Glaciers are very sensitive to global temperature changes, retreating and growing as the Earth warms and cools. Because of this sensitivity, geologists have made careful measurements of the changes in benchmark glaciers in order to monitor the effects of global climate change and have studied the history of glaciers as a means of reconstructing the sequence of temperature fluctuations in Earth's past.

Of the freshwater ice of the world, 99 percent is located in Antarctica and in Greenland: 90 percent of the ice is located in Antarctica (an area of 14 million square kilometers containing 27.6 million cubic kilometers of ice); 9 percent is in Greenland (an area of 1.726 million square kilometers containing 2.85 million cubic kilometers of ice), and 1 percent exists in the glaciers and ice caps scattered throughout the world. The two ice sheets of Antarctica and Greenland represent about 10 percent of the Earth's land area and contain more than threequarters of its freshwater.

Geologists can provide useful information about the history of glaciation by studying the geographic features left by glaciers that have long since melted away. When alpine glaciers form, they produce an amphitheater-like depression called a "cirque" at their highest point. When the compression of snow produces ice in a depth of about 20 meters, the ice begins to flow downward along the valleys of former streams. The ice deepens and widens the stream valley to produce a glacial trough. Valleys of tributary streams are filled with ice as well, but the shorter length of the tributary glaciers and the lesser discharge produce less deepening of the channel. Typically, these tributary glaciers erode their channels only down to the current ice surface of the main glacier, resulting in distinctive "hanging valleys" that end in a precipice at their juncture with the valley produced by the main body of the glacier. The height of these hanging valleys indicates the depth of the ice in the main glacier.

Materials the glacier erodes are pushed to its side, creating lateral moraines, or to its lower end, creating a frontal or end moraine, or accumulate under the glacier, creating a ground moraine, or till. These features identify the greatest breadth (lateral moraines) and furthest extent (terminal moraine) of ancient glaciers. The sediments deposited in glacial lakes--called varves--can be used to determine the length of the deposition process. Distinctive small hills called drumlins are formed under an advancing glacier and, because of their teardrop shape, indicate the direction of the ice and water flow. Meltwater accumulates in lakes found in the cirques; these lakes are called tarns. Other lakes occupy the depressions in the glacial troughs. When the glacier retreats, it uncovers a Ushaped valley instead of the V-shaped valley it had prior to the glaciation.

Continental ice sheets transform the landscape differently. When they retreat, they leave a multitude of lakes, many of which are round (kettle lakes). Others are elongated in the direction of the ice flow--such as the Finger Lakes in New York. By reconstructing the history of glaciation from these alterations made to the landscape, glaciologists and geologists have been able to contribute greatly to the understanding of the nature of Earth's climatic changes.

Glaciers are masses of ice that are produced where the summer temperatures fail to melt the snow that fell during the preceding winter. Over time, this snow is compressed by overlying layers of more recents now, forcing out some of the air that exists around snow crystals. This air escapes toward the surface; the density of this older snow steadily increases, until the snow turns into ice. At that point, the density of the ice varies from 0.85 to 0.91. Ice therefore floats on liquid water, whose density is 1. The transformation of snow into ice is slower in polar areas, where compression is the major mechanism at work, because air temperatures remain low all year round, producing very little, if any, meltwater to be refrozen. In temperate climates, ice forms more rapidly than in the polar areas because there are periods of melting when the temperature is above the freezing point of water. The meltwater soaks the snow and refreezes during the next colder period. This process is much faster than compression to achieve the density of ice.

Glaciers can exist at any latitude, even along the equator. They are present on every continent except Australia. The necessary condition for glaciers is that the air temperature remains low enough to prevent melting of the last winter snow. Because temperature decreases with increasing elevation, glaciers are found on high mountains or volcanoes. In Africa, Mounts Kenya, Kilimanjaro, and Ruwenzori have small glaciers (though they are retreating rapidly). In South America, there are many small tropical glaciers located in the Andes as well. In North America, Europe, and Asia, small glaciers dot the summits of high mountain ranges and volcanoes. Glaciers gain mass--called "accumulation"--by the deposition of snow in the highest elevation, called the cirque--a large amphitheater at the summit of glaciers.

Glaciers lose mass (a process called "ablation") by melting, sublimation, and calving. Melting takes place in a glacier where the temperature is greater than 0œ Celsius. This can occur where the air temperature reaches this value or at the underside of the glacier, where friction of the ice on the ground beneath the glacier causes the temperature to increase, producing meltwater. This water lubricates the underside of the glacier, resulting in faster movement.

The second component of ablation is sublimation. Sublimation is the transformation of ice directly into water vapor, without an intermediate liquid stage. In Antarctica, sublimation is a major contributor to the ablation of ice because the ice sheet is affected by very strong winds which enhance the process. The third form of ablation is calving. It involves the breaking of the end of the glacier when it reaches an ocean or a lake. Calving produces icebergs--masses of ice that float because of the lower density of ice (0.85-0.91) compared to that of water (1). Calving is responsible for about 40 percent of ablation in Greenland and 80 to 90 percent in Antarctica.

