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Th e Int 7th ern Per atio ma Co fro nal nfe st ren ce Yel Jun lowkni 199 e 23- fe 27 8 PERMAFROST - Seventh International Conference (Proceedings), Yellowknife (Canada), Collection Nordicana No 55, 1998 ? FIELD INVESTIGATIONS OF PERMAFROST AND CLIMATIC CHANGE IN NORTHWEST NORTH AMERICA C.R. Burn Department of Geography, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6 Canada e-mail: [email protected] Abstract Yukon Territory, adjacent portions of Northwest Territories, and Alaska contain a continental range of permafrost conditions. The response of permafrost to climatic change is recorded in the cryostratigraphy of late Pleistocene and Holocene sediments, with an early Holocene thaw unconformity being a widespread and prominent feature. More recently, temperature profiles from deep boreholes show an inflection associated with near-surface warming of 2¡ to 4¡C since the Little Ice Age. Simultaneously, the southern limit of permafrost has moved northwards. In order to understand the present climate:ground temperature system, an analytical solution has been verified to relate the annual mean ground surface temperature to the annual mean permafrost surface temperature under equilibrium conditions. Ground surface temperatures have been obtained from air temperatures using n-factors. The solution assumes that heat transfer in the active layer is only by conduction. The relations show that the impact on permafrost temperatures of changes in snow cover and soil moisture conditions may surpass the effect of changes in air temperature per se. Observations from the sporadic permafrost zone indicate the persistence of permafrost despite recent warming. This is due to minimal snow cover on residual peat landforms, and to latent heat in ice-rich ground. The persistence further complicates interpretation of the response of permafrost to climate change. Introduction Permafrost is a geologic manifestation of climate, so permafrost conditions should change over time. Instrumental and paleoenvironmental records indicate that the climate is warming faster in the Arctic than at lower latitudes of the Northern Hemisphere, and that the warming has been greater in the 20th century than in the previous 400 years (Overpeck et al., 1997). The response of permafrost to climate change, a theme of this conference, is the focus of several research projects supported by the International Permafrost Association (e.g., Brown, 1997; Harris, 1997). In Canada the Mackenzie Basin Impact Study, a multi-disciplinary project, recently produced its final report on the potential impact of climate change on the region, and concluded that a principal threat to the landscape was "accelerated erosion and landslides caused by permafrost thaw .... especially in sloping terrain and the Beaufort Sea coastal zone" (Cohen, 1997, 297). Air temperature in parts of Mackenzie Basin has risen by 1.5¼C over the last century (Maxwell, 1997), and, in the popular press, increased geomorphological activity has been attributed to such warming (e.g., Grescoe, 1997). In contrast, the scientific literature has ascribed past mass wasting in Mackenzie Valley to site-specific disturbances (Mackay and Matthews, 1973; Harry and MacInnes, 1988), although the potential for these events to follow climate warming is acknowledged (Mackay, 1975a). On the 1967 Permafrost Map of Canada, R.J.E. Brown (1967) implicitly recognized the importance of climatic change. Although the primary purpose of the map was to indicate the spatial extent of permafrost, Brown chose the Ð5¼C mean annual air temperature isotherm to separate the continuous and discontinuous zones. He recognized that, over the long term, a climatic warming of over 5¼C would be required to degrade permafrost in the continuous zone. A climatic shift of such magnitude is not common during an interglacial period, although smaller fluctuations occur. Therefore, in the continuous permafrost zone, permafrost is continuous in time as well as space, and is discontinuous in these dimensions to the south. The response of permafrost to climate change is shown by changes in the ground temperature profile, or in the depth of the active layer, or both. In this paper, research on permafrost and climate change will be considered under four themes: (1) historical climate: permafrost relations; (2) cryostratigraphic relations; (3) relations between near-surface ground temperatures and present climate; and (4) the impact of climate change on permafrost distribution. The purpose of the paper is to review recent progress in these fields, with emphasis on evidence from northwest Canada and C.R. Burn 107 The region is on the western, climatically-leading edge of the continent, but the Wrangell-St. Elias and Coast Mountains block maritime air masses from entering the region, causing a subarctic, continental climate conducive to permafrost (Wahl et al., 1987; Burn, 1994). Enhancement of temperature inversions by cold-air drainage in the dissected topography of the region results in the coldest temperatures of the North American winter being recorded here (Kalkstein et al., 1990; Burn, 1993), and the presence of discontinuous permafrost (Heginbottom, 1995). Taylor et al. (1998) describe the influence of the inversion on permafrost temperatures in central Mackenzie Valley: valley-bottoms are underlain by permafrost as a result of frigid winters, while, at high elevations there is little thawing under cool summer conditions; in between there may be a permafrost-free zone. In central Yukon, alpine permafrost is found above 1500 m a.s.l., with welldeveloped cryoplanation terraces and patterned ground (Hughes, 1983). Unfortunately, these topographic effects cannot be resolved at the scale of present general circulation models (GCMs), which generate a climate similar to Figure 1. Permafrost map of Yukon and adjacent Northwest Territories (after Heginbottom, 1995). adjacent areas of Alaska. Field data are presented to illustrate relations discussed in the literature. The review builds on a summary prepared in 1992 (Burn and Smith, 1993), and focuses on material published since then. The review is regionally-restricted, in contrast with the paper presented at the Beijing Permafrost Conference on this subject (Nelson et al., 1993). The Yukon and adjacent Northwest Territories, permafrost, and climate change A north-south transect across northwest Canada or adjacent Alaska from the Beaufort Sea to the Pacific Ocean covers a continental range in permafrost conditions (Figure 1; Smith et al., 1998, this conference). At Tuktoyaktuk, N.W.T., the mean annual air temperature (MAAT) is -10.5¼C, while at Whitehorse, Y.T., MAAT is 1.0¡C, and on the coast at Juneau, AK, MAAT is 4.5¼C (Arctic Environmental and Data Center, 1986; Environment Canada, 1993). Continuous permafrost over 600 m thick, and near-surface ground temperatures below Ð8¼C are found near the Beaufort Sea coast of Alaska, Yukon Territory, and the Mackenzie Delta area, N.W.T. (Mackay, 1974; Lachenbruch and Marshall, 1986; Judge et al., 1987). In contrast, the mean annual ground temperature in the sporadic discontinuous permafrost of southern Yukon Territory is above Ð1¼C, and permafrost is less than 20 m thick (Burn, 1998). 