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An Ice Age is a period of time, typically about 30 million but occasionally as long as 300 million years, during which ice sheets cover at least the Earth's polar areas. Individual Ice Ages have sub-Ice Ages, called glacials (when cold) or interglacials (when warmer) that operate in cycles of 40,000 and 100,000 years. When the term "Ice Age" is used colloquially, it often refers to these shorter glacials, periods when the ice caps extend significantly beyond the poles and into the hearts of continents such as North America and Eurasia. In this sense, "the last Ice Age" refers to what is formally called "the last glacial period", which began about 70,000 BP and ended between 15,000 and 10,000 BP. This is the Ice Age that was experienced by early man.
Scientists cannot say exactly what causes Ice Ages, although there are a number of inter-implicated variables at play. These include atmospheric composition (greenhouse gases), slight changes in the Earth's orbit known as Milankovitch cycles, the Earth's albedo (reflectivity), changes in the location and amount of crust at different points on the Earth's surface, variations in solar output, large meteor impact, the release of methane clathrates, and supervolcanism. The short-term (40,000/100,000 year) cycles are known to be caused by variations in the Earth's orbit.
Because there is ice covering Antarctica and Greenland, we are in the middle of an Ice Age, one that began 40 million years ago. Ice Ages are fairly atypical circumstances for the Earth; aside from six Ice Ages, the Earth's poles have been largely ice-free. Fossils of trees have been found at land which was just a few hundred miles from the poles at the time they lived. Ice Ages are about as rare as mass extinctions, occurring once every 100 million years or so.
The typical average global temperature when the Earth is not in an Ice Age is about 22° C (71.6° F). During an Ice Age, it dips about 10° C to an average of 12° C (53.6° F). At the poles, the temperature is far below freezing practically all the time.
During Ice Ages, large quantities of water get locked up in ice sheets, lowering the global sea level. During the most recent Ice Age, the global sea level was about 100 ft less than it is now, opening up large sections of land such as the North Sea, and connecting Papua New Guinea to the Southeast Asian mainland and Russia to Alaska via the Bering land bridge. Because of the Ice Age, our ancestors could cross over into the Americas. It would be over 10,000 years before humans who traveled to the Americas would be reunited with their distant cousins from Europe, Asia, and Africa.