Thermokarst

In permafrost regions, frozen ground may contain substantial ice within soil or as larger bodies of ground ice. When that ice melts, the ground loses volume and may settle or collapse. Water can collect in the lowered terrain, while changes in drainage may redirect or remove water and sediment. The resulting features vary with ground-ice content, terrain, vegetation, water flow and the scale of thaw. Examples include thermokarst lakes and ponds, pits, sinkholes and collapsed pingos. Thermokarst is not a single landform but a family of landscape responses linked to thaw and ground deformation.

The term is used for characteristic relief and the processes that produce it through thaw of ice-rich permafrost or melting of massive ice. It is distinct from thermal erosion, in which thermal and mechanical action erodes the land surface, and from thermal abrasion of shore bluffs. Slope failures and thaw slumps may also arise through thaw and gravity-driven movement; they can accompany thermokarst but are not automatically the same process. In real landscapes, different thaw, erosion and transport mechanisms may interact.

Subsidence changes local topography and can modify soil moisture, water flow, vegetation and the decomposition of organic matter. Whether a thawed area becomes wetter or drains depends on local terrain and hydrological connections. Thermokarst lakes can promote further thaw because water and lake heat transfer affect frozen ground below and around them. Changes in soil and vegetation can influence carbon storage and release, but the net ecosystem outcome depends on site conditions and processes. Uneven settlement can also damage roads, buildings and other infrastructure built on ice-rich permafrost.

Researchers document thermokarst using field measurements, repeat photography, elevation surveys, satellite imagery and records of ground temperature and active-layer depth. Combining topographic and ecological observations helps distinguish thaw-related surface change from other landscape processes and assess its consequences. A 2019 study in the Canadian High Arctic reported up to 90 centimetres of subsidence at monitored sites over a 12-year observation period, illustrating that very cold, ice-rich permafrost can undergo substantial change under anomalously warm summers. That result describes the studied sites and period, not a universal rate.