FIGURE 10-2 Records of sunspot number and cosmogenic isotope fluctuations in tree rings and ice cores associated with solar activity over the past 2,000 years. SOURCE: Updated from Fröhlich and Lean (2004). Reprinted with permission; copyright 2004.

galactic cosmic rays that are pushed away from Earth by the solar wind during times of strong solar activity. The relation between irradiance and cosmogenic isotopes, though, is complex and is not necessarily linear (Wang et al. 2005b).

Proxy records of solar irradiance have been used, often in combination with recent satellite measurements and solar modeling experiments, to obtain reconstructions of total solar irradiance over the last two millennia. Early reconstructions adopted a long-term trend in solar irradiance based on cosmogenic isotope records and comparisons to Sun-like stars, yielding a solar irradiance estimate for the Maunder Minimum (1645–1715) of about 0.2–0.4 percent below contemporary solar minima (Hoyt and Schatten 1993, Lean et al. 1995). The mechanisms proposed to explain this trend have been questioned recently, and newer reconstructions assume that solar output during the Maunder Minimum was closer to that of the present-day solar minima (Foukal et al. 2004). The most recent reconstructions (Fröhlich and Lean 2004, Wang et al. 2005a) indicate that solar irradiance has varied by about 0.1 percent over the last 2,000 years, which is equivalent to variations in radiative forcing of 0.1–0.2 W·m^–2 between periods of low and high solar activity (Figure 10-2). Extended intervals of low