glaciology IS THE study of the formation and movement of glaciers and their response to climate change. It included studying the past positions of glacial ice, monitoring changes in present-day ice, and forecasting the future behavior of the world's glaciers and ice sheets. Because glaciers respond directly to changes in air temperature, they are one of the key indicators of climate change on the planet. Understanding the behavior of glaciers is important to understanding the effects of climate change. Although climate change is a central focus of glaciology, this discipline also incorporates aspects of geophysics, geology, climatology, hydrology, and geomorphology.
glaciers as tools to reconstruct the past
The landscapes left behind by retreating glaciers can be valuable tools for glaciologists to reconstruct the past positions and characteristics of glacial ice. End moraines, for example, are formed when sediment that is eroded and transported by glacial ice is deposited at the front of the glacier in piles of rock and soil. These moraines usually mark the maximum extent of the glacial terminus, or snout. Often, glacial valleys contain numerous sets of end moraines that correspond to either the last Ice Age or smaller, more recent advances, such as the Little Ice Age in that ended in the mid-1800s. Wood that is buried in these moraines can be carbon dated to help scientists determine the age of these landforms.
Glaciers also retain evidence of past climates within layers of accumulated ice. Because snow in the accumulation zone is progressively buried, glacial ice is older toward the bed of the glacier. Glaciologists drill cores through glaciers and ice sheets to sample ice of different ages. They measure the ratios of the isotopes of oxygen and hydrogen in the ice to gain information about past atmospheric temperatures. They also extract ancient air that is trapped in small bubbles deep within the ice. This can tell scientists about concentrations of greenhouse gases, such as carbon dioxide, in past climates. The levels of methane also reveal information about the level of biological productivity in the past, while traces of sulfur are indications of volcanic activity. If the time of the eruptions is known, these layers of volcanic material can be used to help date ice cores.
The oldest ice exists deep within polar ice sheets and many ice cores have been drilled in Antarctica and Greenland. The Vostok ice core from central Antarctica, for example, dates back more than half a million years and has provided a wealth of information about past atmospheric conditions. Ice cores from alpine glaciers in the world's major mountain ranges are not as old because this ice tends to flow more quickly and is much shallower than ice in polar ice sheets. These cores however, have provided glaciolo-gists with a wealth of information about past climate change at lower latitudes (closer to the Equator). This information is invaluable because it provides proxy climate records for many centuries prior to the first recorded measurements of atmospheric conditions.
studying present-day ice
Glaciers are often referred to as global barometers because they respond rapidly to changes in air temperature. They are one of the most visible indicators of climate change on the planet and, thus, glaciolo-gists can learn a lot about the world's climate changes by monitoring the way that land-based ice is growing or shrinking.
The most common way glaciologists monitor the health of a glacier is to measure the mass balance, or water budget. The mass balance of a glacier is found by measuring the rates of ablation (melting) compared to accumulation. The difference between the accumulation and ablation for a given year describes the annual net mass balance, which corresponds to the change in glacier volume. During cooler periods, the rate of accumulation usually exceeds the rate of ablation, and during warm intervals, the reverse occurs. Glaciers respond to these changing temperatures to try to attain a neutral mass balance, where accumulation equals ablation. They do this by retreating and thinning in warmer climates and advancing and thickening in colder climates. This ongoing fluctuation in glacial ice volume has a profound effect on the amount of liquid water that is available to the hydro-logical cycle. For this reason, mass balance measurements are a key tool for forecasting the water supply in mountainous regions.
Ground-penetrating radar is another commonly used technique to study glaciers. Glaciologists drag the radar over the ice surface to get a picture of the shape of the topography under the glacier. Because conditions under the ice play a huge role in the flow rate and behavior of glaciers, this technique is very useful for learning about the characteristics of the ice-bedrock boundary. Scientists often measure the speed of ice flow by placing markers (such as vertical stakes) in the ice and using a global positioning system (GPS) to track their movement.
Glaciologists have developed other tools to monitor ice over very large areas, such as the Greenland and Antarctic Ice Sheets. These include the use of satellite data to measure the changing surface area of the world's ice, and laser altimetry, which is used to measure changing surface elevations. Together, these tools can be used to remotely assess the mass balance of major ice sheets. While they will not completely replace ground-based observations, these techniques are effective because they allow a large amount of data to be collected in a short time. They also cost a fraction of the amount that is required to conduct field-based measurements in remote areas.
response of glaciers to global WARMING
Glaciologists use their knowledge of glacial dynamics to understand how the planet's ice is responding to climate change, and to predict how this response may change in the future. Because Antarctica and Greenland house a large proportion of the world's ice, these regions are a key focus for glaciological research. These glacial systems are the largest on the planet and, while they respond more slowly to changes in atmospheric temperature than alpine glaciers, the sheer volume of ice that they contain means that they could play a central role in future sea level rise. Glaciologi-cal research in polar regions focuses on large outlet glaciers that drain ice from the interior of the giant ice sheets to the coast. Scientists monitoring these glaciers have found that the flow rate has increased over the past few decades in areas such as the Siple Coast in Antarctica. This has contributed to the loss of ice sheet mass, and the collapse of ice shelves and floating glacier tongues. It has also led to concern that these glaciers may play a major role in any future collapse of the entire Antarctic ice sheet.
Although polar regions dominate the Earth's cryo-sphere (snow and ice), there is also a significant amount of ice stored at lower latitudes, closer to the Equator. These alpine glaciers are concentrated in the major mountain belts of the world, such as the Andes, the
Rocky Mountains, the Himalayas, and the European Alps. Because these systems are smaller than those in polar regions, they respond more quickly to changes in the climate. For this reason, glaciologists also monitor these glacial systems closely and have been making mass balance measurements for more than 100 years in alpine regions. With only a few exceptions, glaciolo-gists have found that alpine glaciers in both the northern and southern hemispheres are experiencing widespread retreat, and some have disappeared completely.
Glaciologists also use their knowledge of the physical laws that control the behavior of glaciers, along with past observations, to try to predict how glaciers will respond to different global warming scenarios. This is usually done using physical models, which incorporate natural laws. These models cannot fully represent all of the processes that occur in glacial environments, but they aim to produce a good approximation of glacial behavior. The models are tested to see if they can mimic the behavior of present day glaciers before they are used to predict future changes.
SEE ALSo: Antarctic Ice Sheets; Climatic Data, Ice Observations; Glaciers, Retreating; Greenland Ice Sheet; Ice Ages; Little Ice Age; Vostok Core.
BIBLioGRAPHY. D.I. Benn and D.J.A. Evans, Glaciers and Glaciation (Wiley, 1998); Michael Hambrey and Jurg Alean, Glaciers (Cambridge University Press, 2004); Intergovernmental Panel on Climate Change, Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, 2007); Johannes Oerlemans, Glaciers and Climate Change (Balkema, 2001).
Kate E. Sinclair University of Calgary
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