Earth's climate undergoes variations on a wide range of time scales, from seasonal and interannual to glacial-interglacial and beyond. These variations are revealed in a variety of proxy records collected from ice-cores, ocean sediments and other sources (Bradley, 1999; Duplessy, 1999). The glacial-interglacial cycles are among the most striking phenomena in climate dynamics. While we are now at an interglacial, the last ice age was at its peak only about 20,000 years ago. Ice sheets covered much of North America and Europe, global average temperature was about 6° colder than today, and sea level was about 120 meters lower below today's. Various paleoclimate proxies (Petit et al., 1999; Imbrie et al., 1984) indicate that during the past 800,000 years or so (the late Pleistocene), the glacial-interglacial cycles were characterized by a pronounced 100,000 year (100 kyr) time scale, with additional weaker spectral peaks at 41 and 23 kyr (Figure 1) and an asymmetric saw-tooth structure (the slow build-up of the land glaciers and the relatively abrupt melting).
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