See also

Air–Sea Interaction: Momentum, Heat and Vapor Flux-es; Sea Surface Temperature. Boundary Layers: Com-plex Terrain; Convective Boundary Layer; NeutrallyStratified Boundary Layer; Stably Stratified Boundary Lay-er; Surface Layer. Energy Balance Model, Surface.Land–Atmosphere Interactions: Overview. Numeri-cal Models: Methods.

Further Reading

Beljaars ACM (1995) The impact of some aspects of theboundary layer scheme in the ECMWF model. ECMWFseminar proceedings,Parameterization of Sub-Grid ScalePhysical Processes, September 1994, Reading.

Brutsaert WA (1982) Evaporation into the Atmosphere.Dordrecht: Reidel.

Garratt JR (1992) The Atmospheric Boundary Layer.Cambridge University Press.

Holtslag AAM and Duynkerke PG (eds) (1998) Clear andCloudy Boundary Layers. Royal Netherlands Academyof Arts and Sciences, PO Box 1921, Amsterdam.

Lock AP (1998) The parametrization of entrainment incloudy boundary layers. Quarterly Journal of the RoyalMeteorological Society 124: 2729–2753.

Mellor GL and Yamada T (1982) Developmentof a turbulence closure model for geophysical fluidproblems. Review of Geophysics and Space Physics 20:851–875.

Smith RNB (1990) A scheme for predicting layer clouds andtheir water content in a general circulation model.Quarterly Journal of the Royal Meteorological Society116: 435–460.

Stull RB (1988) An Introduction to Boundary LayerMeteorology. Dordrecht: Kluwer.

European Centre for Medium-Range Weather Forecasting(1993) Workshop proceedings, Parameterization of theCloud Topped Boundary Layer. Shinfield Park, Reading:ECMWF.

PERMAFROST

TE Osterkamp, University of Alaska, Fairbanks, AK,USA

C R Burn, Carleton University, Ottawa, ON, Canada

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Introduction

Permafrost, ground that remains at or below 01Cfor two or more years, underlies about a fifthof the land surface of the Earth. Permafrost terrainconsists of an active layer at the surface that freezesand thaws each year, underlain by perennially frozenground. The top of permafrost is at the base of theactive layer, and the base of permafrost occurs wherethe ground temperature rises above 01C at depth(Figure 1).

The first scientific reports on permafrost werepublished in the 1830s by the Royal GeographicalSociety of London. These papers reported the thick-ness of frozen ground in awell at Yakutsk, Russia, andprovided instructions to officers of the Hudson’s BayCompany on describing the phenomenon. The firstsystematic study published in English of perenniallyfrozen ground was prompted by strategic considera-tions in World War II, when the Alaska Highway wasbuilt through northernBritishColumbia andYukon toAlaska, with the associated Canol Pipeline. At thattime, the term ‘permafrost’ was coined by S.W.Mulleras a contraction of ‘permanent frost’.

Top of permafrost

Minimumtemperature

Maximumtemperature

Depth of zeroannual amplitude

0Tps

Temperature

Dep

th

Active layer

Geothermalgradient

Base of ice-bearingpermafrost

Base of permafrost

Freezing-point-depression

Figure 1 Schematic temperature profiles in permafrost, illustrat-

ing the annual maximum and minimum temperatures. Annual

mean permafrost surface temperature, Tps, is obtained by

extrapolating the common linear portion of the profiles upwards.

Soil particleeffects, solutes, andhydrostatic pressuredecrease the

phase equilibrium temperature so that a layer just above the base

of the permafrost does not contain ice. The change in slope of the

temperature profile at the base of ice-bearing permafrost is caused

by the difference in thermal conductivities between the frozen and

unfrozen ground.

PERMAFROST 1717

Permafrost grows by freezing from its base down-ward or, when new material is added to the groundsurface, from the top upwards, or by a combination ofthese processes. The first permafrost on Earth likelyformed prior to or during the first ice age about 2.3billion years ago, and its occurrence, distribution, andthicknesses have varied in response to repeated ice agesthroughout Earth’s history.

There is scientific and geotechnical interestin permafrost principally because it contains ice,is close to its thawing point, and is sensitive tochanges in surface conditions caused by humanactivities and climate. Temperatures in permafrostpresent retrievable records of past climate, andclimatic change commonly leaves an imprint on thestratigraphy of ground ice. Ice-rich permafrost con-tains ice in excess of the water content at saturation,with the ice masses commonly distributed as lenses,millimeters to centimeters in thickness, within theground.Massive ground ice with dimensions typicallyfrommeters to tens ofmeters also occurs in permafrost(Figure 2).

