Permafrost – Definition, Formation, and Climate Change

Permafrost is ground that remains at or below 0 °C (32 °F) for at least two consecutive years. Despite its name, permafrost does not have to be permanently frozen, and it is not necessarily made mostly of ice. It may consist of soil, sediment, peat, or solid rock, with ice content ranging from almost none to large masses of ground ice.

Permafrost is an important part of Earth’s cryosphere, particularly in the Arctic and in high mountains. It influences landscapes, ecosystems, water movement, construction, and the global carbon cycle. Some permafrost formed recently, while other deposits have remained frozen for tens of thousands or even hundreds of thousands of years.

Today, rising temperatures are warming and thawing permafrost in many regions. Thaw changes ecosystems and drainage patterns, damages infrastructure, and allows microorganisms to decompose previously frozen organic matter, releasing carbon dioxide (CO₂) and methane (CH₄). Because these greenhouse gases contribute to additional warming, permafrost is an important component of the climate system.

Key Takeaways: Permafrost

• Permafrost is defined by temperature and time: Ground must remain at or below 0 °C (32 °F) for at least two consecutive years.
• Permafrost may consist of soil, sediment, peat, rock, organic matter, ice, or mixtures of these materials.
• Most permafrost occurs at high northern latitudes, but it also occurs in Antarctica, high mountains, and beneath parts of the Arctic Ocean.
• Permafrost is commonly classified as continuous, discontinuous, sporadic, or isolated according to how much of an area it underlies.
• Freezing and thawing produce characteristic features such as ice wedges, patterned ground, pingos, thermokarst, and thaw lakes.
• Permafrost stores enormous amounts of organic carbon that accumulated when cold conditions slowed decomposition.
• Climate warming causes permafrost to warm and thaw, altering landscapes, ecosystems, infrastructure, and the carbon cycle.

Why Is Permafrost Important?

Permafrost is much more than frozen ground. It is a major environmental control in cold regions.

Frozen ground affects drainage because water does not readily infiltrate deeply into frozen soil. As a result, areas underlain by permafrost can support ponds, lakes, wetlands, and waterlogged soils even where precipitation is relatively low.

Permafrost also affects ecology. Its presence limits rooting depth, controls soil moisture and nutrient availability, and influences which plants and microorganisms can survive. Changes in permafrost can therefore transform entire ecosystems.

For people living in cold regions, permafrost is an important engineering consideration. Buildings, roads, pipelines, airports, and other structures may depend on frozen ground remaining mechanically stable. Ice-rich permafrost can lose volume when it thaws, causing the surface to settle unevenly.

Permafrost is also important to the global carbon cycle. Northern permafrost contains vast quantities of frozen organic matter accumulated over thousands of years. When this material thaws, microorganisms can decompose it and release carbon to the atmosphere as CO₂ and CH₄.

What Is Permafrost?

Permafrost is any ground that stays at or below the freezing point of water, 0 °C (32 °F), for at least two consecutive years.

The definition is based entirely on ground temperature. It does not specify:

• How much ice the ground contains
• What type of material makes up the ground
• Whether snow covers the surface
• Whether the location is Arctic
• How long beyond two years the ground has remained frozen

For example, solid bedrock that remains below 0 °C for several years is permafrost even if it contains almost no ice.

Similarly, ground that freezes every winter but completely thaws during summer is seasonally frozen ground, not permafrost.

The Active Layer

In many permafrost regions, the surface layer thaws during summer and freezes again during winter. This seasonally thawed zone is called the active layer.

The active layer lies above the permafrost table, which is the upper surface of the perennially frozen ground.

Its thickness varies greatly. Climate, vegetation, snow cover, soil composition, moisture, slope direction, and local drainage all affect how deeply summer warmth penetrates the ground.

A warming climate often increases the depth of seasonal thaw. Scientists therefore monitor active-layer thickness as one indicator of changing permafrost conditions.

What Is Permafrost Made Of?

Permafrost is not a particular substance. It describes the thermal state of the ground.

Permafrost may contain:

• Mineral soil
• Sand and gravel
• Silt and clay
• Peat
• Organic matter
• Sedimentary deposits
• Bedrock
• Liquid water in some circumstances
• Ground ice

Ice occurs in several forms. Water may simply freeze within pores between soil particles, or ice may form larger structures such as lenses, layers, veins, and wedges.

Some permafrost contains little ice. Other deposits are ice-rich, containing enough ground ice that thaw causes dramatic loss of volume and collapse of the surface.

Can Permafrost Contain Liquid Water?

Yes. Although permafrost has a temperature of 0 °C or lower, small amounts of liquid water can remain within it.

Dissolved salts lower the freezing point of water, and water confined in very small soil pores can remain unfrozen below 0 °C. Therefore, “frozen ground” does not necessarily mean every molecule of water is solid ice.

How Does Permafrost Form?

Permafrost forms when the ground loses more heat during cold periods than it gains during warm periods. If this negative heat balance persists long enough, ground temperatures remain below freezing from one year to the next.

Several factors influence its formation and persistence.

Air temperature: Cold annual temperatures favor permafrost.

Snow cover: Snow is an effective insulator. Thick snow can actually keep winter soil warmer by reducing heat loss from the ground.

Vegetation: Moss, peat, shrubs, and other vegetation alter heat transfer between the atmosphere and soil.

Soil moisture: Water requires considerable energy to freeze and thaw, so moisture content affects the ground’s thermal behavior.

Soil and rock type: Different materials conduct and store heat differently.

Topography: Elevation, slope, and exposure to sunlight affect ground temperature. In the Northern Hemisphere, north-facing slopes often remain cooler than comparable south-facing slopes.

Surface water: Rivers, lakes, and wetlands modify local ground temperatures and can create gaps in otherwise continuous permafrost.

