In 1840, the Swiss scientist Louis Agassiz published a book on glaciers—huge frozen rivers of ice, filling valleys and slowly creeping downslope to lakes of meltwater below. Glaciers etch and grind down rocks, carry loads of gravel and big boulders, leaving them far from their origins, wherever the ice finally melts. Agassiz noted that such imprints existed all over northern Europe, and suggested that parts of Germany, Poland, and Russia used to be covered by enormous glaciers.
Shortly after Agassiz's book, evidence for past glaciations was found in North America as well. Later geological studies found evidence that such glaciers advanced and retreated several times in the last million years. At the peak of the last ice age 18,000 years ago, more than 30% of the Earth's land surface was covered with ice. In the U.S., ice sheets extended as far south as southern Wisconsin and Long Island, NY. In Europe, the ice extended as far south as northern Germany and Poland.
Watch "How the Ohio River was Formed" from PBS LearningMedia™ to learn more about North American glaciations.
The evidence for the Ice Ages is clear, but what could cause such a huge change? One thing we know for sure-any change in climate, whether warming or cooling, has to be the result of a change in the amount or distribution of solar energy in the Earth system.
When radiometric dating emerged in the 1950's, we were able to date the advances and retreats of glaciers. These dates caused scientists to take a second look at the calculations of Milutin Milankovitch, a scientist who contributed an explanation for the Ice Ages that had, until then, been widely ignored.
Milankovitch developed a comprehensive mathematical model that calculated latitudinal differences in insolation and surface temperature at different latitudes for 600,000 years prior to the year 1800. He then attempted to correlate these changes with the growth and retreat of the Ice Ages. In this module, you will explore the evidence for astronomical/orbital forcing of climate during the Ice Ages—also known as Milankovitch cycles.
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