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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