108 Figure 2. Relations between thawing degree-days and distance from the Beaufort Sea, 1994-96, along a transect from Pelly Island to Inuvik, N.W.T. (see also Burn, 1997, Figure 12b). Two sites were not occupied in 1994. The 7th International Permafrost Conference Scandinavia for the region. In Scandinavia there is relatively little permafrost, and so the GCM results are unsuitable for investigations of potential climate change in northwest North America (Stuart and Judge, 1991). Field experiments by SeppŠlŠ (1982) demonstrated the importance of snow cover on permafrost distribution in the discontinuous zone, and this critical variable is poorly represented for the region in GCMs, because the rainshadow caused by the coastal mountains is not reproduced (Burn, 1994). In a similar fashion, a steep summer climatic gradient in the Beaufort Sea coastal zone, due to the presence of proximal pack ice offshore (Haugen and Brown, 1980; Zhang et al., 1996a), is not apparent at the scale of present GCMs. However cooler winter temperatures inland offset this gradient on an annual basis, so that MAAT changes little with distance from the coast. Nevertheless, the summer gradient controls the development of vegetation communities, which, in turn, impact snow accumulation and, hence, near-surface ground temperature and active-layer development (Clebsch and Shanks, 1968; Mackay, 1974; Romanovsky and Osterkamp, 1995; Nelson et al., 1997). Changes in conditions along the gradient may not be uniform under future climatic change, as suggested by the varying range in interannual variability of summer climate along a transect across treeline in the Mackenzie Delta area (Figure 2; see Burn, 1997), and in the scattered covariance of air and ground surface temperature series on the Alaskan coastal plain (Romanovsky and Osterkamp, 1995). Ecological changes following climate change, such as northward treeline migration, may compound ground temperature increases (Gavrilova, 1993; Burn, 1997). upwards, at a rate less than 2 cm a-1, with heat supplied by the geothermal flux, so the response of permafrost thickness is over glacial time scales (Osterkamp and Gosink, 1991). By carefully monitoring sites near the north coast of Alaska between 1983 and 1993, Osterkamp et al. (1994) detected a cycle of about 10 years in ground temperatures. The derived amplitude at the permafrost surface decreased inland from 2¼C at the coast. The magnitude of fluctuation near the coast may represent sensitivity to maritime effects, particularly sea ice, on air temperature and snow cover, although Osterkamp et al. (1994) drew attention to the coincidence of the period with sunspot activity. Subsequently, Osterkamp and Romanovsky (1996) supplemented these data with measurements taken between 1986 and 1993 from the upper 20 m of permafrost, which were consistent with the original interpretation. However, they were unable to judge whether the near-surface temperature series formed part of an overall warming or was the rising limb of a cyclic fluctuation. The ground temperature profile in the discontinuous permafrost zone has also responded to the 20th century Historical ground temperature: Permafrost relations Pioneering work by Lachenbruch and Marshall (1969, 1986) established the use of ground temperature profiles from arctic Alaska to infer past changes in the annual mean permafrost surface temperature (AMPST). Warming of permafrost occurs as a result of either warmer summers and/or winters, or, in particular, winters with thicker or more persistent snow cover (Smith, 1975; Zhang et al., 1997). This means that increases in permafrost temperatures are not necessarily associated with increases in MAAT or in the depth of the active layer, and vice-versa. Osterkamp et al. (1994) report that in northern Alaska near-surface temperatures have varied by 4¼C over a decade, on the same order as the last century, but variation in active-layer thickness has shown little correlation with the ground temperature fluctuation. In thick permafrost, where AMPST remains below 0¼C, degradation occurs from the bottom Figure 3. Temperature profiles in permafrost, summer, 1990 and, 1997, at a site near Mayo, Y.T. (Site A1 of Burn, 1992). The change in gradient at 18 m depth is associated with a change in lithology from ice-rich glaciolacustrine sediments (above) to sand (below). The annual temperature range at 5 m is 0.25¼C. C.R. Burn 109 within 5 km of Mayo in July 1997 all indicated recent cooling of the ground, and none showed the near-surface isothermal zone evident in 1990. Warming during the last three decades has led to eradication of some thin permafrost and an apparent northward displacement of the southern boundary of the discontinuous zone in northwest Canada. Thawing of permafrost in peat bogs at the southern margins of the permafrost zone in Manitoba began at the end of the Little Ice Age (Thie, 1974), and continues today (French and Egorov, 1998). Similar permafrost degradation has been reported for northern Alberta (Vitt et al., 1994), where in 1988, a field survey of permafrost conditions following the route taken in 1962 by Brown (1964), concluded that the southerly limit of permafrost had moved 120 km northwards (Kwong and Gan, 1994). Less than 20% of the ground in northern Alberta is underlain by permafrost (Heginbottom, 1995), and under such marginal conditions, we must expect thin permafrost to aggrade and degrade periodically. Zoltai (1993) demonstrated such cyclic behaviour in the marginal permafrost of peatlands in northwestern Alberta, due to wild fires. Figure 4. Snow accumulation at Mayo Airport, Y.T. (a) in two winters of the 1990Õs; (b) mean accumulation in March 1955 - 1997. Data in (b) for 1955 1988 supplied by Atmospheric Environment Service, and for 1989 - 1997 from station weather record at Mayo Airport. climate warming, but this has received less attention. For the same increase in heat flux, the temperature of ÒwarmÓ permafrost may change less than that of ÒcolderÓ permafrost due to latent heat effects (Riseborough, 1990). Several temperature profiles measured at Norman Wells by Imperial Oil Ltd in the late 1940Õs and early 1950Õs indicated curvature to depths of 50 m and near-surface warming of 3¼C (Hemstock, 1953; Mackay, 1975a), following the warmest air temperatures of this century. Curvature was still apparent in the upper portions of temperature profiles recorded in central Mackenzie Valley during the early 1970Õs (Kurfurst et al., 1974), even though air temperatures had declined. Climate variation in central and southern Yukon is dominated by fluctuations in winter conditions, with snow accumulation being positively associated with winter air temperature (Burn, 1990). In 1990, the temperature profile in permafrost at Mayo (Figure 1) was consistent with a warming, since the early 1970s, of 1.25¼C in AMPST (Burn, 1992). Subsequently, the ground has cooled by 0.2¼C or more in the upper 15 m of permafrost (Figure 3), in association with reduced snow accumulation (Figure 4). Five temperature profiles through permafrost measured at different sites 110 In all cases discussed above, changes in the temperature profile have been driven by variations in AMPST, which integrates both summer and winter climate. AMPST is a thermal variable which only summarizes variations in hydroclimate implicitly. For explicit evidence of changes in seasonal and moisture regimes, we must turn to the cryostratigraphy. Cryostratigraphic relations Two recent sets of cryostratigraphic observations from the region relate to climatic conditions during the last glacial maximum (c. 20,000 yr. BP). The first are observations of sand and ice wedges on and around Richards Island, which indicate that the region experienced cold, dry conditions conducive to eolian activity (Murton and French, 1993; Murton et al., 1997). The second concern data from the Klondike area, where Fraser and Burn (1997) determined that the ice-rich, unconsolidated silt deposits, which mantle auriferous gravel in valley bottoms, are of Late Wisconsinan age. The absence of massive icy bodies from the lowermost parts of most silt sections suggests this area, too, was arid during the glacial maximum. Data on the cryostratigraphy of these unconsolidated materials are presented by Kotler and Burn (1998), who have determined that most of the ground ice in the silts formed at the very end of the Wisconsinan, after 12,000 14C years BP. The principal cryostratigraphic data relevant to our topic have been observations of truncated ice wedges associated with an early Holocene thaw unconformity The 7th International Permafrost Conference occur due to progressive deepening of the active layer, and