Occurrence, Distribution, andThickness

The spatial extent of permafrost generally changeswith climate, but there can be considerable regionalvariation because of snow cover and other factors.Over half of Canada and Russia, most of Alaska, andnorth-east China are underlain by continental perma-frost, while alpine permafrost is found at highelevations in middle and low latitudes (e.g., thesummit of Mauna Kea in Hawaii). Some permafrostis found in Scandinavia, but the spatial extent is muchless than at corresponding latitudes ofNorthAmerica,because of the warming influence of the Gulf Stream.Permafrost containing water ice and other ices isknown to exist on Mars and on other bodies in oursolar system.

Permafrost regions are divided into zones withvarying spatial extent of perennially frozen ground. Inthe continuous permafrost zone (Figure 3), more than90%of the ground is underlain by permafrost, and it isusually absent only beneath rivers and lakes that do

Figure 2 Massive ground ice exposed at the Beaufort Sea coast near Tuktoyaktuk, Northwest Territories. The ice was formed during

permafrost aggradation after deglaciation, with water supplied from the underlying sands. Banding indicates variations in the

concentration of sediment suspended in the ice. Undulation of the banding indicates displacement of the ice subsequent to formation.

(Photograph by J. R. Mackay. See Mackay JR and Dallimore SR (1992) Massive ice of the Tuktoyaktuk area, western Arctic coast,

Canada. Canadian Journal of Earth Sciences 29: 1235–1249.)

1718 PERMAFROST

not freeze to their bottom in winter. At continentalscale, the limiting annual mean air isotherm forcontinuous permafrost is about �81C. At warmertemperatures, variations due to microclimatic effectslead to a zone ofwidespread discontinuous permafrostwhere 50 to 90% of the ground is underlain bypermafrost. In the sporadic discontinuous permafrostzone, 10 to 50% of the ground is underlain bypermafrost.

Subsea permafrost formed during glacial periods,when lower sea levels exposed large areas of the polarcontinental shelves. The cold air temperatures andlong periods of exposure allowed permafrost to growthere to great depths. Thicknesses of several hundredsof meters are known to exist in these shelves and, atpresent, this permafrost is slowly degrading beneaththe relatively warm and salty ocean. However, severaltens of thousands of years are required to thaw itcompletely.

The distribution and thickness of permafrost arecontrolled by factors that influence its heat balanceand heat flow within it. Since pore spaces inpermafrost are generally blocked by ice, heat flow isby conduction.While the presence of permafrost is dueprimarily to climate, considerable modification of thetemperature between the atmosphere and permafrost

may occur, owing to vegetation, energy exchanges atthe snow–ground surface, transfer of heat through theactive layer, and local geological and hydrologicalconditions. In particular, the winter snow coverbuffers the ground from frigid air temperatures, andthe annual mean ground surface temperature, Ts, iscommonly 2 to 41C warmer than the annual mean airtemperature, Ta. In the summer months, shade fromvegetation and a supply of soil moisture for evapora-tion are two significant site variables that reduce theground surface temperature. Ta and Ts tend to besimilar at windswept sites where there is little snowaccumulation. Significant changes in ground tempera-tures occur across the continental tree line, for withinthe forest there is less wind at ground level than on thetundra, and hence the snow is deeper, less dense, and abetter insulator.

Anorganic horizon at the ground surface commonlyassists the development and persistence of permafrost.Dry moss and peat have low thermal conductivities,reducing heat flow into the ground in summer.However, autumn rain characteristically increasesthe water content of the moss and peat, increasingtheir conductivity (particularly when frozen) andfacilitating extraction of heat from the groundin winter. These principles indicate why, near the

ExplanationZone of sub-seapermafrost

Zone ofcontinuouspermafrost

Zone of discontinuouspermafrost

Zone of alpine permafrost

20°30°

40°

30°

30°100°30°

30°

40°

50°180°

140°

30°

40°

120°

20°

140° 60°

TL. PÉWÉ 1981

Figure 3 Generalized map of the approximate distribution of permafrost in the north circumpolar region of the Earth. Direct data

(probing, drilling, sampling, temperature measurements) are scarce, so the map is somewhat unreliable, especially at local scales in

discontinuous permafrost. Subsea permafrost typically exists in the continental shelves of the Arctic Ocean where seabed temperatures

remain negative. (Adapted fromPewe TL (1983) Alpine permafrost in the contiguousUnited States: a review.Arctic and Alpine Research

15: 145–156, by permission of the Regents of the University of Colorado).

