Permafrost is defined as ground (soil or rock, including ice or organic material) that remains at or below 0°C for at least two consecutive years.
Lowland permafrost regions are traditionally divided into several zones based on estimated geographic continuity in the landscape. A typical classification recognizes continuous permafrost (underlying 90-100% of the landscape); discontinuous permafrost (50-90%); and sporadic permafrost (0-50%). In the Northern hemisphere, regions in which permafrost occurs occupy approximately 25% (23 million km²) of the land area. In the discontinuous and sporadic zones, permafrost distribution is complex and patchy, and permafrost-free terrain is common. The thickness of permafrost varies from less than one metre to more than 1500 metres.
Most of the permafrost existing today formed during cold glacial periods, and has persisted through warmer interglacial periods, including the Holocene (last 10,000 years approximately). Some relatively shallow permafrost (30 to 70 meters) formed during the second part of the Holocene (last 6,000 years) and some during the Little Ice Age (from 400 to 150 years ago).
In continental interiors, permafrost temperatures at the boundaries between continuous and discontinuous are generally about -5°C, corresponding roughly with the -8°C mean annual air temperature. Permafrost in mid- and low- latitude mountains is warm and its distribution is closely related to characteristics of the land surface, such as slope gradient and orientation, vegetation patterns, and snow cover.
Subsea permafrost occurs close to 0°C over large areas of the Arctic continental shelf, where it formed during the last glacial period on the exposed shelf landscapes. Permafrost is geographically continuous beneath the ice-free regions of the Antarctic continent and also occurs beneath areas in which the ice sheet is frozen to its bed.
Permafrost can be used as a paleothermometer—fluctuations of air temperature from the late 19th and 20th centuries can be obtained by measuring temperature in deep permafrost boreholes. Warming since the late 1960s has been observed in permafrost temperature profiles from many locations. Over the past several decades, permafrost temperatures have generally increased in lowlands and mountains; exceptions are in some newly exposed drained lake basins and aggrading shorelines where permafrost is forming. Thawing of permafrost has been observed in many lowland and mountain locations in recent decades—much of the evidence is indirect, and is based on changes in forest and tundra vegetation, differential subsidence of the ground surface, and loss of lakes. Increases in active-layer thickness have been observed in warm summers (for western North America; 1989, 1998, 2004), resulting in increased slope failures, ground subsidence in ice-rich terrain, increased lake drainage. At regional and global scales, changes in permafrost zonal boundaries are difficult to identify due to 3-dimensional irregularities in permafrost distribution. Degradation of permafrost and changes in its distribution are associated with increased formation of “taliks”. Open taliks penetrate through the permafrost and closed taliks or thawed depressions occur under deep lakes and rivers.
21th Century Changes
Changes in zonal permafrost “boundaries” modeled using climate-change scenarios are usually based on predictions of increased active-layer thickness and temperature changes at relatively shallow permafrost depths, not on the complete disappearance of permafrost. Warm permafrost degrades from both the top and bottom, increasing the extent of talik formation. The southern limit of permafrost moves northward in an irregular pattern, and is governed by localized factors that include peatland distribution, soil moisture, vegetation patterns, and snow cover. Movements of the “boundary” between the sporadic and discontinuous permafrost zones are largely governed by the development and extent of open taliks. In areas of ice-rich permafrost, the southern “boundary” of the continuous permafrost zone remains relatively stable as complete disappearance of permafrost may take centuries to millennia, making it difficult to determine geographic changes except where permafrost is thin. Rapid coastal erosion, although sustained by storms and related wave intensity, is highly dependent on the amount and type of ground ice. Changes in permafrost distribution predicted by models require extensive field- or remote-sensing based verification over extended time periods (snapshots of permafrost temperature over decadal intervals). Monitoring of the thermal state of permafrost (TSP) at the global scale is required to understand hydrologic connections, future changes in permafrost distribution, and to serve as validation global and regional models.