Field Investigations of Permafrost and Climatic Change in Northwest North America

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 to follow climate warming is acknowledged (Mackay, 1975a). Permafrost is a geologic manifestation of climate, so permafrost conditions should change over time. On the 1967 Permafrost Map of Canada, R.J.E. Brown Instrumental and paleoenvironmental records indicate (1967) implicitly recognized the importance of climatic that the climate is warming faster in the Arctic than at change. Although the primary purpose of the map was lower latitudes of the Northern Hemisphere, and that to indicate the spatial extent of permafrost, Brown the warming has been greater in the 20th century than chose the Ð5¼C mean annual air temperature isotherm in the previous 400 years (Overpeck et al., 1997). The to separate the continuous and discontinuous zones. He response of permafrost to climate change, a theme of recognized that, over the long term, a climatic warming this conference, is the focus of several research projects of over 5¼C would be required to degrade permafrost in supported by the International Permafrost Association the continuous zone. A climatic shift of such magnitude (e.g., Brown, 1997; Harris, 1997). In Canada the is not common during an interglacial period, although Mackenzie Basin Impact Study, a multi-disciplinary smaller fluctuations occur. Therefore, in the continuous project, recently produced its final report on the poten- permafrost zone, permafrost is continuous in time as tial impact of climate change on the region, and con- well as space, and is discontinuous in these dimensions cluded that a principal threat to the landscape was to the south. "accelerated erosion and landslides caused by permafrost thaw .... especially in sloping terrain and the The response of permafrost to climate change is Beaufort Sea coastal zone" (Cohen, 1997, 297). Air tem- shown by changes in the ground temperature profile, or perature in parts of Mackenzie Basin has risen by 1.5¼C in the depth of the active layer, or both. In this paper, over the last century (Maxwell, 1997), and, in the popu- research on permafrost and climate change will be con- lar press, increased geomorphological activity has been sidered under four themes: (1) historical climate: per- attributed to such warming (e.g., Grescoe, 1997). In con- mafrost relations; (2) cryostratigraphic relations; (3) trast, the scientific literature has ascribed past mass relations between near-surface ground temperatures wasting in Mackenzie Valley to site-specific distur- and present climate; and (4) the impact of climate bances (Mackay and Matthews, 1973; Harry and change on permafrost distribution. The purpose of the MacInnes, 1988), although the potential for these events 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 enter- ing 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 well- developed 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). 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.

108 The 7th International Permafrost Conference Scandinavia for the region. In Scandinavia there is rela- upwards, at a rate less than 2 cm a-1, with heat supplied tively little permafrost, and so the GCM results are by the geothermal flux, so the response of permafrost unsuitable for investigations of potential climate thickness is over glacial time scales (Osterkamp and change in northwest North America (Stuart and Judge, Gosink, 1991). 1991). Field experiments by SeppŠlŠ (1982) demonstrated the importance of snow cover on permafrost distrib- By carefully monitoring sites near the north coast of ution in the discontinuous zone, and this critical vari- Alaska between 1983 and 1993, Osterkamp et al. (1994) able is poorly represented for the region in GCMs, detected a cycle of about 10 years in ground tempera- because the rainshadow caused by the coastal moun- tures. The derived amplitude at the permafrost surface tains is not reproduced (Burn, 1994). decreased inland from 2¼C at the coast. The magnitude of fluctuation near the coast may represent sensitivity In a similar fashion, a steep summer climatic gradient to maritime effects, particularly sea ice, on air tempera- in the Beaufort Sea coastal zone, due to the presence of ture and snow cover, although Osterkamp et al. (1994) proximal pack ice offshore (Haugen and Brown, 1980; drew attention to the coincidence of the period with Zhang et al., 1996a), is not apparent at the scale of pre- sunspot activity. Subsequently, Osterkamp and sent GCMs. However cooler winter temperatures Romanovsky (1996) supplemented these data with inland offset this gradient on an annual basis, so that measurements taken between 1986 and 1993 from the MAAT changes little with distance from the coast. upper 20 m of permafrost, which were consistent with Nevertheless, the summer gradient controls the devel- the original interpretation. However, they were unable opment of vegetation communities, which, in turn, to judge whether the near-surface temperature series impact snow accumulation and, hence, near-surface formed part of an overall warming or was the rising ground temperature and active-layer development limb of a cyclic fluctuation. (Clebsch and Shanks, 1968; Mackay, 1974; Romanovsky and Osterkamp, 1995; Nelson et al., 1997). Changes in The ground temperature profile in the discontinuous conditions along the gradient may not be uniform permafrost zone has also responded to the 20th century 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).

