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Although our planet is characterized by water, the extent to which water in its
frozen state plays a role in the functioning of the Earth is perhaps underappreciated.
Snow and ice cover about a sixth of the Earth’s surface, as snow
lying on the ground, as glaciers and larger masses of terrestrial ice, including
the huge ice sheets of Antarctica and Greenland, as sea ice, river and lake ice,
and as icebergs. Together with the subsurface ice of permafrost, these forms
of ice constitute the Earth’s cryosphere. The cryosphere plays a significant
function in controlling the global climate system, and has an obvious role as an
indicator of change in the system, but aspects of it are important at all spatial
scales, to a wide range of topics that includes ecology, hydrology, economics,
transport, and recreation.
Most of the Earth’s snow and ice is located in remote places where access
is difficult, climate is extreme, and the polar night complicates the task of
collecting information. For these reasons, and because of the enormous areas
represented by snow and ice, the observational techniques of remote sensing
from the air and especially from satellites are particularly useful as a means of
gathering data about the cryosphere. These techniques offer the possibility of
collecting large amounts of information in a short time. Rates of viewing the
Earth’s surface can be as high as 100,000 km2 per second for some spaceborne
systems, easily capable of giving a synoptic view of large parts of the Earth’s
surface, although the most useful systems have superficial rates typically a
factor of 10 or 100 less than this. Some remote sensing systems can view the
Earth’s surface through cloud cover; some are unaffected by the absence of
daylight and so are able to function during the polar night. Virtually all remote
sensing systems now produce digital data, which are suitable for storage and
analysis by computer. These virtues were recognized early by researchers
concerned with snow and ice. Since the emergence of spaceborne technologies
for monitoring the cryosphere (around 1970), new methods have been
developed, the number of sensors that are used simultaneously has increased,
and progressively longer runs of self-consistent data have become available. |
