Permafrost

"Permafrost is defined as ground (soil or rock and included 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 meter to more than 1500 meters." (International Permafrost Association, 2019)

Sources

International Permafrost Association. "What is permafrost?". Accessed March 13, 2019. Available at: https://ipa.arcticportal.org/publications/occasional-publications/what-… 

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