are not tied to any particular climate scenario. Figure 5 indicates relatively large subsidence for a small increase in active-layer depth, due to the ice-rich zone at the top of permafrost (Cheng, 1983; Burn, 1986). The evidence from cryostratigraphy and ground temperature profiles indicates how permafrost has responded to historical climate change. We now turn to the relations between permafrost and climate derived from present conditions. Present ground temperature: Climate relations Figure 5. Potential terrain subsidence (cm) at two sites on Pelly Island, N.W.T. The two profiles represent material between the base of the present active layer and the early Holocene thaw unconformity; the short profile is from the crest of a slope. The potential subsidence was determined from the excess ice content of near-surface permafrost obtained by core drilling, using the method outlined by Mackay (1970, Figure 3). If the excess ice content of a 20 cm interval is 50%, then if this interval is incorporated in the active layer, there will be at least 10 cm of subsidence and at most 10 cm of activelayer thickening. Subsidence is given in (a) for an absolute increase in active layer thickness; and in (b) for a relative increase in active-layer thickness. in many parts of the region (Mackay, 1975b, 1978, 1992; Burn et al., 1986; Burn, 1997). The unconformity records the maximum development of the active layer around 9000 cal. years BP. Usually, segregated ice is abundant between the unconformity and the base of the present active layer (e.g., Pollard and French, 1980), complicating estimates of paleoactive-layer thickness. Burn (1997, Figure 9) presented two profiles from Richards Island in which the depth of the paleoactive layer was estimated by removing the excess ice which postdates the unconformity. The method requires analysis as outlined by Mackay (1970, Figure 3). The reconstructed paleoactive-layer depth for the coastal region of the Canadian western Arctic was about twice the present thickness. A considerable portion of the increase in temperature relative to the present regime generating the deeper summer thaw is from the paleogeography of the region rather than climatic change per se. The sites examined were further from the coast 8000 to 9000 years ago than they are today, and therefore benefited from the regional summer temperature gradient (Burn, 1997; see Figure 2). Similar data on the paleoactive layer from Pelly Island have been rearranged to estimate the subsidence that may occur if the present active layer deepens following climate warming (Figure 5). These data provide an indication of the extent of terrain disturbance that may Air and ground temperatures are being monitored simultaneously in the region, in order to determine the response of ground temperatures to climatic variation, and to detect the impact of climatic change on permafrost (e.g., Nixon et al., 1995; Zhang et al., 1997). In Takhini River valley, near Whitehorse, Y.T. (Figure 1), ground temperatures have been monitored monthly or more frequently since 1991 (Burn, 1998). At the Takhini site, which is in a spruce forest, the AMPST is -0.8¼C, the active layer is 1.4 m thick, the base of permafrost is at 18.5 m, and the temperature gradient in the permafrost is linear. The site is 2.3¼C cooler than Whitehorse Airport, but air temperature series from these locations have a coefficient of determination (r2) of 0.98, indicating that fluctuation is synchronous and of proportional magnitude. As with other sites in southern and central Yukon, monthly mean air temperatures at Whitehorse exhibit little variation during summer, in contrast with the winter. As a result, the active layer depth has remained almost constant during the period of observation. Figure 6 indicates monthly mean air temperatures measured at Whitehorse Airport and ground temperatures at 1.5 m depth from the Takhini valley site. There is relatively little snow accumulation at the site (maximum 35 cm), because of arid conditions in the valley, and interception by the forest canopy. Overall, the ground temperature series has decreased by 0.07¼C a-1, in response to recent relatively cool winters. A similar trend (cooling at 0.02¼C a-1) summarizes temperature variation at 50 cm depth in the active layer. Data collected at irregular intervals, mostly in winter, for 1982 1990 show similarly little temporal trend (Burn, 1998). Instead, a great variation in ground temperatures occurs in this area between vegetation units, and permafrost degrades once the spruce forest is cleared. At the Fifth International Conference on Permafrost, A.H. Lachenbruch pointed out that assessment of the response of permafrost to climate change involves moC.R. Burn 111 Figure 6. Ground temperatures at the surface of permafrost, 1.5 m depth, in Takhini River valley, Y.T., and monthly mean air temperatures at Whitehorse Airport, 50 km to the east, 1991 - 1997. Ground temperature data collected manually from thermistor cables. delling three environmental systems: the atmospheric climate; the ground surface and active layer; and permafrost (Lachenbruch et al., 1988). Critical relations are those between air temperature and ground surface temperature, and between the ground surface temperature and permafrost surface temperature. Permafrost is a medium within which energy is transferred dominantly by conduction, and there are well-known methods for determining the response of the temperature profile in permafrost to fluctuations in AMPST (Lachenbruch et al., 1988). Outstanding problems are the coupling of atmospheric temperature changes to changes in ground surface temperature, and transmission of changes in the ground surface temperature through the active layer to permafrost. Currently there are several approaches contributing to progress on these aspects. CONVECTIVE EFFECTS Of fundamental significance to the energy transfer through the active layer is the mode of such exchange. Many formulations of these relations assume that the heat flow is exclusively by conduction (e.g., Goodrich, 1982; Kane et al., 1991). However, Hinkel and Outcalt (1994) have demonstrated that convective transfer occurs within the active layer during snowmelt and during infiltration of precipitation in summer (Hinkel et al., 1997). Sudden increases in soil temperature that cannot be generated by conduction have been recorded, and similar conclusions have been drawn from spectral analyses of soil temperature time series (Hinkel and Outcalt, 1993). In addition, minimal conduction may also occur during the zero-curtain period of freeze-up. Soil tempera- 112 tures measured at intervals of one hour during this period exhibit variations which have been attributed to evaporation, vapour flow, and condensation, driven by osmotic gradients (Outcalt et al., 1990; Outcalt and Hinkel, 1996). In some areas of arctic Alaska, the zero curtain period may last two months or more, so the potential total transfer may be a considerable portion of the annual budget. This portion is not estimated precisely, and may differ between soil types. However, Osterkamp and Romanovsky (1997) found little evidence of non-convective effects in seven yearsÕ data collected at three sites on the North Slope of Alaska, and concluded that the thermal regime could be modelled adequately by conductive processes alone. Convective transport is clearly important in gravel and coarse sand, because iron-stained gravel, well below the present active layer, and associated with the early Holocene thaw unconformity, has been observed by J.R. Mackay (personal communication, 1996) at several sites in the Mackenzie delta area. Baker and Osterkamp (1988) have reported direct observations of convective heat transport in coarse-grained subsea permafrost. Similarly, in organic soils the potential for vapour flow is significant and demonstrated (Outcalt and Hinkel, 1996). In fine-grained mineral soil the influence of convective transfer should be much less. Figure 7 