Ice Ages

Actually there have been at least four major advances of glacial ice sheets in the last billion years. The geologic times for these were:

1. Late Proterozoic (800 to 600 MYA)
2. Pennsylvanian-Permian (320 to 250 MYA)
3. Late Neogene to Quaternary (the last 4 MY)
4. Ordovician to Silurian (460 to 430 MYA)

Many glacial advances and retreats have occurred during these periods, but we will focus on the ones that took place over large periods of time, tens to hundreds of millions of years ago.

What Causes the Ice Ages?

Scientists have done extensive studies of glaciers and their movement, both past and present. This includes features created by glaciers, both erosional and depositional. However, a widely accepted theory for the causes of the ice ages has not yet been established. They do agree that there are some very good possibilities, including:

1. Changing of continental positions (plate techtonics);
2. Uplift of continental blocks;
3. Reduction of CO₂ in the atmosphere; and
4. Changes in the earth's orbit.

For more on these possibilities click here

Because the moving of plates is extremely gradual, it cannot explain the glacial and non-glacial climates that took place during the Pleistocene epoch.

Today, scientists think that the changes in climate during the Pleistocene may have been caused by variations in the earth's orbit. This idea or hypothesis was first developed by a Yugoslavian scientist, Milutin Milankovitch. He believed that the amount of solar radiation is the most important factor in controlling the earth's climate. The amount of solar radiation would vary based on the following:

• Variations in the shape of the earth's orbit around the sun (referred to as eccentricity)
• Changes in the angle that the axis makes with the plane of the earth's orbit (referred to as obliquity)
• The wobbling of the earth's axis (called precession)

Milankovitch used these factors to figure the amount of solar energy in connection with the earth's surface temperature to correlate changes in the climate during the Pleistocene epoch. Refer to Figures 1 and 2 above to see glacial coverage during the Pleistocene Epoch.

It is worth noting that these variables cause little or no change in the amount of solar energy hitting the earth. The impact is caused by the degree of contrast between the seasons. For instance, a milder winter in the middle to high latitudes means greater snowfall, while cooler summers would mean a reduction in the snowmelt. Although Milankovitch did his studies in 1938, modern scientists currently back up his theories. The studies were very detailed and mathematically complex, but the conclusions were in agreement with the theory. They found that the changes in the earth's climate over the last several hundred thousand years are directly related to changes in the geometry of the earth's orbit. Specifically, they stated "It is concluded that changes in the earth's orbital geometry are the fundamental causes of the succession of the Quaternary ice ages."

For more information on ice ages click here
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References:

• Tarbuck, Edward J. and Lutgens, Frederick K.,Earth, An Introduction to Physical Geology, 1996, Prentice-Hall, Inc., pp. 273-302
  
• Illinois State University Museum, Ice Ages, 1995, http://www.museum.state.il.us/exhibits/ice_ages/index.html