Causes of Glaciation

Climate change and the cause of glaciation over at least the last 2 million years has fol lowed a distinctive pattern termed the Astronomical Theory, developed by James Croll in 1875 and elaborated on by Milutin Milankovitch in 1924. The Milankovitch Theory states that there is a regularity and frequency of climatic fluctuations based on the: (1) eccentricity of the earth's orbit (change from circular to elliptical orbit around the sun), (2) obliquity of the ecliptic (the angle of tilt of the plane of the elliptic path), and (3) the precession of the earth's axis on which it rotates (ranging from 0 to 23.5 degrees). Together, these act as the primary driving mechanisms, also known as orbital forcing, of global climate change; in the Pleistocene and Holocene, they produced long-term periods of cooling and short, rapid periods of warming at cycles of 100,000, 43,000, 24,000, and 19,000 years. Evidence of astronomical variables and, hence, the cycles of climatic fluctuation, have been collected from coral reefs, pollen records, deep-sea cores, loess sequences, ice cores, and tropical lake records.

The cause of the onset of North Hemisphere glaciation is likely to have been related to the connection of North America to South America by Panama and the rise of the Isthmus of Panama in the latest Pliocene. The so-called oceanic conveyor belt in the North Atlantic Ocean was created by the restriction of ocean water flow into the Pacific from the Atlantic and caused the cooling and sinking of those warm surface waters, which deprived the Arctic Ocean of heat. The waters of the Atlantic are slightly more saline than the Pacific Ocean, because of the evaporative effects of the hot, dry trade winds from the Sahara Desert in Africa. Prior to the closing of the Isthmus of Panama, the Atlantic Ocean water mixed with that of the Pacific Ocean. After the closing, the slightly denser, more saline warm-water ocean currents of the Atlantic were diverted along the western mar

Table 1

Glacial and Interglacial Stages

of the Pleistocene Epoch for

North America and the Alpine

Region of Europe

Alpine

Years before

North America

Region

Present

Wisconsinian

Würm

75,000

Sangamon

Riss-Würm

125,000

Illinoian

Riss

265,000

Yarmouth

Mindel-Riss

300,000

Kansan

Mindel

435,000

Aftonian

Günz-Mindel

500,000

Nebraskan

Günz

1 or\r\ r\r\r\

Pre-Nebraskan

Pre-Günz

1,800,000

Source: Levin, Harold L. 1999. The Earth through Time, 6th ed. Fort

Worth: Saunders College Publications. (This material is used by per

mission of John Wiley and Sons.)

gin of the Atlantic Ocean northward toward the Arctic, where they cooled and sank before reaching the Arctic Ocean. The Arctic Ocean was effectively isolated from warm water inputs from the Atlantic Ocean currents, eventually cooling the whole Arctic region. The Northern Hemisphere was plunged into the "Ice Age" without the mediating effects of the warm water currents that originated from the equatorial region.

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