presents ground temperatures measured over a year, beginning on 1 August 1996, at 1 m depth in the floor of a thaw slump near Mayo, Y.T. (Figure 1), where permafrost is degrading. A similar series is also presented from an undisturbed site nearby. The soil at both The 7th International Permafrost Conference an equation for the offset (Romanovsky and Osterkamp, 1995, eq. 12): Offset = Figure 7. Daily mean ground temperature series at a disturbed site in a retrogressive thaw slump, and an undisturbed site in permafrost near Mayo, Y.T., for one year beginning on 1 August 1996. The data are daily means of observations collected every 4.8 hours from 1 m depth by data logger. sites is a glaciolacustrine silty clay, and the permafrost table is now 4.5 m below the surface at the site in the slump. The active layer at the undisturbed site is about 80 cm thick. There is scant evidence of convective effects at the undisturbed site, where the sensor is in permafrost. At the site in the slump, convective transfer may be interpreted in early May, during snowmelt, and in late July as a result of rainfall. In the rest of the year, the smooth temperature transitions suggest that the heat flow follows a conductive regime. Although the time interval of data collection (4.8 hours, presented in Figure 7 as a daily mean temperature) may be insufficient to detect convective activity, the net proportion of the annual thermal regime attributed to convection appears small. The result is that the thermal regime may be modelled adequately as a conductive system. Similarly, data on long-term (1958-1997) permafrost degradation collected at a site in Takhini valley burned by forest fire are consistent with the Stefan solution, and indicate that, in similar fine-grained sediments, the thermal regime may be considered conductive (Burn, 1998). Any convective effects are masked by conduction because the sediments are fine-grained. THERMAL OFFSET Analysis of soil temperatures, collected at several sites on a transect southward from the Alaskan coast at Prudhoe Bay between 1986 and 1992, has provided the basis for estimating the "thermal offset" (Romanovsky and Osterkamp, 1995). The offset is the relation between annual mean ground surface temperature (AMGST) and AMPST (Goodrich, 1982; Burn and Smith, 1988). The soil temperatures were recorded at the ground surface and at various depths within the active layer and near-surface permafrost every four hours. The sites were in fine-grained sediments, with overlying organic horizons and a high degree of saturation. From these data, Romanovsky and Osterkamp (1995) were able to determine the range in thermal offset registered at various sites and under different annual climatic regimes. Assuming only conductive transfers of heat within the active layer, they derived analytically ö DDTs æ Kt - 1÷ ç P è Kf ø [1] where kf and kt are the thermal conductivities of the active layer in frozen and thawed states, DDTs is the annual total thawing degree-days at the ground surface, and P is the period of the temperature cycle, 1 year. The equation was compared with determinations of the thermal offset from a numerical model (Goodrich, 1982), and with the field data. In both cases the agreement was excellent, indicating the applicability of a conductive model at this scale in saturated, finegrained soils. The greatest difference between field observations and equation [1] occurred in the warmest year at the warmest site. This suggests that while the equation may be effective for cold permafrost, its application may be more limited in the discontinuous zone, or under transient conditions. It may be inappropriate at warmer sites with fine-grained soil to characterize thermal conductivity as a bimodal function over the annual cycle. To date a validation of the model against field data from a region with warm permafrost, e.g., an AMPST close to -1¼C, has not been published. Romanovsky and Osterkamp (1995, eq. 13) also derived the equation presented by Kudryavtsev (1981) for the AMPST in terms of the thermal regime at the ground surface: AMPST = kt DDTs - k f DDFs kf P [2] where DDFs is the annual total freezing degree-days at the ground surface. The outstanding value of equation [2] is the succinct expression of a fundamental relation. It identifies critical summary values for the temperature regime at the ground surface (DDFs, DDTs) and lithology (kf, kt) governing the difference between AMGST and AMPST. The equation is rapidly applied, in comparison with numerical simulations of the annual active-layer thermal regime (Smith, 1977; Smith and Riseborough, 1983). As a result, equation [2] is a valuable tool for regional assessments of the impact of changes in ground surface temperature on ÒcoldÓ permafrost. For studies of specific sites, numerical simulations are required to provide the details often necessary. THE N-FACTORS The data analysed by Romanovsky and Osterkamp (1995) were combined with records of air temperature and snow cover characteristics collected simultaneously C.R. Burn 113 to assess the influence of climate variation on AMPST (Zhang et al., 1997). While summer ground temperatures and active-layer depth responded to variations in air temperatures and the duration of the thaw season (Romanovsky and Osterkamp, 1997), subsurface conditions in winter were relatively insensitive to these variables, but not to characteristics of the snow pack. These effects are due in part to the inverse relation between air temperature and snow depth on the North Slope of Alaska over the period examined, in contrast with central Yukon. On a regional basis, however, the variation in winter ground temperatures is dominated by the systematic increase in snow accumulation (Zhang et al., 1996b), due to trapping by microrelief or vegetation, which increases in height with distance from the coast. The data from northern Alaska clearly separate air and ground temperature relations into two seasonal regimes. These relations have been summarized through determination of n-factors. The n-factor is defined as the ratio of the ground surface temperature index for the thaw (or freezing) season to the air temperature index for the same season, usually expressed as accumulated thawing (or freezing) degree-days (Lunardini, 1978,, 1981). This method offers potential for summarizing microclimatic exchanges within generalized vegetation units. In general, the n-factors are determined empirically, by collecting air and ground temperatures simultaneously at sites representative of ecological units. Various n-factors for natural surfaces in Mackenzie valley are presented by Taylor (1995), adding to the inventory of Jorgenson and Kreig (1988) and Shur and Slavin-Borovskiy (1993). The n-factors are generally greater in the thawing season than the freezing season and at tundra sites than in the boreal forest (Taylor, 1995; Smith et al., 1998). For sites with permafrost, the freezing season n-factor varies mostly with snow cover characteristics, associated with the vegetation community. As a result, Smith and Riseborough (1996, eq. 5) presented a modified form of equation [2] to relate the air temperature regime to AMPST, using n-factors to convert the air temperature index to that of the ground surface: kt × n1 × DDTa - n f × DDFa [3] kf AMPST = P where nt and nf are the thawing and freezing season n-factors, and DDTa and DDFa are the thawing and freezing indices for air temperature. Equation [3] allows exploration of changes in equilibrium AMPST in response to climatic change, as summarized by either fluctuations in DDTa, DDFa, and/or nf. Burn (1998) calculated the n-factors for sites with equilibrium and degrading permafrost in Takhini valley, southern Y.T., and noted that while the thaw season n-factors were consistent with data from other areas, the n-factor in winter varied with subsurface conditions. The freezing season n-factor is lowered at sites without permafrost, or with a very deep active layer, because of the continuing contribution of latent heat released during frost penetration. During the period with Òzero curtainÓ, nf may be close to zero. In contrast, once freeze-up has occurred at sites with permafrost, the ground surface temperature may readily decline. The difference in winter soil temperatures between sites in forested and burned areas, where permafrost is degrading, is shown in Figure 8. Air temperatures are cooler by 1.1¼C at the forested site, while the average daily soil temperature difference is 2.45¼C. Equation [3] is restricted in consideration of permafrost degradation by the dependence of nf on the thickness of the active layer, and by the delay in response of AMPST to climate change caused by energy exchanges within permafrost (e.g., Riseborough, 1990). Projecting permafrost distribution after climate change Figure 8. Daily mean soil temperatures at 20 cm depth, 1 November 1994 31 March 1995, at forested and burned sites, Takhini River valley, Y.T. Observations were taken every 4.8 hours by data logger. The least-squares regression line for the data is TB = 0.43TF - 0.88 (r2 = 0.90), for soil temperatures (¼C) in the forest (TF) and the burned area (TB). 