ICE

Ice, both on land and in the sea, affects the exchange of energy continuously taking place at the Earth's surface. Ice and snow are among the most reflective of naturally occurring Earth surfaces. In particular, sea ice is much more reflective than the surrounding ocean, so that if it were to increase in extent, for instance because of large-scale cooling, then more solar energy would be reflected back to space and less would be absorbed at the surface. This would tend to cool the local region further, with the likelihood that more ice would be formed and still more cooling would occur.



Distribution of Earth's Water

Over 75% all of the Earth's fresh water is contained in the ice sheets of Antarctica and Greenland.

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Ice Sheets


Antarctic

The Earth's polar ice sheets, which cover most of Greenland and Antarctica, contain about 77 percent of the world's fresh water. If these ice sheets were to melt, sea levels would rise worldwide and flood hundreds of coastal cities. Even small changes in global ice sheets can have serious effects on global sea levels and, therefore, coastal regions.

Temperature records in the Antarctic Peninsula indicate a warming trend over the past few decades. Some scientists are concerned that the West Antarctic ice sheet, which lies between the Antarctic Peninsula and the Transantarctic Mountains, may suddenly increase its discharge of ice into the oceans as a result of the warming. Such an increase could cause rapid increases in sea levels.

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Greenland

Here you can see the process of ice flow. As snow falls on the ice pack, the weight of the ice increases. Resistance from the bedrock causes the ice to push outward at the sides of the ice pack. Where ice sheets extend outward to the ocean, the ice tends to move out over the surrounding water, forming "ice shelves". As the diagram illustrates, ice also flows over land.

There is a concern that, with global warming, the water under the ice shelves would be warmer and cause them to break up more readily, forming very large icebergs. If the ice shelves of West Antarctica were to break up, this would release more inland ice in an irreversible process, perhaps leading to sea level rises of several meters.

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Polar Sea Ice


Global Sea Ice

Polar sea ice is one of the most variable features of the Earth's climate, changing considerably from summer to winter and from one year to another. At any given time, global sea ice covers an area approximately the size of the North American continent. The presence of the ice restricts the transfer of heat between the ocean and the atmosphere. The ice also restricts evaporation into the atmosphere and affects the circulation of the ocean.

Future plans include the use of data from the Special Sensor Microwave/Imager (SSM/I), currently on board Defense Meteorological Satellite Program (DMSP) satellites, and the Advanced Microwave Scanning Radiometer (AMSR), scheduled for flight on the EOS satellites.


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Arctic Sea Ice (Winter and Summer)

Figure 1 shows average sea ice concentrations in the north polar region for March 1994. This is near the yearly maximum sea ice coverage, calculated from data from the Special Sensor Microwave Imager (SSM/I), an instrument on board the F11 satellite of the Defense Meteorological Satellite Program (DMSP).

Figure 2 depicts the average sea ice concentrations in the north polar region for September 1994. This is near the yearly minimum sea ice coverage, as calculated from data from the DMSP SSM/I.

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Antarctic Sea Ice (Winter and Summer)


Figure 1 shows average sea ice concentrations in the south polar region for February 1994. This is near the yearly minimum sea ice coverage as calculated from data from the DMSP SSM/I.

Figure 2 depicts the average sea ice concentrations in the south polar region for September 1994. This is near the yearly maximum sea ice coverage, as calculated from data from the DMSP SSM/I.

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Global Snow Cover

The amount and depth of snow cover affect climate on regional and global scales. When snow is deposited on a land surface, it typically increases the degree to which the surface reflects incoming solar energy. In so doing, snow affects the Earth's radiation balance and, in turn, its climate. More information about the properties and effects of snow cover, will aid in the development of more accurate models to help us understand and predict the role of snow in the Earth's climate.

The Special Sensor Microwave / Imager (SSM/I) on board Defense Meteorological Satellite Program (DMSP) satellites continues to provide important information, as will the Advanced Microwave Scanning Radiometer (AMSR) and the Moderate-Resolution Imaging Spectroradiometer (MODIS) scheduled for flight on the EOS satellites.

In the image above (from February 1979) sea ice is indicated by white, ice sheets by gray, and snow by purple.

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Northern Hemisphere Snow Cover

The two images on the left are derived from data from the DMSP SSM/I in 1993. They show the difference between the amount of snow and ice in the Arctic region on January 3 and August 16. Gray is snow cover, light blue is sea ice, green is open land and dark blue is open sea.

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Snow Cover Example: Northern Montana

Landsat TM color-composite image of northern Montana, including Glacier National Park, March 14, 1991. Snow-covered areas are blue, clouds are white and vegetation is rust-colored.

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Glaciers

A glacier results from a multi-year surplus accumulation of snowfall in excess of snowmelt on land, creating a mass of ice at least 0.1 square kilometer in area that shows some evidence of movement in response to gravity. Glaciers may terminate in land or in water, and are classified as continental ice sheets, ice caps, ice fields, outlet glaciers, valley glaciers, mountain glaciers, glacierets, ice shelves, and rock glaciers. At present, glaciers cover about 10.7 percent of the land area and 3.1 percent of the Earth's surface, and are found on all continents except Australia.

Glaciers play a major role in the global hydrologic cycle, especially the volumetric transfer of water between glacier ice and the oceans. The approximate sea level rise potential for glaciers, including ice sheets, is 80.5 meters. During the last ice age, the sea level was 125 meters lower than today and the total land area was 37.1 percent of the Earth's surface, compared to today's land area of 29.1 percent. During the last glacial minimum, sea levels were 6 meters higher than today's and the Greenland ice sheet is believed to have been almost completely melted away.

NASA high-alititude, false-color infrared aerial photograph of the Alsek River and part of the terminus of the Alsek Glacier, south-east of Yakutat, Alaska.

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This is an oblique aerial photograph of Columbia Glacier, Alaska, in July 1976, before it began a catastrophic retreat, as predicted by the U.S. Geological Survey.

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Retreat of the Muir Glacier

Between September 12, 1973 and September 6, 1986, the Muir Glacier retreated to the northwest more than 7 kilometers. Other nearby glaciers have been shrinking over a period of several decades as well. Vegetation encroaches quickly as deglaciation occurs. This can be seen when comparing the amount of vegetation (shown in red) in the images.

Burroughs Glacier (B) is near the Muir Glacier (A), as you can see in the image on the left. By 1986, the Burroughs Glacier had become a melting ice field, having been cut off from its source of replenishment to the northwest because of glacier shrinkage.

Changes in glaciers on a global basis will be monitored using such instruments as the Enhanced Thematic Mapper Plus (ETM+), currently in flight on Landsat 7, and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), provided by Japan and currently in flight on the Terra satellite.

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