). While the primary radiative perturbation of ± 3 percent is indisputably the changing insolation generated by the annual cycle of the Earth's elliptical orbit around the Sun, intrinsic changes in the Sun's radiative output also occur on decadal and possibly longer time scales.
The Sun's total radiative input to Earth decreased by about 0.1 percent during 1980–1986 then increased by about the same amount during 1986–1990 (Willson and Hudson, 1991). A 0.1 percent solar irradiance change produces a climate forcing of 0.24 W/m2. For comparison, the climate forcing by increasing greenhouse gases from 1980 to 1986 was about 0.25 W/m 2 (Hansen et al., 1990). Concomitant increases in atmospheric aerosols may have reduced the net anthropogenic climate forcing to almost half that arising from greenhouse gases alone (Hansen et al., 1993). Thus, during the recent descending phase of the 11-year solar activity, solar forcing canceled much of the net anthropogenic forcing.
The climate system's response to various forcings depends on the history, altitude, and latitude of the forcing and the climate sensitivity, l-1. While the change in equilibrium global surface temperature associated with a steady climate forcing of 0.24 W/m2 is estimated to be in the range 0.1° to 0.2°C, the transient response to a periodic 11-year forcing of the same magnitude is assumed to be much less than the equilibrium response because the response time of the climate system is of the order of decades or more (Hansen and Lacis, 1990). However, the true extent to which the climate system's response diminishes or amplifies solar forcing compared with anthropogenic forcings is uncertain.
As records of paleoclimate and historical solar activity have improved, the possibility that variations in solar radiative forcing played a role in past climate change continues to be raised (see, for example, The Royal Society, 1990). There is now clear corroborating evidence from 14C in tree rings and 10Be in ice cores that solar activity during past millennia