Ice age

A period of time with temperatures low enough for ice sheets to form and periodically expand.

The term "ice age" is commonly used about periods of extensive ice expansion. However, it is more accurately defined as a period characterized by the presence of large ice sheets. Which means that we are presently still living in an ice age, since ice sheets still exist in Greenland and the Antarctic.

Within an ice age there are periods of extra cold climate called glacials. During these periods the glaciers and ice sheets expand extensively, and cover a large part of the Earth's surface. The sea level drops as water is stored in the ice sheets, and the circulation in the oceans is probably disturbed.

Between these cold periods, glacials, are warmer periods, interglacials, where the ice retreats. The most recent glacial period ended about 11,000 years ago. This is often referred to as the last "ice age". Since then we have been living in an interglacial period.

The Earth has experienced several ice ages, and the latest started about 2 million years ago. The Earth is free of ice in between ice ages.

There are several possible causes of ice ages. Changes in the atmospheric content of greenhouse gases, Milankovitch cycles, solar variation and changes in the arrangement of the landmasses are all potential causes.