PERMAFROST IN CANADA
Except for relatively few areas where adequate observational data exist, maps which show the distribution of permafrost have been based on broad extrapolations and an assumed relationship between air temperature and ground temperature.
According to Figure 2 (Heginbottom et al. 1995), the permafrost regions encompass about 50% of the nation’s landmass. At the national scale, permafrost distribution has been conventionally represented in terms of broad latitudinal zones in which permafrost occurrence is spatially continuous, widespread (discontinuous) or scattered (sporadic). Most published maps of permafrost in Canada show a similar continental scale pattern (Nelson 1989).
On this map, latitudinal bands (zones) of permafrost occurrence are seen to extend across the northern part of the continent, reaching furthest south near James Bay (approximate latitude 50oN), with boundaries trending northwest towards the Alaska border, and east to northeast toward the Atlantic coast of Quebec-Labrador. There is a discontinuity in the pattern of permafrost occurrence west and east of James Bay evident in Figure 2. Permafrost zones appear to be displaced some 300-km northwards east of James Bay. This is, in part, due to the climatology of Hudson Bay. According to Rouse (1993), pronounced regional summer cooling attributable to Hudson Bay/James Bay can help to explain the southward displacement of the arctic tree line and zone of continuous permafrost in this region.
Permafrost zones are intended to convey the relative areal dominance of permafrost and permafrost-free conditions in a region. While the concept of permafrost zones tends to suggest distinct kinds of terrain, in reality there is a gradual transition from the seasonally frozen ground of temperate regions of the south to the extensive perennially frozen ground of the far north.
Knowledge about permafrost conditions within any particular zone is limited by the scarcity of field measurements of ground thermal conditions. This is not surprising given the vast extent of the Canadian North, and the difficulty and expense of sampling the landscape at appropriate intervals.
The discontinuous zone describes a band of transition between conditions where permafrost is ubiquitous (spatially continuous) to conditions where it is absent. The extent of the discontinuous zone is the spatial consequence of the range of local ground thermal conditions that develops in response to the regional climate. According to Figure 2, the width of the discontinuous zone ranges from about 500 km west of James Bay to about 1200 km in northern Quebec.
|Permafrost Zones and Climate|
Field observations over the years have indicated a broad relationship between mean annual air temperature and permafrost distribution at the continental scale. Permafrost maps have frequently portrayed the limits of spatial occurrence and continuity in association with isotherms of mean annual air temperature (MAAT), notwithstanding that climate varies and that permafrost is never truly in equilibrium.
The pattern of MAAT in Canada relates to the influence of the Western Cordillera on the upper airflow, and the subsequent persistence of arctic air masses for longer periods in eastern Canada, together with the summertime influence of persistent ice in Hudson Bay (Rouse 1993). Brown (1970) observed that the permafrost temperature can be as much as several degrees warmer than MAAT. On this basis, he stated that the southern limit of permafrost corresponds roughly with the 1oC mean annual air isotherm. He placed the continuous boundary on the -8oC isotherm but noted that the band of continuous permafrost along the coast of Hudson Bay west of James Bay appeared to be anomalous in this regard. French and Slaymaker (1993) state that southern limit of continuous permafrost corresponds with a MAAT of –6° to –8°C, with the southern discontinuous limit at about –1°C. (For more on permafrost zones and temperatures, click here.)
Over the years, discussions about climate and permafrost (e.g. Brown 1960, Williams and Smith 1989) have recognized that regional climate parameters (such as screen air temperature or freezing/thawing indices) have a more complex relationship to permafrost temperatures.
The IPCC (1990) suggested that research be directed toward understanding the climate-permafrost system, including the effects of temperature forcing from climatic change.