Permafrost

Permafrost is ground that remains at or below 0°C for at least two consecutive years. It occurs in soil, rock, or sediment and may contain ice, unfrozen water, or both.

Ground ice: Ice within soil or rock, including pore ice, ice lenses, and massive ice. Its amount and distribution determine how much the ground settles when it thaws.
Thermokarst: Irregular landforms produced when ice-rich permafrost thaws and the ground subsides. It is the visible landscape response to melting ground ice.
Yedoma: Ice-rich, carbon-bearing permafrost deposits formed during the Pleistocene in parts of Siberia and Alaska. Its abundant ground ice and organic carbon make thaw especially consequential.
Seasonally frozen ground: Ground that freezes during part of each year and thaws during the warmer season. Unlike permafrost, it does not remain frozen for two consecutive years.
Taliks: Unfrozen zones within or beneath permafrost that persist through the year. These pockets interrupt frozen ground and can transmit heat and water through it.
Permafrost carbon cycle: The storage, transformation, and release of carbon in permafrost-affected soils. Thaw can expose ancient organic matter to decomposition and greenhouse-gas release.
Peat plateau: An elevated peatland landform underlain by permafrost in northern regions. Its frozen, raised surface can collapse into wetland when permafrost thaws.
Periglacial environment: A cold environment shaped by freeze–thaw processes, whether or not permafrost is present. Permafrost is one possible feature of these broader cold landscapes.
Cryoturbation: The mixing and deformation of soil caused by repeated freezing and thawing. Freeze–thaw movement creates patterned ground and reshapes soils above permafrost.
Permafrost methane emissions: Methane released from permafrost landscapes through microbial activity and geological pathways. Waterlogged thaw zones can produce methane as frozen carbon decomposes without oxygen.

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