"There is another important cycle that has the potential to affect the Earth's climate; it is a 41,000-year variation in obliquity, the tilt of the Earth's axis with respect to a direction perpendicular to its orbital plane. This variation is different from precession - the two motions are at right angles to each other - and astronomically is a much smaller effect. The obliquity varies by only a few degrees back and forth, and the current value of 23.4° is near the middle of the range. However, climatologically, the obliquity variation has the potential to have a fairly direct effect on seasonal extremes. After all, it is the obliquity that causes our seasons in the first place - if the Earth's axis were perpendicular to its orbital plane, there would be no seasons at all. The astronomical cycles described above are called Malankovitch cycles after Milutin Malankovitch, a Serbian scientist who provided a detailed theory of their potential influence over climate in the 1920s. Milankovitch's work was an attempt at explaining the ice ages, and it built upon previous astronomical theories of climate variation postulated by Joseph Adhemar and James Croll in the 19th century. Although the Malankovitch theory is well-grounded astronomically, it remains controversial. The theory predicts different effects at different latitudes, and thus its use as a predictor of global (or at least hemispheric) climate change is not unambiguous. The exact mechanisms by which the relatively modest variations in the Earth's orbit and axis direction might result in such large effects as the ice ages are not well established. The theory's popularity has tended to vary depending on the type of long-term climatological data that has been available and the method used to establish a time scale for the data. The 21,000-year perihelion cycle and the 41,000-year obliquity cycle do in fact appear to be present in the climatological record. But the dominant climate cycle that is seen has a period of about 100,000 years. Although this coincides with the period of change in the eccentricity of the Earth's orbit, the theory outlined above does not predict that we should see this period directly - the effect of eccentricity should appear only as a modulation of the 21,000-year perihelion cycle. The mechanism by which the Earth's orbital eccentricity could affect the climate in such a direct and important way is not known, and some researchers believe that the 100,000-year climate cycle is not due to orbital variations at all. Thus, many questions remain about long-term climate variations and their relationship, if any, to astronomical causes."

There is a web page about the Milankovitch Cycle at NASA Goddard. They cite recent research into the correlation between these cycles and certain deep-sea sediment deposits:

The magnitude of the "Milankovich effect" depends on the difference between largest and smallest distances from the Sun. That, in its turn, depends on the eccentricity of the Earth's orbit, which varies with a 100,000-year cycle, on which a 413,000-year cycle is superposed. J. Rial (Univ. of North Carolina) found signatures of those cycles in the oxygen isotope content of deep-sea sediments, in full agreement with the Milankovich theory. His work is in "Science," vol. 285, p. 564, 23 July 1999; a non-technical explanation "Why the Ice Ages Don't Keep Time " is on pages 503-504 of the same issue."

Another Milankovitch resource at Scott Rutherford's page also gives good info on the predicted impacts. The figure above shows the changes in the solar 'insolation' at the Earth from Milankovitch variations.

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