The term "mass balance" refers to the difference in mass between accumulation and ablation. A glacier will grow longer if accumulation is greater than ablation over a period of time. This is called a glacial advance. A glacier will get shorter if the amount of ice removed by ablation exceeds the amount of snow that accumulates in the coldest part, the cirque. When glaciers become shorter they are said to retreat. This does not mean that the ice stops moving downward from the cirque to the lower end of the glacier, called the front. Glacial retreat indicates only that the front of the glacier will be found closer to the cirque.

Today, most glaciers in the world are retreating. Although there are a few exceptions where glaciers are advancing, the worldwide trend is a steady retreat in response to the general raising of the Earth's temperatures.

The science that studies the cryosphere is called glaciology. One of the first important glaciologists was Louis Agassiz, a native of Switzerland, who studied the glaciers of the Alps in the nineteenth century. Building on Agassiz's work, modern glaciologists discovered that glaciers not only are a good source of information about the global impact of recent environmental changes but also provide valuable data about the long history of climate change on this planet. Modern study of ice cores, cylinders of ice retrieved from glaciers, has shown that ice records the temperatures of the Earth atmosphere at the time the snow fell. The paleotemperatures are inferred from the composition of the water that makes up the ice.

In nature, water is made of two atoms of hydrogen and one atom of oxygen; however, oxygen has three isotopes (elements that have the same number of protons but different numbers of neutrons). The lightest and most abundant of the three is O16. O18 is the heaviest but exists in much smaller quantities. Higher Earth temperatures make it easier for the heavier O18 molecule to evaporate, resulting in snow--and therefore glacial ice--that has an increased proportion of it. During glaciations, the lower temperatures lead to a depletion of O18 in the ice of glaciers. The same principle is applied to ice when two isotopes of hydrogen are measured. Because the ice sheets of Antarctica and Greenland are very thick, ice cores obtained by drilling into these ice sheets are very long. They therefore provide a very long record of the climates of the past (known as "paleoclimates"). Based on the cores retrieved so far, scientists have been able to identify a sequence of glacial and interglacial cycles that covers the last 800,000 years.

The Earth has had many ice ages in its 4.5-billion-year history. Most recently, beginning about one million years ago, the Great Ice Age occurred during a time period called the Pleistocene. This glacial period formed an ice sheet in North America, Northern Europe, Northern Asia, and Antarctica that expanded until it reached its maximum extent about twenty thousand years ago. The Pleistocene Glaciation was not uniformly cold; short interglacial periods of warming occurred several times. Finally, about ten thousand years ago, the warming trend continued, melting the ice sheets and uncovering the northern continents. The northern part of Canada became free of ice about six thousand years ago. The mountain glaciers attached to the high mountain ranges of the American West are remnants of the Pleistocene period. In more recent centuries, the Earth experienced a shorter period of cold temperatures, called the Little Ice Age, beginning about 1650 and ending approximately in 1850, during which the Earth cooled by about 1œ Celsius. (The term "Little Ice Age" is used differently by different writers. Many use it to refer to the climate cooling from about 1300 to 1850, while others use it for the latter half of that interval, when cooling was greatest, beginning around 1550 or 1600.) The mountain glaciers that advanced during this period are currently retreating in response to today's higher temperatures.

At their present rate of melting and retreating, glaciers are having a major impact on the populations living in their vicinity. The increase in meltwater can increase the production of hydroelectricity for a short while but at the same time may impact the population's well-being in the very near future by decreasing the amount of available water for irrigation or electricity production when the glaciers will have completely disappeared. Monitoring glacier changes and drawing scientific conclusions about their retreat is not something new. As early as 1894 scientists began cataloging glaciers and their changes. These findings were published by the World Glacier Monitoring Service. Maximum extents of glaciers were computed by using the position of their terminal moraines, and their volumes were estimated by measuring the height of their lower end since it corresponds to the height of ice that used to occupy the glacial trough.

In the 1970's, during the International Hydrological Decade declared by United Nations Educational, Scientific, and Cultural Organization, the Temporal Technical Secretariat for the World Glacier Inventory was created and began making a comprehensive inventory of more than 100,000 glaciers worldwide. Since then, with the help of satellite instruments, it has been determined that there are about 160,000 glaciers, thousands of whose outlines, retreats, and advances are readily mappable. These measurements allow scientists to rapidly assess the impact of the warming of the Earth on the cryosphere, and their study has proven a valuable tool in monitoring their reaction to the warming of the Earth's atmosphere.

The work of the Intergovernmental Panel on Climate Change and the research conducted for the Fourth International Polar Year (2007-2008) organized by the International Council for Science in conjunction with the World Meteorological Organization are focused on understanding the extremely complex relationships between glaciers and climates. One of the goals of the United States National Committee for the International Polar Year is the creation of a network of observation platforms to monitor glaciers in order to provide reliable data by which scientists will be able to assess the impact of global warming both on the glaciers themselves and upon the global ecosystem of which they are an essential part.

Bibliography:

1) Benn, Douglas I., and David J. A. Evans. Glaciers and Glaciation. New York: Arnold, 1998.

2) Bennet, Matthew, and Neil Glasser. Glacial Geology: Ice Sheets and Landforms. New York: Cambridge University Press, 1997.

3) Hambrey, Michael, and Jurg Alean. Glaciers. New York: Cambridge University Press, 2006.

4) Trewby, Mary. Antarctica: An Encyclopedia from Abbott Ice Shelf to Zooplankton. Toronto: Firefly Books, 2002.

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