114 In North America three principal approaches have been adopted towards modelling the impact of climate change on permafrost distribution. The first, the Nelson-Outcalt frost index (Nelson and Outcalt, 1987), is based entirely on climatic statistics, and has been The 7th International Permafrost Conference used with GCM output to project permafrost distribution under various climate scenarios (Anisimov and Nelson, 1996, 1997). The second, the TTOP model (equation [3], Smith and Riseborough, 1996), explicitly recognizes the impact of surface and soil conditions on ground temperatures, and therefore has potential application over smaller areas. The third is the use of calibrated numerical models for site-specific applications (Romanovsky et al., 1997). The Nelson-Outcalt frost index is a normalized ratio of frost penetration to thaw penetration on a regional basis, using the climatic thawing index and a modified freezing index, to accommodate snow cover, derived from climatic data. A ratio of 0.50 represents the equatorward boundary of permafrost, and 0.67 represents the boundary between continuous and discontinuous permafrost zones. The model has been verified by comparing projections of the index with extant permafrost maps (Anisimov and Nelson, 1997). The model is suited to application at hemispherical scale, and has been used to forecast changes in the extent of permafrost associated with various scenarios for global climatic warming (Anisimov and Nelson, 1996, 1997). Nelson and Outcalt (1987) explicitly indicated that the model applied only to equilibrium permafrost, and did not accommodate degrading permafrost. The result is that the model's utility is limited for consideration of transient conditions, such as may be expected over the next century. Nevertheless, results from GCMs have been used with the model to examine the area where changes in permafrost distribution may occur as a result of a changed climate, and a scenario of permafrost distribution during the early Holocene climatic optimum has been presented (Anisimov and Nelson, 1996). This latter case illustrates well the difficulties implicit in the technique for considering transient conditions. The model indicates that, relative to present conditions, permafrost was considerably restricted in spatial extent during the early Holocene, with the discontinuous and continuous permafrost zones smaller by about 25% and 67% respectively. However, in the discontinuous permafrost of central Yukon there is field evidence of permafrost persistence during this period from the x18O concentration in ground ice (Burn et al., 1986; Kotler and Burn, 1998). Permafrost in Takhini Valley, only 15 m thick, is presently degrading as a result of forest fire, but will likely require over 1200 years for its eradication (Burn, 1998). Furthermore, a large portion of the permafrost extant during the Little Ice Age in the peatlands of the western Canadian provinces has been mapped from aerial photographs taken between 1949 and 1952, a century after climate warming began (Halsey et al., 1995). The persistence of perennially- frozen ground is due to elevation of permafrost-cored landforms in peatlands and the associated reduction in snow cover (SeppŠlŠ, 1982), and to the high ice content of frozen peat. These observations led Halsey et al. (1995) to the counter-intuitive result that permafrost is more extensive where MAAT is presently between 0¼ and -3.5¼C, than it was in regions of similar climatic regime 150 years ago. Anisimov et al. (1997) conceded this point in attempting to estimate active-layer thickness rather than permafrost distribution per se, but suggested that active layer response to climate change may be rapid. They used the method presented by Kudryavtsev et al. (1974) to couple GCM output to forecasts of active-layer development, which, unfortunately, cannot accommodate variations in the ice content of near-surface permafrost. Persistence of active-layer thickness is provided by the characteristically ice-rich zone at the surface of permafrost. Where the active layer is relatively deep, further deepening requires evacuation of latent heat from melting near-surface ground ice along a gentle temperature gradient. Maximum active-layer thickness occurred about 1000 calendar years after the period of maximum solar insolation in western Arctic Canada (10,000 cal. years BP), but initiation of thermokarst lakes coincided with maximum insolation (see Burn, 1997). The persistence of permafrost was illustrated by Riseborough and Smith (1993) in simulations of climatic variability over periods of a millennium. The model coupled a randomly varying climate based on the record from Fort Simpson, N.W.T., to the TONE ground thermal simulator written by L.E. Goodrich (1982, modified after Steven, 1982), via explicit consideration of the thermal regime in snow. A key result is that while permafrost may form rapidly during several cold winters, numerous warm years are required to thaw the same thickness of ground. The development and thawing of excess ice was not simulated, and this would tend to further stretch the periods with permafrost. Smith and Riseborough (1996) and Riseborough and Smith (1998, this conference) have conducted a series of sensitivity analyses on the critical variables responsible for the AMPST, as represented by equation [3]. The analyses have included consideration of the importance of changes in snow cover and variations in the thermal properties of ground materials. These data indicate that changes in precipitation regime will influence the response of permafrost to climate change, potentially swamping the effects of changes in air temperature per se. Increases in soil moisture content tend to reduce AMPST, while the effect of snow depth is directly related to AMPST (Riseborough and Smith, 1998). In combination, these relations make more explicit the response of permafrost to climate change. C.R. Burn 115 Finally, for specific sites, calibrated numerical simulations have continued to provide effective models of the ground thermal response to climatic change (Lachenbruch et al., 1988; Burn, 1992; Zhang and Osterkamp, 1993). Such analyses of ground temperature profiles has allowed reconstruction of the late Pleistocene and Holocene environmental history of Mackenzie Delta area (Taylor et al., 1996). Changes in surface temperatures due to the glacial/interglacial transition, submergence during post-glacial sea level rise, and emergence during delta progradation are amenable to modelling as step functions, whose magnitudes can be estimated independently. The general environmental history of the region is known (Rampton, 1988), so Taylor et al. (1996) were able to use the temperature profiles to test specific hypotheses about permafrost evolution in the area, and determine