Ice Ages

Ice ages are times during which significant portions of Earth's surface are covered by glaciers and extensive ice fields. Scientists sometimes use more specific terms for an ice age depending on the length of time it lasts. Geologic evidence suggests that seven major periods of cooling and ice accumulation have occurred during the history of Earth. These periods are often known as ice eras and, except for the last of these, are not well understood.

What is known is that 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 to low enough levels for fields of ice to grow and cover...