PERMAFROST AND CLIMATE
Permafrost is a thermal condition of the ground, and is, ultimately, a climatic phenomenon. However, while maps of permafrost distribution portray a broad zonal distribution, it is well known that the actual relationship between permafrost and climate is not simply a function of latitude.
According to Gold and Lachenbruch (1973), in northern latitudes the mean annual ground surface temperature (MAGST) is 1o to 6oC warmer than the corresponding MAAT, largely as a result of snow cover. Variations in vegetation, topography, snow cover and soil conditions can produce variations of several degrees in mean ground temperatures over even a small area (e.g. Judge 1973, Smith 1975).
Where mean annual air temperatures are within a few degrees of 0oC, local factors result in a discontinuous patchwork of frozen ground. Whether permafrost exists in a particular place or not, therefore, depends on the interplay of regional climatic conditions and various local factors (e.g. Smith and Riseborough 1983).
Following Lachenbruch et al. (1988), the climate-permafrost system can be represented by the temperature regime at three levels:
- the air temperature (as measured at standard screen height);
- the temperature at the ground surface;
- the temperature at the top of permafrost (TTOP).
As a general rule, the temperatures at each of these levels will differ on a mean annual basis, according to the influences of surface cover and ground thermal properties, as depicted in Figure 3.
Figure 3. The Climate-Permafrost System (schematic)
On an annual basis, the effect of winter snow cover is generally greater than that of vegetation cover in summer (e.g. Annersten 1964, Nicholson and Granberg 1973, Smith 1975, Rouse 1984). Thus, the MAGST is likely to be warmer than the MAAT in most cases.
Between the ground surface and the top of permafrost, heat transfer by conduction varies seasonally between frozen and thawed states. This produces a difference between MAGST and TTOP (Goodrich 1978). The mean annual ground temperature becomes progressively colder through the active layer (see Burn and Smith 1988, Romanovsky and Osterkamp 1995) and thus the lowest value occurs at the permafrost table (TTOP). As a result of this thermal offset, permafrost can exist even where the MAGST is above 0oC.
Within a region of uniform climate one can visualize a range of local ground temperature, reflecting the variation of local environmental conditions. In particular, the effect of snow cover and the ground thermal properties significantly modulate the relation between mean air temperature and mean permafrost temperature.
This means that extrapolating climate change to changes in permafrost conditions is not altogether straightforward. Smith and Riseborough (1996) showed that air, surface and ground temperatures will change differentially under climate warming, depending on the interplay of climatic and local factors. Smith and Burgess (1998, 1999) recognised this but did not manage to quantify the temperature response of permafrost to climate change.
Smith and Riseborough (1996) developed a general formulation of the permafrost-climate system that offers a functional framework for analyzing the influence of climate and local factors on the temperature of permafrost.