In addition to visual indications of recent warming from the near-surface curvature of thermal profiles (see above), direct measurements over two to three decades at locations in Alaska, the Mackenzie corridor, the Canadian High Arctic and northern Québec, allow quantification of trends in permafrost thermal state across North America in the years leading up to the IPY.
Western Canada and Alaska
Climate records indicate that rates of warming during the last century in western North America are higher than in other circum-Arctic regions (e.g. Serreze et al., 2000; Warren and Egginton, 2008). Air temperature records in the Mackenzie corridor, for example, show a decline in MAAT from the late 1940s through to the early 1960s and a general increase over the past 50 years (Figure 12A). Ground temperatures measured at sites in discontinuous permafrost during the past 25 years have also warmed (Figure 12B). Where MAGT values are <−1°C, such as at Norman Wells (site 84-2B), ground temperatures have increased on average at about 0.2°C per decade, but the absolute rate has slowed in more recent years consistent with the trends in air temperature (Figure 12A). Records for Norman Wells also indicate a decrease in snow cover since winter 2005–2006 (Environment Canada, 2009a), which could also be a factor in the reduced rate of change in ground temperature. Increases in permafrost temperatures are smaller or insignificant for warmer ice-rich permafrost (Figure 12B) due to the phase change that occurs as permafrost temperatures approach 0°C (Smith et al., 2005). Consequently permafrost in the southern portion of the discontinuous permafrost zone can persist for extended periods even under a warming climate, especially if there is an insulating peat layer (Smith et al., 2008a). Due to these latent heat effects, crossing the 0°C threshold with resultant permafrost thaw is difficult. The diversity in permafrost thermal state within the discontinuous zone is also decreasing over time with, for example, the temperature difference between the warmest and coldest site in Figure 12B decreasing from 1.4°C to 1.0°C.
Figure 12. (A) Mean annual air temperature from Environment Canada stations in the central (Norman Wells 65.3°N 126.8°W), and southern (Fort Simpson 61.8°N 121.2°W and High Level 58.6°N 117.2°W) Mackenzie Valley. The 5-year running mean is shown by the heavy solid line. (B) Mean annual ground temperature at depths of 10 to 12 m between 1984 and 2008 for monitoring sites in the central and southern Mackenzie Valley (updated from Smith et al., 2005). Coordinates for 84-5B and 85-7A are 59.7°N 119.5°W and 63.5°N 123.6°W respectively. See Figure 6A for location of other sites.
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Discontinuous permafrost in Alaska has experienced similar change. Warming ranging from 0.3°C to 0.6°C was observed between 1985 and 2000 (Figure 13A) with larger increases at colder sites. However, some sites such as Livengood show little change. Permafrost temperature dynamics became more complex during the 2000s, with only the northernmost site at Coldfoot showing a noticeable increase of 0.3°C between 2000 and 2009. The Old Man site, located about 65 km to the south, also showed a noticeable increase (0.4°C) between 2000 and 2007 but temperatures then decreased by 0.2°C between 2007 and 2009. Similar cooling was observed at the Birch Lake and College Peat sites. A slight cooling in the 2000s was also observed at Healy and Gulkana (Figure 13A). This recent cooling of permafrost in the Interior Alaska can be explained by the general decrease in air temperature and simultaneous decrease in the snow thickness over the past few years (Figure 14).
Figure 13. Time series of permafrost temperatures measured in Alaska at: (A) 15 m depth measured at several sites across the discontinuous permafrost zone and (B) at 20 m depth measured at several sites across the continuous permafrost zone. Location of sites provided in Figure 5 except for Healy 63.9°N 149.2°W, Coldfoot 67.2°N 150.2°W and Happy Valley 69.1°N 148.8°W.
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Figure 14. Daily snow on the ground (A) and mean annual air temperatures (B) measured at the Fairbanks Alaska meteorological station.
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The longest records of change in Alaska have been obtained by reactivating sites, such as Barrow in northern Alaska, where high quality permafrost temperature records were obtained decades ago. The establishment by UAF of a site within Barrow Environmental Observatory in the early 2000s allowed comparison of present permafrost temperatures with high quality measurements (precision 0.01°C) obtained during the 1950s and early 1960s by the USGS (Brewer, 1958). Comparison of permafrost temperature profiles obtained on 9 October 1950 (Max Brewer, personal communication) and by the UAF on 9 October 2001 shows that the permafrost temperature at 15 m (which is slightly above the depth of zero annual amplitude) is now more than 1°C higher (Romanovsky et al., 2002). This noticeable, but still moderate, increase over such a long period reflects the fact that the air temperature and snow-cover thickness were relatively low in Barrow during the 1960s and 1970s (see Figure 4). As a result, much colder permafrost temperatures at the permafrost table (up to 2–3°C colder) were typical for Barrow during the 1970s (Romanovsky et al., 2002) and recent rapid permafrost warming (Figure 5B) reflects at least in part the recovery from that colder period.
Three decades of permafrost temperature continuously recorded by the UAF and USGS in northern Alaska show that a major rapid warming occurred (Figures 13B and 15) during the late 1980s and 1990s (Clow, 2008a; Clow and Urban, 2008; Osterkamp, 2008). Especially strong warming was observed north of the Brooks Range, where temperatures increased between 1985 and 2000 at 20 m depth by 1.5 to 2.5°C. Permafrost warming continued on the North Slope during the 2000s but at a modest rate. The warming was site specific, with some sites not showing any warming while others warmed by as much as 0.6°C during this time. The mean warming across the USGS deep borehole array during this decade was 0.4°C.
