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FIGURE 2.1 Contemporary solar activity variations as indicated by the sunspot number (top panel) and changes in total solar radiative output (bottom panel) recorded by the ERB radiometer on the Nimbus 7 satellite, ACRIM I on the SMM satellite and ACRIM II on the UARS, and by the ERBE program (NOAA9 and ERBS). Total solar irradiance is increased during times of maximum solar activity (e.g., 1980 and 1990) and decreased during the intervening minimum. The differences in irradiance levels between the different measurements are of instrumental origin and reflect absolute inaccuracies in the measurements. Proposed future programs to measure total solar irradiance are indicated. Courtesy of J. Lean.

is compared in Figure 2.2 with anthropogenic radiative forcing of climate by increased greenhouse gases and aerosols and by ozone decreases. During the first half of the 1980s, forcing of the climate system by declining solar radiative output was more than sufficient to offset the estimated net anthropogenic forcing.

Despite the similarity of the climate forcings over the decadal time scales shown in Figure 2.2, the magnitude of the climate system's response to solar forcing could be greater or less than its response to anthropogenic forcing. This is because the translation of radiative forcing to surface

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