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Ice Shelf: definition.
An ice shelf is the floating extension of the ice sheets that have formed on land
from thousands or millions of years of snowfall. It is therefore made up entirely
of fresh water, in contrast to sea ice, which still
has some salt in it from the ocean. Most of the world’s ice is located in
Antarctica and Greenland. The ice sheet slowly flows downhill due to gravity,
usually as glaciers which can also become ice streams.
When the ice sheet reaches the coast, it starts to float when the depth of the
floating ice is less than the water depth.
Who cares? There have been many occasions in the last decade when large
icebergs have calved (broken off) from the front of Antarctic ice shelves. In
some cases the ice shelves have rapidly and completely disintegrated (see http://nsidc.org/iceshelves/).
Some of these events are expected, since iceberg calving is something that happens
regularly (every 10-100 years or so) on most shelves. However, some events are
clearly tied to changing regional climate, like significant warming of the region
around the Antarctic Peninsula. When ice shelves melt or break up, they do not
raise sea level (try floating an ice block in a glass of water: the level of water
in the glass does not change once the ice melts). However, what scientists worry
about is that the ice shelves “buttress” the flow of ice off the continent.
Recent cases where ice shelves have disappeared indicate that the flow of glaciers
down to the ocean speeds up (see http://nsidc.org/news/press/20031219_speed_up.html).
Thus, melting ice shelves might lead to the rapid flux of ice from Antarctica
and Greenland into the ocean. Losing even a fraction of Antarctica’s ice
to the ocean could raise global sea level by several feet, flooding much of the
US East Coast, areas of Europe and the UK, and many other densely inhabited regions.