For many years Scientist have known that the Earth has experienced numerous Ice Ages. They could demonstrate this with core samplings from deep within glaciers. The questions still unresolved are the triggering mechanisms, duration, intensity and intervals.
The Wisconsin Ice Age
Beginning About 100,000 Ago
Peaked 18,000 Ago
During the last Ice Age, called The Wisconsin because that was about the southern limit of its' progress, close to 5% of Earth's water was frozen in the form of glacial ice. This caused sea levels to drop 400 feet and exposed vast amounts of land which today are sea floors. One of these exposures occurred in the Bering Straits providing a land "bridge" from Siberia to Alaska.
Scientist believe that these huge quantities of ice recede much more quickly than they form. One theory has the beginning at about 100,000 years ago; Bering land bridge formed about 38,000; migration of people and animals beginning about year 38,000; ice age peaking around 18,000; and water covering the bridge once more about 8,000 years ago. Therefore, they think that during The Wisconsin Ice Age inhabitants of Asia had about 30,000 years to cross into North America.
When Earth's climate cools for a long period of time the ocean cools and polar ice sheets expand. Water accumulates in the form of glaciers and snow causing the sea level to drop. Periods of warm climate cause the reverse to happen. The melting continental ice returns to the ocean causing the sea level to rise. Land that was visible and usable during the ice age is reclaimed by the rising ocean.
Glaciers form where climates are so cold that snow will not melt during summers. The snow is transformed, by recrystallization, into ice. As snow accumulates, the ice thickens at the top of mountain valleys or at the domed center of ice sheets. As the weight of these sheets increase, gravity starts to pull them downhill. As the ice is pulled down, new accumulations take place at the top.
Scientist have discovered through geologic records that dramatic changes in climate forced the ice sheets to retreat toward polar regions over the past 8,000 years. It has been challenging for scientists to find the causes of such dramatic changes in climate over the past million years or so. "What turns ice ages on and off?" is only one of the questions that scientists are seeking answers to.
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