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 per- mafrost 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 solu- tion 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 per- mafrost 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 interpre- tation 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 per- mafrost 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-bot- toms 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 per- mafrost 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 mo- dels (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 con- trast 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 condi- tions (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 tempera- tures 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 per- mafrost 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) demonstrat- ed 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 vary- ing 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).