Permafrost: Types, Extent, and Climate Impacts

Definition and temperature criterion

Permafrost is defined as soil, rock, or sediment that remains frozen for more than two consecutive years. In practice, it occurs where the mean annual temperature is at or below 0 °C (32 °F). Permafrost typically lies beneath an “active layer” that freezes and thaws seasonally, and the boundary between the active layer and permafrost is sometimes called the permafrost table. Temperature criteria also determine how continuous permafrost is across a region. Permafrost formation is favored when mean annual air temperatures are below freezing, though local factors (such as snow insulation in some boreal forests) can create exceptions. If mean annual soil surface temperatures are only slightly below 0 °C, permafrost may form only in sheltered spots, producing discontinuous permafrost; colder mean annual soil temperatures support continuous permafrost. In addition, permafrost depth is limited by geothermal heat: it extends downward until geothermal warming and the mean annual surface temperature reach an equilibrium at 0 °C, defining the base depth of permafrost.

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Active layer and permafrost table

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