Outlook The form and rate of permafrost degradation will differ
between regions, depending on geographical location
Permafrost warming has not yet resulted in widespread and on specific environmental settings. On the Arctic
permafrost thawing on a landscape or regional scale. tundra, the ground temperatures are generally cold and
Long-term thawing of permafrost starts when the active no widespread permafrost thawing is expected during the
layer of soil above the permafrost, which thaws during 21st century, with the possible exception of the European
the summer, does not refreeze completely even during tundra where temperatures are closer to zero. However,
the most severe winter. Year-round decomposition of or- location of ground ice close to the surface makes the Arc-
ganic matter can then occur, and permafrost continues tic tundra surfaces extremely sensitive to thawing, as only
to thaw from the top down. Predicted further changes a small amount of thawing can lead to development of
in climate will eventually force high latitude natural sys- thermokarst. In contrast, in boreal forests ground ice is
tems to cross this very important threshold. typically located at a greater depth below the surface. Thus,
although warming of permafrost will soon lead to exten-
When permafrost starts to thaw from the top down, many sive permafrost thawing because of the relatively high
processes, some of them very destructive, can be triggered temperature of permafrost in boreal forests, the thawing
or intensified. These changes may impact ecosystems, in- will not immediately lead to destructive processes.
frastructure, hydrology and the carbon cycle, with the larg-
est impacts in areas where permafrost is rich in ground ice. Future changes in permafrost will be driven by changes
One of the most significant consequences of ice-rich per- in climate (primarily by air temperature and precipita-
mafrost degradation is the formation of thermokarst, land tion changes), changes in surface vegetation and chang-
forms in which parts of the ground surface have subsid- es in surface and subsurface hydrology. At present, there
ed
33
. Thermokarst forms when ground ice melts, the result- is no coupled climate model that takes into account all
ing water drains and the remaining soil collapses into the of these driving forces. However, by choosing a future
space previously occupied by ice. In addition to its impacts climate scenario and assuming certain changes in veg-
on ecosystems and infrastructure, thermokarst often leads etation and/or hydrology, it is possible to specify and ap-
to the formation of lakes and to surface erosion, both of ply an equivalent forcing to a permafrost model in order
which can significantly accelerate permafrost degradation. to project future permafrost dynamics on a regional or
Effects of thermokarst
Figure 7.4: Modelled permafrost temperatures (mean annual
on a railway track.
temperature at the permafrost surface) for the Northern Hemi-
Photo: US Geological Survey
sphere, derived by applying climatic conditions to a spatially
distributed permafrost model
34,35
.
(a) Present-day: temperatures averaged over the years 1980–
1999. Present-day climatic conditions were based on the CRU2
data set with 0.5° x 0.5° latitude/longitude resolution
36
.
(b) Future: projected changes in temperatures in comparison
with 1980–1999, averaged over the years 2080–2099. Future cli-
mate conditions were derived from the MIT 2D climate model
output for the 21st century
37
.
Source: Permafrost Laboratory of the Geophysical Institute, University of
Alaska Fairbanks
186 GLOBAL OUTLOOK FOR ICE AND SNOW
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