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 Figure 3. Temperature profiles in permafrost, summer, 1990 and, 1997, at a fluctuation. In thick permafrost, where AMPST remains 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 glaciolacus- below 0¼C, degradation occurs from the bottom trine 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.

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 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 - variations in hydroclimate implicitly. For explicit evi- 1988 supplied by Atmospheric Environment Service, and for 1989 - 1997 dence of changes in seasonal and moisture regimes, we from station weather record at Mayo Airport. must turn to the cryostratigraphy. climate warming, but this has received less attention. For the same increase in heat flux, the temperature of Cryostratigraphic relations ÒwarmÓ permafrost may change less than that of Òcold- erÓ permafrost due to latent heat effects (Riseborough, Two recent sets of cryostratigraphic observations from 1990). Several temperature profiles measured at the region relate to climatic conditions during the last Norman Wells by Imperial Oil Ltd in the late 1940Õs and glacial maximum (c. 20,000 yr. BP). The first are obser- early 1950Õs indicated curvature to depths of 50 m and vations of sand and ice wedges on and around Richards near-surface warming of 3¼C (Hemstock, 1953; Mackay, Island, which indicate that the region experienced cold, 1975a), following the warmest air temperatures of this dry conditions conducive to eolian activity (Murton century. Curvature was still apparent in the upper por- and French, 1993; Murton et al., 1997). The second con- tions of temperature profiles recorded in central cern data from the Klondike area, where Fraser and Mackenzie Valley during the early 1970Õs (Kurfurst et Burn (1997) determined that the ice-rich, unconsolidat- al., 1974), even though air temperatures had declined. ed silt deposits, which mantle auriferous gravel in valley bottoms, are of Late Wisconsinan age. The absence Climate variation in central and southern Yukon is of massive icy bodies from the lowermost parts of most dominated by fluctuations in winter conditions, with silt sections suggests this area, too, was arid during the snow accumulation being positively associated with glacial maximum. Data on the cryostratigraphy of these winter air temperature (Burn, 1990). In 1990, the tem- unconsolidated materials are presented by Kotler and perature profile in permafrost at Mayo (Figure 1) was Burn (1998), who have determined that most of the consistent with a warming, since the early 1970s, of ground ice in the silts formed at the very end of the 1.25¼C in AMPST (Burn, 1992). Subsequently, the Wisconsinan, after 12,000 14C years BP. ground has cooled by 0.2¼C or more in the upper 15 m of permafrost (Figure 3), in association with reduced The principal cryostratigraphic data relevant to our snow accumulation (Figure 4). Five temperature pro- topic have been observations of truncated ice wedges files through permafrost measured at different sites associated with an early Holocene thaw unconformity

