Quaternary Glaciation - Causes - Astronomical Cycles

Astronomical Cycles

Main articles: Milankovitch cycles and orbital forcing See also: 100,000-year problem

The role of Earth's orbital changes in controlling climate was first advanced by James Croll in the late 19th century. Later, Milutin Milanković, a Serbian geophysicist, elaborated on the theory and calculated these irregularities in Earth's orbit could cause the climatic cycles known as Milankovitch cycles. They are the result of the additive behavior of several types of cyclical changes in Earth's orbital properties.

Changes in the orbital eccentricity of Earth occur on a cycle of about 100,000 years. The inclination, or tilt, of Earth's axis varies periodically between 22° and 24.5°. (The tilt of Earth's axis is responsible for the seasons; the greater the tilt, the greater the contrast between summer and winter temperatures.) Changes in the tilt occur in a cycle 41,000 years long. Precession of the equinoxes, or wobbles on Earth's spin axis, complete every 21,700 years. According to the Milankovitch theory, these factors cause a periodic cooling of Earth, with the coldest part in the cycle occurring about every 40,000 years. The main effect of the Milankovitch cycles is to change the contrast between the seasons, not the amount of solar heat Earth receives. These cycles within cycles predict that during maximum glacial advances, winter and summer temperatures are lower. The result is less ice melting than accumulating, and glaciers build up.

Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that sufficiently long and detailed chronology of the Quaternary temperature changes was worked out to test the theory adequately. Studies of deep-sea cores, and the fossils contained in them indicate that the fluctuation of climate during the last few hundred thousand years is remarkably close to that predicted by Milankovitch.

A problem with the theory is that the astronomical cycles have been in existence for billions of years, but glaciation is a rare occurrence. Actually, astronomical cycles perfectly explain glacial and interglacial periods, and their transitions, inside an ice age. Other factors such as the position of continents and the effects this has on the earth's oceanic currents, or long term fluctuations inside the core of the sun must also be involved that caused Earth's temperature to drop below a critical threshold and thus initiate the ice age in the first place. Once that occurs, Milankovitch cycles will act to force the planet in and out of glacial periods.