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Snow
Glaciers
Permafrost
Sea Ice
Cryospheric Trends
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Snow
Seasonal snow cover, the largest component of the cryosphere, covers up to
33 percent of the Earth's total land surface. About 98 percent of the total
seasonal snow cover is located in the Northern Hemisphere. Although snowdrifts
and avalanches often pose hazards to humans, snow also provides much of
the world's water. For example, snowfall accounts for 60 to 70 percent
of annual precipitation in the U.S. Sierra Nevada and Rocky Mountains,
and is later released as water during spring snow melt and river runoff.
Glaciers
Glaciers and ice sheets cover about 10 percent of the Earth's land area. Glaciers,
large, thickened masses of ice, accumulate from snowfall over long periods
of time. When these ice masses reach a critical thickness, they begin to
move, or flow. A body of ice that covers a large area of land and flows outward
in all directions is called an ice cap or ice sheet. Ice caps form in high
mountain summit and plateau regions. Two ice sheets exist on Earth now, one
in Greenland and one in Antarctica. All continents except Australia bear
ice in the form of mountain glaciers, ice sheets, or ice caps. Today, glaciers
and ice sheets store about 75 percent of the world's freshwater.
Permafrost
Nearly 24 percent of the exposed land of the Northern Hemisphere contains permafrost.
Permafrost prevails over tundra regions and in the Siberian boreal forest.
Extensive regions also undergo seasonal freezing and thawing. Because the
melting of frozen ground produces unstable surfaces, understanding permafrost
is important to civil engineering and architecture in cold regions. Freezing
and thawing processes also have a significant impact on ecosystem diversity
and productivity.
Sea
Ice
Floating ice includes sea ice and frozen lake and river water. Sea ice typically
covers 14 to 16 million square kilometers of the Arctic Ocean, and 17 to 20
million square kilometers of the Southern Ocean around Antarctica, during their
respective winter seasons. The seasonal cycle of sea ice extent influences
both human activities and biological habitats. Many polar mammals, such as
penguins, polar bears, and seals, depend entirely on sea ice for habitat. Shipping
companies must time navigation activities to coincide with periods of low sea
ice concentration.
Since snow and sea ice can influence global climate, and glaciers and
ice sheets directly affect sea level, the role of the cryosphere within
the global climate system should not be underestimated.
Cryospheric
Trends
Because much of the cryosphere occurs in generally remote locations, taking
measurements in the field can be both difficult and dangerous. Fortunately,
the last several decades have seen the development of increasingly sophisticated
satellite technology that has enabled researchers to monitor the cryosphere
on a routine basis. In recent years, scientists have discovered striking trends.
Satellite data indicate that during the past 30 years, annual snow cover in
the Northern Hemisphere and Arctic sea ice extent have decreased at a rate
of about 3 percent per decade.
Are these changes part of a natural cycle of climate variability? Or
are they a result of human influences on the climate system? These questions
underscore the critical need for more cryospheric data and research.
For more information, see the State of the Cryosphere.
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