cross-section of permafrost shows an ice wedge hiding just below the surface
U.S. Fish and Wildlife Service
Permafrost is not defined by soil
moisture content, overlying snow cover, or location; it's defined solely by
temperature. Any rock or soil remaining at or below 0°C for two or more years
is permafrost. Permafrost can contain over 30 percent ice, or practically no ice
at all. It can be overlain by several meters of snow, or little or no snow.
Understanding permafrost is not only important to civil engineering and
architecture, it's also a crucial part of studying global change and protecting
the environment in cold regions.
What are the types of
frozen ground? Frozen ground is can be either seasonally frozen ground or
permafrost. Seasonally frozen ground freezes in the winter and thaws in the
summer. More than half of the land in the Northern Hemisphere has some
seasonally frozen ground.
Permafrost is a type of
frozen ground that stays at or below 0° Celsius (32° Fahrenheit) for at
least two years. Permafrost does not have to contain water or ice. As long
as the temperature of the ground stays below freezing, it is still
considered frozen ground, even if it is completely dry. If permafrost
begins to warm significantly, it thaws.
These two types of frozen
ground can occur separately, or together. A layer of ground that freezes
and thaws every year may sit on top of permafrost. This is called the
active layer. The active layer is seasonally frozen ground, and is not
part of the permafrost. Permafrost begins where the seasonally frozen
Continuous permafrost exists under almost the entire land surface in an
area. Areas with continuous permafrost often have permafrost layers more
than 100 meters (330 feet) thick. The deepest permafrost ever found is in
Siberia, a region in northern Russia. One area in Siberia has a permafrost
layer that extends down 1,650 meters (5,413 feet).
Discontinuous permafrost exists under a large portion of a particular area
or only in a few specific places. Alpine permafrost is discontinuous
permafrost that exists on the tops of mountains where the ground stays
very cold. In areas with discontinuous permafrost, the permafrost layer
may extend as deep as ten meters (thirty-three feet) underground. Taliks
are sections of unfrozen ground within permafrost.
can be isolated or sporadic. It is called isolated if less than ten
percent of the surface has permafrost under it. Sporadic means ten to
fifty percent of the surface has permafrost under it.
Cold permafrost -
Remains below 30° F, and which may be as low as 10° F as on the North Slope;
tolerates introduction of considerable heat without thawing.
Ice-rich - 20% to 50%
Thaw-stable - Permafrost
in bedrock, in well drained, coarse-grained sediments such as glacial outwash
gravel, and in many sand and gravel mixtures. Subsidence or settlement when
thawed is minor, foundation remains essentially sound.
Thaw-unstable - Poorly
drained, fine grained soils, especially silts and clays. Such soils generally
contain large amounts of ice. The result of thawing can be loss of strength,
excessive settlement and soil containing so much moisture that it flows.
Warm permafrost -
Remains just below 32° F. The addition of very little additional heat may
Most permafrost in the Northern
Hemisphere occurs between latitudes of 60°N and 68°N. (North of 67°N,
permafrost declines sharply, as the exposed land surface gives way to the Arctic
Ocean.) There is also a significant amount of permafrost around 35°N, in the
Qinghai-Xizang (Tibet) Plateau, and in the mountains of southwest Asia and the
U.S. Rocky Mountains.
This map shows the
location of different permafrost types in Alaska. Darker shades of blue indicate
larger percentages of permanently frozen ground. Lighter blues, and the terms
isolated and sporadic, refer to lower percentages of frozen ground. White
indicates areas where permafrost formed during the last ice age, in areas where
conditions would not allow it to form today. This permafrost is called relict
Most frozen ground
in the United States is seasonally frozen ground. The shaded areas on the map,
between the dotted lines, mark the average depth that the soil freezes to in the
winter. The actual depth that the ground freezes to in any particular place may
vary due to local climate, soil type, elevation, and terrain. - Credit: Neil
Proximity to large water
bodies tends to reduce temperature extremes, which affects the distribution of
permafrost. Scandinavia and Iceland, for instance, have relatively little
permafrost . About 37 percent of Northern Hemisphere permafrost occurs in
western North America, mainly in Alaska and northern Canada between 165°W and
60°W. Most permafrost occurs in the Eastern Hemisphere, mainly in Siberia and
the Far East of Russia, northern Mongolia, northeastern China, the
Qinghai-Xizang (Tibet) Plateau, and surrounding mountains between 60°E and
of permafrost and ground ice in the Northern Hemisphere, based on the EASE-Grid
version of the IPA map. "High," "Med" and "Low"
refer to ice content, and "T" and "t" refer to thick and
thin overburden, respectively. National
Snow and Ice Data Center
Despite its name, permafrost is
characterized by its instability. It is often covered by an active layer that
regularly melts. Although permafrost can be thousands of years old, it is
sometimes newly formed or about to melt, and it often exists close to its
melting point . As permafrost thaws, it jeopardizes both man-made structures and
natural features. Thawing permafrost on mountain slopes can lead to landslides .
Approximately 55 percent of the Northern Hemisphere's land surface is covered by
seasonally frozen ground, which can last for a few weeks in the middle and lower
latitudes, and for several months at high latitudes and high elevations .
illustration of the range in temperatures experienced at different depths in the
ground during the year. The active layer (shown in grey) thaws each summer and
freezes each winter, while the permafrost layer remains below 0°C.
Much of the Northern Hemisphere
frozen ground is overlain by evergreen boreal forest. These boreal forests
comprise both a source and a sink of carbon. In fact, the Arctic contains nearly
one-third of the Earth's stored soil carbon. If the high northern latitudes were
to have a significant temperature increase, the regional soils would begin to
release carbon into the atmosphere, which could lead to increased plant growth,
carbon aspiration, and possibly a temperature drop or stabilization.
Alternately, it could lead to higher temperatures, fueling the cycle of carbon
release and temperature rise .
Frozen ground's widespread
distribution makes it a substantial component of the cryosphere. Likewise, its
role in the storage and release of carbon make it a major factor in future
General circulation models
predict that, for a doubling of atmospheric concentrations of carbon dioxide due
to anthropogenic sources, mean annual air temperatures may rise up to several
degrees over much of the Arctic. In the discontinuous permafrost region, where
ground temperatures are within 1-2 degrees of melting, permafrost will likely
ultimately disappear as a result of ground thermal changes associated with
global climate warming. Where ground ice contents are high, this permafrost
degradation will have associated physical impacts. Of greatest concern are soils
with the potential for instability upon thaw (thaw settlement, creep or slope
failure). Such instabilities may have implications for the landscape,
ecosystems, and infrastructure.
Frozen ground is worth watching closely because its fate is tied
to our own.
feels the heat of global warming Russia Today Video
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