Frozen Ground
Permafrost extent
within the exposed
Summary
land areas of the
Arctic and Antarctic
Permafrost: perennially frozen ground – rock, sediment or
Permafrost temperatures have increased during the last
any other earth material with a temperature that remains be-
20–30 years in almost all areas of the Northern Hemi-
low 0°C for two or more years.
sphere. An increase in the depth of the active layer above
the permafrost, which thaws in the summer, is less cer-
Permafrost (Northern Hemisphere):
tain. Further increases in air temperatures predicted
Area Covered (million square km) 22.8
Ice Volume (million cubic km) 4.5
for the 21st century are projected to initiate widespread
Potential Sea Level Rise (cm) ~7
permafrost thawing in the subarctic and in mountain
regions in both hemispheres. Widespread thawing of
Source: IPCC 2007
1a
permafrost will speed up the decomposition of organic
material previously held frozen in permafrost, emitting There are two permafrost zones: continuous permafrost
large amounts of greenhouse gases into the atmosphere. and discontinuous permafrost (Figure 7.1). In the contin-
Thawing of ice-rich permafrost may also have serious uous permafrost zone, permafrost lies beneath the entire
consequences for ecosystems and infrastructure, and in surface except beneath large rivers and deep lakes. Most
mountain regions, may reduce the stability of slopes and continuous permafrost formed during or before the last
increase the danger of rock falls and landslides. glacial period. In the discontinuous permafrost zone, per-
mafrost lies beneath 10 to 90 per cent of the surface. Most
discontinuous permafrost is much younger and formed
Introduction to permafrost within the last several thousand years. Permafrost ranges
from very cold (–10° C and lower) and very thick (from
Permafrost zones occupy up to 24 per cent of the ex- 500 to 1400 metres) in the Arctic, to warm (one or two
posed land area of the Northern Hemisphere
1
(Figure degrees below the melting point) and thin (from several
7.1). Permafrost is also common within the vast conti- metres or less to 150 metres) in the subarctic.
nental shelves of the Arctic Ocean. This subsea perma-
frost formed during the last glacial period when global The main feature that distinguishes permafrost from un-
sea levels were more than 100 m lower than at present frozen ground is the presence of ground ice. The amount
and the shelves were exposed to very harsh climate con- of ground ice in permafrost varies from a few tenths of a
ditions. Subsea permafrost is slowly thawing at many per cent to 80 or 90 per cent of the total permafrost vol-
locations. Permafrost of various temperatures and con- ume. The mechanical strength of frozen soil with ice in
tinuity also exists in mountainous areas, due to the cold it is close to the strength of bedrock, while the strength of
climate at high elevations. Permafrost exists throughout unfrozen soil is much lower. The stability of ecosystems in
ice-free areas of the Antarctic, as well as underneath permafrost regions depends on the stability of the ground
some areas of the Antarctic Ice Sheet
2
. ice; loss of permafrost means a loss of system stability.
182 GLOBAL OUTLOOK FOR ICE AND SNOW