Permafrost is a thickness soil or bedrock beneath the surface that has been permanently frozen for at least 2 years, with mean ground temperatures less than 0ᵒC. (Holden, 2008; Davis, 2001). Continuous and Discontinuous are the two main types. Washburn (1979) indicates that up to 26% of the Earth is affected by permafrost. Most of this exists where annual average temperatures are less than 0ᵒC and as such is highly susceptible to changes in surface climate conditions (IPCC, 2007).
Overlaying the permafrost are two uniform layers: active and organic (Summerfield, 1991). The organic layer is dominated by the presence of large amounts of organic material in varying stages of decomposition.
The Active Layer is important ecologically; as most exchanges of energy, moisture and gasses occur through it (Anisimov, et al., 1997). The active layer annually, or on occasion diurnally, freezes between 1 and 2m deep in winter and in summer thaws when temperatures rise above 0ᵒC (Davis, 2001). At temperatures greater than zero, enough thermal energy is present to thaw the permafrost. The greater the thermal energy the greater the thickness of the active layer due to the heat penetrating further down into the soil thawing at greater depths. This is becoming a great concern as more permafrost is now being destroyed, and hence active layer thicknesses increasing, due to climatic changes (Davis, 2001).
Future climate change will have significant impacts. Predictions are for the Earth’s temperatures to rise between 0.8 -3.5 0ᵒC by 2100 AD. The greatest changes occur in the spring and winter (Fosberg, 1998). These predictions are based on a variety of global circulation models (GCMs) based on current scientific understanding and predetermined starting conditions (Boer, et al., 1992; Wilson, et al., 1987; Cubasch, et al., 1990; Meehl, et al., 1993). With greater temperatures all year round more permafrost will melt in the summer and less form in the winter producing a thicker layer that actively freeze-thaws.
A further active layer thickening initiator has been identified in forest fires (Mackay, 1995). Forest fires remove the organic layer through combustion. This increases the thermal conductivity of the soil; a factor determining active layer thickness (Anisimov, et al., 1997), and reduces local albedo (Bolton, et al., 2002). The result is most thermal energy being retained in the soil and it transferring to greater depths.
Fire activity is strongly influenced by four factors; human activities; ignition agents, fuels and weather and climate (Johnson, 1992; Swetnam, 1993; Wein, 1976). Temperatures have been identified to be the most important predictor of total area burned in Canada. The greater the daily temperature the greater the area burned (Duffy, et al., 2005; Flannigan, et al., 2005).
Canada is a nation overriding upon both types of permafrost, experiences forest fires and will be subject to climatic change. Canadian forest fires currently burn more than 2 million hectares annually, 50% of these in the Northern Territories (Flannigan, et al., 2009). It is predicted that that warmer summer temperatures will increases the fire ignition risk by 10-30% and increases the total area burned by 74-118% by 2100 AD (IPCC, 2007). The Fire season length will increase by 30 days or more (Flannigan, et al., 1993). Lightning activity; an ignition factor, is also expected to increase (Price, et al., 1994)
Inuvik is a small town in the Northwest Territories of Canada. The Inuvik region is in a zone of discontinuous permafrost 300m deep (Mackay, 1995; Holden, 2008) and has suffered numerous fires over the last 50 years. The most notable is the fire of 1968 which burned 350,000 km² of forest in the Northern Mackenzie Valley Region. Another fire in 1999 burned 175,000 km² (Wein, 2002).
After the 1968 fire Mackay (Mackay, 1995) showed that the active layers had thickening by 24.1cm by the end of the first year and by 34.8cm by the following autumn. This was calculated by placing 152cm long wooden stakes into the ground. They were pushed down to refusal; around the 0ᵒC isotherm, and knife cuts made to provide a record of ground level. It is indicated that the forest fire of 1968 led to the increase in active layer depth.
However Viereck (1982) found that after the 1971 Wickersham Dome fire in Fairbanks, Alaska, there was ‘no significant difference in active layers’ between burned and unburned sites in the first summer following. This would seem to contradict the body of evidence indicating the opposite to be true.
Inuvik has, as previously stated, suffered two major forest fires; one in 1968 and the other in 1999. Using the Inuvik region as the location for a study the questions posed above can be tackled. Figure 1 shows that there are sites where the two fires extended. There are clear locations of overlap and areas where the fires never met. Measurements of four locations; an area of just 1968 fire, just 1999 fire, both fires and no fire, using the methods outlined by Mackay (Mackay, 1995) will help to answer the issued raised by this critique.
One of the key points in current literature is that with fire activity, active layer depth increases. However the sites that had been studied at Inuvik had been free from fire activity for a hundred years (Wein, 2002). The organic layer was dense and burned severely resulting in extreme temperatures above the surface which conducted through the active layer to the permafrost below. This resulted in a large increase in the active layer by the year’s end (149%). Would this have been the case if a fire had broken out 6 months, a year, 5 years ago? Did lack of any recent fire activity increase the size of the active layer through a greater degree of organic layer combustion? That is to say; do sites that have been burned more than once in close temporal proximity increase their active layer depths to the same degree as single burn locations? These questions have not been answered or attempted in current schools of thought or literature and this presents an opportunity.
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