Permafrost is ground that remains at or below zero degrees Celsius for at least two consecutive years, although the definition concerns ground temperature rather than the simple presence of frozen water. It can occur beneath soil, sediment, rock, peat and other terrestrial materials and may contain varying quantities of ice. The uppermost layer above permafrost that freezes and thaws seasonally is called the active layer. Permafrost can be continuous, discontinuous, sporadic or isolated depending on geographical distribution and climate.
It is widespread across high latitude regions of the Northern Hemisphere and also occurs at high elevations. Permafrost is not necessarily permanently frozen on geological timescales because its existence depends on climatic and environmental conditions. Changes in snow cover, vegetation, soil moisture, surface albedo and ground insulation can alter the transfer of heat between atmosphere and subsurface.
Permafrost thermal conditions are governed by the balance of energy entering and leaving the ground. Air temperature is important, but ground temperature does not simply equal air temperature because snow can insulate the surface during winter, vegetation can shade the ground during summer and soil moisture affects thermal conductivity and latent heat exchange. The phase change between ice and water consumes or releases latent heat, buffering temperature changes while substantial ice remains available to melt or freeze.
Permafrost thickness can range from relatively shallow layers to hundreds of metres in regions that have remained cold for long periods. Boreholes provide direct temperature measurements and allow researchers to identify warming trends at depth. Ground temperature records can reveal climatic changes because thermal disturbances propagate downward over time. The active layer commonly thickens when thaw conditions intensify, but its thickness also responds to local soil, vegetation and hydrological conditions.
Permafrost contains substantial quantities of organic carbon accumulated from incompletely decomposed plant and animal material over long periods. When permanently frozen ground thaws, previously protected organic matter can become accessible to microorganisms. Under oxygen rich conditions, decomposition tends to produce carbon dioxide, while oxygen limited saturated environments can produce methane. Both gases are greenhouse gases, although their atmospheric behaviour differs substantially. The release of carbon from thawing permafrost is therefore an important component of climate feedback research.
Not all thawed carbon is immediately released to the atmosphere, because some can be transported laterally into rivers and lakes or remain stored in soils and sediments. Carbon response also depends on vegetation recovery, hydrology, soil chemistry and the type of permafrost disturbance. The magnitude and timing of emissions are consequently uncertain and vary considerably between locations and thaw mechanisms.
Permafrost thaw can occur gradually as the active layer deepens or abruptly through processes such as thermokarst development. Thermokarst forms when ground ice melts and the surface subsides, producing irregular terrain that can include ponds, depressions, drained basins and collapsing ground. Ice rich permafrost is especially susceptible to subsidence because melting removes the physical volume previously occupied by ground ice.
Thaw slumps and retrogressive thaw failures can expose large quantities of frozen sediment and organic material, particularly on slopes and coastal margins. Infrastructure built on permafrost can be affected by uneven settlement, changing bearing capacity and altered drainage. Roads, pipelines, buildings and airstrips therefore require engineering approaches that account for local ground temperature, ice content and future thermal change. The consequences of thaw depend strongly on whether ground ice is massive, pore filling, segregated or relatively scarce.
Permafrost has significant relationships with hydrology because frozen ground can restrict infiltration and alter how water moves through soil and sediments. In some environments, permafrost acts as a relatively impermeable layer that promotes lateral surface flow and shallow subsurface drainage. Thawing can deepen the seasonally unfrozen zone and create new pathways for groundwater movement. Conversely, thaw settlement can alter drainage patterns and cause water to accumulate in depressions.
Changes in soil saturation can influence vegetation, microbial decomposition and methane production, creating interactions among hydrology, ecology and carbon cycling. Lakes and wetlands within permafrost landscapes can expand, drain or change chemistry as frozen ground changes. These processes demonstrate that permafrost is not simply a frozen geological material but a control on landscape structure, water movement and ecosystem function.
Permafrost has a long geological and climatic history. During past cold periods, extensive regions of the Northern Hemisphere were permanently frozen and supported landscapes quite different from those found in modern temperate regions. Ancient permafrost features can survive after climate warming and provide evidence of former environmental conditions. Ice wedges form when thermal contraction cracks develop in frozen ground and fill repeatedly with snow or meltwater that freezes, creating distinctive vertical structures.
Yedoma is a particularly carbon rich type of ice rich permafrost deposit associated with late Pleistocene environments of parts of Siberia and Alaska. Permafrost also occurs beneath Arctic continental shelves where former terrestrial areas were flooded following postglacial sea level rise. These relict subsea permafrost systems respond differently from terrestrial permafrost and are studied as part of the wider cryosphere.
Permafrost research uses borehole thermometry, ground penetrating radar, remote sensing, active layer measurements, soil sampling, geophysical surveys and numerical modelling. Remote sensing can identify surface deformation and thermokarst development over large areas, while interferometric radar can measure millimetre to centimetre scale ground movement in suitable conditions. Field measurements provide direct information about soil temperature, moisture, ice content and active layer thickness. Models integrate climate forcing with snow, vegetation, soil and thermal properties to estimate future permafrost conditions.
Important uncertainties include the spatial distribution of ground ice, future precipitation and snow conditions, abrupt thaw processes and interactions between hydrology and vegetation. Monitoring is particularly challenging because permafrost can change below the surface before obvious surface indicators appear. Long term observation therefore requires both direct subsurface measurements and landscape scale remote sensing.
Permafrost is commonly confused with seasonally frozen ground, which freezes during winter but does not remain frozen for at least two consecutive years. It is also distinct from glaciers because permafrost refers to a thermal condition of ground rather than a moving mass of ice. Ground within permafrost can contain liquid water as well as ice, depending on temperature, salinity and pore conditions. The term does not imply that every component of the ground is frozen.
Its significance extends across climate science, geology, ecology, hydrology, engineering and environmental management. Changes in permafrost can influence greenhouse gas emissions, landscape stability, river sediment loads, infrastructure, ecosystems and freshwater systems. Accurate interpretation therefore requires consideration of temperature, ice content, soil properties, geomorphology and hydrological conditions rather than assuming that all frozen ground behaves in the same way.
