Another important component of the cryosphere is permafrost, or frozen ground. The majority of permafrost is found in the Arctic (Fig 22.214.171.124), though the small area of land in Antarctica that is not covered by ice sheets also has permafrost. Regions containing permafrost are often categorized by the quantity of total area of the region that is frozen. Continuous permafrost, as its name implies is nearly entirely frozen with 90-100% of the land area of the region containing permafrost. Discontinuous permafrost, as the name implies, is not a single, solid sheet of frozen land (less than 90% of the total area is frozen), and can be sporadic (10-50% frozen area) or isolated (less than 10% frozen area).
As with other components of the cryosphere, permafrost is particularly sensitive to warming. One mechanism for tracking changes in permafrost extent is to measure the depth of the active layer, or the unfrozen soil that overlays permafrost. The Circumpolar Active Layer Monitoring (CALM) program tracks soil temperature and active layer depth in over 200 permafrost sites in both hemispheres. The resulting data on active layer depth for nine regions is shown in Fig 126.96.36.199 and indicates a consistent trend of soil warming, active layer depth, and permafrost loss across the Arctic.
Degradation and thawing of permafrost has serious consequences for human communities and future climate change scenarios. Thawing of permafrost destabilizes soil profiles, which can lead to sinkholes, mudslides, and collapse of infrastructure built atop permafrost. Additionally, thawing of permafrost can directly release greenhouse gases that are frozen into the soil, and increases the respiratory activity of soil organisms, increasing the release of CO2 in Arctic ecosystems. This represents a concerning feedback loop by which warming caused by atmospheric greenhouse gases leads to thawing of permafrost, which releases more greenhouse gases into the atmosphere, exacerbating warming trends and thawing of permafrost.