110 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

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 Figure 5. Potential terrain subsidence (cm) at two sites on Pelly Island, Takhini River valley, near Whitehorse, Y.T. (Figure 1), N.W.T. The two profiles represent material between the base of the present ground temperatures have been monitored monthly or 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 more frequently since 1991 (Burn, 1998). At the Takhini excess ice content of near-surface permafrost obtained by core drilling, using site, which is in a spruce forest, the AMPST is -0.8¼C, 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 the active layer is 1.4 m thick, the base of permafrost is layer, there will be at least 10 cm of subsidence and at most 10 cm of active- at 18.5 m, and the temperature gradient in the per- layer 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. mafrost is linear. The site is 2.3¼C cooler than Whitehorse Airport, but air temperature series from in many parts of the region (Mackay, 1975b, 1978, 1992; these locations have a coefficient of determination (r2) Burn et al., 1986; Burn, 1997). The unconformity records of 0.98, indicating that fluctuation is synchronous and the maximum development of the active layer around of proportional magnitude. As with other sites in sou- 9000 cal. years BP. Usually, segregated ice is abundant thern and central Yukon, monthly mean air tempera- between the unconformity and the base of the present tures at Whitehorse exhibit little variation during sum- active layer (e.g., Pollard and French, 1980), complicat- mer, in contrast with the winter. As a result, the active ing estimates of paleoactive-layer thickness. layer depth has remained almost constant during the period of observation. Burn (1997, Figure 9) presented two profiles from Richards Island in which the depth of the paleoactive Figure 6 indicates monthly mean air temperatures layer was estimated by removing the excess ice which measured at Whitehorse Airport and ground tempera- postdates the unconformity. The method requires tures at 1.5 m depth from the Takhini valley site. There analysis as outlined by Mackay (1970, Figure 3). The is relatively little snow accumulation at the site (maxi- reconstructed paleoactive-layer depth for the coastal mum 35 cm), because of arid conditions in the valley, region of the Canadian western Arctic was about twice and interception by the forest canopy. Overall, the the present thickness. A considerable portion of the -1 increase in temperature relative to the present regime ground temperature series has decreased by 0.07¼C a , generating the deeper summer thaw is from the paleo- in response to recent relatively cool winters. A similar geography of the region rather than climatic change per trend (cooling at 0.02¼C a-1) summarizes temperature se. The sites examined were further from the coast 8000 variation at 50 cm depth in the active layer. Data collec- to 9000 years ago than they are today, and therefore ted at irregular intervals, mostly in winter, for 1982 - benefited from the regional summer temperature gradi- 1990 show similarly little temporal trend (Burn, 1998). ent (Burn, 1997; see Figure 2). Instead, a great variation in ground temperatures occurs in this area between vegetation units, and per- Similar data on the paleoactive layer from Pelly Island mafrost degrades once the spruce forest is cleared. have been rearranged to estimate the subsidence that may occur if the present active layer deepens following At the Fifth International Conference on Permafrost, climate warming (Figure 5). These data provide an indi- A.H. Lachenbruch pointed out that assessment of the cation of the extent of terrain disturbance that may response of permafrost to climate change involves mo-

C.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 tures measured at intervals of one hour during this climate; the ground surface and active layer; and per- period exhibit variations which have been attributed to mafrost (Lachenbruch et al., 1988). Critical relations are evaporation, vapour flow, and condensation, driven by those between air temperature and ground surface tem- osmotic gradients (Outcalt et al., 1990; Outcalt and perature, and between the ground surface temperature Hinkel, 1996). In some areas of arctic Alaska, the zero and permafrost surface temperature. Permafrost is a curtain period may last two months or more, so the medium within which energy is transferred dominantly potential total transfer may be a considerable portion of by conduction, and there are well-known methods for the annual budget. This portion is not estimated pre- determining the response of the temperature profile in cisely, and may differ between soil types. However, permafrost to fluctuations in AMPST (Lachenbruch et Osterkamp and Romanovsky (1997) found little evi- al., 1988). Outstanding problems are the coupling of dence of non-convective effects in seven yearsÕ data col- atmospheric temperature changes to changes in ground lected at three sites on the North Slope of Alaska, and surface temperature, and transmission of changes in the concluded that the thermal regime could be modelled ground surface temperature through the active layer to adequately by conductive processes alone. permafrost. Currently there are several approaches con- tributing to progress on these aspects. Convective transport is clearly important in gravel and coarse sand, because iron-stained gravel, well CONVECTIVE EFFECTS below the present active layer, and associated with the Of fundamental significance to the energy transfer early Holocene thaw unconformity, has been observed through the active layer is the mode of such exchange. by J.R. Mackay (personal communication, 1996) at sev- Many formulations of these relations assume that the eral sites in the Mackenzie delta area. Baker and heat flow is exclusively by conduction (e.g., Goodrich, Osterkamp (1988) have reported direct observations of 1982; Kane et al., 1991). However, Hinkel and Outcalt convective heat transport in coarse-grained subsea per- (1994) have demonstrated that convective transfer mafrost. Similarly, in organic soils the potential for occurs within the active layer during snowmelt and vapour flow is significant and demonstrated (Outcalt during infiltration of precipitation in summer (Hinkel and Hinkel, 1996). In fine-grained mineral soil the influ- et al., 1997). Sudden increases in soil temperature that ence of convective transfer should be much less. cannot be generated by conduction have been recorded, and similar conclusions have been drawn from spectral Figure 7 presents ground temperatures measured over analyses of soil temperature time series (Hinkel and a year, beginning on 1 August 1996, at 1 m depth in the Outcalt, 1993). floor of a thaw slump near Mayo, Y.T. (Figure 1), where permafrost is degrading. A similar series is also present- In addition, minimal conduction may also occur dur- ed from an undisturbed site nearby. The soil at both ing the zero-curtain period of freeze-up. Soil tempera-

