Permafrost, or permanently frozen ground, is ground (soil, sediment, or rock) that remains at or below 0°C for at least two consecutive years. It occurs both on land and beneath offshore arctic continental shelves, and underlies approximately 25% (23 million km²) of the Earth's land surface. The thickness of permafrost varies from less than one meter to more than 1500 meters.
In the general model, the permafrost region of the northern hemisphere is commonly divided into broad zones, on the basis of the proportion of the ground that is underlain by permafrost. The northernmost zone, wherein permafrost is effectively ubiquitous, forms the continuous permafrost zone. To the south of this is the discontinuous permafrost zone, wherein some proportion of the ground does not contain permafrost. Over the years, different authors have subdivided the zone of discontinuous permafrost into several different subzones.
The earliest classification schemes, developed in Russia and followed in North America, defined the permafrost zones on the basis of ground temperature. In Russia, later works have used a combination of temperature and thickness to define the permafrost zones, sometimes with annotations of the spatial continuity of permafrost. In North America, Heginbottom relied heavily on spatial data in compiling his permafrost maps of Canada, and this approach was followed also in the compilation of the International Permafrost Association (IPA) northern hemisphere map.
in Heginbottom et al. 1995 the following distinctions are used:
"Mountain permafrost" is somewhat off these categories since its defintion is exclusively based on its location at high altitudes.
Permafrost zones by ground temperature categories.
90-100%. Continuous permafrost is associated with the Arctic and Polar zones. In this zone, permafrost lies beneath the entire surface except beneath large rivers and deep lakes. Most continuous permafrost formed during or before the last glacial period. The southern limit of continuous permafrost corresponds closely to a mean annual isotherm(which is derived from air temperatures) of –8ºC.
50-90%. Discontinuous permafrost is associated with the Subarctic zone. Most discontinuous permafrost is much younger and formed within the last several thousand years. The southern limit of discontinuous permafrost corresponds closely to a mean annual isotherm of –1ºC.
10-50%. There is no commonly acknowledged thermal criteria for the southern boundary of sporadic permafrost. The presence of sporadic permafrost is often dependent on the presence of organic soils that help preserve the permafrost under milder climates.
0-10%. The distribution of permafrost and patchy, and permafrost-free terrain is common.
Permafrost at high elevations where the mean annual air temperature is below -3°C. It often exists far below the altitudes to which glaciers extend, and even below the tree line in continental areas. Mountain permafrost exists in different forms - in steep bedrock, in rock glaciers, in debris deposited by glaciers or in vegetated soil, and contains variable amounts of stored fresh water in the form of ice. The distribution and characteristics of permafrost in mountain regions are very patchy. Areas with mountain permafrost include the Himalaya in Asia, the Alps in Europe, the Rocky Mountains in North America, and other high altitude mountain ranges.
No permafrost present.
Bachelor of Circumpolar Studies 311, Module 1: Biophyical Parameters.
Brown, J., O.J. Ferrians, Jr., J.A. Heginbottom, and E.S. Melnikov. 1998, revised February 2001. Circum-arctic map of permafrost and ground ice conditions. Boulder, CO: National Snow and Ice Data Center. Digital media.
Heginbottom, J.A., Dubreuil, M-A. and Harker,P.A. 1995: Canada – Permafrost. Ottawa, Canada: Natural Resources Canada, National Atlas of Canada, 5th edition, Plate 2.1 (MCR No. 4177; scale 1:7 500 000).
Heginbottom, J.A. 2002. Permafrost mapping: a review. Progress in Physical Geography 26: 623.
Leopold, M., Voelkel, J., Dethier, D., Williams, M., and Caine. N. 2010: Mountain Permafrost - A Valid Archive to Study Climate Change? Examples from the Rocky Mountains Front Range of Colorado, USA. Nova Acta Leopoldina NF 112, Nr. 384, 281-289 (2010).
Permafrost zones and permafrost temperatures. http-server.carleton.ca/~msmith2/permafrost_zones.htm (accessed: 15 April 2014)
Romanovsky, V.E., Gruber, S., Instanes, A., Jin, H., Marchenko, S.S., Smith, S.L., Trombotto, D., & Walter, K.M. 2007. Frozen Ground, Chapter 7, In: Global Outlook for Ice and Snow, Earthprint, UNEP/GRID, Arendal, Norway, pp. 181-200. UNEP, 2007. Global Outlook for Ice and Snow. In: J. Eamer (ed). Norway: UNEP/GRID-Arendal, 235 pp.
The International Permafrost Association website: What is Permafrost? http://ipa.arcticportal.org/resources/what-is-permafrost (accessed: 15 April 2014).