Geoscience Reference
In-Depth Information
increasing coverage of shrubs, a process known as shrubbification (Mynemi et al.,
1997
; Sturm, Racine, and Tape,
2001
; Zhou et al.,
2001
; Nemani et al.,
2003
; Tape
et al.,
2006
; Bhatt et al.,
2010
).
2.2.3
Frozen Ground
Perennially frozen ground, known as permafrost, underlies nearly all of the Arctic
land area. Permafrost is said to be present whenever ground temperatures are below
freezing through two summer seasons. Ground ice need not be present, although in
sediments ice may be present either in segregated form throughout, or as lenses and
wedges (French,
1996
). The upper part of the ground in permafrost regions, termed
the active layer, thaws seasonally. The active layer depth depends on air tempera-
ture, vegetation cover, soil material, moisture content, and the amount of insulation
provided by winter snow cover. In the Low Arctic the average active layer depth
varies from 80-90 cm in the south to 50-60 cm in the north. Active layer depths
are greater on heated slopes and in coarse-textured soils, and least in peat bogs
(Chernov and Matveyeva,
1997
).
In acting as an impermeable barrier, the presence of permafrost plays a major
role in the hydrology of the Arctic. Following snowmelt, the ground becomes water-
logged unless there is sufficient topographic relief for drainage channels to form.
Hence there are numerous shallow lakes. Permafrost is generally absent beneath
large deep lakes, especially in the Sub-Arctic. As outlined in
Chapter 6
, in areas
with sufficient drainage, the impermeable permafrost barrier fosters rapid channel-
ing of precipitation and meltwater into streams, which impacts strongly on the sea-
sonality of river discharge and evaporation rates.
Zones in which permafrost may be present account for 24 percent of the exposed
Northern Hemisphere land surface (Zhang et al.,
1999
). However, the actual area
underlain by permafrost is smaller if we take account of the fractional coverage of
different permafrost zones (continuous = 90 to 100 percent; discontinuous = 50 to
90 percent; sporadic = 10 to 50 percent; isolated = less than 10 percent of the ground
underlain by permafrost). On this basis, 12.8-17.8 percent of the exposed land sur-
face is underlain by permafrost (Zhang et al.,
2001
). The area mapped as continuous
permafrost represents 47 percent of the entire permafrost region of the Northern
Hemisphere. In addition, there is frozen ground below much of the Greenland Ice
Sheet and the sub-polar ice caps.
The Northern Hemisphere permafrost distribution, shown in simplified form
in
Figure 2.15
, was compiled by the International Permafrost Association (IPA)
(Brown et al.,
1997
). Permafrost is spatially continuous where the mean annual SAT
is below approximately −7°C, and is discontinuous for an SAT of −1°C to −3°C.
The permafrost thins toward the southern borders, near the −1°C SAT isotherm
(Ives,
1974
). In the zone of continuous permafrost, the thickness exceeds 600 m in
northeastern Siberia and in northern Canada and Alaska. Extreme depths of 1,200-
1,470 m are found in the Anabar Plateau of north-central Siberia (72-73
o
N, 113
o
E)
(Tumel,
2002
). This is attributed to ice-free conditions through the Quaternary, low
