Antarctic Ice sheets

ICE covers much of the polar regions of Earth, and is a critical component to the planet's climate. An ice sheet is defined as a mass of ice that is greater than 31,067 mi. (50,000 km.) in area, such as those of Antarctica and Greenland. Ice sheets should not be confused with ice caps, which are masses of ice covering less than 31,067 mi. (50,000 km.) in area. It is estimated that approximately 90 percent of the Earth's total ice mass, or 27 million cu. km., is located in the Antarctic Ice Sheet. Scientists are concerned that increasing temperatures of air and sea will cause significant melting of the world's ice, including the Antarctic Ice Sheets, causing sea level rise.

Ice sheets, glaciers, ice caps, permafrost, snow, and sea ice are all components of the Earth's cryosphere (portions of the Earth's surface that are frozen over land or water). In the Southern Hemisphere, the Antarctic Ice Sheet covers approximately 98 percent of the Antarctic continent and is the single largest mass of ice on the planet. Ice sheets form as snow and frost build up in an area, compressing the previously fallen snow into ice. The total area of ice sheets is changed regularly by melting, primarily where the ice comes into contact with water or warmer dry land at its base, and by calving, or the falling off of large pieces of the ice sheet, which become known as icebergs. The Antarctic ice sheet covers the major landmass of the continent in the eastern Antarctic and extends over the ocean in western Antarctica. In western Antarctica, the ice sheet is as deep as 8,202 ft. (2,500 m.) below sea level.

The Antarctic continent is cold year round, and is extremely dry (desert; little to no annual precipitation). Because of this, the Antarctic ice sheet has historically experienced very little melting from the surface. Typically, the summertime melt that is experienced by the ice sheet is focused around the northern Antarctic Peninsula and the northeasternmost regions of the ice sheet. Most ice from the Antarctic ice sheet is lost by calving of glaciers from the protruding ice shelves of the sheet.

With climate in a constant state of flux on Earth, scientists have begun to give considerable attention to the warming trends that appear to exceed normal climate oscillations. While gradual melting and refreezing of ice is common anywhere on Earth, sudden large-scale melting of the polar ice sheets may have significant implications for local and global ecosystems. It is believed that the interaction of warming ocean waters and increasing air temperatures is contributing to the thinning and breaking up of the ice sheets. This disturbance of the ice sheets, and glacier tongues (extension of glaciers projecting seaward, typically afloat), primarily in western Antarctica at the ocean interface, has increased floating ice (drift ice), changing the ecosystem structure of this sensitive region. Long term, such disturbances may have a significant impact on macro-fauna, such as seals and whales, which are dependent on the ice sheet-water interface for feeding.

The contribution of melting ice to seawater will have a multitude of effects on global climate. While natural fluctuations of sea level do occur from gradual melting and thermal expansion, mean sea level has been on the rise in recent years. Data show that 1870-2001, mean sea level rose by almost 7.87 im. (20 cm.), whereas 1993-2006 global sea level raised an average of .12 in. (3.1 mm.) per year, or 2.7 in. (4.3 cm.) in 13 years. Experts are concerned that melting of the Antarctic ice sheet across its entire surface, not just minimally at the base and edges as is normally seen, accompanied by an increase in calving, may contribute to sea level rise. The Antarctic ice sheet accounts for about 61 percent of all of Earth's freshwater, whereas, together, the polar ice sheets of Antarctica and Greenland account for 98-99 percent of the freshwater ice on Earth. If the Antarctic Ice Sheet were to melt, over the next thousands of years there would be a 183.7 ft. (56 m.) increase in sea level, causing catastrophic scenarios. If the Western Antarctic Ice Sheet (WAIS) alone were to melt, there would likely be a five to six m. increase in mean sea level.

Approximately 150 million people live within one meter of high tide, and 250 million live within five meters of high tide. Melting of glaciers and ice caps is second only to thermal expansion of the oceans as the primary contributors to sea level rise. The immediate effects of melting are submergence and increased flooding of coastal zones. Under current coastal protection scenarios, and with the predicted 40 cm. rise in sea level by 2080, more than 100 million people will be flooded annually.

Beyond sea level rise, experts are concerned that the influx of freshwater to the oceans with the melting of the Antarctic ice sheet will significantly alter the salinity and temperature of the oceans, causing unpredictable effects on the ecosystems in this region. Such changes are known to affect the evaporation of the oceans to the atmosphere, potentially altering the precipitation events that contribute to the build up and maintenance of the Antarctic ice sheet. However, this rise in sea level will also force more saline waters further inland than has been experienced in recent times, flooding mangroves and coastal flood lands with seawater.

The breakup and collapse of the Larson B ice shelf, in 2002, off of the northern Antarctic Peninsula drew worldwide attention to the thinning and melting of the polar ice sheets. Experts report from sediment core records that a breakup of the ice sheet of this magnitude was unprecedented since the last Ice Age. Over the last 100 years, nine ice shelves around the Antarctic Peninsula have broken up. Reports by the National Aeronautics and Space Administration (NASA) indicated in 2002 that the overall mass of the Antarctic Ice Sheets were on the increase. Since then, satellite data compiled at the University of Colorado at Boulder suggests that, to the contrary, the amount of ice in Antarctica is actually decreasing.

SEE ALSO: Climatic Data, Ice Observations; Glaciers, Retreating; Glaciology; Little Ice Age; Radiation, Ultraviolet; Sea Level, Rising; Vostok Core.

BIBLIOGRAPHY. R.B. Alley, The Two Mile Time Machine (Princeton University Press, 2000); C.R. Bentley, R.H. Thomas, and Isabella Velicogna, Ice Sheets (United Nations Environmental Programme, 2007).

Sandra Connelly Rochester Institute of Technology

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