112 The 7th International Permafrost Conference an equation for the offset (Romanovsky and Osterkamp, 1995, eq. 12):

[1] DDTstæ K ö Offset =-ç 1÷ P è K f ø

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 determina- Figure 7. Daily mean ground temperature series at a disturbed site in a ret- rogressive thaw slump, and an undisturbed site in permafrost near Mayo, tions of the thermal offset from a numerical model Y.T., for one year beginning on 1 August 1996. The data are daily means of (Goodrich, 1982), and with the field data. In both cases observations collected every 4.8 hours from 1 m depth by data logger. the agreement was excellent, indicating the applicabili- sites is a glaciolacustrine silty clay, and the permafrost ty of a conductive model at this scale in saturated, fine- table is now 4.5 m below the surface at the site in the grained soils. The greatest difference between field slump. The active layer at the undisturbed site is about observations and equation [1] occurred in the warmest 80 cm thick. There is scant evidence of convective year at the warmest site. This suggests that while the effects at the undisturbed site, where the sensor is in equation may be effective for cold permafrost, its appli- permafrost. At the site in the slump, convective transfer cation may be more limited in the discontinuous zone, may be interpreted in early May, during snowmelt, and or under transient conditions. It may be inappropriate in late July as a result of rainfall. In the rest of the year, at warmer sites with fine-grained soil to characterize the smooth temperature transitions suggest that the thermal conductivity as a bimodal function over the heat flow follows a conductive regime. Although the annual cycle. To date a validation of the model against time interval of data collection (4.8 hours, presented in field data from a region with warm permafrost, e.g., an Figure 7 as a daily mean temperature) may be insuffi- AMPST close to -1¼C, has not been published. cient to detect convective activity, the net proportion of the annual thermal regime attributed to convection Romanovsky and Osterkamp (1995, eq. 13) also appears small. The result is that the thermal regime derived the equation presented by Kudryavtsev (1981) may be modelled adequately as a conductive system. for the AMPST in terms of the thermal regime at the Similarly, data on long-term (1958-1997) permafrost ground surface: degradation collected at a site in Takhini valley burned by forest fire are consistent with the Stefan solution, kts DDT- kf DDFs [2] and indicate that, in similar fine-grained sediments, the AMPST = thermal regime may be considered conductive (Burn, kPf 1998). Any convective effects are masked by conduction because the sediments are fine-grained. where DDFs is the annual total freezing degree-days THERMAL OFFSET at the ground surface. The outstanding value of equa- Analysis of soil temperatures, collected at several sites tion [2] is the succinct expression of a fundamental rela- on a transect southward from the Alaskan coast at tion. It identifies critical summary values for the tem- Prudhoe Bay between 1986 and 1992, has provided the perature regime at the ground surface (DDFs, DDTs) basis for estimating the "thermal offset" (Romanovsky and lithology (kf, kt) governing the difference between and Osterkamp, 1995). The offset is the relation AMGST and AMPST. The equation is rapidly applied, between annual mean ground surface temperature in comparison with numerical simulations of the annu- (AMGST) and AMPST (Goodrich, 1982; Burn and al active-layer thermal regime (Smith, 1977; Smith and Smith, 1988). The soil temperatures were recorded at Riseborough, 1983). As a result, equation [2] is a valu- the ground surface and at various depths within the able tool for regional assessments of the impact of active layer and near-surface permafrost every four changes in ground surface temperature on ÒcoldÓ per- hours. The sites were in fine-grained sediments, with mafrost. For studies of specific sites, numerical simula- overlying organic horizons and a high degree of satura- tions are required to provide the details often necessary. tion. From these data, Romanovsky and Osterkamp (1995) were able to determine the range in thermal off- THE N-FACTORS set registered at various sites and under different annu- The data analysed by Romanovsky and Osterkamp al climatic regimes. Assuming only conductive transfers (1995) were combined with records of air temperature of heat within the active layer, they derived analytically and snow cover characteristics collected simultaneously

