Permafrost

Permafrost is defined by temperature: ground that remains at or below 0°C for at least two consecutive years. The term includes soil, sediment and rock. It does not mean that a place is frozen only during winter, and it is not defined by how much ice or snow it contains. Some permafrost has abundant ground ice, while the defining test is the temperature of the ground itself.

Permafrost is most extensive in the Arctic and other high northern latitudes, but it also occurs in parts of Antarctica, in high mountains, and beneath some Arctic continental shelves. Its boundaries can be patchy. Air temperature alone does not determine whether permafrost is present: ground temperatures also respond to soil, water, vegetation, snow cover and local terrain.

In many permafrost landscapes, the upper ground thaws during the warmer part of the year and freezes again as temperatures fall. This seasonally thawing and refreezing layer is called the active layer; permafrost lies beneath it. Its depth varies with local climate and ground conditions.

Permafrost should be distinguished from seasonally frozen ground, which freezes for only part of a year and does not remain at or below freezing for two consecutive years. The distinction matters for mapping cold-region landscapes and for understanding how seasonal weather interacts with ground that remains frozen over much longer periods.

Permafrost can preserve organic material from plants and other organisms in cold ground. When organic-rich permafrost thaws, microbes can decompose material that had remained frozen, releasing carbon dioxide and methane. These gases retain heat in the atmosphere, so emissions from thawing permafrost can reinforce warming and form a climate feedback. The amount and timing of emissions depend on how thaw unfolds and on local ground and ecosystem conditions.

Permafrost soils are a large, climate-sensitive carbon store, but they are not a single uniform reservoir: carbon content and thaw patterns vary across landscapes. Assessing the feedback therefore requires observations of ground temperature, carbon exchange and ecosystem change over time.

When ground ice melts, soil can settle or shift, destabilizing slopes and altering drainage. Such changes can affect habitats, waterways and infrastructure built on or near permafrost, including roads and buildings. Thaw can also change watershed hydrology and the movement of materials through soil and water. These effects vary by location; permafrost conditions, ground ice and the design of local infrastructure all matter.

Observations from Arctic Alaska illustrate that consequences can extend into streams and rivers. A NOAA Arctic Report Card essay describes recent water-quality changes in some watersheds and identifies thaw-related exposure of minerals as a leading explanation under study. This is one regional example, not a universal outcome of permafrost thaw.

Permafrost is difficult to map precisely because frozen ground can occur discontinuously and because near-surface air temperatures do not always match temperatures underground. Researchers combine field measurements with maps and remote observations to track ground conditions and thaw. Long-term records are important: a single warm season is not enough to determine whether ground meets the two-year definition or how a landscape is changing.

Scientists study permafrost because it links the cryosphere, land ecosystems, water systems, infrastructure and the global carbon cycle. Its response to warming is therefore relevant both to regional planning and to projections of future climate.