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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,
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