Glaciers

Glaciers are perennial land-based accumulations of ice and snow, as well as debris such as rock and sediment, that flow slowly due to their own weight. They 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.

Background

Only about 3 percent of the Earth's water is freshwater. Of the freshwater ice of the world, 99 percent is located in the North Pole, Antarctica, and Greenland. Ninety percent of the ice was located in Antarctica (an area of 14 million square kilometers containing 27.6 million cubic kilometers of ice); 9 percent was in Greenland (an area of 1.726 million square kilometers containing 2.85 million cubic kilometers of ice), and 1 percent existed in the glaciers and ice caps scattered throughout the world. The two ice sheets of Antarctica and Greenland represented about 10 percent of the Earth's land area and contained more than three-quarters of its freshwater.

Ice Formation and Location

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 recent snow, 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. 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 (though they do exist on New Zealand and Papua New Guinea in Oceania). The necessary condition for glaciers is that the average air temperature remains low enough to prevent complete melting of the last winter snow. Because temperature decreases with increasing elevation, glaciers are typically 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. Glacial melting takes place in a glacier where the temperature is greater than 0° 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). In the first decades of the twenty-first century, calving was 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. This process is known as deglaciation.

Glacial Landscape

Geologists can discover 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 and other glacial deposits left 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 U-shaped valley instead of the V-shaped valley it had prior to the glaciation.

The ice sheets known as continental glaciers 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.

Glaciology and Paleoclimates

The science that studies the frozen portions of earth's surface, or 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 (the Last Glacial Maximum) 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 planet cooled by about 1° 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.

Context

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 1970s, during the International Hydrological Decade declared by United Nations Educational, Scientific, and Cultural Organization (UNESCO), 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 almost 200,000 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.

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 have beneficial short-term effects, such as an increase in the production of hydroelectricity, but in the longer term is expected to negatively impact populations around the world. The loss of vital freshwater reserves held in glaciers will reduce the amount of available water for irrigation, electricity production, and human consumption. Therefore, the study of glaciers has taken an urgent turn in the late twentieth and early twenty-first centuries.

The work of the Intergovernmental Panel on Climate Change and the research conducted for the Fourth International Polar Year (2007–8), organized by the International Council for Science in conjunction with the World Meteorological Organization, 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 was the creation of a network of observation platforms to monitor glaciers in order to provide reliable data by which scientists can 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.

The forecast for glaciers has not been positive. As anthropogenic climate change continues, scientists warn that most glaciers around the world are at risk of disappearing completely. For example, researchers for the International Union for Conservation of Nature (IUCN) in 2019 released a study suggesting that twenty-one of forty-six UNESCO World Heritage sites featuring glaciers would likely be devoid of ice by 2100. Even when projecting a drastic cut in global fossil-fuel emissions, the scientists noted at least eight of the sites would still completely lose their glaciers. The study further noted the importance of glaciers as unique ecosystems, indicating that a chain reaction of other ecological effects will be felt with the widespread loss of glaciers. For example, melting ice will contribute to expected rising sea levels that threaten coastal communities around the world. Ice loss not only continued at a rapid pace during the first decades of the twenty-first century; in fact, it quickened. While the Earth lost an average of 760 billion tons of ice per year for most of the 1990s, by 2021 annual ice loss had risen to roughly 1.2 trillion tons per year. In 2024, Antarctica was losing ice mass at an average rate of 150 billion tons per year, while Greenland's glaciers were melting at about 270 billion tons per year, according to NASA.

Key Concepts

• alpine glaciers: large masses of ice found in valleys, on plateaus, and attached to mountains
• cryosphere: the portion of the Earth's surface that is composed of frozen water
• firn: the intermediary stage between snow and ice
• glacial ice: ice created by the compression of snow, sometimes saturated with meltwater that is refrozen
• ice caps: masses of ice covering areas smaller than 50,000 square kilometers
• ice sheets: masses of ice covering large landmasses

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