C.R. Burn 113 to assess the influence of climate variation on AMPST generally greater in the thawing season than the freez- (Zhang et al., 1997). While summer ground tempera- ing season and at tundra sites than in the boreal forest tures and active-layer depth responded to variations in (Taylor, 1995; Smith et al., 1998). air temperatures and the duration of the thaw season (Romanovsky and Osterkamp, 1997), subsurface condi- For sites with permafrost, the freezing season n-factor tions in winter were relatively insensitive to these vari- varies mostly with snow cover characteristics, associa- ables, but not to characteristics of the snow pack. These ted with the vegetation community. As a result, Smith effects are due in part to the inverse relation between and Riseborough (1996, eq. 5) presented a modified air temperature and snow depth on the North Slope of form of equation [2] to relate the air temperature regime Alaska over the period examined, in contrast with cen- to AMPST, using n-factors to convert the air tempera- tral Yukon. On a regional basis, however, the variation ture index to that of the ground surface: in winter ground temperatures is dominated by the sys- tematic increase in snow accumulation (Zhang et al., k t ××n DDT - n × DDF 1996b), due to trapping by microrelief or vegetation, k 1 a f a [3] which increases in height with distance from the coast. AMPST = f P The data from northern Alaska clearly separate air and ground temperature relations into two seasonal where nt and nf are the thawing and freezing season regimes. These relations have been summarized n-factors, and DDTa and DDFa are the thawing and through determination of n-factors. The n-factor is freezing indices for air temperature. Equation [3] allows defined as the ratio of the ground surface temperature exploration of changes in equilibrium AMPST in index for the thaw (or freezing) season to the air tem- response to climatic change, as summarized by either perature index for the same season, usually expressed fluctuations in DDT , DDF , and/or n . as accumulated thawing (or freezing) degree-days a a f (Lunardini, 1978,, 1981). This method offers potential for summarizing microclimatic exchanges within gener- Burn (1998) calculated the n-factors for sites with alized vegetation units. In general, the n-factors are equilibrium and degrading permafrost in Takhini val- determined empirically, by collecting air and ground ley, southern Y.T., and noted that while the thaw season temperatures simultaneously at sites representative of n-factors were consistent with data from other areas, ecological units. Various n-factors for natural surfaces the n-factor in winter varied with subsurface condi- in Mackenzie valley are presented by Taylor (1995), tions. The freezing season n-factor is lowered at sites adding to the inventory of Jorgenson and Kreig (1988) without permafrost, or with a very deep active layer, and Shur and Slavin-Borovskiy (1993). The n-factors are 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

In North America three principal approaches have been adopted towards modelling the impact of climate 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. change on permafrost distribution. The first, the Observations were taken every 4.8 hours by data logger. The least-squares Nelson-Outcalt frost index (Nelson and Outcalt, 1987), 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). is based entirely on climatic statistics, and has been

