Ice Ages
Ice ages were periods in Earth's history when glaciers and vast ice sheets covered large portions of Earth's surface. Earth's average annual temperature varies constantly from year to year, from decade to decade, and from century to century. During some periods, that average annual temperature has dropped low enough to allow fields of ice to grow and cover large regions of Earth.
The most recent ice age
Over the last 2.5 million years, about two dozen ice ages have occurred. That means that Earth's average annual temperature greatly shifted upwards and downwards about two dozen times during that time. In each case, a period of...
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