times of emergence and submergence for various sites. Conclusion The paper has attempted to summarize research on permafrost and climatic change in northwest Canada and Alaska, emphasizing insights gained from field and theoretical studies. Within the region, the ground temperature profile in permafrost shows the impact of climatic warming over the last 30 to 150 years, and cryostratigraphy records the effect of the warmest climatic period of the Holocene. Research on heat transfer has provided a model for the translation of the thermal regime at the ground surface into the temperature at the surface of permafrost, while n-factors are used to obtain ground surface temperatures from the air temperature. Field evidence from the discontinuous permafrost zone indicates the considerable persistence of permafrost following climate change, due to latent heat contained in ground ice. GCMs indicate that future climate change at these latitudes may be most apparent in winter, and will therefore affect both nf and DDFa. The active layer will respond to such effects, but its most rapid response will still be to changes in surface conditions such as those following forest fire. At this point, then, some suggestions are offered on outstanding problems and future work. From a practical perspective, it is essential that various monitoring programs, which have emphasized this region, continue to collect data on a consistent basis. In particular, the value of the CALM program (Brown, 1997), which includes a transect of Mackenzie Valley (Nixon and Taylor, 1998), and key sites in Alaska (Nelson et al., 1997), increases as time passes, the record is extended, and trends, cycles and unusual events are recognized (Burt, 1994). Similarly, the value of near-surface ground temperature monitoring increases with time, but the significance of such records can only be evaluated if they are continuous and of quality. Recent 116 changes in emphasis for Federal Departments in Canada mean that such programs become difficult to continue. Instead, recognition of the potential for spatially extensive investigations using information technology is growing (Nelson et al., 1997). It would be dangerous, though attractive, to assume that computer modelling may be substituted for field investigations. Within such programs, however, the issue raised by Smith and Riseborough (1983) of the impact of microclimatic modulation on the response of permafrost to climate change remains outstanding. The model of air temperature - permafrost relations developed by Romanovsky and Osterkamp (1995) has been validated for cold permafrost and may be widely applied to estimate equilibrium conditions in such terrain, if the n-factors are known. The variation in n-factor with snow conditions is not well described, but appears to change abruptly across treeline (Smith et al., 1998). Within vegetation units there is, as yet, little assessment of the variation in n-factor from site to site, or from year to year at the same site. The model has yet to be validated for warm permafrost in fine-grained soil, and is formulated for equilibrium conditions. The transient response of permafrost to climate change is not easily estimated because of the number of compounding variables set in a context of a naturally varying climatic system. At a site scale, calibrated numerical models may provide precise predictions of the thermal regime of the active layer and ÒwarmÓ permafrost, even under transient conditions. Efficient extrapolation of these results in a regional context is a significant challenge, one which I doubt will be overcome without continuing conscientious efforts from field workers. Acknowledgments The research program has been supported by the National Sciences and Engineering Research Council of Canada, the Polar Continental Shelf Project (PCSP) and the Geological Survey of Canada, Natural Resources Canada, the Inuvik Research Centre of Aurora College, the Northern Research Institute of Yukon College, the Atmospheric Environment Service, Environment Canada, and the Northern Affairs Program of Indian Affairs and Northern Development Canada. Assistance from many people in the Yukon and Mackenzie delta area, particularly Jim and Shann Carmichael of Mayo, Scott Smith of Whitehorse, and Les Kutny and Alan Fehr of Inuvik is acknowledged with gratitude. I thank J.R. Mackay and M.W. Smith for constant stimulation and encouragement, and Joan Ramsay Burn for her support of these endeavours. J. Brown, H.M. French, J.R. Mackay, T.E. Osterkamp, D.W. Riseborough and M.W. Smith provided helpful comments on the manuscript. PCSP contribution 00498. The 7th International Permafrost Conference References Anisimov, O.A. and Nelson, F.E. (1996). Permafrost distribution in the northern hemisphere under scenarios of climatic change. Global and Planetary Change, 14, 59-72. Anisimov, O.A. and Nelson, F.E. (1997). Permafrost zonation and climate change in the northern hemisphere: results from transient general circulation models. Climatic Change, 35, 241-258. Anisimov, O.A., Shiklomanov, N.I. and Nelson, F.E. (1997). Global warming and active-layer thickness: results from transient general circulation models. Global and Planetary Change, 15, 61-77. Arctic Environmental Information and Data Center (1986). Alaska climate summaries. Alaska Climate Center, Technical Note 3. Baker, G.C. and Osterkamp, T.E. (1988). Implications of salt fingering processes for salt movement in thawed coarsegrained subsea permafrost. Cold Regions Science and Technology, 15, 45-52. Brown, J. (1997). Circumpolar active layer monitoring (CALM) network - November 1997. Frozen Ground, 21, 2223. Brown, R.J.E. (1964). Permafrost investigations on the Mackenzie Highway in Alberta and Mackenzie District. National Research Council of Canada, Division of Building Research, Technical Paper 175. Brown, R.J.E. (1967). Permafrost in Canada. Geological Survey of Canada, Map 1246A. Burt, T.P. (1994). Long-term study of the natural environment Ð perceptive science or mindless monitoring? Progress in Physical Geography, 18, 475-496. Burn, C.R. (1986). The development of near-surface ground ice during the Holocene at sites near Mayo, Yukon Territory, Canada. Journal of Quaternary Science, 3, 31-38. Burn, C.R. (1990). Implications for paleoenvironmental reconstruction of recent ice-wedge development at Mayo, Yukon Territory. Permafrost and Periglacial Processes, 1, 3-14. Burn, C.R. and Smith, C.A.S. (1988). Observations of the "thermal offset" in near-surface mean annual ground temperatures at several sites near Mayo, Yukon Territory, Canada. Arctic, 41, 99-104. Burn, C.R. and Smith, M.W. (1993). Issues in Canadian permafrost research. Progress in Physical Geography, 17, 156172. Cheng, G. (1983). The forming process of thick-layered ground ice. Cold Regions Science and Technology, 8, 57-66. Clebsch, E.E.C. and Shanks, R.E. (1968). Summer climatic gradients and vegetation near Barrow, Alaska. Arctic, 21, 161-171. Cohen, S.J. (1997). What if and so what in northwest Canada: Could climate change make a difference to the future of the Mackenzie Basin? Arctic, 50, 293-307. Environment Canada. (1993). Canadian climate normals, 19611990, Yukon and Northwest Territories. Canadian Climate Program, Environment Canada, Ottawa. Fraser, T.A. and Burn, C.R. (1997). On the nature and origin of "muck" deposits in the Klondike area, Yukon Territory. Canadian Journal of Earth Sciences, 34, 1333-1344. French, H.M. and Egorov, I.E. (1998). Twentieth century variations in permafrost, Thompson, northern Manitiba, Canada. In Proceedings 7 th International Conference on Permafrost, Yellowknife, Canada. Nordicana, QuŽbec, p. 297-304. Gavrilova, M.K. (1993). Present human induced change and cryoecology. In Proceedings 6th International Conference on Permafrost, Beijing, China. South China University of Technology Press, Wushan Guangzhou. Vol. 2, pp. 10061009. Goodrich, L.E. (1982). The influence of snow cover on the ground thermal regime. Canadian