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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