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Last edited 02 May 2025

Permafrost

Climate Change 2021, The Physical Science Basis, published by the Intergovernmental Panel on Climate Change in 2021, defines permafrost as: ‘Ground (soil or rock, and included ice and organic material) that remains at or below 0°C for 21 at least two consecutive years (Harris et al., 1988). Note that permafrost is defined via temperature rather 22 than ice content and, in some instances, may be ice-free.’

Climate Change 2021 – The Physical Science Basis, Annex VII: Glossary, written by the Intergovernmental Panel on Climate Change (IPCC) and published by Cambridge University Press in 2023, defines permafrost as:

Ground (soil or rock, and included ice and organic material) that remains at or below 0°C for at least two consecutive years (Harris et al., 1988). Note that permafrost is defined via temperature rather than ice content and, in some instances, may be ice-free.

Near-surface permafrost: Permafrost within about 3–4 m of the ground surface. The depth is not precise, but describes what commonly is highly relevant for people and ecosystems. Deeper permafrost is often progressively less ice-rich and responds more slowly to warming than near-surface permafrost. The presence or absence of near-surface permafrost is not the only significant metric of permafrost change, and deeper permafrost may persist when near-surface permafrost is absent.
Permafrost degradation: Decrease in the thickness and/or areal extent of permafrost.

Permafrost thaw: Progressive loss of ground ice in permafrost, usually due to input of heat. Thaw can occur over decades to centuries over the entire depth of permafrost ground, with impacts occurring while thaw progresses. During thaw, temperature fluctuations are subdued because energy is transferred by phase change between ice and water. After the transition from permafrost to non-permafrost, ground can be described as thawed.