Milankovitch Cycles
The Serbian astronomer Milutin Milankovitch (1879–1958) developed a theory that explained climatic variations in astrophysical terms. He was particularly concerned with the origin of an ice age during the Pleistocene. Through observations of the stars, Milankovitch found that the basic elements that govern the earth's orbit around the Sun are not constant. First, he noticed that the eccentricity of the elliptical path of the earth's revolution around the Sun changes with cycles of roughly 100,000 and 400,000 years. Second, he found that the obliquity, that is the angle of the earth's spin axis with the plane of its eccentric orbit changes with a frequency of roughly 41,000 years between 22 and 25 degrees. Third, he took into consideration that the earth's axis of revolution behaves like the spin axis of a top that is winding down. The spin axis traces a circle on the celestial sphere over a period of approximately 22,000 years. This motion, which is called the precession of the equinoxes, was probably detected by Hipparch of Nikaia (about 150 B.C.). It is the reason why a person then Aries-born, is born under the sign of Pisces today.
In 1920, Milankovitch calculated the effect of each of these cycles on the total summer insulation at a latitude of 65 degrees North. He reasoned that at this latitude, small insulation changes might have a big effect because a decrease of the summer insulation allowed the snow and ice of winter to persist through the summer months, and into the following winter. In this way, big ice sheets can develop and accelerate in a positive feed-back through enhancing the northern latitude albedo.
It required a few decades for the theory to have its break-through. Today, however, it is generally accepted that Milankovitch's theory describes the main causes for the waxing and waning of the Pleistocene ice sheets correctly. Proof for this theory is derived from cyclic variations of the chemical and paleontological composition of marine, lacustrine, and terrestrial sediments. Investigations into the temporal variation of the ratio of oxygen isotopes in particular have shown that Milankovitch cycles do indeed influence the climate greatly on time scales covering thousands to hundreds of thousands of years, although continental distribution, ocean patterns, the total solar irradiance, and other factors play an additional role. The Sun, for instance, is obviously a prominent factor for climatic variations on a time scale between years to a few thousand years, which is summarized in the socalled Athenian hypothesis.
Ice Ages
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