Arctic and Alpine Permafrost
Definition: Permafrost is a layer of permanently
frozen ground, that is, a layer in which the
temperature has been continuously below 0oC for
at least two years.
This means that moisture in the form of either
water or ice may or may not be present.
Permafrost may therefore be unfrozen, partially
frozen, or frozen depending on the state of the
ice/water content.
Seasonally frozen ground, or active layer, is
usually a layer above the permafrost that freezes
in winter and thaws in summer where depth of
thawing from the surface is usually less than a
metre or so in thickness.
Central to the operation of most cold-climate
processes are freezing and thawing of the ground
surface.
1
These may occur either diurnally, as in many
temperate and subtropical regions, or seasonally,
as in much of northern Canada.
The depth of frost penetration depends mainly on
the intensity of the cold, its duration, thermal and
physical properties of the soil and rock, and
overlying vegetation.
Where the depth of seasonal frost exceeds that of
thaw during the summer following, a zone of
frozen (i.e. temperature < 0oC) ground persists
throughout the year and is commonly referred to
as permafrost, or perennially cryotic ground.
All three conditions - diurnal frost, seasonal
frost, and permafrost - influence the nature and
extent of cold-climate processes.
The seasonal (i.e. annual) rhythm of ground
freezing and thawing dominates much of northern
Canada where long, cold winters are typical.
2
Usually, spring thaw occurs quickly and over
three-quarters of the soil thaws during the first
four to five weeks in which air temperatures are
above 0oC. Ground thermal regimes are closely
related to snow thickness and density.
Autumn freeze-back is equally complex - in
regions underlain by continuous permafrost,
freezing is two-sided, occurring both downward
from the surface and upward from the perennially
frozen ground beneath, and the freezing period is
much longer and may persist for 6 to 8 weeks.
During most of this period the soil remains in a
near-isothermal conditions as a result of the
release of latent heat on freezing that retards the
drop in temperature.
Permafrost is found in the Arctic and subarctic, in
high mountain ranges, and in ice-free regions of
Antarctica.
There is broad zonation of permafrost conditions
in Canada according to climate.
3
Zones of either continuous or discontinuous
permafrost are recognized, in addition to alpine
permafrost or subsea permafrost.
In total, approximately 50% of Canada's land
surface is underlain by permafrost of some sort.
The southern limit of the zone of continuous
permafrost correlates well with the approximate
position of the -6 to -8oC mean annual air
temperature isotherm, and this relates to the -5oC
isotherm of mean annual ground temperatures.
The discontinuous zone is further subdivided into
areas of widespread permafrost and scattered
permafrost; at its extreme southern fringes,
permafrost exists as isolated “islands” beneath
peat and other organic sediments.
In certain areas of the western Canadian Arctic
underlain by unconsolidated sediments, ground
ice may comprise at least 50% by volume of the
upper 1-5 m of permafrost.
4
Although many types of ground ice can be
recognized, pore ice, segregated ice, and wedge
ice are the most significant in terms of volume
and widespread occurrence.
There is a tendency to regard a frozen soil as one
in which the water has been replaced by ice; in
fact, at most temperatures of interest, frozen soils
contain ice and water.
Soil and rock do not automatically freeze at 0oC,
especially if percolating groundwater is highly
mineralized or under pressure.
As a result, significant quantities of unfrozen
porewater may continue to exist at temperatures
below zero.
The more fine-grained a soil is, the greater is the
amount of water remaining at a given
temperature.
5
As the water content is reduced by progressive
formation of ice, the remaining water is under an
increasing suction that develops by freezing.
Intimately associated with ground freezing are
the phenomena of frost heaving and ice
segregation, which take place wherever moisture
is present within the soil.
Frost heaving caused by ice segregation occurs
throughout much of Canada.
Annual ground displacements of several
centimeters are common, with cyclic differential
ground pressures of many kilopascals per square
centimeter.
Field studies in the Mackenzie Delta region
indicate that heave occurs not only during
autumn freeze-back, but also during winter when
ground temperatures are below 0oC.
Geomorphic evidence of frost heaving includes
upheaval of bedrock blocks, upfreezing of objects
6
and tilting of stones, and sorting and migration of
soil particles.
