The Great Ice Age
About a million years ago, the Earth entered a period of cooling and widespread glaciation. Ice sheets gripped most of northern Europe and North America and left ample evidence of their presence as they finally retreated about 11,000 years ago. Naturalists began piecing together geologic evidence of what they would dub the Great Ice Age as long ago as the 18th Century. Ice ages seem to occur at regular intervals in geologic history - about every 200 million years or so - and it's likely that a number of factors interact to trigger their onset. First, changes in the Earth's orbit around the sun, orientation to the sun, or changes within the sun itself could lead to significant changes in the Earth's climate.

A second factor is the distribution of Earth's landmasses. Changes in the world's geography alter ocean and air mass circulation patterns. For example, mountain ranges can block moisture-laden air masses, encouraging glaciation on one side of the range, while discouraging it on the other. Also, since temperatures are lower at high altitudes, glaciers can cover mountains at low latitudes where they would not form at sea level. Finally, once ice sheets start to form, they reflect more and more sunlight back into space, causing the Earth to cool further. This positive feedback loop has caused some scientists to speculate that the planet could switch from temperate to ice-bound in just a few years or decades. So the mild planet we know today could turn cold and inhospitable without much warning.

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