Permafrost

by

Ellen Wohl

• LAST REVIEWED: 28 July 2021
• LAST MODIFIED: 28 July 2021
• DOI: 10.1093/obo/9780199363445-0132

Introduction

Permafrost is permanently frozen ground that remains continuously below 0 °C for two or more years. The upper level of permafrost, the permafrost table, can occur within a centimeter of the ground surface or at a depth of several meters. The active layer, which thaws each summer, overlies permafrost. Permafrost underlies about a quarter of the northern hemisphere and can form in sediment or bedrock and on land or under the ocean. Permafrost forms incrementally and, in the regions where it is up to 1 km thick, permafrost can represent thousands of years of formation. Permafrost is present at high latitudes and high altitudes. In these regions, permafrost can be described as continuous, discontinuous, sporadic, or isolated. Continuous permafrost forms at mean annual air temperatures below -5 °C and is laterally continuous, regardless of surface aspect or material. Discontinuous permafrost forms where the mean annual air temperature is between -2 and -4 °C, allowing permafrost to persist in 50 to 90 percent of the landscape. Permafrost is sporadic where 10 to <50 percent of the landscape is underlain by permafrost and mean annual air temperature is between 0 and -2 °C. Permafrost is considered isolated where less than 10 percent of the landscape is underlain by permafrost. When it is present, permafrost creates unique conditions. Permafrost forms an impermeable layer beneath the active layer, for example, which limits the rooting depth of plants and prevents infiltration by water during the summer. The lack of deep infiltration can facilitate formation of extensive wetlands in high-latitude areas that receive relatively little precipitation. Permafrost degradation (thaw) creates diverse environmental hazards, including instability of the ground surface that affects infrastructure and fluxes of water, sediment, and organic matter entering rivers, lakes and oceans. Permafrost degradation releases frozen microbes, some of which are pathogens, and organic carbon. Permafrost degradation also influences the geographic range of plants and animals and thus ecosystem processes and biotic communities. The greatest concern with permafrost degradation at present, however, is the potential for releasing significant carbon into the atmosphere. Globally, soils are the largest terrestrial reservoir of carbon and permafrost soils are the single largest component of the carbon reservoir. Carbon released by degrading permafrost can enter the atmosphere as the greenhouse gases carbon dioxide and methane, or the carbon can be taken up by plants or transported by rivers to the ocean and buried in marine sediments. The balance among these different pathways is largely unknown, but carbon release to the atmosphere presents a serious threat as a mechanism to enhance global warming.

General Overviews

The three works cited in this section provide a general overview of some aspect of permafrost and periglacial environments. Dobinski 2011 provides the broadest summary. Margesin 2009 focuses on soils in permafrost regions. Rowland, et al. 2010 focuses on the effects of warming climate on permafrost-related processes.

• Dobinski, W. 2011. Permafrost. Earth-Science Reviews 108.3–4: 158–169.

  DOI: 10.1016/j.earscirev.2011.06.007

  A comprehensive overview of the history of permafrost research, the definition and characteristics of permafrost, distinctive physical processes in areas underlain by permafrost, and the geographic distribution of permafrost.

• Margesin, R., ed. 2009. Permafrost soils. Berlin: Springer.

  A thorough reference to all aspects of soils in permafrost regions, with chapters on soils in Arctic, Antarctic, and mountain permafrost regions, and sections on biodiversity in permafrost, biological activity in permafrost, contaminants in permafrost, and the effects of warming climate on permafrost soils.

• Rowland, J. C., C. E. Jones, G. Altmann, et al. 2010. Arctic landscapes in transition: Responses to thawing permafrost. EOS, Transactions of the American Geophysical Union 91.26: 229–236.

  DOI: 10.1029/2010EO260001

  Provides a brief but thorough review of evidence of recent permafrost degradation in Arctic regions, including increased coastal erosion; expansion of river networks; enhanced river bank erosion; changes in extent of lakes, ponds, and wetlands; and increased hillslope soil erosion and landslides.