GROUND THAW CONSIDERATIONS IN PERMAFROST AREAS UNDER PRESENT AND PROJECTED CLIMATES

Field investigations in Arctic Canada indicate that vertical heat transfer into frozen soils is mainly through conduction. Heat convection is generally less effective except during the snowmelt period when infiltrating meltwater quickly raises the near-surface ground temperature to 0oC. During the thawing season, most of the ground heat flux is consumed by latent heat for ground ice melt and for conduction to below the permafrost table, leaving a small amount to warm the active layer. Thus, the ice content and the thermal conductivity of soil materials play the crucial roles in determining the depth of ground thaw.

Sensitivity analyses confirm quantitatively the significance of moisture content and soil organic fraction in affecting the rates of freeze-thaw. The common presence of peat at the top of subarctic and tundra soils should be emphasized in the studies of permafrost degradation. A thick organic soil cover retards thaw penetration so that ground thaw is shallower and the range of maximum thaw depth is smaller than for soils with a thin surface organic layer. Above ground, the snow buffers the soil from extreme atmospheric coldness but long snow duration shortens the season of ground thaw.

Estimates of maximum seasonal thaw in permafrost regions have major applications such as the planning and installation of infrastructures. The scarcity of measured data over the vast Arctic and subarctic region necessitates the use of models that range from the computationally intensive land surface schemes to semi-empirical algorithms. Projected ground thaw can also be evaluated for scenarios of climate warming, using atmospheric data produced by Global Climate Models (GCM) as inputs to ground freeze-thaw simulation schemes. Caution must be exercised in generalizing regional thaw depths because local soil, snow and vegetation factors give rise to considerable variability.