114 The 7th International Permafrost Conference used with GCM output to project permafrost distribu- frozen ground is due to elevation of permafrost-cored tion under various climate scenarios (Anisimov and landforms in peatlands and the associated reduction in Nelson, 1996, 1997). The second, the TTOP model snow cover (SeppŠlŠ, 1982), and to the high ice content (equation [3], Smith and Riseborough, 1996), explicitly of frozen peat. These observations led Halsey et al. recognizes the impact of surface and soil conditions on (1995) to the counter-intuitive result that permafrost is ground temperatures, and therefore has potential appli- more extensive where MAAT is presently between 0¼ cation over smaller areas. The third is the use of cali- and -3.5¼C, than it was in regions of similar climatic brated numerical models for site-specific applications regime 150 years ago. (Romanovsky et al., 1997). Anisimov et al. (1997) conceded this point in attempt- The Nelson-Outcalt frost index is a normalized ratio ing to estimate active-layer thickness rather than per- of frost penetration to thaw penetration on a regional mafrost distribution per se, but suggested that active basis, using the climatic thawing index and a modified layer response to climate change may be rapid. They freezing index, to accommodate snow cover, derived used the method presented by Kudryavtsev et al. (1974) from climatic data. A ratio of 0.50 represents the equa- to couple GCM output to forecasts of active-layer torward boundary of permafrost, and 0.67 represents development, which, unfortunately, cannot accommo- the boundary between continuous and discontinuous date variations in the ice content of near-surface per- permafrost zones. The model has been verified by com- mafrost. Persistence of active-layer thickness is provi- paring projections of the index with extant permafrost ded by the characteristically ice-rich zone at the surface maps (Anisimov and Nelson, 1997). The model is suited of permafrost. Where the active layer is relatively deep, to application at hemispherical scale, and has been used further deepening requires evacuation of latent heat to forecast changes in the extent of permafrost associat- from melting near-surface ground ice along a gentle ed with various scenarios for global climatic warming temperature gradient. Maximum active-layer thickness (Anisimov and Nelson, 1996, 1997). occurred about 1000 calendar years after the period of maximum solar insolation in western Arctic Canada Nelson and Outcalt (1987) explicitly indicated that the (10,000 cal. years BP), but initiation of thermokarst lakes model applied only to equilibrium permafrost, and did coincided with maximum insolation (see Burn, 1997). not accommodate degrading permafrost. The result is that the model's utility is limited for consideration of The persistence of permafrost was illustrated by transient conditions, such as may be expected over the Riseborough and Smith (1993) in simulations of climatic next century. Nevertheless, results from GCMs have variability over periods of a millennium. The model been used with the model to examine the area where coupled a randomly varying climate based on the changes in permafrost distribution may occur as a record from Fort Simpson, N.W.T., to the TONE ground result of a changed climate, and a scenario of per- thermal simulator written by L.E. Goodrich (1982, mo- mafrost distribution during the early Holocene climatic dified after Steven, 1982), via explicit consideration of optimum has been presented (Anisimov and Nelson, the thermal regime in snow. A key result is that while 1996). This latter case illustrates well the difficulties permafrost may form rapidly during several cold win- implicit in the technique for considering transient con- ters, numerous warm years are required to thaw the ditions. same thickness of ground. The development and thawing of excess ice was not simulated, and this would The model indicates that, relative to present condi- tend to further stretch the periods with permafrost. tions, permafrost was considerably restricted in spatial extent during the early Holocene, with the discontinu- Smith and Riseborough (1996) and Riseborough and ous and continuous permafrost zones smaller by about Smith (1998, this conference) have conducted a series of 25% and 67% respectively. However, in the discontinu- sensitivity analyses on the critical variables responsible ous permafrost of central Yukon there is field evidence for the AMPST, as represented by equation [3]. The of permafrost persistence during this period from the analyses have included consideration of the importance x18O concentration in ground ice (Burn et al., 1986; of changes in snow cover and variations in the thermal Kotler and Burn, 1998). Permafrost in Takhini Valley, properties of ground materials. These data indicate that only 15 m thick, is presently degrading as a result of changes in precipitation regime will influence the forest fire, but will likely require over 1200 years for its response of permafrost to climate change, potentially eradication (Burn, 1998). Furthermore, a large portion swamping the effects of changes in air temperature per of the permafrost extant during the Little Ice Age in the se. Increases in soil moisture content tend to reduce peatlands of the western Canadian provinces has been AMPST, while the effect of snow depth is directly relat- mapped from aerial photographs taken between 1949 ed to AMPST (Riseborough and Smith, 1998). In combi- and 1952, a century after climate warming began nation, these relations make more explicit the response (Halsey et al., 1995). The persistence of perennially- of permafrost to climate change.

