220.127.116.11 Permafrost Degradation
Permafrost degradation refers to a naturally or artificially caused decrease in the thickness and/or areal extent of permafrost. Evidence of change in the southern boundary of the discontinuous permafrost zone in the past decades has been reported. In North America, the southern boundary has migrated northward in response to warming since the Little Ice Age, and continues to do so today (Halsey et al., 1995). In recent years, widespread permafrost warming and thawing have occurred on the Tibetan Plateau, China. Based on data from ground penetration radar and in situ measurements, the lower limit of permafrost has moved upward about 25 m from 1975 through 2002 on the north-facing slopes of the Kunlun Mountains (Nan et al., 2003). From Amdo to Liangdehe along the Qinghai-Xizang Highway on the Tibetan Plateau, areal extent of permafrost islands decreased approximately 36% over the past three decades (Wang, 2002). Areal extent of taliks (areas of unfrozen ground within permafrost) expanded about 1.2 km on both sides of the Tongtian River (Wang, 2002). Overall, the northern limit of permafrost retreated about 0.5 to 1.0 km southwards and the southern limit moved northwards about 1.0 to 2.0 km along the Qinghai-Xizang (Tibet) Highway (Wang and Zhao, 1997; Wu and Liu, 2003).
When the warming at the top of permafrost eventually penetrates to the base of permafrost and the new surface temperature remains stable, thawing at the base of the ice-bearing permafrost occurs (i.e., basal thawing), especially for thin discontinuous permafrost. At Gulkana, Alaska, permafrost thickness is about 50 to 60 m and the basal thawing of permafrost has averaged 0.04 m yr–1 since 1992 (Osterkamp, 2003). Over the Tibetan Plateau, basal thawing of 0.01 to 0.02 m yr–1 was observed since the 1960s in permafrost of less than 100 m thickness (Zhao et al., 2003). It is expected that the basal thawing rate will accelerate over the Tibetan Plateau as the permafrost surface continues to warm.
If ice-rich permafrost thaws, the ground surface subsides. This downward displacement of the ground surface is called thaw settlement. Typically, thaw settlement does not occur uniformly and so yields a chaotic surface with small hills and wet depressions known as thermokarst terrain; this is particularly common in areas underlain by ice wedges. On slopes, thawing of ice-rich, near-surface permafrost layers can create mechanical discontinuities in the substrate, leading to active-layer detachment slides (Lewkowicz, 1992), which have the capacity to damage structures similar to other types of rapid mass movements. Thermokarst processes pose a serious threat to arctic biota through either oversaturation or drying (Hinzman et al., 2005; Walsh et al., 2005). Extensive thermokarst development has been discovered near Council, Alaska (Yoshikawa and Hinzman, 2003) and in central Yakutia (Gavrilov and Efremov, 2003). Significant expansion and deepening of thermokarst lakes were observed near Yakutsk (Fedorov and Konstantinov, 2003) between 1992 and 2001. The largest subsidence rates of 17 to 24 cm yr–1 were observed in depressions holding young thermokarst lakes. Satellite data reveal that in the continuous permafrost zone of Siberia, total lake area increased by about 12% and lake number rose by 4% during the past three decades (L.C. Smith et al., 2005). Over the discontinuous permafrost zone, total area and lake number decreased by up to 9% and 13%, respectively, probably due to lake water drainage through taliks.
The most sensitive regions of permafrost degradation are coasts with ice-bearing permafrost that are exposed to the Arctic Ocean. Mean annual erosion rates vary from 2.5 to 3.0 m yr–1 for the ice-rich coasts to 1.0 m yr–1 for the ice-poor permafrost coasts along the Russian Arctic Coast (Rachold et al., 2003). Over the Alaskan Beaufort Sea Coast, mean annual erosion rates range from 0.7 to 3.2 m yr–1 with maximum rates up to 16.7 m yr–1 (Jorgenson and Brown, 2005).