It is thought that during the past billion years or so, the Earth's climate has fluctuated between warm periods (even completely ice-free at times) and cold periods, when massive glaciers and ice sheets covered huge portions of the planet.

During the cold periods, or Ice Ages, the entire Earth gets colder. The polar regions are very cold, there are large differences in temperature between the equator and the poles, and more snow falls in winter than melts in the summer. Gradually, glaciers form and slowly grind their way across the continents.

The last Ice Age ended about 20,000 years ago, but during its peak, the massive Laurentide ice sheet covered over 5 million sq. miles / 13 million sq. km of North America and stretched from the Arctic through eastern Canada to the northern half of the United States.

Its weight was so great that it depressed the Earth's crust by as much as 700 meters / 2,300 feet, and so much water was deposited on the land in the form of ice and snow that sea levels dropped dramatically.

Although the exact causes have not been proven, an Ice Age is most likely the result of a complicated combination of such things as solar output, tilt of the Earth's axis, distance of the Earth from the Sun, position and height of the continents, ocean circulation, and the composition of the atmosphere.

PICTURE: Glaciation during the last Ice Age.