CRYSYS: Learn more about permafrost - CRYSYS - CRYosphere SYStem in Canada - [Meteorological Service of Canada

LEARN MORE ABOUT PERMAFROST!

Permafrost or perennially frozen ground is soil or rock that remains below 0°C for at least two consecutive years. Permafrost underlies about 50% of the ground surface of Canada (see Figure 1 below) and it may be more than 500 metres thick in the far north of the country. The spatial distribution, thickness and temperature of permafrost is highly dependent on the temperature at the ground surface, which in turn is influenced by several other environmental factors such as vegetation type, snow cover, drainage, and soil type. Permafrost is continuous across the arctic regions and underlies essentially all exposed land areas, with unfrozen ground found only beneath larger lakes and rivers and in small areas of newly aggraded ground. To the south of this zone lies a broad zone of discontinuous permafrost, in which the proportion of frozen ground decreases southward. At the southern limits of the permafrost zone, permafrost is confined to isolated patches of terrain such as areas of elevated organic terrain.

Within the permafrost regions, the presence of perennially frozen ground and associated ground ice strongly influence the properties and performance of ground materials and also controls to a significant degree the development of the land. This is because of ground stability problems associated with the thawing of ice-rich permafrost as a result of disturbance of the ground surface or climate change.

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Figure 1: Canadian permafrost distribution. Source: National Atlas of Canada

GROUND THERMAL REGIME

A typical example of ground temperatures in a permafrost region is shown in Figure 2 (from Yellowknife, Northwest Territories). The warmest and coolest temperatures at depths between 0 and 50 metres are depicted. With increasing depth, the seasonal difference in temperature decreases. The point at which there is no discernable change in temperature is termed the depth of zero annual amplitude, occurring in this example at about 15 metres. Below this depth, temperatures increase with depth according to the geothermal gradient. The permafrost base in this example is about 50 metres. The portion of the ground which rises above 0°C for part of the year is termed the active layer, freezing and thawing with the changing seasons (about 3 metres depth at this site). The thickness of permafrost can be altered by changes in the climate or a disturbance of the surface. Permafrost thins and the active layer thickens when ground temperatures increase.

Figure 2: An example of ground temperatures in a permafrost region (Courtesy of GSC).

PERMAFROST AND MEAN AIR TEMPERATURE

Field observations over the years have indicated a broad relationship between mean annual air temperature (MAAT) and permafrost distribution (on the continental scale), although permafrost temperature is generally 2 to 4°C higher than MAAT - see national map and Figure 3 below for the Mackenzie valley. The difference between air and ground temperature and the local variation in ground temperature are dependent on several factors such as snow cover, surficial materials, vegetation cover, the presence of an organic layer, moisture conditions and exposure to wind. Also, the insulating effect of a snow cover in winter can be much greater than that of vegetation cover in summer. Hence, local environmental factors can produce variations of several degrees in mean ground temperature, over a small area even though the air temperature may show little variation. Presently, the southern limit of continuous permafrost corresponds roughly with a MAAT of -6° to -8°C, with the southern discontinuous limit at about -1°C. Where mean annual air temperatures are within a few degrees of 0°C, local factors result in a discontinuous patchwork of frozen ground.

Air and ground temperatures in the Mackenzie valley

Figure 3: The variation in mean annual air and ground temperature along the Mackenzie valley. In the northern area ground temperatures contrast markedly between the Mackenzie Delta and the adjacent Tuktoyaktuk uplands (Courtesy of GSC).

Additional general information on permafrost distribution, thickness and ground ice can be found on the Geological Survey of Canada's web site under What is permafrost and Where does it occur.

Most of the material on this page was provided by: Margo Burgess and Sharon Smith (Geological Survey of Canada)

Useful References:

Burgess, M.M. and Smith, S.L. 2000. Shallow ground temperatures in the Mackenzie valley; in The Physical Environment of the Mackenzie River Valley, Northwest Territories: a Base Line for the Assessment of Environmental Change, (ed.) L.D. Dyke and G.R. Brooks; Geological Survey of Canada, Bulletin 547.
Heginbottom, J.A., Dubreuil, M-A., and Harker, P.A. (Comp.), 1995. Canada - Permafrost; National Atlas of Canada (fifth edition), Plate 2.1; Geomatics Canada, National Atlas Information Service, and Geological Survey of Canada, MCR 4177, scale 1:7 500 000.
Smith, S.L., Burgess, M.M., and Heginbottom, J.A., (in press). Permafrost in Canada, a challenge to northern development; in A Synthesis of Geological Hazards in Canada, (ed.) G.R. Brooks; Geological Survey of Canada, Bulletin 548.
Wolfe, S. A. (ed.), 1998. Living with frozen ground. A field guide to permafrost in Yellowknife, Northwest Territories; Geological Survey of Canada, Miscellaneous Report 64, 71 p.