4.7.2.2 Changes in Permafrost Temperature

Permafrost in the NH has typically warmed in recent decades (Table 4.5), although at a few sites there was little warming or even a cooling trend. For example, measurements (Osterkamp, 2003) and modelling results (see Hinzman et al., 2005; Walsh et al., 2005) indicate that permafrost temperature has increased by up to 2°C to 3°C in northern Alaska since the 1980s. Changes in air temperature alone over the same period cannot account for the permafrost temperature increase, and so changes in the insulation provided by snow may be responsible for some of the change (Zhang, 2005). Data from the northern Mackenzie Valley in the continuous permafrost zone show that permafrost temperature between depths of 20 to 30 m has increased about 1°C in the 1990s (S.L. Smith et al., 2005), with smaller changes in the central Mackenzie Valley. There is no significant trend in temperatures at the top of permafrost in the southern Mackenzie Valley, where permafrost is thin (less than 10 to 15 m thick) and warmer than –0.3°C (S.L. Smith et al., 2005, Couture et al., 2003). The absence of a trend is likely due to the absorption of latent heat required to melt ice. Similar results are reported for warm permafrost in the southern Yukon Territory (Haeberli and Burn, 2002). Cooling of permafrost was observed from the late 1980s to the early 1990s at a depth of 5 m at Iqaluit in the eastern Canadian Arctic. This cooling, however, was followed by warming of 0.4°C yr^–1 between 1993 and 2000 (S.L. Smith et al., 2005). This trend is similar to that observed in Northern Quebec, where permafrost cooling was observed between the mid-1980s and mid-1990s at a depth of 10 m (Allard et al., 1995) followed by warming beginning in 1996 (Brown et al., 2000). Warming of permafrost at depths of 15 to 30 m since the mid-1990s has also been observed in the Canadian High Arctic (Smith et al., 2003).

There is also evidence of permafrost warming in the Russian Arctic. Permafrost temperature increased approximately 1°C at depths between 1.6 and 3.2 m from the 1960s to the 1990s in East Siberia, about 0.3°C to 0.7°C at a depth of 10 m in northern West Siberia (Pavlov, 1996) and about 1.2°C to 2.8°C at a depth of 6 m from 1973 through 1992 in northern European Russia (Oberman and Mazhitova, 2001). Fedorov and Konstantinov (2003) reported that permafrost temperatures from three central Siberian stations did not show an apparent trend between 1991 and 2000. Mean annual temperature in Central Mongolia at depths from 10 to 90 m increased 0.05°C to 0.15°C per decade over 30 years (Sharkhuu, 2003).

At the Murtèl-Corvatsch borehole in the Swiss Alps, permafrost temperatures in 2001 and 2003, at a depth of 11.5 m in ice-rich frozen debris, were only slightly below –1°C, and were the highest since readings began in 1987 (Vonder Mühll et al., 2004). Analysis of the long-term thermal record from this site has shown that in addition to summer air temperatures, the depth and duration of snow cover, particularly in early winter, have a major influence on permafrost temperatures (Harris et al., 2003). Results from six years of ground temperature monitoring at Janssonhaugen, Svalbard, indicate that the permafrost has warmed at a rate of about 0.5°C per decade at a depth of 20 m (Isaksen et al., 2001). Results from Juvvasshøe, in southern Norway, indicate that ground temperature has increased by about 0.3°C at a depth of 15 m from 1999 to 2006. At both these sites, wind action prevents snow accumulation in winter and so a close relationship is observed between air, ground surface and ground subsurface temperatures, which makes the geothermal records from Janssonhaugen and Juvvasshøe more direct indicators of climate change.

Permafrost temperature increased about 0.2°C to 0.5°C from the 1970s to 1990s over the hinterland of the Tibetan Plateau (Zhao et al., 2003), up to 0.5°C along the Qinghai-Xizang Highway over a period from 1995 to 2002 (Wu and Liu, 2003; Zhao et al., 2004) and about 0.2°C to 0.4°C from 1973 to 2002 at depths of 16 to 20 m in Tianshan Mountain regions (Qiu et al., 2000; Zhao et al., 2004). Over the Da Hinggan Mountains in north-eastern China, permafrost surface temperature increased about 0.7°C to 1.5°C over a period from 1978 through 1991 from the valley bottom to the north-facing slopes (Zhou et al., 1996). Permafrost temperature at the depth of the zero annual temperature variation increased about 2.1°C on the valley bottom, 0.7°C on the north-facing slopes and 0.8°C on south-facing slopes. In areas of the south-facing slopes where no permafrost exists, soil temperature at depths of 2 to 3 m increased about 2.4°C (Zhou et al., 1996).