Permafrost: Types, Extent, and Climate Impacts

Definition and temperature criterion for permafrost

Permafrost is defined as soil, rock, or underwater sediment that remains frozen continuously below 0°C (32°F) for at least two consecutive years. In practice, this corresponds to permafrost occurring where the mean annual temperature is at or below 0°C. The permafrost zone typically lies beneath an overlying active layer that freezes and thaws seasonally, while the deeper permafrost remains frozen year-round. The temperature criterion for permafrost is therefore the 0°C threshold: the vertical boundary of the permafrost zone is associated with the average annual temperature line at 0°C. Permafrost also has a base depth where geothermal heat and the mean annual surface temperature reach an equilibrium at 0°C, marking the lowest extent of frozen ground.

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Active layer and permafrost table

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