PERMAFROST 1719

southerly limit for continental permafrost in NorthAmerica, permafrost generally occurs in peat landsand under thick moss layers.

At regional scale, permafrost distribution may becontrolled by topography, in terms both of aspect,modifying the receipt of radiation, and of coldconditions brought by high elevation. However, inthe mountain and plateau regions of Alaska andadjacent Canada, strengthening of winter inversionsby cold-air drainage into valley bottoms enhancesconditions for permafrost there.

The thickness of permafrost is commonly influencedby geothermal heat flow and bedrock stratigraphy. Atcontinental scale, heat flow varies by a factor of fourbetween stable craton, where the flow is low, andtectonically active terranes. This geothermal heat isconducted through the permafrost where the tem-perature gradient varies with the thermal conductivityof the bedrock. At local and regional scales, themovement of groundwater carries heat that canmodify the spatial distribution and thickness ofpermafrost. Under equilibrium conditions where aconstant annual mean permafrost surface tempera-ture, Tps, has existed for a long time, the thickness ofhomogeneous permafrost, Ze, is governed by Tps, thethermal conductivity of permafrost, Kp, and thegeothermal heat flow, J, where (from Fourier’s law)

Ze � Kp

JTps ½1�

Equation [1] provides reasonable estimates ofZe whenTps remains near its long-term value or has been closeto it for a sufficient period. While Ta is the principalvariable governing Tps, the values differ owing to theeffects of snowcover, the active layer, andother factorsmentioned above.

Properties and Processes

Unfrozen Water and Ice

Seasonally frozen ground and permafrost containunfrozen water (Figure 4) and ice in equilibrium attemperatures less than 01C as a result of the effects ofsoil particles and solutes. In the absence of solutes,temperature, T, and the soil’s specific surface area, S,are the primary determinants of the amount ofunfrozen water, yu, and are empirically related by

ln yu ¼ 0:2618þ 0:5519 ln S

� 1:449S�0:264 ln jTj ½2�where yu is in percent. Unfrozen water reduces groundthermal conductivity and distributes latent heat over arange of temperatures, so that temperature changes

require freezing or thawing throughout the frozenground. These effects retard the thermal response ofthe active layer and permafrost.

Frost Heave

Unfrozen water is also responsible for the develop-ment of ice lenses in the ground during freezing,leading to local uplift (heave) of the surface. Thewateroccurs in mobile films on particle surfaces, where it isheld in tension. Water flows along the tension andthermal gradients in the films to cooler regions of theground. When the water content in the freezing soilexceeds saturation, the excess volume separates soilparticles to form layers, or lenses, of segregated ice. Icesegregation and frost heave characteristically occur infine-grained soils, where the unfrozen water content issufficiently large to conduct water into the freezingground. In coarse-grained soil, the unfrozen watercontent is small and the permeability is too low toallow water migration during freezing. In saturated,coarse-grained soil, the expansion of water duringfreezing is accommodated by expelling the excesswater into unfrozen ground ahead of the freezing

−4 −2 0

Sand

Clay

Silt

0.0

0.1

0.2

0.3

Unf

roze

n w

ater

con

tent

(vo

lum

e fr

actio

n)

0.4

0.5

Unfrozen water content

Temperature (°C)

Figure 4 Representative values for the temperature depen-

dence of unfrozen water contents in sand, silt, and clay. Unfrozen

water contents typically increase with temperature and finer-

grained soil and are small in moss and peat. The presence of

solutes increases unfrozen water contents.

1720 PERMAFROST

front. When the freezing system is closed, pore-waterexpulsion may also lead to heave as hydrostaticpressure deforms the overlying frozen ground.

Active Layer

Permafrost is separated from the atmosphere by aboundary layer consisting of the active layer andvegetation in summer with the snow cover added inwinter. The active layer transmits heat to and frompermafrost, reduces the amplitude of thermal varia-tions at the top of permafrost compared with theground surface, is the medium through which moist-ure and gases are exchanged between the permafrostand the atmosphere, and provides water and nutrientsfor biological processes. In permafrost terrain, theactive layer supports plant and animal communitiessince virtually all biological activity below groundoccurs within it.

Permafrost immediately below the active layer ischaracteristically ice-rich. This ice-rich zone acts as animpermeable barrier to drainage, so much permafrostterrain is wet. The ice-rich zone is the reasonpermafrost terrain is considered sensitive to distur-bance, for deepening of the active layer commonlyleads to subsidence (thaw settlement) and acceleratederosion as the ice melts.

