Polar Ice Fact Sheet
 

Polar ice consists of sea ice formed from the freezing of sea water, and ice sheets and glaciers formed from the accumulation and compaction of falling snow. Both types of ice extend over vast areas of the polar regions. Global sea-ice coverage averages approximately 25 million km2, the area of the North American continent, whereas ice sheets and glaciers cover approximately 15 million km2, roughly 10% of the Earth's land surface area.

Composition of Polar Ice

Effects on Energy Exchange
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.

On the other hand, if global warming occurs, then more ice would be expected to melt, reducing the energy reflected back to space and increasing the energy absorbed at the surface. The affected portions of the Earth would become still warmer. Scientists refer to this kind of reinforcing process as a "positive feedback."

next: Global Warming & Land Ice

 

by Steve Graham
April 24, 1999

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Polar Ice
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Global Warming & Land Ice


  Global Warming and Land Ice
Over the past century, sea level has slowly been rising. This is in part due to the addition of water to the oceans through either the melting of or the "calving" off of icebergs from the world's land ice. Many individual mountain glaciers and ice caps are known to have been retreating, contributing to the rising sea levels. It is uncertain, however, whether the world's two major ice sheets-Greenland and Antarctica-have been growing or diminishing. This is of particular importance because of the huge size of these ice sheets, with their great potential for changing sea level. Together, Greenland and Antarctica contain about 75% of the world's fresh water, enough to raise sea level by over 75 meters, if all the ice were returned to the oceans. Measurements of ice elevations are now being made by satellite radar altimeters for a portion of the polar ice sheets, and in the future they will be made by a laser altimeter as part of NASA's Earth Observing System (EOS). The laser altimeter will provide more accurate measurements over a wider area.

The Greenland ice sheet is warmer than the Antarctic ice sheet and as a result, global warming could produce serious melting on Greenland while having less effect in the Antarctic. In the Antarctic, temperatures are far enough below freezing that even with some global warming, temperatures could remain sufficiently cold to prevent extensive surface melting.

Where ice sheets extend outward to the ocean, the ice tends to move out over the surrounding water, forming "ice shelves." There is 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, possibly leading to sea level rises of several meters.

Ice Flow Movement
In addition to increasing the amount of melting, global warming would also be expected to increase the amount of precipitation in the polar regions. There are three reasons for this: 1) warmer air can carry more moisture than colder air; 2) warmer wates would encourage increased evaporation from the ocean; and 3) lessened sea ice would also lead to more evaporation from the ocean, as more ocean area would be exposed directly to the atmosphere. Global warming could therefore be expected initially to increase both melting and snowfall. Depending on which increase dominates, the early result could be either an overall decay or an overall growth of the ice sheets.

Global Warming Detection and Sea Ice
The melting and growth of sea ice, in contrast to land ice, does not affect sea level, because the sea ice is floating on the ocean already and is in equilibrium with it. Sea ice is nonetheless still important in the context of climate change. Sea ice, with its high reflectance and the insulation it provides between the polar atmospheres and oceans, is a key part of the climate system. Furthermore, sea ice responds to changes in the atmosphere and oceans, and hence changes in it could be a clue to broader climate change, such as global warming. However, the record to date is not clear enough to make any definitive conclusions about long-term climate trends based on the sea-ice observations alone. Sea ice varies significantly from season to season and from year to year, and the extent of its natural variability is not yet fully known.

We need to continue to monitor the location and extent of sea ice and its changes seasonally and interannually. We also need additional studies to determine ice thicknesses and reflectivities. This kind of information can be fed into climate models to attempt to simulate future climate conditions. The same information will also serve as a check on models to see if they are properly simulating existing sea-ice amounts and distributions.

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