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longer-term irradiance variations superimposed on the 11-year activity cycle, such as the speculated long term increase in irradiance from the Maunder Minimum to the present Modern Maximum.

Lacking a detailed modeling capability for, and adequate knowledge of, solar processes on which to base predictions, researchers have utilized spectral analysis to develop predictive tools. Phenomena such as sunspot numbers have periodicities on the order of 100, 55, and 11 years, along with the solar magnetic cycle of 22 years (e.g., Berger et al., 1990). Ice core records as well as other climatic data suggest periods of about 80 and 180 years (Johnsen et al., 1970), possibly related to solar activity (Otaola and Zenteno, 1983). Extrapolation into the future of two cycles evident in the 14 C record, at 208 years (the Suess cycle) and 88 years (the Gleissberg cycle), suggests that the increasing solar activity that has followed the Maunder Minimum may continue into the early twenty-first century (Damon and Sonett, 1991), with a decline commencing around 2040. But extrapolation of these cycles into the future and prediction of solar effects is a highly questionable procedure, given our lack of knowledge of the fundamental processes involved (see Chapter 6).

Wigley and Kelly (1990) have attempted to assess limits on the role that solar forcing of climate change may play, relative to that of greenhouse gases, during the next 200 years. Analogous to their approach, and consistent with their results, the predictions shown in Figure 2.5 indicate that were the Sun to experience a period of inactivity such as the Maunder Minimum, commencing in the year 2000, and accompanied by reduction in its radiative output of 0.25 percent, the resultant climate forcing would indeed modulate, but not counter, the predicted anthropogenic climate forcing. As noted previously, determining the actual climate impact of the forcings shown in Figure 2.5 (and Figure 2.2) is difficult because of the specific nature expected for the climate system's response to each of the individual forcings.

Solar Activity Cycles and the Weather

There have been many studies of the possible relationships between weather phenomena and the 11-year solar sunspot cycle or the 22-year solar magnetic cycle. Summaries of the results of these studies prior to the early 1980s have been published by Herman and Goldberg (1978) and NAS