Permafrost

Permafrost is ground, including soil, rock, and the ice that binds them, that remains at or below 0°C for at least two consecutive years. It underlies large portions of the Arctic and subarctic regions and occurs in high mountain areas around the world.

Permafrost forms when surface temperatures stay cold enough for extended periods to freeze and preserve soil and organic material. A layer near the surface that thaws seasonally is called the active layer; beneath it, the frozen ground is the permafrost. Ground ice content can vary widely, producing features such as ice wedges and thermokarst in thawing landscapes. Permafrost is classified as continuous, discontinuous, or sporadic, depending on how much of the ground remains frozen. Thickness ranges from meters to several hundred meters in the deepest deposits.

Permafrost stores large amounts of organic carbon, with estimates commonly cited around 1,400 to 1,600 gigatons. When permafrost thaws, microbial decomposition releases carbon dioxide and methane, with methane more likely in waterlogged soils. Thawing also causes ground subsidence and the development of thermokarst terrain, including lakes and altered drainage, which affects ecosystems and human infrastructure.

Climate warming is intensifying thaw and reducing permafrost extent and stability in many regions, potentially triggering abrupt thaw and feedbacks to the climate system. Monitoring relies on borehole temperature measurements, active-layer observations, and remote sensing to track thaw depth, extent, and ground deformation.

Permafrost is a key component of high-latitude geology and the global climate system, shaping hydrology, vegetation, and infrastructure in cold regions.