Figure 15. Change in mean annual ground temperature at 20 m depth on the North Slope of Alaska from 1989 to 2007–2008. Although temperature measurements were made in all the boreholes in this region during the International Polar Year, measurements were not made at the 20 m depth in some of the wells during 1989, preventing us from reporting the MAGT change in these instances.
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Permafrost research in the Mackenzie Delta area has been largely inspired by the work of J.R. Mackay. Investigations in the Mackenzie Delta region have used Mackay's (1974) map of ground temperatures in the region from the late 1960s and early 1970s as a benchmark from which to measure changes that have occurred in the past four decades. Burn and Kokelj (2009) present a map of current ground temperatures in the region and the original map for comparison. In the tundra areas near the delta, the MAGT has increased by 1 to 2°C during this period, with the greatest changes being recorded in the outer delta area. At Garry Island (Figure 8A) where current MAGT is −6.7°C, records from the early 1970s indicate the MAGT was then −8.0°C (Mackay and MacKay, 1974). The greatest permafrost warming in the region appears to have occurred in the outer delta plain, where increases of more than 2°C have been recorded. Within the delta south of the treeline the ground is considerably warmer (presently about −2°C) than at tundra sites (presently about −4°C), largely due to the influence of the water bodies that cover about 40% of the surface. The thermal regime of these lakes and channels may not be sensitive to climate warming in winter because development of the ice cover prevents fluctuations in bottom temperatures. As a result, ground temperatures at 15 m depth have only increased by about 0.5°C since 1970 at sites where comparable data are available (Kanigan et al., 2008).
Analysis of the ground temperature profile to 42 m depth at Herschel Island (Figure 8B) indicates that warming of permafrost has been occurring throughout the 20th century (Burn and Zhang, 2009). This conclusion is possible due to climate data collected between 1899 and 1905 at the site and historical accounts, which indicate climate warming since then, particularly in autumn and early winter (October to January). The total warming has been about 2°C. To the east at Paulatuk, the inclination of the temperature envelope observed in a borehole drilled to 28 m depth (Figure 8B) also provides evidence of permafrost warming. However, it is not possible to determine the extent of the warming to present conditions of −6°C, because there are no previous ground temperature records from Paulatuk, and the climate record is too short to estimate antecedent equilibrium conditions.
Active-layer conditions have been monitored at more than 50 sites representing a variety of terrain conditions in the Mackenzie valley since the early 1990s (e.g. Smith et al., 2009b). The active layer responds to short-term fluctuations in climate, especially to summer air temperature conditions. Smith et al. (2009c) found that there was no definite trend in active-layer thickness over the period of record and that for several sites active layers were thinner following 1998, including during the IPY (Table 2). Results from long-term monitoring at Illisarvik indicate that there was an increase of 8 cm in thaw depth between 1983 and 2008 but that the maximum was in 1998 and since then, thaw depths have generally been shallower (Burn and Kokelj, 2009).
Table 2. Active-layer thickness (AL) for Canadian CALM sites reporting data for 2007. C3 to C14 are located in the Mackenzie region of NWT (from Smith et al., 2009b) and C20 is Baker Lake, Nunavut. A > or < sign means that the active layer is greater or less than the value reported.
|CALM ID||Location (Lat °N Long °W)||Period||2007 AL (cm)||Maximum (cm, Year)||Minimum (cm, Year)||Mean (cm)|
|C3||69.7°N 134.5°W||1991–2007||64||70, 1998||59, 2000 and 2003||62|
|C4||69.4°N 134.9°W||1992–2007||132||>132, 1999||101, 2005||>117|
|C5||69.2°N 134.1°W||1991–2007||73||91, 1998||64, 2000||76|
|C7||67.8°N 134.1°W||1992–2007||128||140, 1998||115, 2005||131|
|C8||67.8°N 134.1°W||1992–2007||113||116, 1998||102, 1992||109|
|C13||63.5°N 123.7°W||1993–2007||67||67, 2007||<58, 1993||<63|
|C14||62.7°N 123.1°W||1993–2007||88||91, 1999||79, 1993||86|
|C20||64.2°N 95.5°W||1997–2007||229||229, 2007||125, 1997||191|
Synthesis of Permafrost Temperature Trends
Permafrost temperatures measured across northern North America have almost all increased over the past two to three decades. The magnitude of the change varies, being less in warmer permafrost (>−2°C) than in colder permafrost. Based on these trends, it will take decades to centuries for colder permafrost to reach the thawing point while warmer permafrost is already undergoing internal thaw at temperatures below 0°C.
Permafrost at tundra sites and in bedrock is more sensitive thermally to changes in climate than sites below the treeline or in ice-rich soils. Warming of permafrost in western North American has occurred essentially continuously over the past 20–30 years, with a slowing in the rate of warming at many locations in the past decade. In northern Québec and the eastern Arctic, however, warming did not begin until 1993 and has continued to present. These changes represent only the latest to affect permafrost temperatures, which results from modelling studies suggest have been warming since the Little Ice Age (Taylor et al., 2006; Chouinard et al., 2007; Burn and Zhang, 2009).