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and possible carbon dioxide changes are believed to cause
significant variations in the Earth's climate.
Processes within the Earth system regulate the solar energy
inputs through numerous feedback mechanisms that influence the
greenhouse warming of the Earth. Some of these feedbacks include
variations in cloudiness and ice cover that determine the planetary
albedo and hence affect the portion of the incoming solar radiation
that is available to the Earth system.
Variations in solar energy related to the activity of the Sun
can also generate natural changes in the Earth system: assessing
the extent of this latter effect is the topic of this report.
There is no doubt that solar variability alters the energy input
to the global Earth system, which is considered here in the
broadest sense to extend from the biosphere, where weather and
climate are experienced, to the Earth's near-space environment,
some 1000 km above. Both the short-wavelength ultraviolet (UV)
radiation and the solar wind and energetic particles from the Sun
undergo large changes related to the presence of active regions in
the solar atmosphere. These changes cause dramatic variability in
the Earth's upper atmosphere, ionosphere, and magnetosphere. Only
recently have spacecraft observations revealed that small
variations (about 0.1 percent) also occur in the total
electromagnetic energy radiated by the Sun. These radiative
variations are also connected to the presence of active regions in
the solar atmosphere (dark sunspots and bright faculae), and they
occur on all time scales observed thus far, from minutes to the
Sun's 11-year activity cycle.
The spectrum of the radiant energy incident on the top of the
Earth's atmosphere and the change in this radiation during the
solar activity cycle are shown in Figure 1.1. Some of the Sun's
radiant energy is reflected back into space by the Earth's surface,
by clouds, and by aerosols; the remaining portion is absorbed by
the Earth's surface and within the Earth's atmosphere. Figure 1.2
illustrates the altitude of unit optical depth. This is the mean
altitude at which solar spectral energy is reduced by the Earth's
atmosphere to roughly 1/e of its value at the top of the
atmosphere. This curve is determined by the concentrations of
radiatively absorbing gases in the Earth's atmosphere. Figures 1.1
and 1.2 indicate that the more variable, shorter wavelength solar
energy is absorbed at higher altitudes in the atmosphere. Radiation
at wavelengths shorter than