11.2.1.5. Cryosphere and Permafrost

Permafrost is highly responsive to climatic fluctuations at several temporal and spatial scales (Nelson and Anisimov, 1993). Evidence of spatially extensive episodes of permafrost thawing and poleward contraction has been documented (Halsey et al., 1995; Anisimov and Nelson, 1996b; Anisimov and Nelson, 1997; WASI, 1997). Depending on regional climate and local biological, topographic, and edaphic parameters, pronounced warming in the high latitudes of Asia could lead to thinning or disappearance of permafrost in locations where it now exists (Anisimov and Nelson, 1996a). Poleward movement of the southern boundary of the sporadic permafrost area is likely in Mongolia and northeast China. Large-scale shrinkage of the permafrost region in boreal Asia also is likely. In northern regions of boreal Asia, the mean annual temperature of permafrost, hence the depth of seasonal thawing (active layer thickness), will increase (Izrael et al., 1999). The perennially frozen rocks will completely degrade within the present southern regions (ICRF, 1998). The development of thermokarst and thermal erosion because of perennial thawing and increase in the depth of seasonal thawing of ice-rich grounds and monomineral ice accumulations is a critical process in permafrost regions of boreal Asia (Izrael et al., 1999). The change in rock temperature will result in a change in the strength characteristics, bearing capacity, and compressibility of frozen rocks, generation of thermokarst, thermal erosion, and some other geocryological processes (Garagulia and Ershov, 2000). In response to projected climate change, four main economic sectors in permafrost regions—surface and underground construction, the mining industry, heating energy demand, and agricultural development— will be affected (ICRF, 1995, 1998; Anisimov, 1999). Because large quantities of carbon are sequestered in the permafrost of boreal peatlands and tundra regions (Botch et al., 1995; Ping, 1996), changes in distribution of frozen ground and systematic increase in the thickness of seasonally thawed layer are likely to result in the release of large amounts of CO₂ and possibly methane (CH₄) into the atmosphere.

The permafrost area on the Tibetan Plateau has an average altitude of about 5,000 m and is one of the several regions not significantly affected by direct human activities. Because of this area's thermal and moisture conditions are on the edge of the ecological limitations of vegetation, it is believed to be highly sensitive to global warming (Zhang et al., 1996). The boundary between continuous and discontinuous (intermittent or seasonal) permafrost areas on the Tibetan Plateau are likely to shift toward the center of the plateau along the eastern and western margins (Anisimov and Nelson, 1996b).

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