Permafrost is permanently frozen ground that remains at or below 0°C for at least two consecutive years. It is found in polar regions and high-altitude areas and plays a critical role in the Earth’s climate system, particularly in the carbon cycle, as it stores large amounts of organic carbon that can be released as greenhouse gases when thawed.
Reference: Brown, J., et al. (1998). “Circumpolar Active Layer Monitoring (CALM) Program: Description and Data.” CD-ROM. National Snow and Ice Data Center.