Geotechnical Journal, 19, 421-432. Grescoe, T. (1997). Temperature rising. Canadian Geographic, 117, 36-44. Burn, C.R. (1992). Recent ground warming inferred from the temperature in permafrost near Mayo, Yukon Territory. In Dixon, J.C. and Abrahams, A.D. (eds.), Periglacial geomorphology. Wiley, Chichester, pp. 327-350. Halsey, L.A., Vitt, D.H. and Zoltai, S.C. (1995). Disequilibrium response of permafrost in boreal continental western Canada to climate change. Climatic Change, 30, 57-73. Burn, C.R. (1993). Comments on "Detection of climatic change in the western North American Arctic using a synoptic climatological approach". Journal of Climatology, 6, 1473-1475. Harris, C. (1997). Permafrost and climate in Europe (PACE). Frozen Ground, 21, 3-4. Burn, C.R. (1994). Permafrost, tectonics, and past and future regional climate change, Yukon and adjacent Northwest Territories. Canadian Journal of Earth Sciences, 31, 182-191. Burn, C.R. (1997). Cryostratigraphy, paleogeography, and climate change during the early Holocene warm interval, western Arctic coast, Canada. Canadian Journal of Earth Sciences, 34, 912-925. Burn, C.R. (1998). The response (1958 to 1997) of permafrost and near-surface ground temperatures to forest fire, Takhini River valley, southern Yukon Territory. Canadian Journal of Earth Sciences, 35, 184-199. Burn, C.R., Michel, F.A. and Smith, M.W. (1986). Stratigraphic, isotopic, and mineralogical evidence for an early Holocene thaw unconformity at Mayo, Yukon Territory, Canada. Canadian Journal of Earth Sciences, 23, 794-803. Harry, D.G. and MacInnes, K.L. (1988). The effect of forest fires on permafrost terrain stability, Little Chicago Travaillant Lake area, Mackenzie Valley, N.W.T. In Current Research, Part D, Geological Survey of Canada Paper, 88-1D, pp. 91-94. Haugen, R.K. and Brown, J. (1980). Coastal-inland distributions of summer air temperature and precipitation in northern Alaska. Arctic and Alpine Research, 12, 403-412. Heginbottom, J.A. (1995). Canada- Permafrost. In National Atlas of Canada, 5th ed. Natural Resources Canada, Ottawa. Plate 2.1 Hemstock, R.A. (1953). Permafrost at Norman Wells, N.W.T. Imperial Oil Ltd., Calgary. Hinkel, K.M. and Outcalt, S.I. (1993). Detection of nonconductive heat transport in soils using spectral analysis. Water Resources Research, 29, 1017-1023. C.R. Burn 117 Hinkel, K.M. and Outcalt, S.I. (1994). Identification of heattransfer processes during soil cooling, freezing, and thaw in central Alaska. Permafrost and Periglacial Processes, 5, 217235. Hinkel, K.M., Outcalt, S.I. and Taylor, A.E. (1997). Seasonal patterns of coupled flow in the active layer at three sites in northwest North America. Canadian Journal of Earth Sciences, 34, 667-678. Hughes, O.L. (1983). Surficial geology and geomorphology, Janet Lake, Yukon Territory. Geological Survey of Canada Map 4-1982. Jorgenson, M.T. and Kreig, R.A. (1988). A model for mapping permafrost distribution based on landscape component maps and climatic variables. In Proceedings 5th International Conference on Permafrost, Trondheim, Norway. Tapir Publishers, Trondheim, Vol. 3, pp. 176-182. Judge, A.S., Pelletier, B.R. and Norquay, I. (1987). Permafrost base and distribution of gas hydrates. In Pelletier, B.R. (ed.), Marine science atlas of the Beaufort Sea: geology and geophysics. Geological Survey of Canada, Miscellaneous Report 40, Map 39. Kalkstein, L.S., Dunne, P.C. and Vose, R.S. (1990). Detection of climatic change in the western North American Arctic using a synoptic climatological approach. Journal of Climate, 3, 1153-1167. Kane, D.L., Hinzman, L.D. and Zarling, J.P. (1991). Thermal response of the active layer to climatic warming in a permafrost environment. Cold Regions Science and Technology, 19, 111-122. Kotler, E. and Burn, C.R. (1998). The cryostratigraphy of unconsolidated material overlying auriferous creek gravel, Klondike area, Yukon Territory, Canada. In Program, Abstracts and IPA Reports, 7th International Conference on Permafrost, Yellowknife, Canada Kudryavtsev, V.A., Garagulya, L.S., Kondratyeva, K.A. and Melamed, V.G. (1974). Fundamentals of frost forecasting in geological engineering investigations. (In Russian.) Nauka, Moscow. Translated in United States Army Cold Regions Research and Engineering Laboratory Draft Translation 606. Kurfurst, P.J., Isaacs, R.M., Hunter, J.A. and Scott, W.J. (1974). Permafrost studies in the Norman Wells region, Northwest Territories. In Aitken, J.D. and Glass, D.J. (eds.), Proceedings Symposium on the Geology of the Canadian Arctic, Saskatoon, Canada. Geological Association of Canada Canadian Society of Petroleum Geologists, pp. 277-299. Kwong, Y.T.J. and Gan, T.Y. (1994). Northward migration of permafrost along the Mackenzie Highway and climatic warming. Climatic Change, 26, 399-419. Lachenbruch, A.H. and Marshall, B.V. (1969). Heat flow in the Arctic. Arctic, 22, 300-311. Lachenbruch, A.H. and Marshall, B.V. (1986). Changing climate: geothermal evidence from permafrost in the Alaskan arctic. Science, 234, 689-696. Lachenbruch, A.H., Cladouhos, T.T. and Saltus, R.W. (1988). Permafrost temperatures and the changing climate. In Proceedings 5 th International Conference on Permafrost, Trondheim, Norway. Tapir Publishers, Trondheim, Vol. 3, pp. 9-17. Lunardini, V.J. (1978). Theory of n-factors and correlation of data. In Proceedings 3 rd International Conference on Permafrost, Edmonton, Alberta. National Research Council of Canada, Ottawa, Vol. 1, pp. 40-46. Lunardini, V.J. (1981). Heat transfer in cold climates. Van Nostrand Reinhold, New York. 118 Mackay, J.R. (1970). Disturbances to the tundra and forest tundra environment of the western Arctic. Canadian Geotechnical Journal, 7, 420-432. Mackay, J.R. (1974). Seismic shot holes and ground temperatures, Mackenzie Delta area, Northwest Territories. In Geological Survey of Canada Paper, Part A, 74-1, pp. 389-390. Mackay, J.R. (1975a). The stability of permafrost and recent climatic change in the Mackenzie Valley, N.W.T. In Geological Survey of Canada Paper, Part B, 75-1, pp. 173-176. Mackay, J.R. (1975b). Relict ice wedges, Pelly Island, N.W.T. (107 C/12). In Geological Survey of Canada Paper, Part A, 751, pp. 469-470. Mackay, J.R. (1978). Freshwater shelled invertebrate indicators of paleoclimate in northwestern Canada during late glacial times: Discussion. Canadian Journal of Earth Sciences, 15, 461-462. Mackay, J.R. (1992). Lake stability in an ice-rich permafrost environment: examples from the western arctic coast. In Robarts, R.D. and Bothwell, M.L. (eds.), Aquatic ecosystems in semi-arid regions: implications for resource management. Environment Canada, Saskatoon, Sask. National Hydrology Research Institute Symposium Series 7, pp. 126. Mackay, J.R. and Matthews, W.H. (1973). Geomorphology and Quaternary history of the Mackenzie River valley near Fort Good Hope, N.W.T., Canada. Canadian Journal of Earth Sciences, 10, 26-41. Maxwell, B. (1997). Responding to global climate change in Canada's Arctic. The Canada Country Study: Climate Impacts and Adaptation, Vol. 2. Environment Canada, Ottawa. Murton, J.B. and French, H.M. (1993). Sand wedges and permafrost history, Crumbling Point, Pleistocene Mackenzie delta, Canada. In Proceedings 6th International Conference on Permafrost, Beijing, China. South China University of Technology Press, Wushan Guangzhou. Vol. 1, pp. 482487. Murton, J.B., French, H.M. and Lamothe, M. (1997). Late Wisconsinan erosion and eolian deposition, Summer Island area, Pleistocene Mackenzie Delta, Northwest Territories: optical dating and implications for glacial chronology. Canadian Journal of Earth Sciences, 34, 190-199. Nelson, F.E. and Outcalt, S.I. (1987). A computational method for prediction and regionalization of permafrost. Arctic and Alpine Research, 19, 279-288. Nelson, F.E., Shiklomanov, N.I., Mueller, G.R., Hinkel, K.M., Walker, D.A. and Bockheim, J.G. (1997). Estimating active-layer thickness over a large region: Kuparuk River basin, Alaska, U.S.A. Arctic