C.R. Burn 115 Finally, for specific sites, calibrated numerical simula- changes in emphasis for Federal Departments in tions have continued to provide effective models of the Canada mean that such programs become difficult to ground thermal response to climatic change continue. Instead, recognition of the potential for spa- (Lachenbruch et al., 1988; Burn, 1992; Zhang and tially extensive investigations using information tech- Osterkamp, 1993). Such analyses of ground tempera- nology is growing (Nelson et al., 1997). It would be ture profiles has allowed reconstruction of the late dangerous, though attractive, to assume that computer Pleistocene and Holocene environmental history of modelling may be substituted for field investigations. Mackenzie Delta area (Taylor et al., 1996). Changes in Within such programs, however, the issue raised by surface temperatures due to the glacial/interglacial Smith and Riseborough (1983) of the impact of microcli- transition, submergence during post-glacial sea level matic modulation on the response of permafrost to cli- rise, and emergence during delta progradation are mate change remains outstanding. amenable to modelling as step functions, whose magni- tudes can be estimated independently. The general The model of air temperature - permafrost relations environmental history of the region is known developed by Romanovsky and Osterkamp (1995) has (Rampton, 1988), so Taylor et al. (1996) were able to use been validated for cold permafrost and may be widely the temperature profiles to test specific hypotheses applied to estimate equilibrium conditions in such ter- about permafrost evolution in the area, and determine rain, if the n-factors are known. The variation in n-fac- times of emergence and submergence for various sites. tor with snow conditions is not well described, but appears to change abruptly across treeline (Smith et al., Conclusion 1998). Within vegetation units there is, as yet, little assessment of the variation in n-factor from site to site, The paper has attempted to summarize research on or from year to year at the same site. The model has yet permafrost and climatic change in northwest Canada to be validated for warm permafrost in fine-grained and Alaska, emphasizing insights gained from field and soil, and is formulated for equilibrium conditions. The theoretical studies. Within the region, the ground tem- transient response of permafrost to climate change is perature profile in permafrost shows the impact of cli- not easily estimated because of the number of com- matic warming over the last 30 to 150 years, and cryos- pounding variables set in a context of a naturally vary- tratigraphy records the effect of the warmest climatic ing climatic system. At a site scale, calibrated numerical period of the Holocene. Research on heat transfer has models may provide precise predictions of the thermal provided a model for the translation of the thermal regime of the active layer and ÒwarmÓ permafrost, even regime at the ground surface into the temperature at under transient conditions. Efficient extrapolation of the surface of permafrost, while n-factors are used to these results in a regional context is a significant chal- obtain ground surface temperatures from the air tem- lenge, one which I doubt will be overcome without con- perature. Field evidence from the discontinuous per- tinuing conscientious efforts from field workers. mafrost zone indicates the considerable persistence of permafrost following climate change, due to latent heat Acknowledgments contained in ground ice. GCMs indicate that future climate change at these latitudes may be most apparent in The research program has been supported by the winter, and will therefore affect both nf and DDFa. The National Sciences and Engineering Research Council of active layer will respond to such effects, but its most Canada, the Polar Continental Shelf Project (PCSP) and rapid response will still be to changes in surface condi- the Geological Survey of Canada, Natural Resources tions such as those following forest fire. At this point, Canada, the Inuvik Research Centre of Aurora College, then, some suggestions are offered on outstanding the Northern Research Institute of Yukon College, the problems and future work. Atmospheric Environment Service, Environment Canada, and the Northern Affairs Program of Indian From a practical perspective, it is essential that vari- Affairs and Northern Development Canada. Assistance ous monitoring programs, which have emphasized this from many people in the Yukon and Mackenzie delta region, continue to collect data on a consistent basis. In area, particularly Jim and Shann Carmichael of Mayo, particular, the value of the CALM program (Brown, Scott Smith of Whitehorse, and Les Kutny and Alan 1997), which includes a transect of Mackenzie Valley Fehr of Inuvik is acknowledged with gratitude. I thank (Nixon and Taylor, 1998), and key sites in Alaska J.R. Mackay and M.W. Smith for constant stimulation (Nelson et al., 1997), increases as time passes, the record and encouragement, and Joan Ramsay Burn for her is extended, and trends, cycles and unusual events are support of these endeavours. J. Brown, H.M. French, recognized (Burt, 1994). Similarly, the value of near-sur- J.R. Mackay, T.E. Osterkamp, D.W. Riseborough and face ground temperature monitoring increases with M.W. Smith provided helpful comments on the manu- time, but the significance of such records can only be script. PCSP contribution 00498. evaluated if they are continuous and of quality. Recent

116 The 7th International Permafrost Conference References

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