Polar ice

The polar ice caps cover the North and South Poles and their surrounding territory, including the entire continent of Antarctica in the south, the Arctic Ocean, the northern part of Greenland, parts of northern Canada, and bits of Siberia and Scandinavia also in the north. Polar ice caps are dome-shaped sheets of ice that feed ice to other glacial formations, such as ice sheets, ice fields, and ice islands. They remain frozen year-round, and they serve as sources for glaciers that feed ice into the polar seas in the form of icebergs . Because the polar ice caps are very cold (temperatures in Antarctica have been measured to −126.8°F [−88°C]) and exist for a long time, the caps serve as deep-freezes for geologic information that can be studied by scientists. Ice cores drawn from these regions contain important data for both geologists and historians about paleoclimatology and give clues about the effects human activities are currently having on the world.

Polar ice caps also serve as reservoirs for huge amounts of the earth's water . Geologists suggest that three-quarters of the world's fresh water is frozen at the North and South Pole. Most of this freshwater ice is in the Southern Hemisphere. The Antarctic ice cap alone contains over 90% of the world's glacial ice, sometimes in huge sheets over 2.5 mi (4 km) deep and averaging 1.5 mi (2 km) deep across the continent. It has been estimated that enough water is locked up in Antarctica to raise sea levels around the globe over 200 ft (61 m), drowning most of the world's major cities, destroying much of the world's food-producing capacity, and ending civilization.

Although the polar ice caps have been in existence for millions of years, scientists disagree over exactly how long they have survived in their present form. It is generally agreed that the polar cap north of the Arctic Circle, which covers the Arctic Ocean, has undergone contraction and expansion through some 26 different glaciations in just the past few million years. Parts of the Arctic have been covered by the polar ice cap for at least the last five million years, with estimates ranging up to 15 million. The Antarctic ice cap is more controversial; although many scientists believe extensive ice has existed there for 15 million years, others suggest that volcanic activity on the western half of the continent it covers causes the ice to decay, and the current south polar ice cap is therefore no more than about three million years old.

At least five times since the formation of the earth, because of changes in global climate , the polar ice has expanded north and south toward the equator and has stayed there for at least a million years. The earliest of these known ice ages was some two billion years ago, during the Huronian Epoch of the Precambrian Era. The most recent ice age began about 1.7 million years ago in the Pleistocene Epoch . It was characterized by a number of fluctuations in North polar ice, some of which expanded over much of modern North America and Europe , covered up to half of the existing continents, and measured as much as 1.8 mi (3 km) deep in some places. These glacial expansions locked up even more water, dropping sea levels worldwide by more than 30 ft (100 m). Animal species that had adapted to cold weather , like the mammoth, thrived in the polar conditions of the Pleistocene glaciations, and their ranges stretched south into what is now the southern United States.

The glaciers completed their retreat and settled in their present positions about 10,000–12,000 years ago. There have been other fluctuations in global temperatures on a smaller scale, however, that have sometimes been known popularly as ice ages. The 400-year period between the fourteenth and the eighteenth centuries is sometimes called the Little Ice Age. Contemporaries noted that the Baltic Sea froze over twice in the first decade of the 1300s. Temperatures in Europe fell enough to shorten the growing season, and the production of grain in Scandinavia dropped precipitously as a result. The Norse communities in Greenland could no longer be maintained and were abandoned by the end of the fifteenth century. Scientists believe that we are currently in an interglacial period, and that North polar ice will again move south some time in the next 23,000 years.

Scientists believe the growth of polar ice caps can be triggered by a combination of several global climactic factors. The major element is a small drop (perhaps no more than 15°F [9°C]) in average world temperatures. The factors that cause this drop can be very complex. They include fluctuations in atmospheric and oceanic carbon dioxide levels, increased amounts of dust in the atmosphere, heightened winds—especially in equatorial areas—and changes in surface oceanic currents. The Milankovitch theory of glacial cycles also cites as factors small variations in Earth's orbital path around the Sun , which in the long term could influence the expansion and contraction of the polar ice caps. Computer models based on the Milankovitch theory correlate fairly closely with observed behavior of glaciation over the past 600 million years.

Scientists use material preserved in the polar ice caps to chart these changes in global glaciation. By measuring the relationship of different oxygen isotopes preserved in ice cores, they have determined both the mean temperature and the amount of dust in the atmosphere in these latitudes during the recent ice ages. Single events, such as volcanic eruptions and variations in solar activity and sea level, are also recorded in polar ice. These records are valuable not only for the information they provide about past glacial periods; they serve as a standard to compare against the records of more modern periods.

The process of global warming , which has been documented by scientific investigation, is also reflected in the ice caps. Should global warming continue unchecked, scientists warn, it could have a drastic effect on polar ice. Small variations over a short period of time could shrink the caps and raise world sea levels. Even a small rise in sea level could affect a large percentage of the world's population, and it could effectively destroy major cities like New York. Ironically, global warming could also delay or offset the effects of the coming ice age.

See also Glacial landforms