In a dry active layer,Ts � Tps; however, in awet one,there is an asymmetry in heat flow because the frozenthermal conductivity, Kf, exceeds Kt, the thawedconductivity. This makes Ts warmer than Tps and thedifference is the thermal offset (Figure 5)

Tps � Ts ¼ ItP

Kt

Kf� 1

� �½3�

where It is the thawing index at the ground surface andP is the period (365 days).

Once the snow melts, the ground surface warmsabove 01C and thawing of the active layer begins(Figure 6). In a simplified model, its maximumthickness is

X �ffiffiffiffiffiffiffiffiffiffiffi2KtIth

r½4�

where h is the volumetric latent heat of the ground,which depends on the ice content. The thickest activelayers (1 to 2m or more) develop in dry materials,characteristically bedrock, sand, and gravel. Thinactive layers are common in wet organic soils, wherethere may be considerable amounts of ice at thebeginning of summer, and where the thermal con-ductivity of a dry surface layer is very low.The thinnestactive layers, less than 30 cm thick, occur in the HighArctic, owing to short, cool summers.

The active layer typically reaches X and beginsfreezing upward from the bottom when Ts >0�C, oneor two weeks before freezing downward from thesurface. Temperature changes during freezing areretarded by the presence of unfrozen water for a fewweeks in cold permafrost and formuch of thewinter inwarm permafrost. The rate of freezing down from thesurface depends on the amount and timing of the earlywinter snow cover and on the thermal conductivityand moisture content of the active layer. Freezing maybe slow if a deep snow cover is established early inwinter on a wet active layer. Temperatures in theportion of the active layer that remain thawed areconstrained near 01C, a phenomenon termed the ‘zerocurtain’, until freezing is complete.

Thermal Regime

A temperature profile in permafrost that is homo-geneous, does not contain unfrozen water, and is in

1.0

0.8

0.6

0.4

0.2

0.0

Dep

th (

m)

−1.0 −0.5 0.0 0.5 1.0 1.5 2.0 2.5

Mean temperature (°C)

Hogan Hillannual meantemperature

profiles

86/8787/8888/8989/9090/9191/92

Figure 5 Annual mean temperature profiles in the active layer

showing the effects of thermal offset. In the context of climatic

change, thermal offset allows permafrost to survive changes that

produce annual mean ground surface temperatures warmer than

01C over multiyear periods as shown at this site. (Adapted with

permission from Osterkamp TE and Romanovsky VE (1999)

Evidence for warming and thawing of discontinuous permafrost in

Alaska. Permafrost and Periglacial Processes 10: 17–37.)

PERMAFROST 1721

equilibriumwith a constant long-termTps, is a straightline described by Fourier’s law (seasonal variationsnear the surface and freezing point depression near thebase cause deviations from a straight line). However,the thermal regime of permafrost is often character-istically different during its formation and growth,after surface temperatures change, and during thaw-ing. Prior to permafrost formation, interannualvariability in Ts and in conditions within the season-ally frozen layer can cause the depth of freezing toexceed the depth of thawing during the followingsummer, resulting in a temporary layer of frozenground. Ts can be significantly positive when thisoccurs, owing to thermal offset. A long-term shift inconditions allows such a layer to persist and grow,creating a thickening layer of permafrost with time.When the permafrost freezes from its base downward,the solution of the Stefan problem yields the approx-imate depth to the bottom of the growing permafrostat time t:

Z �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2Kpð�TpsÞt

h

r½5�

In eqn [5], Kp and h depend on ground properties.Equation [5] neglects geothermal heat flow and theeffects of freezing point depression near the base ofpermafrost (Figure 1). It reduces toZ � constant n

ffiffit

p,

where the constant is typically about 1 to 5 for a widerange of conditions,where t ismeasured in years andZin meters.

In principle, permafrost grows untilZ ¼ Ze, when itis in equilibrium with Tps. However, in reality, Tps isvariable over the long time scales needed to grow evenrelatively thin permafrost, resulting in permafrostthicknesses that vary with long-term variations in Tps

(Figure 7).A change in Tps (warming or cooling) produces a

curvature in the temperature profile that penetratesdeeper with time (Figure 8). The magnitude of thechange at any depth can be obtained from themeasurements and the timing can be calculated fromthe maximum depth of penetration of the thermalsignal. For permafrost with thicknessZ, the time scalerequired for the temperature profile to respond to anew surface condition is

t � Z2

4D½6�

where D is the thermal diffusivity of the permafrost.Freezing or thawing at the bottom of the permafrostbegins once the thermal disturbance has penetratedthere, with calculated rates in thick continuouspermafrost that are typically millimeters per year(Figure 7).

