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For the purposes of the U.S. Global Change Research Program, discussed below are three times scales associated with different mechanisms for solar variability: the relatively recent climate (a hundred to a few thousand years), the weather (tens of years), and orbital variations (many thousands of years).

Solar Irradiance Changes and the Relatively Recent Climate

There has been much speculation that climate changes over the past few thousand years have been the product of variations in the Sun's radiative output. Eddy (1976) pointed out the coincidence in time between the Maunder Minimum of solar activity and the lowest temperatures of the Little Ice Age in Europe and North America (see Figure 1.3). He also presented qualitative evidence that other century-scale variations in climate over the past millennium coincided roughly with variations in solar activity deduced from anomalies in the 14C cosmogenic isotope record. Whereas the long term trend in records of cosmogenic isotopes such as 14C and 10Be reflects, primarily, changes in the Earth's magnetic field that affect the interaction of cosmic rays with the Earth's atmosphere, the wiggles superimposed on the smooth long term trend are believed to occur because of the modulation of the local cosmic ray intensity by magnetic fields embedded in the solar wind, which varies in response to solar activity (Damon and Sonett, 1991; Beer et al., 1991; Stuiver and Braziunas, 1993). Thus, enhanced solar activity corresponds to 14C minima, and the mechanism proposed by Eddy for the apparent relationship between climate and the 14C wiggles involved changes in the total solar irradiance linked to the long term envelope of the 11-year sunspot cycle and reflected in the 14C record.

The extent to which cosmogenic isotope variations really indicate terrestrially relevant variations in solar energy outputs, either radiative or particle, and the scaling of the relationship over long times, is poorly known; the paleoclimate record is similarly somewhat uncertain (Bradley and Jones, 1993). Although results are mixed (Wigley and Kelly, 1990; Crowley and Howard, 1990; Damon and Jirikowic, 1994), there is some suggestion of a relationship; during the past 10,000 years, six of the seven strongest maxima in the 14C wiggles correspond closely to climate minima,