|Figure One - Ice Age Man. Courtesy of Manhattan's|
Museum of Natural History.
During the Pleistocene, massive sheets of ice flowed southward across Canada into the northern United States about seventeen times. This lasted approximately 1.65 million years.
Imagine what Canada and other northern hemisphere countries were like with a mile or so of ice on top of the land for thousands of years. The ice sheets were heavy enough to push the Earth's crust down about 1000 feet. Where did all the water for the ice come from. Answer: the oceans! Sea levels dropped substantially.
Do we know what caused these Ice Ages? Let me present one theory.
|Figure Two - An estimate of the depth of ice in meters|
from the Wisconsin glaciation around 21,000 years ago.
During the last Ice Age, ice sheets spread and shrank on a 100,000 year-long-cycle. Glaciers dominated the land from 60,000 to 90,000 years during the cold phase of the cycle, and then mostly disappeared for 10,000 to 40,000 years during the warm phase of the cycle (Bonnicksen 2000; p. 5).
The Wisconsin glaciation began about 100,000 year ago in North America and ice sheets reached maximum thickness around 18,000 years ago. A warming trend started around 17,000 years ago and the ice sheets began to melt. There was a lot of ice to melt. Figure Two represents an estimate of how thick the ice sheets were in meters on four future North American cities. It took another 11,000 years for Canada to completely thaw out, right around 6,000 years ago. Earth has been in an interglacial climate ever since.
|Figure Three - CLICK to BUY the ENTIRE SERIES|
Researchers have found that the sun’s output varies, and that the amount of sunlight that reaches certain parts of the globe is affected by how the Earth orbits the Sun. A Serbian astronomer-Eccentricity. Milankovitch defined the first circumnavigational cycle as eccentricity or the shape of the Earth’s orbit around the sun. Due to gravitational pull from other planets, the Earth does not orbit the Sun in a perfect circle. The Earth has an elliptical orbit around the Sun that varies from five percent to zero percent ellipticity over a 100,000 year-long-cycle (Figure Five). An elliptical orbit reduces or increases solar radiation during the various seasons. When the Earth is in its most elliptical orbit, it receives twenty to thirty percent more solar energy at its perihelion (Earth is closest to the Sun) than its aphelion (Earth is farthest from the Sun). Currently, Earth is in an interglacial cycle and its eccentricity is at a minimum.
mathematician by the name of Milutin
Milankovitch (1879-1958) hypothesized that past glacial cycles correlated to cyclical
changes in insolation, and that the Earth’s circumnavigation around the Sun were the main cause of these cyclical changes in insolation. Milankovitch and others claimed that the Earth’s orbital path had a huge impact on past global
cooling and warming cycles. Milankovitch tested his theory against temperature data
from the paleoclimate records, and then he proposed a 100,000 year-cycle between ice sheets.
He claimed that the ice sheets were not created by dramatic changes in the amount of insolation reaching Earth, but how solar energy was distributed on Earth. He identified three circumnavigation cycles that were responsible for the Ice Ages. The cycles were eccentricity, axial tilt or obliquity, and wobble or precession.
|Figure Four - Milutin|
|Figure Five - Eccentricity. The Earth moves in a slightly elliptical|
path during its annual revolution around the Sun.
|Figure Six - Axial Tilt or Obliquity. The Earth is tilted on its rotational axis|
23.4 degrees from a plane perpendicular to the surface over
which it moves during its revolution around the Sun.
|Figure Seven - Precession or wobble. The effect of the wobble is to|
systematically change the timing of the solstices and equinoxes
relative to the extreme positions the Earth occupies on
its elliptical path around the Sun.
Bottom Line. Milutin Milankovitch suggested a combination of conditions that were conducive to glaciation. The first condition is minimal axial tilt or obliquity. Changes in axial tilt has very little effect from solar radiation at lower latitudes, but increases the effect toward the poles. As axial tilt increases, summer radiation increases significantly. Therefore, minimal axial tilt is conducive to ice sheet buildup The second condition is a relatively high eccentricity. Eccentricity variations affect the intensity of the seasons because it alters the distance the Earth is from the Sun. The third condition is the Northern Hemisphere summer should coincide with aphelion (Earth is farthest from the Sun) creating cooler summers which means less melting of the existing ice sheets. When all of these conditions converge, we have what is often referred to as a "cold orbit" and there is a good chance that ice sheets will expand.2000 Bonnicksen, Thomas M. America’s Ancient Forests from the Ice Age to the Age of Discovery. John Wiley and Sons. New York.
2015 Bradley, Raymond S. Paleoclimatology - Reconstructing Climates of the Quaternary. Third Edition. Elsevier Publishing. New York.