Franklin Bluffs

Ground surface10

−10

−20

0

Active layerbegins to freezefrom top downwards

Active layerbegins to thaw

Tem

pera

ture

(°C

)

10

−10

−20

0

Tem

pera

ture

(°C

)

Permafrostbeginsto cool

9/1/87 11/1/87 1/1/88 3/1/88 5/1/88 7/1/88 9/1/88Date

Permafrost surface

Figure 6 Time series of temperatures at the ground and permafrost surfaces for the annual cycle. Freezing of the active layer from the

top downward beganabout 26September 1987, and from the bottomupward about ten days earlier. The zero curtain persisted until about

12 November 1987, the date of freeze-up of the active layer. At this time, the lower portion of the active layer and upper portion of the

permafrost began to cool. Ground surface temperatures during spring, 1988, remained near 01C from 4 to 11 June (the period of

snowmelt), when the active layer began to thaw.

1722 PERMAFROST

When Tps � 0�C, interannual variability in Ts andconditions within the active layer may cause the depthof summer thaw to exceed the depth ofwinter freezing,resulting in a temporary residual thaw layer betweenseasonally frozen ground and permafrost. Ts can besignificantly positive when this occurs, and interann-ual variabilitymayallow the layer to refreeze and thawrepeatedly. If the conditions that caused the warmingpersist, the layer may become too thick to refreeze.Then, the permafrost is decoupled from the atmo-sphere and warms continuously throughout the year.An equation similar to [5] can be obtained for thethickness of the thawed layer above permafrost.Thawing rates at the top of the permafrost dependon the temperatures at the bottom of the former activelayer, Kt, and h and can be a few tenths of a meter peryear for near-surface permafrost. While the perma-frost is thawing, temperatures within it warm veryslowly as they approach 01C (Figure 9) because of theeffects of unfrozen water.

Geomorphic Features

There are three principal geomorphic features uniqueto permafrost terrain: pingos (conical ice-cored hills)and ice-wedge polygons, both associated with perma-frost aggradation; and thermokarst terrain, associatedwith ground thawing. Pingos may form in theunfrozen sandy sediments of drained-lake bottoms

that are completely surrounded by permafrost. Afterdrainage, lake sediments freeze primarily from the topdownward and from the sides and bottom of the talik(the unfrozen layer) inward. Freezing of the sandsresults in pore-water expulsion into the enclosed talik,increasing hydrostatic pressure there. The increasedpressure lifts the permafrost, and the water freezes inplace, to create a core of ice in the mound (Figure 10).Pingos grow until the talik freezes completely. Thereare 1350 pingos in Canada’s western Arctic, and thelargest, Ibyuk Pingo, is 50mhigh, over 1200 years old,and is growing about 2 cm taller annually. Pingos arealso found at the base of hill slopes, where the pressureto lift permafrost may be provided by confinedgroundwater flowing down slope.

Ice-wedge polygons are ubiquitous in continuouspermafrost terrain, where cracks occur in the perma-frost as a result of thermal contraction in winter. Thecracks relieve thermal stress approximately normal totheir axes, yielding a polygonal network that maysubdivide further as more cracks form. Ice wedgesbegin to grow in the cracks when spring snowmeltinfiltrates themand freezes to formnear-vertical sheetsof ice. Repetition of the process over many years leadsto development of wedges of ice, which commonlyextend downwards from the base of the active layer forabout 4m (Figure 11), although some exceed 10m indepth. Polygons are clearly expressed in lowlands, butmovement of the active layer commonly obscures

Finite element model

640

590

540

690

Per

maf

rost

thic

knes

s (m

)

0 50 000 100 000 150 000 200 000 300 000250 000

Time before present (yr)

Figure 7 Calculated permafrost thickness variations in response to changes in paleoclimate. Thicknesses varied about 84m (562

m � Z � 646m) with maximum thawing rates of 5mm yr� 1 and freezing rates of 2 to 3mm yr� 1. (Adapted with permission from

Osterkamp TE and Gosink JP (1991) Variations in permafrost thickness in response to changes in paleoclimate. Journal of Geophysical

Research 96: 4423–4434. Copyrightr by the American Geophysical Union.)

PERMAFROST 1723

them on hill slopes. Ice-wedge growth deforms thesurrounding ground to accommodate the additionalvolume. Growth of the ice wedges often forces theadjacent ground upwards and laterally, creating atrough above the ice wedge.

