The Last Four Glacial Cycles
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During the last 400,000 years, considerable parts of North America, Europe and Asia were covered by large ice-sheets. At the most recent maximum of the glaciation about 21,000 years before present the sea level was approximately 120 m (394 ft) lower than today due to the large quantities of water locked in the ice-sheets, and surface temperatures in many regions of the Earth were significantly colder. The animation shows the extent of the continental ice-sheets and sea-ice as well as the distribution of land and ocean that is altered by changes in sea-level.
A truly satisfactory explanation of the dynamics of glacial cycles remains elusive to this day. Even though there is compelling evidence that changes in the strength and distribution of solar insolation caused by variations in the Earth's orbit around the sun are mainly responsible for the build-up and melting of ice-sheets. However, the way in which these variations interact with e.g. winds, ocean currents and the terrestrial and marine biosphere is still a matter of current research.