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How long must ground remain frozen to be classified as permafrost?
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permafrost
Distribution of permafrost in the Northern Hemisphere.
To be classified as permafrost, ground must maintain a temperature of 0 °C (32 °F) or colder continuously for a period of two or more years.1
This classification is based exclusively on temperature rather than the presence of ice, meaning that ground can be considered permafrost even if it contains no ice, provided it meets the temperature requirement.1