Warming or disturbance to permafrost terrainusually leads to deepening of the active layer andthawing of the ice-rich zone at the top of permafrost orof near-surface massive ground ice, causing localsubsidence. This thaw settlement produces a pittedrelief, called thermokarst terrain. Drainage conditionsdetermine whether standing water will be present ornot. When depressions in thermokarst terrain collectwater, the disturbance to permafrost is enhanced,leading to growth of thermokarst lakes. ‘Beaded’streams occur when a series of pools (beads) formalong the stream as a result of thawing ice-richpermafrost or large ice masses. Most features ofthermokarst terrain range from 1 to 100m in lateraldimensions, although thermokarst lakes are oftenlarger. Thermokarst depressions up to 100 km2 in area

and 5 to 20m in depth form part of the landscape ofSiberia, where they are called ‘alasses’.

Thermokarst terrain also includes retrogressivethaw slumps (Figure 12) that commonly developwhere ice-rich permafrost is exposed by erosion inriverbanks, lakeshores, and along the coast, or byother processes. These features, with a near-verticalretreating headwall and a low-angled foot slope, arethe commonest form of landslide in permafrostterrain. Landslides involving only the thawing activelayer also occur, particularly in areas of fine-grained,ice-rich soil, when the active layer and vegetationdetach from the underlying frozen material. If ice-richpermafrost or massive ground ice is exposed, the slopefailure may develop into a thaw slump.

Impacts of Climatic Warming

The Earth’s climate has generally warmed since themid-to late 1800s. In the Arctic and Sub-Arctic,

Profileat t = τ

Depth of penetration

Initialprofile

Warming

TemperatureD

epth

Figure 8 Schematic thermal response of permafrost to warming

of its surface temperature. Seasonal variations near the surface

are not shown. The magnitude of the warming at any depth can be

obtained directly from the measurements and the timing of the

warming can be calculated from the depth of penetration of the

warming signal. (Adapted with permission from Lachenbruch AH,

SassJH,MarshallBV, andMosesTH (1982)Permafrost, heat flow,

and the geothermal regime at Prudhoe Bay, Alaska. Journal of

Geophysical Research 87: 9301–9316. Copyright r by the

American Geophysical Union.)

1981

1990

24

22

20

12

10

8

6

18

16

14

Dep

th (

m)

−0.2 −0.1 0.0 0.1 0.2

Temperature (°C)

Figure 9 Temperature profiles in thawing permafrost near

Fairbanks, Alaska obtained 9 years apart. The presence of

unfrozen water retards temperature changes and produces the

observed curvature below 8m depth. (Adapted with permission

from Osterkamp TE and Romanovsky VE (1999) Evidence for

warming and thawing of discontinuous permafrost in Alaska.

Permafrost and Periglacial Processes 10: 17–37.)

1724 PERMAFROST

permafrost has also warmed, particularly in Russia,China, Mongolia, Alaska, and western Canada. InAlaska, Tps for continuous permafrost warmed 2 to41C over the last century followed by a cooling in theearly 1980s and then a warming of up to 31C sincethen. In central Alaska and the Yukon Territory ofCanada, discontinuous permafrost has warmed since1970 as a result of changes in air temperatures andsnow cover. While it has warmed typically 1 to 21Csince the late 1980s in Alaska, the ground has beencooling in the Yukon owing to a reduction in snowcover. In the Northwest Territories, warming ofpermafrost by about 11C has been observed since theearly 1970s, but in northern Quebec there has beencooling. Such regional variations are to be expected asa result of spatial climatic variability. However, thegeneral signal from the Northern Hemisphere indi-cates permafrost warming, and in some areas perma-frost is presently thawing at both top and bottom, thesouthern boundary of the permafrost is movingnorthward, and the occurrence of thermokarst terrainis increasing.

Current global circulation models predict that airtemperatures in the Arctic will rise 2 to 51C during thenext half-century, with the warming in winter larger

than in summer and with more precipitation through-out the year.While there is considerable uncertainty inthese predictions, warming of Tps by a few degreeswould have serious consequences for permafrostregions, especially for discontinuous and sporadicdiscontinuous permafrost.

In the continuous zone, the active layer, thaw lakes,coastal processes, landscape processes, eolian activity,and vegetation would be sensitive to climatic warm-ing. The effect on the permafrost would be to warm itand, possibly, to change the depth of the active layer.Thawing at the base of the permafrost would startseveral centuries or more later.

Since most of the discontinuous and sporadicdiscontinuous permafrost is within a few degrees ofthawing, it can be expected to warm, to begin thawingfrom the top and bottom with an increase in theincidence of thermokarst terrain, and, eventually, todisappear. Although many centuries would berequired for the permafrost to disappear, thawing ofthe warmest permafrost from the top would beginimmediately.