and Alpine Research, 29, 367-378. Nelson, F.E., Lachenbruch, A.H., Woo, M-K., Koster, E.A., Osterkamp, T.E., Gavrilova, M.K. and Cheng, G. (1993). Permafrost and changing climate. In Proceedings 6 th International Conference on Permafrost, Beijing, China. South China University of Technology Press, Wushan Guangzhou. Vol. 2, pp. 987-1005. Nixon, F.M. and Taylor, A.E. (1998). Regional and temporal active layer monitoring transect from the sporadic to continuous permafrost zone, Mackenzie Valley, northwest Canada. In Proceedings 7 th International Conference on Permafrost, Yellowknife, Canada. Nordicana, QuŽbec, pp. 815-820 Nixon, F.M., Taylor, A.E., Allen, V.S. and Wright, J.F. (1995). Active layer monitoring in natural environments, lower Mackenzie Valley, Northwest Territories. In Current Research,Geological Survey of Canada, 1995-B, pp. 99-108. The 7th International Permafrost Conference Osterkamp, T.E. and Gosink, J.P. (1991). Variations in permafrost thickness in response to changes in paleoclimate. Journal of Geophysical Research, 96, 4423-4434. Osterkamp, T.E. and Romanovsky, V.E. (1996). Characteristics of changing permafrost temperatures in the Alaskan Arctic. Arctic and Alpine Research, 28, 267-273. Osterkamp, T.E. and Romanovsky, V.E. (1997). Freezing of the active layer on the coastal plain of the Alaskan arctic. Permafrost and Periglacial Processes, 8, 23-44. Osterkamp, T.E., Zhang, T. and Romanovsky, V.E. (1994). Evidence for a cyclic variation of permafrost temperatures in northern Alaska. Permafrost and Periglacial Processes, 5, 137-144. Outcalt, S.I. and Hinkel, K.M. (1996). Thermally-driven sorption, desorption, and moisture migration in the active layer in central Alaska. Physical Geography, 17, 74-90. Outcalt, S.I., Nelson, F.E. and Hinkel, K.M. (1990). The zerocurtain effect: heat and mass transfer across an isothermal region in freezing soil. Water Resources Research, 26, 15091516. Overpeck, J., Hughen, K, Hardy, D., Bradley, R., Case, R., Douglas, M, Finney, B., Gajewski, K, Jacoby, G., Jennings, A., Lamoureux, S., Lasca, A., MacDonald, G., Moore, J., Retelle, M., Smith, S., Wolfe, A. and Zielinski, G. (1997). Arctic environmental change of the last four centuries. Science, 278, 1251-1256. Pollard, W.H. and French, H.M. (1980). A first approximation of the volume of ground ice, Richards Island, Pleistocene Mackenzie Delta, Northwest Territories, Canada. Canadian Geotechnical Journal, 17, 509-516. Rampton, V.N. (1988). Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories. Geological Survey of Canada Memoir, 423. Riseborough, D.W. (1990). Soil latent heat as a filter of the climate signal in permafrost. In Proceedings 5 th Canadian Permafrost Conference, QuŽbec City, QuŽbec. Nordicana, No. 54, QuŽbec, pp. 199-205. Riseborough, D.W. and Smith, M.W. (1993). Modelling permafrost response to climate change and climate variability. In Lunardini, V.J. and Bowen, S.L. (eds.), Proceedings 4th International Symposium on Thermal Engineering and Science for Cold Regions, Hanover, New Hampshire. US Army Cold Regions Research and Engineering Laboratory, Special Report 93-22, pp.179-187. Riseborough, D.W. and Smith, M.W. (1998). Defining the limits of permafrost. In Proceedings 7 th International Conference on Permafrost, Yellowknife, Canada. Nordicana, QuŽbec, pp. 935-942. Romanovsky, V.E. and Osterkamp, T.E. (1995). Interannual variations of the thermal regime of the active layer and near-surface permafrost in northern Alaska. Permafrost and Periglacial Processes, 6, 313-335. Romanovsky, V.E. and Osterkamp, T.E. (1997). Thawing of the active layer on the coastal plain of the Alaskan Arctic. Permafrost and Periglacial Processes, 8, 1-22. Romanovsky, V.E., Osterkamp, T.E. and Duxbury, N.S. (1997). An evaluation of three numerical models used in simulations of the active layer and permafrost temperature regimes. Cold Regions Science and Technology, 26, 195203. SeppŠlŠ, M. (1982). An experimental study of the formation of palsas. In French, H.M. (ed.), Proceedings 4th Canadian Permafrost Conference, Calgary, Alberta. National Research Council of Canada, Ottawa, pp. 36-42. Shur, Y.L. and Slavin-Borovskiy, V.B. (1993). N-factor maps of Russian permafrost region. In Proceedings 6 th International Conference on Permafrost, Beijing, China. South China University of Technology Press, Wushan Guangzhou. Vol. 1, pp. 564-568. Smith, C.A.S., Burn, C.R., Tarnocai, C. and Sproule, B. (1998). Air and soil temperature relations along an ecological transect through the permafrost zones of northwestern Canada. In Proceedings 7 th International Conference on Permafrost, Yellowknife, Canada. Nordicana, QuŽbec, pp. 1009-1016. Smith, M.W. (1975). Microclimatic influences on ground temperatures and permafrost distribution, Mackenzie Delta, Northwest Territories. Canadian Journal of Earth Sciences, 12, 1421-1438. Smith, M.W. (1977). Computer simulation of microclimatic and ground thermal regimes: test results and program description. Indian Affairs and Northern Development Canada, ALUR Report 75-76-72. Smith, M.W. and Riseborough, D.W. (1983). Permafrost sensitivity of climatic change. In Proceedings 4th International Conference on Permafrost, Fairbanks, Alaska. National Academy Press, Washington, DC, Vol. 1, pp. 1178-1183. Smith, M.W. and Riseborough, D.W. (1996). Permafrost monitoring and detection of climatic change. Permafrost and Periglacial Processes, 7, 301-309. Steven, G.P. (1982). The moving interface problem with finite elements. Advances in Water Resources, 5, 29-34. Stuart, R.A. and Judge, A.S. (1991). On the applicability of GCM estimates to scenarios of global warming in the Mackenzie Valley area. Climatological Bulletin, 25, 147-169. Taylor, A.E. (1995). Field measurements of n-factors for natural forest areas, Mackenzie Valley, Northwest Territories. In Current Research, Geological Survey of Canada, 1995-B, pp. 89-98. Taylor, A.E., Dallimore, S.R. and Judge, A.S. (1996). Late Quaternary history of the Mackenzie-Beaufort region, Arctic Canada, from modelling of permafrost temperatures. 2. The Mackenzie Delta - Tuktoyaktuk Coastlands. Canadian Journal of Earth Sciences, 33, 62-71. Taylor, A.E., Nixon, M., Eley, J., Burgess, M. and Egginton, P. (1998). Effect of atmospheric temperature inversions on ground surface temperatures and discontinuous permafrost, Norman Wells, Mackenzie Valley, Canada. In Proceedings 7 th International Conference on Permafrost, Yellowknife, Canada. Nordicana, QuŽbec, pp. 1043-1048. Thie, J. (1974). Distribution and thawing of permafrost in the southern part of the discontinuous permafrost zone in Manitoba. Arctic, 27, 189-200. Vitt, D.H., Halsey, L.A. and Zoltai, S.C. (1994). The bog landforms of continental western Canada in relation to climate and permafrost patterns. Arctic and Alpine Research, 26, 113. Wahl, H.E., Fraser, D.B., Harvey, R.C. and Maxwell, J.B. (1987). Climate of Yukon. Climatological Studies 40, Atmospheric Environment Service, Environment Canada, Ottawa. Zhang, T. and Osterkamp, T.E. (1993). Changing climate and permafrost temperatures in the Alaskan arctic. In Proceedings 6 th International Conference on Permafrost, Beijing, China. South China University of Technology Press, Wushan Guangzhou. Vol. 1, pp. 783-788. Zhang, T., Osterkamp, T.E. and Stamnes, K. (1996a). Some characteristics of the climate in northern Alaska. Arctic and Alpine Research, 28, 509-518. C.R. Burn 119 Zhang, T., Osterkamp, T.E. and Stamnes, K. (1996b). Influence of the depth hoar layer of the seasonal snow cover on the ground thermal regime. Water Resources Research, 32, 2075-2086. Zhang, T., Osterkamp, T.E. and Stamnes, K. (1997). Effects of climate on the active layer and permafrost on the North Slope of Alaska, USA. Permafrost and Periglacial Processes, 8, 45-67. Zoltai, S.C. (1993). Cyclic development of permafrost in the peatlands of northwestern Alberta, Canada. Arctic and Alpine Research, 25, 240-246. 120 The 7th International Permafrost Conference