Engineering
hazards
caused
by
these
displacements and pressures, together with
adverse effects of accumulations of segregated
ice in freezing soil, are widespread and costly.
For instance, foundations for roads and pipelines
in permafrost regions require large quantities of
coarse grained materials to reduce the heaving
during winter.
There are 3 major considerations related to the
water/ice content of permafrost:
1) The freezing of water in the active layer at the
beginning of winter each year results in ice
lensing and ice segregation. The amount of heave
will vary according to the amount and availability
of moisture in the active layer, with poorly
drained silty soils showing the maximum heave
effects as unfrozen water progressively freezes.
This moisture migrates in response to a
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temperature gradient and causes an ice-rich zone
to form in the upper few metres of permafrost.
2) Ground ice is a major component of
permafrost, particularly in unconsolidated
sediments. If ground ice-rich permafrost thaws,
subsidence of the ground results. A range of
processes are associated with permafrost
degradation are summarized under the term
“thermokarst”.
3) The hydrological and groundwater conditions
of permafrost terrain are unique. Subsurface flow
is restricted to unfrozen zones called taliks and to
the active layer.
These are three groups of features whose
formation necessarily involve permafrost and
which therefore are diagnostic of permafrost
conditions: a) patterned ground, including ice
wedge polygons, stone polygons, sorted circles,
sorted stripes, and nonsorted circles; b) palsas,
and c) pingoes.
8
Permafrost terrain is generally regarded as highly
sensitive to thermal disturbance.
Mapping permafrost is not a straightforward
endeavour as remote sensing instruments are
capable of sensing freeze-thaw processes only
within the uppermost 5 cm of soil depth.
The spatial correlation length of permafrost
variability is linked to the surface vegetation and
soil type plus the volumetric water content of the
soil.
Most of the Canadian north is characterized by
permafrost soils at temperatures greater than -2oC
with frozen thicknesses less than 75 m.
Degradation
Degradation of permafrost often involves melting
of ground ice accompanied by local collapse and
subsidence of the ground.
9
These processes are termed thermokarst, a
physical (i.e. thermal) process peculiar to
permafrost regions.
Since thermokarst merely reflects a disruption in
the thermal equilibrium of the permafrost, a range
of conditions can initiate it, including changes in
regional climate, localized slope instability and
erosion, drainage alteration, and either natural
(i.e. fire) or human-induced disruptions to surface
vegetation cover.
In the boreal forest, fire frequently initiates
permafrost degradation and slope failure.
Along the western arctic coastal plain, where
alluvial sediments with high ice contents are
widespread, thermokarst is believed to be one of
the principal processes fashioning the landscape.
Elsewhere, large-scale thermokarst phenomena
include ground-ice slumps and thaw lakes.
10
Active layer
Between the upper surface of permafrost and the
ground surface lies the active layer, a zone that
thaws each summer and refreezes each autumn.
In thermal terms, it is the layer that fluctuates
above and below 0oC during the year. Its
thickness varies from as little as 15-30 cm in the
High Arctic to over 1.5 m in the Canadian
subarctic.
Thickness depends on many factors, including
ambient air temperatures, angle of slope and
orientation, vegetation cover, thickness (depth
and density) and duration of snow cover, soil and
rock type, and ground moisture conditions.
11
Permafrost and climate change
During the past few thousand years, Earth's
climate has been subject of fairly small changes
and world temperatures have fluctuated only
within a couple of degrees.
However, higher levels of carbon dioxide and
other greenhouse gases in the atmosphere may
progressively increase global temperature by as
much as 2 to 4oC over the next century.
In addition to temperature changes, the patterns
of precipitation would undoubtedly change annual totals would likely increase over the arctic
mainland, although current regional projections
are again quite variable between models.
Increase of 10 to 50% in summer and as much as
60% in winter may be anticipated for parts of the
Canadian Arctic.
Such large and rapid climatic changes would
have serious and far-reaching environmental and
12
socio-economic effects in permafrost regions and
for the arctic environment as a whole.
Some might look on the transition to a warmer
Arctic with happy anticipation; in the long term,
it would undoubtedly result in greatly reduced
costs of living and operating there.