Thawing of permafrost as a consequence of humanactivities and as a result of the climatic warming sincethe late 1800s serves as a model for what may be

Figure 10 Pingo near Wolf Lake, Richards Island, western Arctic coast, Canada, about 20m high, with the ice core exposed after

collapse of the central portion. The vertical extent of exposed ice in the far headwall is about 4m. (Photograph by C. R. Burn. SeeMackay

JR (1998) Pingo growth and collapse, Tuktoyaktuk Peninsula area, western Arctic coast, Canada: a long-term field study. Geographie

physique et Quaternaire 52: 271–323.)

PERMAFROST 1725

expected to occur with additional climatic warming.Where permafrost contains massive ground ice or isice-rich, extensive differential thaw settlement hasoccurred, with damage to the natural terrain and toinfrastructure. Human-induced thaw settlement is atpresent responsible for damage to infrastructureon permafrost where the built structures haveraised ground temperatures above 01C (Figure 13).The magnitude of the thaw settlement is typically 1 to3m, but can exceed 5m (vertical settling) of theground surface. Repair of the infrastructure is costly

and some structures, airports, and roads have beenabandoned.

Carbon and Trace Gases

Soils in northern regions may contain one-quarter toone-third of the Earth’s total soil carbon pool, withmuch of it stored in frozen peat in near-surfacepermafrost. When permafrost thaws, the carbonis cycled through terrestrial ecosystems with thegaseous end products (CO2 and CH4) emitted to

Figure 11 Ice wedge exposed in a coastal bluff near Tuktoyaktuk, Northwest Territories, Canada, about 4m tall. The photograph was

taken inMay 1992, when thewinter’s thermal contraction crack was visible. (Photograph byC. R. Burn. SeeMackay JR (2000) Thermally

induced movements in ice-wedge polygons, western Arctic coast: a long-term study. Geographie physique et Quaternaire 54: 41–68.)

1726 PERMAFROST

the atmosphere. While the details of this process arenot well understood and are subject to environmentalconstraints, there is evidence that tundra regions mayhave shifted from being a carbon sink to a carbonsource.

Gas Hydrates

Temperature and pressure conditions within andunder thick permafrost are favorable for the formationand existence of gas hydrates that are a potentiallyabundant source of energy.Warmingof the permafrostwould eventually destabilize these hydrates, produ-cing large quantities of gas (primarily CH4) that mayfind its way into the atmosphere. Long time scales(many centuries or millennia) are required for thisprocess. However, subsea permafrost in the continen-tal shelves of theArcticOcean thatwas submerged andwarmed by seawater more than a few thousand yearsago could be emitting methane at present. Currently,there is too little information to adequately assess thisproblem.

Ecosystems

In areas of ice-rich permafrost, thaw settlement anddevelopment of thermokarst terrain destroys thesubstrate on which the current ecosystems rest,dramatically changing the nature of the ecosystems.The effects have been observed to include:

1. Destruction of trees and reduction in area of borealforest ecosystems.

2. Expansion of thaw lakes, grasslands, andwetlands.3. Destruction of habitat for caribou and terrestrial

birds and mammals.4. Formation of new habitat for aquatic birds and

mammals.5. Coastal and riverbank erosion.6. Clogging of salmon spawning streams with sedi-

ment and debris.7. Slope instabilities, thaw slumps, landslides, and

erosion.8. Talik development, with increasing depth to water

table.9. Increased methane emissions in wet areas.

Figure 12 Headwall of a retrogressive thaw slump in ice-rich glaciolacustrine sediments near Mayo, Yukon Territory, Canada. Lenses

of segregated ice give the exposure its texture. The ice lenses developedwhenpermafrost formed in glacial lake sediments after drainage

of the lake at the endof the last glaciation. Adeepactive layer,which formedduring thewarmest climate of the last 10 000 years, left a thaw

unconformity in the sedimentsmarkedby theabrupt transitionbetween thedarker, lower, ice-richmaterial, and the lighter sediment above.

(Photograph byC.R. Burn. SeeBurnCR (2000) The thermal regime of a retrogressive thaw slumpnearMayo, YukonTerritory.Canadian

Journal of Earth Sciences 37: 967–981.)

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Thermokarst has been observed to result in the partialor complete destruction of some ecosystems and theirconversion to other types of ecosystems. In onelowland area in central Alaska, permafrost degrada-tion is widespread and rapid, causing large shiftsin ecosystems from birch forests to fens and bogs(Figure 14). If current conditions persist, the remain-ing birch forests will be eliminated by the end of thetwenty-first century.

