The Earth has an extensive history of climate fluctuation. It has been through periods of global wide warmth and extreme cold. Some of these fluctuations have led to what we call Ice Ages during which mountain glaciers grow and cover large amounts of the Earth’s surface forming sheets of ice, in some cases nearly a mile thick.
The most recent was The Great Ice Age. It is believed to have started over 1 million years ago during the Pleistocene Era. At its peak, the ice sheet covered nearly 1/3 of the northern hemisphere. There was so much water tied up in the glaciers that ocean levels dropped exposing large tracts of coastal land. It was during this period, about 20,000 years ago, that scientists believe man crossed into North America from what is now known as Siberia. Prior to this period, glaciers advanced and retreated in a cycle that lasted thousands of years. At times the glaciers actually retreated completely. In fact, they only retreated from Canada about 6,000 years ago.
Scientists believe that there were four major periods of glaciations, the Nebraskan, Kansan, Illinioian and Wisconsin. Conversely there were periods of interglacial warming called the Aftonian, Yarmouth and Sangamon. Some believe that we are just experiencing an interglacial warming at the present.
During the last glaciations, in North America the ice sheet advanced as far south as Ohio. Because of the advancement and subsequent retreating of these ice sheets, the land was scraped and smoothed until vast depressions were gouged out of the terrain. When they retreated these depressions filled with melt water and formed lakes, the largest of which became the Great Lakes. In the west a huge body of water called Lake Bonneville dominated the landscape. Formed by glacial melt water, it covered over 20,000 square miles and reached a depth of 1,000 feet. The retreating of the glaciers cut off the water supply and the lake dried up, leaving as a remnant The Great Salt Lake in Utah.
Another effect of glacial retreat is the uplifting of the Earth’s crust. The vast crushing weight of the ice depressed the land so that when the glaciers retreated the land slowly started to spring back in what is called Post Glacial Rebound. The land north of the Great Lakes is still rising today.
Some animals that had adapted to the unique climate of the Ice Age were so specialized in their adaptations that when the last glaciations came to an end they could not survive. Wooly Mammoth, Wooly Rhinoceros, Short-Faced Bears and Saber-Tooth Cats were just some of the animals that went extinct. These animals could not adapt during the extreme fluctuations of temperature and drought that occurred at the end of the last ice age.
Our climate is constantly changing. It has fluctuated for millions of years in cycles of heat and cold. The Ice Ages are a normal product of these changes and the Earth with most likely experience them again. It is just a matter of time.
Louis L, Ray, The Great Ice Age, USGS, U.S. Department of Interior
Post Glacial Rebound, Wikipedia