permafrost

permafrost Permafrost is ground that remains at or below 0°C for at least 2 years. About one-quarter of the Earth's land surface is underlain by permafrost, including over half of Canada and Alaska, the Tibetan Plateau, and north-eastern Russia. Permafrost that formed during the last glacial period when the sea level was lower is also found on the continental shelf of the Arctic Ocean in the Beaufort, Kara, Laptev, and East Siberian Seas. Continental permafrost regions are usually divided into zones of continuous, widespread discontinuous, and scattered discontinuous permafrost, where respectively over 80 per cent, 30–80 per cent, and less than 30 per cent of the terrain is underlain by perennially frozen ground. In the northern hemisphere, the southerly boundaries of these zones are defined by the mean annual air temperature isotherms of –5, –4, and 0 °C, respectively. The zones not only indicate the spatial extent of permafrost, but also its temporal stability, for short-term climatic changes may lead to its eradication or development in the scattered zone, while, further north, permafrost is continuous in space and time. Alpine, or plateau, permafrost is found at high elevations in mountain regions throughout the world.

Permafrost terrain comprises a seasonally thawed active layer underlain by perennially frozen ground. The thickness of the active layer varies from a few centimetres to several metres, depending on summer conditions and the thermal properties of ground materials. The thickness is greatest in dry sand and bedrock, and least in moist, organic soils. The total thickness of permafrost depends on the annual mean temperature at the base of the active layer, the geothermal flux, and the thermal conductivity of ground materials. The heat flux is equal to the product of conductivity and geothermal gradient, represented by the near-surface temperature divided by the depth to the base of permafrost. Permafrost thicknesses vary from a metre or so at the southerly margins of the scattered permafrost zone to over 1000m in Arctic Canada and Russia. In the discontinuous zones, snow depth, soil moisture, and soil organic matter content dominate environmental variables responsible for the presence or absence and temperature of permafrost.

The geotechnical significance of permafrost is derived from the occurrence of ground ice, often close to its melting point. The ice may be buried relict glacier ice, or intrasedimental ice formed either by segregation during permafrost development or by intrusion of water under pressure into frozen ground. Ice wedges may form at the base of the active layer after snow melt infiltrates thermal contraction cracks. Characteristically the uppermost layers of permafrost are ice-rich owing to these wedges, and to water incorporated into permafrost since the Early Holocene, when active layers were generally deeper. The presence of such near-surface ground ice renders the terrain liable to subsidence and ponding or to hill-slope failure if vegetation is disturbed and the active layer deepens. Exceptional care is therefore taken to preserve the integrity of permafrost during construction activities.

Since permafrost is a climatic phenomenon, climatic changes lead to an altered permafrost regime. The depth of permafrost development and degradation follows the square root of time since the change occurred. Ice-rich ground over 10 m thick requires centuries to millennia for complete development or thawing, and for other adjustments to permafrost thickness. The response of permafrost temperatures to climate warming is usually rapid until the ground reaches –2 to −1 °C, at which point thawing of pore water has begun. The latent heat required for further increases in ground temperature then slows the warming. The terrain response to climate warming may, however, be rapid, since thickening of the active layer will lead to melting of the abundant near-surface ground ice. This is a significant issue in considering the response of permafrost terrain to potential global warming.

C. R. Burn