Nomenclature

D thermal diffusivity (m2 s� 1)h Volumetric latent heat of the sediments (J m�3).It Thawing index at the ground surface (usually

stated in 1C days).J Geothermal heat flow, negative for a positive

temperature gradient, (W m� 2).

Kp Thermal conductivity of permafrost, (W m� 1

K� 1).Kf Thermal conductivity of the frozen active layer,

(W m� 1 K� 1).Kt Thermal conductivity of the thawed active layer,

(W m� 1 K� 1).P Period of the annual temperature wave, (365

days).S Soil specific surface area, (m2 g� 1).t Time, (s).T Ground temperature, (1C).Ts Annual mean temperature at the ground surface,

(1C).Tps Annual mean permafrost surface temperature,

(1C).X Maximum thickness of the active layer, (m).Z Thickness of permafrost at time, t, (m).Ze Equilibrium thickness of permafrost, (m).

Figure 13 Longitudinal cracks in a road embankment near Fairbanks, Alaska. Snow removal during winter produces a berm along the

shoulder and slope of the embankment that warms the underlying ice-rich permafrost, causing it to thaw and settle. Settlement under the

shoulder and slope causes the edge of the embankment to tilt outward, putting the top surface of the embankment in tension, which

eventually results in cracks. Patches (darker asphalt) in the pavement are a result of two long cracks and the guardrail on the far side of the

road has sagged as a result of the thaw settlement. (Photograph by T. E. Osterkamp.)

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yu Unfrozen water content, (kg of water per kg ofdry soil, sometimes stated in percent).

t Time scale for a permafrost temperature profileto respond to a new surface temperature,(usually stated in years).

See also

Arctic Climate. Biogeochemical Cycles: CarbonCycle. Cold Air Damming. Energy Balance Model,Surface.Global Change: Biospheric Impacts and Feed-backs; Human Impact of Climate Change; Surface Tem-perature Trends. Hydrology: Soil Moisture. Ice Ages(Milankovitch Theory). Methane. Reflectance andAlbedo, Surface. Satellite Remote Sensing: AerosolMeasurements; Precipitation; Temperature Soundings.Snow (Surface).

Further Reading

Andersland OB and Ladanyi B (1994) An Introduction toFrozen Ground Engineering. New York: Chapman &Hall.

Brown RJE (1970) Permafrost in Canada: Its Influence onNorthern Development. Toronto: University of TorontoPress.

GoldLWandLachenbruchAH(1972)ThermalConditions inPermafrost–A Review of North American Literature.North American Contribution, Permafrost, 2nd Interna-tional Conference, 13–18 July 1973, Yakutsk, USSR, pp.3–23. Washington, DC: National Academy of Sciences.

Johnston GH (ed.) (1981) Permafrost: Engineering Designand Construction. New York: Wiley.

Kudryavtsev VA, Garagula LS, Kondrat’yeva KA andMelamed VG (1974) Osnovy merzlotnogo prognoza,MSU. [VA Kudryavtsev et al. (1977) Fundamentals ofFrost Forecasting in Geological Engineering Investiga-tions. Draft Translation 606. Hanover, NH: US ArmyCRREL.]

Muller SW (1947) Permafrost or Permanently FrozenGround and Related Engineering Problems. Ann Arbor,MI: Edwards.

Tsytovich NA (1975) The Mechanics of Frozen Ground.New York: McGraw-Hill.

Von Baer KE (1838) On the ground ice or frozen sod ofSiberia. Journal of the Royal Geographical Society 8:210–213.

Washburn AL (1979) Geocryology: A Survey of PeriglacialProcesses and Environments. London: Arnold.

Williams PJ and Smith MW (1989) The Frozen Earth:Fundamentals of Geocryology. New York: CambridgeUniversity Press.

Yershov ED (1990) Obshchaya Geokriologiya, Moscow:Nedra. [Williams PJ (ed.) (1998) General Geocryology.Cambridge: Cambridge University Press.]

Figure 14 An area in the Tanana River valley near Fairbanks, Alaska, showing ponds, floating fens and remnant birch forest underlain

by ice-rich permafrost. Thawing is resulting in complete destruction of the trees and forest ecosystem, which is being converted into

floating fenswith ponds. Standing dead birch trees, someon ground that has settled below thewater level, are visible in the center and left

half of the picture. (Photograph by M. T. Jorgenson. Adapted with permission from Osterkamp TE, Viereck L, Shur Y, et al. (2000).

Observations of thermokarst in boreal forests in Alaska.Arctic, Antarctic, andAlpineResearch 32: 303–315 by permission of theRegents

of the University of Colorado).

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