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Figure 15.6. Variation of greenhouse gas concentrations since the 18th century (compiled by J. Chappellaz). These results arc deduced from the analysis of air bubbles trapped in the ice (Blunicr ct al., 1993; Etheridge ct al., 1996) and, for recent years, from direct measurements.

would have no clear information about the impact of anthropogenic activities on the atmospheric concentrations of CO2, CH4, and N2O, inasmuch as direct measurements of these three important greenhouse gases are available only over the past few decades. Ice core data, obtained from high-accumulation sites, provide a unique and reliable record of these anthropogenically induced changes (Figure 15.6).

Ice cores also give indirect access to solar irradiance. This approach is based on the existence of a positive relationship between the 11-year cycle of solar magnetic activity and the output of the Sun and on the fact that magnetic fields of the solar wind deflect the primary flux of charged cosmic particles, something that leads to a reduction of cosmogenic nuclide (such as carbon 14 and beryllium 10). Using a detailed 10Be record measured in an ice core drilled at South Pole Station, Bard et al. (in press) have extended the record of solar irradiance over the past 1200 years. Like others (Lean et al., 1995), these authors suggest that changes in solar forcing may have made a significant contribution to climate variability over the last millcnium. Also, ice cores record the occurrence and magnitude of volcanic eruptions (Hammer ct al., 1980, 1997; Lcgrand

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