The cryosphere includes all forms of frozen water on the Earth's land or sea
surfaces, as well as perennially frozen ground (permafrost).
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 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.
Nearly 24 percent of the
Earth's exposed surface 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.
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 cannot be
underestimated.
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.