New resources could become available, and
mining and agriculture, for example, might
expand; however the terrestrial environment of
the north, in which permafrost plays a major role,
would be profoundly disrupted during the
transition.
The mechanisms of energy exchange at Earth's
surface in cold regions are the same as those
elsewhere on the planet. Together they determine
the surface temperature regime and whether or
not frozen ground will exist.
Let us imagine some change in climatic
conditions which causes the mean annual surface
temperature to fall below 0oC, so that the depth of
13
winter freezing will exceed the depth of summer
thaw.
A layer of permafrost would grow downward
from the base of the seasonal frost, thickening
progressively with each succeeding winter.
Was it not for the effect of heat escaping from
Earth's interior (the geothermal heat flux), the
permafrost would grow to depths in response to
surface temperatures only slightly below 0oC.
However, this outward heat flow results in a
temperature increase of about 30 K km-1, the
figure varying with regional geological
conditions.
Thus the base of permafrost approaches an
equilibrium depth where the temperature increase
caused by this geothermal gradient just offsets
the amount by which the surface temperature is
below freezing.
14
Whereas the base of permafrost is determined by
the mean surface temperature and geothermal
heat flow, the upper layers of permafrost are
influenced more by seasonal and interannual
fluctuations of temperature and snowpack.
The major variation in surface temperature has a
period of one year, corresponding to the annual
cycle of solar radiation (there is also a diurnal
variation corresponding to the daily cycle of
radiation).
Temperature variations experienced with the
passage of the seasons at the surface extend in a
progressively dampened manner to a depth of
some 10-20 m.
Within the layer of annual variation, maximum
and minimum figures form an envelope about the
mean, and the top of permafrost is that depth
where the maximum annual temperature is 0oC.
15
Superimposed on normal periodic variations are
other fluctuations with durations from seconds to
years; causes may included sporadic cloudiness,
variations in weather and changes in climate.
Let us now imagine some change in climatic
conditions which causes mean annual surface
temperature to rise.
The result would be deepening of the active
layer, as both the mean annual temperature and
the
envelope
of
maximum
(summer)
temperatures shift to higher values.
If climatic warming were sustained, the
permafrost table would recede further year by
year and the base of the permafrost would begin
to rise as surface warming propagated to greater
depths.
If the progressive warming were great enough,
then permafrost could eventually disappear
altogether.
16
Since permafrost is a thermal condition, it is
potentially sensitive to changes in climate.
However changes in the thermal regime of the
ground that lead to degradation (or formation) of
permafrost can result from environmental
changes other than fluctuations in climate.
For example, removal, damage, or compaction of
surface vegetation, peat, and soil alters the
balance of surface energy transfers, generally
raising mean summer surface temperature and
thawing the upper layer of permafrost.
In winter, increases in snow cover accumulation,
as can result from barriers, structures, and
depressions or changes in wind patterns, can lead
to significant warming of the ground.
Decreases in snow cover, in contrast, lead to
cooling of the ground, other things being equal.
17
While the effects of surface environmental
changes are usually restricted in areal extent,
climatic change can affect extensive areas of
permafrost.
Even modest climatic warming could have drastic
effects for terrain conditions and northern
engineering, since thousands of square kilometers
of warm permafrost would be directly affected.
While many centuries would be required for
complete degradation of the affected permafrost,
thawing from the surface would begin
immediately, with many potentially serious
results.
There is some evidence that permafrost has been
retreating during the past decades: Syslov (1961)
reports that the permafrost extent at Mezen
(Russia) has retreated northward at an average
rate of 400 m per year since 1837, whereas
similar findings have been reported for the
Mackenzie Valley of Canada.
18
Although permafrost is temperature dependent,
the relation with climate is not straightforward,
since the surface temperature regime does not
depend solely on geographic location.
Local surface conditions such as the type of
vegetation, depth of snow cover, soil type, and
moisture content, profoundly affect the surface
energy regime, being interposed between the
atmosphere and the ground.
Thus myriad local variations of vegetation,
topography, and soil conditions can cause
differences in mean ground temperatures of
several degrees over quite small areas. Wherever
average temperature is within a few degrees of
0oC, such variation mean that permafrost occurs
in patches, or discontinuously.
