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1880 1920 1960 2000 Year

FIGURE 14.54 Annual average number of sunspots from 1880 to 2000, showing the 11-year cycle (adapted from Cliver et al., 1998).

changes due to the solar cycle depends on wavelength, latitude, and altitude (Haigh, 1994). This may have indirect effects on the troposphere by altering stratospheric chemistry (see discussion by Robock (1996) and references therein).

Figure f4.54, for example, shows the annual average number of sunspots from 1880 to the present, which clearly shows this cycle (Cliver et al., 1998). Both the sunspot number and the aa geomagnetic index have been used as proxies for the solar cycle. For the relatively short time period covered by available instrumental temperature records, both the sunspot number and the aa geomagnetic index are correlated to surface temperature (e.g., see Cliver et al., 1998; and Wilson, 1998).

However, there is increasing evidence that longer term solar variations are measurable over the past few centuries as well, and understanding these is very important for discerning anthropogenic effects on global climate (e.g., Lean et al., 1995a; Willson, 1997). Figure 14.55, for example, shows a reconstruction of total solar irradiance from 1610 to the present, in which the ff-year cycle and a component having much longer term variability are both included (Lean et al., 1995a). This two-component model is consistent with the Maunder Minimum that occurred during the years from 1645 to 1715 (Eddy, 1976). During this period, the 11-year cycle did not occur for a number of decades, which was also the coldest period in the "Little Ice a 1369 I _ 1368

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