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The Earth's climate has gone through radical temperature swings in the past several billion years. There have been at least five nearly global glacial advances, in which ice sheets advanced from the poles toward the equator and stayed there for typically a million years. Conversely, during warm periods, the ice sheets retreated, and may not have existed at all. The current trend of global warming will cause the northern polar cap to melt entirely within a hundred years. For more on the history of Earth's polar ice caps click here.