Thermokarst usually commences when a disturbance to the surface energy balance raises the ground temperature sufficiently to thaw excess ice in the underlying permafrost. Many disturbances of local to regional extent initiate thermokarst (Table 13.1). For example, removal of vegetation or peat cover raises the ground surface
Table 13.1 Factors that initiate, retard or counteract thermokarst activity
Retarding or counteracting factors
Vegetation and sur- Damage or removal face organic mat
Artificial heat source
Mean annual air temperature Regional snowfall
Summer weather Continentality Large forest fires
Compaction of peat or organic soil
Ponding on ground surface or underground Flowing surface or groundwater
Wetting of dry peat in summer Thicker snow cover Reduced snow density Early snowmelt in summer Soil erosion exposes ice-rich ground Artificial removal of soil
Laying of gravel pad too thin to contain seasonal freezing and thawing depth e.g., heated buildings, pipelines, utilidors
Thickening snow cover Early accumulation of snow in winter Unusually warm or wet weather Increased continentality Damage vegetation or surface organic mat
Accumulation of peat or organic soil Drainage of ponds, lakes or cavities Refreezing of underground pools Improved drainage or diversion of drainage Drying of peat in summer Thinner snow cover Increased snow density Late snowmelt in summer Deposition of sediment
Burial of ice-rich ground by spoil Thicker gravel pad Insulation placed beneath gravel
Dissipate heat (e.g. allow cold air circulation or use thermosyphons)
Thinning snow cover Later accumulation of snow in winter Cool and dry weather Decreased continentality Regrowth of forest temperature in summer and often initiates thermokarst activity, as resulted from camp construction and drilling activities during the 1940s and 1950s on the tundra of the National Petroleum Reserve, northern Alaska (Lawson 1986). Further south, in the boreal forest near Fairbanks, central Alaska, removal of spruce trees, moss and underlying peat for land development resulted in thermokarst ponds forming within 5 years of site clearance (Nicholas and Hinkel 1996). Because of its low albedo (<10%), water ponding on the ground surface warms rapidly in summer, especially if the water is shallow and darkened by dissolved organic material. With its high heat capacity, water acts as a heat source and promotes thaw of underlying ground ice (Fig. 13.1).
Disturbances that initiate thermokarst are often compound and interactive. For example, fires in the boreal forest and forest-tundra may initiate active-layer deepening and thermokarst subsidence by (a) destroying the shady vegetation canopy, (b) reducing heat loss from evapotranspiration, and (c) lowering the surface albedo due to burning of the organic cover (Mackay 1995; Burn 1998). Both climate warming and regionally extensive fires may raise ground temperatures, and so increase the susceptibility of ice-rich permafrost to thaw by local disturbances (Burn 1992). Recent thermokarst activity in central Alaska reflects increases in both in mean annual air temperature (MAAT) and winter snow cover during the twentieth century (Jorgenson et al. 2001). Furthermore, a link between rising air temperatures and increasing frequency and magnitude of forest fires may accelerate permafrost degradation.
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