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Paleo Slide Set: The Ice Ages
Portrait of Milutin Milankovitch by Paja Jovanovic, 1943, courtesy of Vasko Milankovitch
Croll's arguments electrified the scientific community and provoked a great deal of debate and research. Geologists scoured Europe and the Americas in search of evidence to support or refute Croll's astronomical theory. Initially, the theory seemed to square well with the geologic evidence, but by the 1890s terrestrial field research raised serious questions about its accuracy. Croll's calculations predicted that the end of the last glacial period occurred some 80,000 years ago and yet reasonable estimates placed the age of two North American waterfalls that had formed after the great ice sheets retreated at 6,000-32,000 years (Niagara on the U.S.-Canada border) and 6,000-10,000 years (the Falls of St. Anthony in Minnesota). Clearly, something was wrong with Croll's theory. At the dawn of the twentieth century, few reasonable persons believed that variations in eccentricity and precession caused the ice ages.

However, in the 1910s, the Serbian mathematician Milutin Milankovitch, began to embark on a series of calculations that would eventually revive the orbital theory of climate change. Milankovitch's main contributions were threefold. First, he used new astronomical calculations by the German scientist Ludwig Pilgrim that took into account a third cyclical variation in the earth's orbit, that of obliquity or tilt. Secondly, he reasoned that summer rather than winter temperatures were the main contributors of ice sheet growth and decline. Lastly, he calculated summer radiation curves for the key latitudes of 55, 60, and 65 degrees N that seemed to correlate well with evidence then available from the geologic record.

Photo Credits:
Contributed by John Imbrie
Brown University

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