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the mix of active region emission that generates the solar Ca II irradiance was somewhat different than is currently seen in the Sun, the radiative output of the Sun can be estimated for the Maunder Minimum (Figure 2.3).

Future

Current ability to predict solar activity is at best primitive. Statistical methods predict sunspot numbers and the 10.7 cm radio flux 12 months in the future with moderate success. There are also precursor methods that predict the strength of the next solar cycle from the behavior of polar structure on the Sun and geomagnetic activity in the declining phase of the current cycle (e.g., Schatten and Pesnell, 1993; Thompson, 1993). But there is limited physical understanding of why these precursor methods should be appropriate except that the magnetic fields and corona near the solar poles change near solar maximum and hence may herald the onset of the new cycle before the next generation of sunspots appears.

On century time scales, the periodicities of 11 and 88 years identified in the sunspot record, together with the 208 year periodicity found in the 14C record, provide limited guidance to future solar behavior, such as the occurrence of the next Maunder Minimum. The time span of solar measurement is simply too short for reliable prediction of solar extrema occurring sporadically every 200 years or so. Nevertheless, it has been speculated that the concatenations of the 208 and 88 year periods may have contributed to generally increasing solar activity levels during the twentieth century, with maximum activity predicted to occur during the first half of the twenty-first century (Damon and Sonnet, 1991).

Predictive capability will be substantially improved when a complete understanding is obtained of the mechanisms within the solar atmosphere that produce the emitted radiation, and form sunspots and plages. Predicting the Sun-as-a-star energy quantities needed for global change studies will ultimately require development of a theory for the solar dynamo that can accommodate known solar behavior. Yet, the very nature of solar variability, whether driven by an internal chronometer (Dicke, 1978) or by stochastic or chaotic processes (Mundt et al., 1991; Morfill et al., 1991; Kremliovsky, 1994), remains elusive. Solar activity levels may well defy reliable prediction in the near future.