Polar Ice Caps
Polar Ice Caps
Polar ice caps and geologic history
Investigation of polar ice caps
The polar ice caps cover the territory around the north and south poles of Earth, including almost the entire continent of Antarctica, the Arctic Ocean, most of Greenland, parts of northern Canada, and bits of Siberia and Scandinavia. The ice at the North Pole floats on the ocean in the form of a relatively thin sheet. The Greenland and Antarctic 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 environmental scientists about paleoclimatology (prehistoric climate variations) 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 Earth’s water. Hydrologists suggest that three-quarters of the world’s freshwater is locked up in the ice sitting on Greenland and Antarctica. The Antarctic ice cap alone contains over 90% of the world’s freshwater ice, some in huge sheets over 2.5 mi (4 km) deep and averaging 1.5 mi (2.4 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 240 ft (73 m).
Polar ice caps and geologic history
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 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 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 300 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 to 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 argue that data indicate we are currently in an interglacial period, and that North polar ice will again move south some time in the next 23,000 years.
Investigation of polar ice caps
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 reductions in incoming solar radiation, reflection of that energy back into space, fluctuations in atmospheric and oceanic carbon dioxide and methane levels, increased amounts of dust in the atmosphere such as that resulting from volcanic activity, heightened winds—especially in equatorial areas—and changes in thermohaline circulation of the ocean. 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. Detailed examination of ice cores from the polar regions has shown that the rate of change in Earth’s climate may be much greater that previously thought. The data reflect large climatic changes occurring in periods of less than a decade during previous glacial cycles.
Scientists also use satellites to study the thickness and movements of the polar ice caps. Information is collected through radar, microwave, and even laser instruments mounted on a number of orbiting satellites. Scientists have also utilized similar technology to confirm the existence of polar ice caps on the moon and Mars. These relict accumulations are indicative of the history of these bodies and may prove useful in future exploration efforts as a water and fuel source. The detailed and frequent observations provided by the space-based tools permit scientists to monitor changes in the ice caps to a degree not possible by previous land-based methods.
Recent findings suggest that the ice sheets may be changing much more rapidly than previously suspected. Portions of the ice sheets in Greenland, west Antarctica, and the Antarctic Peninsula are rapidly thinning and, more importantly, losing mass. Scientists are able to document modifications of ice accumulations rates, volume of melt water, and the impact of elevated sea-water temperature and utilize this information in characterizing the movement and evolution of these ice sheets. Glaciers flowing to the ocean in these areas
KEY TERMS
Albedo— The fraction of sunlight that a surface reflects. An albedo of zero indicates complete absorption, while an albedo of unity indicates total reflection.
Glaciation— The formation, movement, and recession of glaciers or ice sheets.
Ice age— An extended period of time in Earth’s history when average annual temperatures were significantly lower than at other times, and polar ice sheets extended to lower latitudes.
Ice core— A cylindrical sample of ice collected by drilling and brought to the surface for analysis.
Ice sheet— A glacial body of ice of considerable thickness and great areal extent, not limited by underlying topography.
Milankovitch theory— Describes the cyclic change of temperature on earth due to variations in Earth’s orbit, including wobble of the axis and ellipticity.
Thermohaline— Said of the vertical movement of seawater, related to density differences due to variations in temperature and salinity.
appear to be accelerating in their advance. Although this acceleration may not be directly related global warming, the potential for their combined impact on sea level is of concern for many observers.
Predicting the future of the ice caps is a difficult task. It is complicated by the interactions of the various factors that control the ice. One example is the possibility that the warming climate will reduce the areal extent of ice in the polar regions. This will decrease the albedo, or tendency to reflect incoming solar radiation, of the polar regions. White ice is particularly good at reflecting much of the sunlight that reaches it and this has a cooling effect on the overall climate. With less albedo, the climate will tend to warm even more. However, the melting ice could impact the thermohaline circulation of the oceans, paradoxically resulting in extensive cooling. These seemingly contradictory results can only be resolved through more detailed scientific observation.
The process of global warming and other forces of climate change will continue to be 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 (which dwells near coastlines). Dislocations of coastal populations could be complicated by disruptions to the climates of food-growing regions, leading to catastrophic global famine.
Resources
PERIODICALS
Alley, Richard B., et al. “Ice-Sheet and Sea-Level Changes.” Science. 310 (2005): 456-460.
Murray, Tavi. “Climate Change: Greenland’s Ice on the Scales.” Nature. 443 (2006): 277-278.
Overpeck, Jonathan T., et al. “Paleoclimatic Evidence for Future Ice-Sheet Instability and rapid Sea-Level Rise.” Science. 311 (2006): 1747-1750.
Peterson, Bruce J, et al. “Increasing River discharge to the Arctic Ocean.” Science (December 13, 2002): 298.
OTHER
National Aeronautics and Space Administration, Jet propulsion Laboratory. “NASA Study Finds Rapid Changes In Earth’s Polar Ice sheets.” August 30, 2002. <http://quest.arc.nasa.gov/news/space/2002/08-30a.txt> (accessed November 15, 2006).
National Ice Core Laboratory. “Why Study Ice cores?” April 13, 2005.ψ<http://nicl.usgs.gov/why.htm>(accessed November 15, 2006).
United States Geological Survey. “Sea Level and climate.” <http://pubs.usgs.gov/fs/fs2-00/> (accessed November 15, 2006).
Kenneth R. Shepherd