Ninth International Conference on Permafrost ... - IARC Research

Historic Change in Permafrost Distribution in Northern British Columbia andSouthern YukonMegan JamesDepartment of Geography, University of Ottawa, Ottawa, CanadaAntoni G. LewkowiczDepartment of Geography, University of Ottawa, Ottawa, CanadaSharon L. SmithGeological Survey of Canada, Natural Resources Canada, Ottawa, CanadaPanya LipovskyYukon Geological Survey, Whitehorse, CanadaIntroductionAs a largely climatically-controlled phenomenon,permafrost conditions are impacted by air temperature andprecipitation, as well as other local surface and subsurfacefactors (Smith & Riseborough 1996, 2002). Numerousstudies have shown that permafrost should diminish inextent with rising air and ground temperatures (ACIA 2004).However, most of these predictions are based on modeling,and there are very few field-based studies of long-termpermafrost change in Canada.The objective of this study is to directly evaluate the impactof recent climate change on permafrost distribution. We wereable to conduct it due to the availability of early baselinepermafrost data, which are rare in Canada. In August 2007,we repeated a 1964 permafrost survey undertaken by the lateRoger Brown along the Alaska Highway from Whitehorse,YT, to Fort St. John, BC (Brown 1967).Study AreaIt is difficult to demonstrate climate change impactson permafrost except in very cold permafrost, wheretemperatures can rise without much thaw, or in areas of verywarm and shallow permafrost, where it may thaw completely(Smith et al. 2005). Even warm, shallow permafrost, likethat found in the study area, can take several decades to thawbecause far more heat is required for phase change than forwarming (Smith et al. 2005).Brown’s sites traverse discontinuous permafrost designatedas sporadic (underlying 10–50% of the landscape) or isolatedpatches (underlying 0–10%) (Heginbottom et al. 1995),which can be expected to be the most sensitive to climatechange, because a small change in temperature can resultTable 1. MAATs of communities along the study transect.Community Latitude Longitude Normal MAAT(1971–2000)Whitehorse, YT 60°42′ N 135°04′ W -0.7°CWatson Lake, YT 60°72′ N 128°49′ W -2.9°CFort Nelson, BC 58°50′ N 122°36′ W -0.7°CFort St. John, BC 56°14′ N 120°44′ W 2.0°C(Environment Canada 2007b)in a transition from a cryotic to a thawed state (Kwong &Gan 1994). According to Smith and Riseborough (2002), thesouthern limit of discontinuous permafrost approximatelycoincides with a mean annual air temperature (MAAT) of-1ºC. Table 1 shows the MAATs of the principal communitiesalong the study transect east of Whitehorse.MethodsBrown described the 60 locations that exhibitedpermafrost in 1964, as well as some nonpermafrost sites, insuch sufficient detail that it was possible to relocate mostof them using milepost information, written descriptions,and photographs. This information was retrieved fromBrown’s 1967 publication, as well as from cartographic andphotographic material archived at the National ResearchCouncil of Canada. The research was conducted in themonth of August so that the depth of the thawed layer wouldbe near its maximum.Using UTM coordinates for Brown’s sites, derived fromarchived 1965 Alaska Highway maps, we drove along theroute until we were within 500 m of the coordinates for a siteand continued for 500 m past. We assessed the landscape oneither side of the highway until we could confidently matchBrown’s photographs and descriptions. At some locationswe had to examine an area several hundred metres aroundthe UTM coordinates and look for sites most likely to havepermafrost (based on vegetation and drainage), which wewould investigate. There were several sites that we wereunable to relocate due to inadequate descriptions or profoundchanges in land cover, such as conversion of spruce forestto farm fields. At each relocated site, 2 ground temperatureprofiles were measured to 1.5 m or until we encountered thefrost table. At sites where a frost table was found, 10 groundprobings, roughly 1 m apart, were performed to measureactive layer thickness.Further information on ground thermal conditions willbe obtained from six climate stations which were set up tomeasure air temperature, ground temperatures, and snowdepths in order to examine the conditions that have allowedpermafrost to endure. These data will be downloaded insummer 2008.115