Geological evidence indicates that throughout most of the Earth's history not only have the climatic zones between polar, temperate, and tropical sectors been much less distinct than they are today, but the polar regions were generally free of permanent ice. There have been three major periods of intense glaciation during the Earth's long history. The first occurred about 650 million years ago (during the Precambrian) and the second was between 250 and 300 million years ago (during the Permian Period). The most recent one began fairly recently, within just the last several million years (during the Pliocene and Pleistocene epochs). One could argue we are still in this latest ice age, but at the moment find ourselves existing in a relatively warmer interval, or interglacial period.
Scientists believe that the polar regions did not become frigid enough for permanent ice formation until continental land masses or landlocked seas disrupted the movements of polar ocean currents. While the poles were located in open water, major ocean currents moved relatively warm waters from low latitudes into high latitudes which would have dispersed seasonal sea ice before it had a chance to become permanent. Today, the north pole is located in the Arctic Ocean which is surrounded and isolated by the North American, Asian, and European continents, and since there is very little mixing with warm water currents, ice is present year round. The south polar region is isolated on the Antarctic continent which not only prevents warm waters from reaching it, but the high elevation of the continent makes it even more conducive to the formation of permanent ice.
Geological evidence shows that cold temperate climates still persisted in the Arctic region until less than three million years ago when permanent ice appeared in central Europe and Asia, and covered the mountains of North America, Greenland, and Iceland. This corresponds to the beginning of the Pleistocene Era. The ice sheets have fluctuated considerably in the northern hemisphere over the last million years, but the Antarctic Ice Cap has remained relatively stable. As much as 30% of the Earth's land surface became covered with permanent ice during the Pleistocene glacial periods. There have been four major glacial periods in the northern hemisphere, and each included several advances and retreats of ice fronts. The most recent glacial period started about 115,000 years ago, and ended about 14,000 years ago. There were several interglacial periods interspersed within those 100,000 years. During the time of greatest ice formation, nearly all of Canada, Greenland, and most of the northern United States were covered with ice up to 2,000 meters (6,600 feet) thick. In Eurasia, the land covered by ice amounted to only about half that found in North America, and was limited to Scandinavia, the Barents Sea (which at that time was land), and northwestern Siberia. Recent geological studies have demonstrated that there have been numerous Arctic "miniglacial" periods lasting but decades within the past 14,000 years, some within historical times.
The northern ice sheets began retreating about 20,000 years ago, and have since uncovered vast areas of land which are now covered with tundra and taiga. Antarctica, on the other hand, is still locked in its ice age. Relatively minor changes in the Antarctic Ice Cap have occurred, however, as evidenced by glacial moraines and icecaused scratches in mountain peaks which are now high above the surface of the ice. Much of the Antarctic continent is surrounded by a ridge of moraine between 100 and 300 kilometers (60 to 180 miles) offshore and in waters up to 500 meters (1,650 feet) in depth, which demonstrates a former edge of the ice mantle. The massive bulk of the ice cap probably would have protected it from changes caused by minor climatic fluctuations, but changes in world sea levels generated by glaciations in the northern hemisphere would expand the coastline of Antarctica considerably, allowing its ice mantle to enlarge respectively. Something to take note of on an Antarctica cruise.
The Greenland Ice Cap contains about 10% of the world's permanent ice, while the Antarctic Ice Cap contains perhaps 88% of the world's ice. These ice caps are selfperpetuating because they gain about the same amount of ice that they lose each year to melting, evaporation, snow being blown out to sea, and of course, tabular bergs and ice bergs breaking away and drifting out to sea.
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