It is generally accepted that variations in the precession and eccentricity of Earth's orbit, and Earth's axial tilt, influence long-term climate fluctuations; the combined variations are referred to as Milankovitch cycles. Precession varies with a mean period of about 23,000 years, axial tilt varies between about 22° and 25° on a 41,000-year cycle, and the small orbital eccentricity variations have cycles of about 100,000 and 400,000 years.
Despite the known regularity of the cycles, many aspects of their manifestation in the geological record are puzzling. For example, why did the Earth slide into a period of ice ages about 3 million years ago? Why, about a million years ago, did the pace of ice ages change from a regular 41,000-year period (the tilt period) to an irregular 100,000-year period whose association with Milankovitch cycles is still unclear (Figure 1)? Was a gradual decrease in atmospheric CO2 the culprit, or was it some other slow change in the climate system? Why did global ice volume have only weak 23,000-year cycles related to Earth's orbital precession during the time period when the 41,000-year pattern dominated, even though those 23,000-year cycles appear prominently during the past million years? Did the north and south polar ice caps oscillate in opposite cycles during the precession period, canceling the precession response [Raymo et al., 2006]? Answering these questions is important because they underscore our incomplete understanding of the climate system.
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