For the most part, an ice sheet moves down slope slowly because the ice is in direct contact with underlying bedrock. In some places, however, ice races along much faster than the rest of the sheet. These areas of fast-moving flow, called ice streams, are believed to be caused by a thin, lubricating layer of water and mud between the ice and the land. In this video segment adapted from NOVA, a team of scientists seeks evidence to support their hypothesis that atmospheric warming -- either now or in the past -- may explain why water has formed beneath the ice sheet.
Antarctica's extremely cold and dry climate preserves its immense, domed ice sheet as snow slowly collects atop the glacial ice. Because the weight of the snow and ice at the top of the dome is so great, gravity forces the ice to spread outward down slope. The rate of movement varies from less than a meter per year near the dome to several hundred meters per year near the edges. Eventually, the glacial ice runs into the sea, forming floating platforms called ice shelves.
The speed with which an ice sheet moves depends on a few variables, including the thickness of the ice and the pitch of the slope on which it lies. Speeds may also vary along different parts of an ice sheet. Ice streams are well-defined areas up to several kilometers wide where rapid ice flow occurs. They typically run from the dome of the sheet out onto the ice shelves and can travel at speeds of up to a kilometer per year, which is roughly 100 times faster than the surrounding ice.
Under normal conditions, an ice sheet is frozen to the bedrock, which significantly slows its movement down slope. Some glaciologists, however, believe that higher temperatures at the base of the ice produce a film of water and mud that separates the ice from the bedrock to form an ice stream. What might be causing this rise in temperature? Possible explanations include the immense pressure of the overlying ice and areas of volcanism under the ice sheet. A third potential heat source is atmospheric warming. Although recent human activities might be to blame, it could also be possible that the increased atmospheric heat following the end of the last ice age has only now penetrated to the base of the thick ice.
Recent studies suggest that Antarctic ice is flowing out to sea faster than it accumulates on land. This is especially the case on the West Antarctic Ice Sheet, much of which lies on land depressed below sea level. Because ice in direct contact with seawater is subject to faster warming, the extensive ice shelves of the West Sheet, where sea temperature has risen by 4.5°C in the past 50 years, could precipitate the collapse of the entire sheet. The amount of water contained in the West Sheet could raise the global sea level by 6 meters.
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