Page 109

dynamo because they provide a way to probe the solar interior, but they will illuminate the dynamo problem only if measurements extend over solar cycle time scales.

Past

In the context of global change, understanding the past behavior of the Sun may well be essential for unraveling the paleoclimate record. Coincidences between climate change in the twelfth and seventeenth centuries and changes in 14C as a proxy for solar activity (corroborated by the 10Be record) suggest that there may be threshold levels of high and low solar activity at which the Sun begins to play a significant role in changing global climate (see Chapter 2). Knowledge of physical conditions on the Sun at these extrema is primitive because modern scientific observations have all been made during an era of high solar activity. Galileo's and others' discovery of sunspots at the beginning of the seventeenth century came at a time to document an ensuing period of low solar activity. Without this documentation, the possible role of the Sun in the Little Ice Age might still not be appreciated. Given the meager and often discontinuous evidence for solar variability in the past, inference of the physical state of the Sun at times of extrema is speculative but necessary. The first steps can be taken with empirical models now available (e.g., Foukal and Lean, 1990; White et al., 1992; Lean et al., 1992a; Hoyt and Schatten, 1993; Nesme-Ribes et al., 1993), but the credibility of such research would be strengthened immeasurably through development of a successful physical model of the solar cycle.

An expanded view of solar variability is provided by knowledge of cyclic behavior in other stars, afforded by long term measurements of stellar Ca II emissions in Sun-like stars. Baliunas and Jastrow (1990) present stellar cycle data that may indicate the presence of Maunder Minimum-type episodes in one-third of the observed stars, but their sample (13 stars) is so small that this conclusion must be regarded as speculative at this time. It is, however, consistent with an independent result from analysis of the radiocarbon data by Damon (1977). Furthermore, the distribution of Ca II emission exhibited by the Sun-like stars does appear to be consistent with the range of Ca II K emission seen in the present-day Sun (White et al., 1992). By assuming that during the Maunder Minimum,



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement