What Is Permafrost?

Permafrost is ground, whether soil, sediment, or rock, that remains at or below 0 degrees Celsius continuously for at least two consecutive years. The definition is thermal rather than compositional: dry bedrock at minus 2 degrees qualifies, while saturated soil at plus 1 degree does not, regardless of how much ice either contains. Permafrost underlies roughly a sixth of the exposed land area of the Northern Hemisphere along with parts of the high mountain ranges, the Antarctic margins, and the shallow shelves of the Arctic Ocean, where it survives as a relic of colder sea levels. Its study belongs jointly to soil science, geocryology, geophysics, and remote sensing.

Permafrost is not permanently frozen from the surface down. A seasonally thawing surface layer, the active layer, sits above it and ranges from a few tens of centimeters in the high Arctic to several meters in warmer zones. Unfrozen zones called taliks can persist within or beneath the frozen ground, particularly under lakes and rivers. The frozen ground overview maintained by the National Snow and Ice Data Center describes how these features determine drainage, vegetation, and the depth to which engineering foundations must reach.

Thermal Structure and Distribution

Permafrost extent is mapped in zones defined by the fraction of the land surface underlain: continuous permafrost covers more than 90 percent of the ground, discontinuous 50 to 90 percent, sporadic 10 to 50 percent, and isolated patches less than 10 percent. Thickness ranges from a meter or two at the southern fringe to more than 1,500 meters in parts of Siberia, set by the balance between geothermal heat flux from below and mean annual ground surface temperature above. Because that balance responds slowly, deep permafrost temperature acts as a low-pass filtered record of past surface climate, which is why borehole thermometry is treated as a climate indicator. Ground temperature series compiled in the NOAA Arctic Report Card assessments of terrestrial permafrost show warming at nearly all monitored sites, with the largest increases in the coldest, continuous zone.

Observation and Remote Sensing

Direct measurement relies on instrumented boreholes with thermistor strings, coordinated internationally through the Global Terrestrial Network for Permafrost, and on mechanical probing of active layer thickness at the end of the thaw season. Because these point measurements are sparse, satellite and airborne techniques carry most of the spatial burden. Interferometric synthetic aperture radar detects the centimeter-scale seasonal heave and subsidence that accompany freeze and thaw, and multi-year interferograms reveal irreversible settlement where excess ground ice has melted. Passive microwave radiometry supplies daily surface freeze and thaw state at coarse resolution, thermal infrared imagery constrains land surface temperature, and electrical resistivity tomography and ground-penetrating radar resolve ice content and permafrost table depth from the surface. These observations are assimilated into ground thermal models that extrapolate borehole records across landscapes.

Carbon Storage and Ground Stability

Permafrost soils hold organic matter that has been protected from decomposition by cold, an estimated stock of well over a thousand petagrams of carbon in the upper three meters and deeper deposits such as Siberian yedoma. When the ground thaws, microbial decomposition releases carbon dioxide under aerobic conditions and methane under waterlogged ones, a feedback quantified in the NOAA assessment of permafrost and the global carbon cycle. Thaw also removes mechanical strength. Melting ice wedges produce thermokarst terrain, hillslopes fail as retrogressive thaw slumps, and coastal bluffs erode rapidly once ice-bonded sediment is exposed to waves. Structures built on permafrost are therefore designed to keep the ground frozen, using elevated pile foundations, insulated gravel pads, and passive thermosiphons that extract heat during winter.

Applications

Permafrost science has applications in a range of fields, including:

• Climate modeling and carbon cycle assessment
• Arctic infrastructure, pipeline, and foundation engineering
• Satellite remote sensing and radar interferometry
• Hydrology and cold regions water resource management
• Ecology and land cover change monitoring
• Planetary science, including studies of Martian ground ice