These circumstances, together with the scattered
nature of direct observations, make precise
mapping of permafrost difficult.
19
While cold is usually seen as the singular feature
of high latitudes, problems resulting from thaw
are generally of greater practical concern.
Where permafrost contains ground ice,
considerable thaw settlement can occur and such
action has been responsible for significant
damage to buildings, roads, runways, etc. and
increased action would undoubtedly cause
additional and severe maintenance and repair
problems.
Special concern might be directed to existing
water-retaining structures, such as reservoirs, and
hydrodams, especially in areas of thaw-sensitive
permafrost.
Erosion of lake, river, and reservoir shorelines
may increase because of permafrost thawing and
a longer open-water season.
Greater sediment transport in rivers could shorten
the operating life of hydro-electric projects, for
example.
20
The expected rise in sea level accompanying
global warming could accelerate coastal retreat in
permafrost regions and combined with thaw
settlement as permafrost melts, could produce
inundation of low-lying areas.
Ground temperatures are strongly influenced by
conductive heat transfer, although localized
circulation of groundwater can occur, particularly
in areas of discontinuous permafrost.
Under steady-state conditions, the mean annual
ground temperature profile is linear with depth
(assuming constant thermal conductivity), and
temperature at any depth Tz is given by:
Tz = Ts + Gz
Where Ts is surface temperature and G is
geothermal gradient (increase in temperature with
depth within the ground).
21
In reality, heat conduction in the ground is more
complex: steady states are rarely achieved, since
surface temperature is continually changing, and
natural variations in soil conditions leads to
differences in thermal properties. In addition,
thermal properties of frozen soils vary with
temperature.
The thermal regime in the upper layers of the
ground is controlled by exchanges of heat and
moisture between the atmosphere and Earth's
surface.
The processes involved in the energy balance
comprise net exchange of radiation (Q*), between
surface and atmosphere, transfer of sensible (QH)
and latent heat (QE) by the turbulent motion of
the air, and conduction of heat into the ground
(QG).
Partitioning of the radiative surplus (or deficit)
among the heat fluxes is governed by the nature
of the surface and the relative abilities of the
22
ground and the atmosphere to transport heat
energy.
Each term affects surface temperature, and thus
the way in which the energy balance is achieved
establishes the surface temperature regime.
Snow profoundly affects the ground thermal
regime, since it presents a barrier to heat loss
from the ground to the air.
In the Mackenzie delta, where mean daily air
temperature is below -20oC for almost six months
in winter, the 1-m ground temperature beneath
120 cm of snow did not fall below -0.2oC.
In marginal areas of permafrost distribution,
snow cover alone may be the critical local factor
determining the presence of permafrost.
In the colder regions of more widespread
permafrost, it influences the depth of the active
layer.
23
Also, in regions of heavy snowfall, lake and river
ice will not be so thick, so that even bodies with
shallow water may not freeze through, as in the
Mackenzie delta where snow cover shapes local
distribution of permafrost.
A study by Goodrich (1982) shows that doubling
of snow cover from 25 to 50 cm increased
minimum ground surface temperature by about
7oC and mean annual surface temperature by
3.5oC.
If the 50 cm of snow accumulates within thirty
days in autumn, mean temperature would rise
above 0oC and permafrost would degrade.
Precipitation increases of as much as 60% in
autumn and early winter projected in some
climate models would therefore help accelerate
permafrost degradation, particularly in marginal
areas.
24
Ultimately there will be longterm changes in the
distribution of permafrost, as tends of thousands
of square kilometers of permafrost may
eventually disappear.
In some areas complete degradation could take
hundreds of years, because of the thermal inertia
of ice-rich materials and the considerable
thicknesses of permafrost affected.
In the short term there would be various rapidonset effects associated with progressive
deepening of the active layer.
Melting of shallow ground ice might lead to
widespread thermokarst and decreased slope
stability, creating in some cases severe
maintenance and repair problems for all manner
of structures.
Differential settlement would cause large internal
stresses in structures, producing distortions and
possibly even failure.
25