Figure 15.4. Normalized proxy NAO index based on western Greenland ice accumulation rates and normalized instrumental NAO index (thick line). The thin line corresponds to the 15-year running median of the proxy index (adapted from Appenzcller et al., 1998).

Polar ice core data are also key to an assessment of the origin of this variability because they allow us to track the increase in the concentration of the most important greenhouse gases since preindustrial times and give indirect access to changes in other climate forcings. One must realize that without the possibility of reconstructing changes in atmospheric composition from the analysis of air bubbles trapped in ice cores, we

¡Composite isotope records from S93 and GRIP

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