Permafrost
Permafrost is ground that remains at or below freezing for at least two consecutive years. It can contain soil, rock, sediment, ice, and ancient organic carbon, making it important for Arctic landscapes, ecosystems, infrastructure, and climate feedbacks.
What permafrost is
Permafrost is defined by temperature, not by whether the ground looks icy. Soil, gravel, bedrock, and sediment can all be permafrost if they remain at or below 0เน€เธเธเน€เธยC for at least two consecutive years. Some permafrost contains little visible ice, while ice-rich permafrost can include large ice wedges, lenses, and frozen organic material.
Active layer
The active layer is the surface layer above permafrost that freezes and thaws each year. Plants, microbes, water flow, and many engineering problems are concentrated here because it is the part of the ground that responds quickly to seasons. A warming climate can deepen the active layer, exposing material that had stayed frozen for decades, centuries, or longer.
Where it is found
Permafrost is most widespread in the Arctic and subarctic, including parts of Alaska, Canada, Greenland, Scandinavia, and Siberia. It also occurs in high mountain areas such as the Tibetan Plateau and some alpine regions. Scientists describe permafrost zones as continuous, discontinuous, sporadic, or isolated depending on how much of the land is underlain by frozen ground.
Ice-rich ground and thermokarst
When ice-rich permafrost thaws, the ground can lose volume and collapse. This process can create thermokarst features such as pits, slumps, uneven terrain, thaw lakes, and eroding coastal bluffs. These landforms matter because they can rapidly change drainage, vegetation, habitat, and human access routes.
Carbon and microbes
Permafrost regions store large amounts of organic carbon in frozen soils. When thaw exposes that material, microbes can decompose it and release carbon dioxide or methane, depending on oxygen and water conditions. Plant growth can take up some carbon, so the balance is complicated, but thawing permafrost is a major reason scientists watch Arctic carbon cycles closely.
Infrastructure and communities
Roads, runways, pipelines, homes, storage tanks, and water systems can be damaged when frozen ground warms, settles, or erodes. In northern communities, permafrost thaw can combine with coastal erosion, flooding, and changing snow cover to raise maintenance costs and relocation pressures. Building on permafrost often requires designs that keep ground cold or allow for movement.
Why it matters
Permafrost is not just frozen dirt. It supports Arctic landforms, influences hydrology, stores carbon, preserves ancient biological material, and affects the safety of infrastructure. Its thaw is both a local problem for people living on frozen ground and a global concern because of possible climate feedbacks.
What remains uncertain
Researchers can measure permafrost temperature at monitoring sites and map broad patterns, but local thaw depends on soil type, ground ice, vegetation, snow, fire, water, and human disturbance. Abrupt thaw is especially hard to scale up because it can transform small areas quickly. Better maps, long-term observations, and models are still needed.