Thermokarst
Thermokarst is irregular terrain formed when ice-rich permafrost thaws, melting ground ice and causing the surface to subside or collapse.
Permafrost: Ground that remains at or below 0°C for at least two consecutive years. Thermokarst begins when permafrost warms enough for its ground ice to thaw.
Thermokarst lake: A lake occupying a depression formed by thaw and subsidence of ice-rich permafrost. It is a common thermokarst form where meltwater fills a subsiding basin.
Permafrost carbon cycle: The storage, transformation, and release of carbon in frozen and thawing permafrost soils. Thermokarst exposes previously frozen organic matter to decomposition and greenhouse-gas production.
Permafrost temperature: The temperature of ground that remains frozen for at least two consecutive years. Warming toward the melting point increases the likelihood of thermokarst initiation.
Abrupt permafrost thaw: Rapid thaw of ice-rich permafrost that causes sudden ground collapse or slope failure. Thermokarst can proceed abruptly, challenging estimates based on gradual thaw alone.
Ground ice: Ice occurring within soil or rock, including pore ice, ice lenses, and massive ice. Its melting removes volume and support, driving thermokarst subsidence.
Alas: A broad, shallow depression in Siberian permafrost terrain, commonly formed after a thermokarst lake drains. Alas landscapes record lake expansion, drainage, and subsequent thaw settlement.
Methane emissions from permafrost: Methane released as thawing permafrost soil decomposes under oxygen-poor conditions. Waterlogged thermokarst terrain can create conditions favorable to methane production.
Ice wedge: A vertical body of ground ice formed as water freezes in cracks in permafrost. Wedge melting can create polygonal troughs and other patterned thermokarst terrain.
Permafrost thaw models: Numerical models that estimate how permafrost responds to climate and surface changes. Representing abrupt subsidence and drainage remains difficult in large-scale projections.