Subsurface soils and bedrock, including the pore fluids they contain (mostly fresh water with trace amounts of various salts and other substances), that remain at temperatures below 0°C for at least two consecutive years are called permafrost (see Permafrost). Widespread continuous permafrost is distinguished from more patchy, sporadic, or discontinuous permafrost. While temperatures at depth remain at subzero temperatures for a long time (centuries to millennia), the zone close to the surface, known as the active layer, experiences seasonal melting and refreez-ing. However, small changes in climate will affect the active layer characteristics significantly and will lead to prolonged thawing of permafrost (see Permafrost Retreat).
This has been observed recently in parts of the Russian Arctic, Canada, and in Alaska (Osterkamp, 1994; Osterkamp and Romanovsky, 1996). The consequence of such thawing actions is the formation of so-called thermokarst and enhanced erosion. Moreover, thawing permafrost will significantly affect man-made structures, including buildings, roads, bridges, and railroad foundations.
Borehole temperatures in continuous permafrost provide a reliable record of past temperatures over the last few decades. Observations in Alaska have revealed a significant warming of up to 2-4°C over the last 50 years (Lachenbruch and Marshall, 1986).
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