| | |  | Landsat satellite images showing retreating glaciers in the Andes of Peru, South America | |
Background
The mass balance of a glacier is the difference between the mass of ice accumulated (e.g. from snowfall) and the mass of ice lost (e.g. through melting or iceberg calving) over the course of one year.
During the seasons, glaciers shrink at the terminus from calving and ice melt in the summer, whilst in the winter solid precipitation accumulates and provides supply to the snow and ice that did not melt during the summer. If more snow and ice accumulate over one year in the accumulation area than melt in the ablation area, the glacier will have a positive mass balance and will grow. If the ablated mass exceeds the accumulated mass, the glacier will have a negative mass balance and shrink. Changes in the mass balance of a glacier are expressed as changes in length, volume, and surface area. The volume of a glacier that has adjusted to the current climate (= in balance) will not change significantly over the years.
Model of a glacier with accumulation area, ablation area, terminal lake and moraine
On a much longer time scale, changes in the orbit of the Earth around the Sun can have an influence on the Earth’s climate and glaciations, leading to ice ages.
The movement of the Earth in relation to its eccentricity, tilt, and precession is known as the Milankovitch cycle. When the Milankovitch cycle favours a cooler Earth, substantial accumulation on ice surfaces worldwide enhances the cooler climate. This feedback is called the albedo effect.
When solar energy hits a bright surface, such as snow or ice, up to 90% of this energy is reflected back to the atmosphere. In contrast, darker land areas absorb a much higher percentage of solar energy and warm up. So if the solar energy is mainly reflected on ice and snow due to its high albedo, the Earth surface will be cooler than for conditions with less snow and ice, eventually leading to a colder climate.
This global scale albedo effect on climate change is mainly caused by the Greenland and Antarctic ice sheets, as well as sea ice. The albedo of smaller glaciers only affects local conditions.
The last glacial maximum was seen about 18,000 years ago. The growth of ice sheets and glaciers in the north of Europe and North America caused the sea level to lower by about 120 metres.
Presently, in most regions of the world the ablated mass of glaciers exceeds the accumulated mass. This is largely due to the increasing atmospheric temperatures. These increasing temperatures, in turn, are to a large extent caused by the increased atmospheric greenhouse effect, caused by greenhouse gas emissions from industrial countries. Consequently, most glaciers on Earth are shrinking.
The Steigletscher (1994) in the very east of the Berne canton in Switzerland
The Steigletscher (2004) in the very east of the Berne canton in Switzerland
By comparing the 1994 and 2004 images of the Steigletscher, you can see the result of unbalanced accumulation and ablation, which has led to glacier retreat. (See GoogleEarth file location 3.12)
Search the internet for information on shrinking glaciers. Write down the region and continent they belong to, and compare your findings to those of your classmates.
Impressive photographs of shrinking glaciers can be found at:
http://www.swisseduc.ch/glaciers/big_melt/index-en.html, and
http://www.gletscherarchiv.de/en/fotovergleiche
Study the GoogleEarth file found in the right menu, and look for signs of glacier retreat, such as terminus lakes or glacier tongues that are significantly smaller than the area they once covered (this is seen in the delineation of their moraines).
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