While variability of even longer duration than discussed above (i.e., on time scales of millions of years) is largely controlled by long-term changes in solar luminosity and shifts in the land-ocean distribution on the surface of the Earth as a result of plate tectonics, the 100,000-10,000-year variations in the Earth's climate described above appear to be driven by periodic changes in solar insolation (the amount of incoming solar radiation over a unit area of the Earth's surface) at high latitudes (the orbital variations have a relatively minor effect at lower latitudes). These insolation changes correspond to (Figure 2) the precession of the rotation axis of the Earth around a reference axis (similar to the phenomenon seen with a spinning top), which operates on a period of 23,000 years, changes in the tilt or obliquity of the rotation axis relative to the orbital plane that varies between 22.5° and 35° with a 41,000-year periodicity, and the variation of the eccentricity of the Earth's orbit (from nearly circular to slightly elliptical), which has a 100,000-year periodicity. The magnitude of the insolation change is significant for the 23,000- and 41,000-year cycles, whereas the absolute change in incoming solar radiation over the 100,000-year cycle is of only minor magnitude. Although insolation variations affect both northern and southern high latitudes, insolation changes at northern latitudes are most significant because of the greater continental area, which allows a greater accumulation of ice and snow. This may contribute to feedback mechanisms such as that due to albedo (described below).
In the 1920s, the Serbian mathematician Milutin Milankovitch calculated, based on theories postulated
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41 D00 years
41 D00 years
by James Croll in the late 19th century, that the periodicities in the Earth's orbital parameters and the predicted insolation changes at northern latitudes could explain cycles of glacial and interglacial periods observed in the climate record. This mechanism is known as Milankovitch forcing. Subsequent statistical analyses of paleoclimate data have largely supported the Milankovitch theory by showing that ice sheets of the Northern Hemisphere have developed and retreated with the Milankovitch cycles (23,000, 41,000, and 100,000 years). In particular, the 100,000-year cycle corresponds to the shift between glacial and interglacial periods (ice ages and warm periods, respectively) for at least the last one million years. It has been shown that the climate system at other latitudes shows similar periodicities of similar magnitude, although with significant time lags of 5000-15,000 years relative to the insolation forcing
at high northern latitudes. What may be the cause for this time lag? Various investigations have shown that the large northern ice sheets that grow with time constants of similar magnitude may have been responsible (Imbrie et al., 1992, 1993). They transmit and amplify changes that correspond to insolation changes at high latitudes due to orbital periodicities to other parts of the world through the global climate system. Thus, interactions between Northern Hemisphere ice sheets and other elements of the climate system may cause the transition from regional insolation changes to global climate variability at time scales of 10,000-100,000 years, with millennial-scale variations causing secondary variations in climate parameters (Bond and Lotti, 1995; Imbrie et al., 1992, 1993).
However, despite these intriguing arguments, there are a number of more specific questions that need to be addressed. What are the underlying mechanisms that nearly synchronize the climate of the Southern and the Northern Hemisphere despite asynchronous insolation forcing? What is the reason for the dominant 100,000 cycle of the last 1.2 million years, despite the fact that the insolation varies significantly at 23,000- and 41,000-year periods and trivially at the 100,000-year period?
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