Once established, thick permafrost responds slowly to environmental change because heat moves gradually through the ground.

When Did Permafrost Form?

There is no single age for Earth’s permafrost.

Some permafrost is relatively young and formed during recent centuries or millennia. Much of the deep permafrost found today developed during the cold climatic conditions of the Pleistocene Epoch, particularly during glacial periods.

During the last ice age, colder temperatures allowed permafrost to extend much farther south than it does today. Some of this ancient permafrost survived the warmer conditions that followed.

Deep deposits in parts of Siberia and other Arctic regions may have remained frozen for hundreds of thousands of years. In contrast, permafrost near its present southern or lower-elevation limits may be much younger and more vulnerable to thaw.

This means permafrost is not necessarily the same age as the material it contains. Sediment may have been deposited first and frozen later, while other sediments accumulated while permafrost was already present.

Where Does Permafrost Occur?

Permafrost occurs wherever ground temperatures remain sufficiently cold, so it is controlled by climate rather than latitude alone.

Its greatest extent is in the Northern Hemisphere, particularly across:

• Siberia and the Russian Arctic
• Alaska
• Northern Canada
• Greenland
• Scandinavia

Permafrost also occurs in Antarctica and on subantarctic islands.

At lower latitudes, it occurs at high elevations in mountain ranges such as the:

• Rocky Mountains
• Andes
• Alps
• Himalayas
• Tibetan Plateau

This high-elevation frozen ground is called alpine or mountain permafrost.

Permafrost also exists beneath portions of the Arctic continental shelf. During glacial periods, sea level was much lower and some present-day seafloor was exposed land. Permafrost formed there before rising seas submerged it. This material is called subsea permafrost.

How Deep Is Permafrost?

Permafrost thickness ranges from less than 1 meter (3.3 feet) to more than 1,500 meters (4,900 feet) in exceptionally cold regions.

Near the southern boundary of permafrost, frozen ground may occur only in scattered patches and may be relatively thin. Farther north, it becomes more continuous and generally thicker.

The surface of the permafrost is called the permafrost table. Beneath sufficiently thick permafrost, heat from Earth’s interior eventually raises the ground temperature above freezing. This lower boundary is called the permafrost base.

Classification of Permafrost

One common classification describes the percentage of a landscape underlain by permafrost.

The boundaries between these zones are not sharp. Local conditions such as snow depth, vegetation, slope orientation, lakes, and soil properties produce considerable variation.

Continuous Permafrost

Continuous permafrost occurs in the coldest regions, especially at high Arctic latitudes. Nearly all of the landscape is underlain by frozen ground, although unfrozen areas may occur beneath large lakes and rivers.

Discontinuous Permafrost

Farther south, average temperatures rise and permafrost becomes discontinuous. Frozen and unfrozen ground occur together across the landscape.

Sporadic and Isolated Permafrost

Near the climatic limit of permafrost, frozen ground survives mainly where local conditions keep the soil unusually cold. Examples include shaded slopes, peatlands, and locations with favorable vegetation or snow conditions.

Taliks: Unfrozen Ground Within Permafrost Regions

An important feature of permafrost landscapes is the talik, a layer or body of ground that remains unfrozen within or beneath a permafrost region.

Taliks commonly occur beneath deep lakes and rivers because the water prevents the underlying ground from becoming as cold as surrounding land.

Some taliks penetrate completely through the permafrost, potentially allowing groundwater to move between shallow and deeper aquifers.

Permafrost Landforms

Repeated freezing and thawing, together with the formation and melting of ground ice, produces distinctive periglacial landforms.

Ice Wedges and Polygonal Ground

Winter contraction causes frozen soil to crack. During spring and summer, meltwater enters these cracks and freezes. Repeated cracking and freezing over many years gradually builds large wedges of ice.

Networks of ice wedges divide the surface into recognizable geometric shapes called ice-wedge polygons or polygonal ground.

Patterned Ground

Freeze-thaw processes can sort stones and soil particles into circles, polygons, stripes, and other repeating patterns.

This patterned ground is especially common in environments where vegetation is sparse.

Pingos

A pingo is a hill with a core of massive ice.

Pingos form when groundwater freezes and expands beneath the surface, pushing overlying sediment upward. Some pingos grow tens of meters high.

Palsas

A palsa is a smaller mound containing a core of frozen peat and mineral soil, often with ice lenses. Palsas commonly occur in peatlands within discontinuous permafrost regions.

Solifluction Lobes

When the active layer thaws but deeper ground remains frozen, water cannot readily drain downward. Waterlogged soil may slowly move downslope.

This process, called solifluction, produces sheets, terraces, and lobes of displaced material.

Thermokarst

Thermokarst develops when ice-rich permafrost thaws and the ground surface subsides or collapses.

The resulting terrain may include:

• Depressions
• Pits
• Irregular hummocks
• Gullies
• Wetlands
• Lakes

The term comes from its superficial resemblance to karst terrain produced by dissolution of limestone, although the processes are entirely different.

Thaw Lakes

Surface depressions produced by thaw can collect water and form thermokarst lakes, also called thaw lakes.

Water absorbs and transfers heat efficiently, so lake formation can promote additional thaw. Some thaw lakes grow, drain, or disappear as the surrounding permafrost landscape evolves.

Permafrost Ecology

Permafrost strongly influences Arctic and alpine ecosystems.

Because frozen ground restricts drainage, the active layer can become saturated during summer. This helps explain why tundra may contain abundant wetlands despite receiving relatively little precipitation.

Permafrost also limits how deeply plants can root. Tundra vegetation therefore includes many shallow-rooted grasses, sedges, mosses, lichens, dwarf shrubs, and other species adapted to cold soils and short growing seasons.

Microorganisms remain important even in extremely cold environments. During the brief thaw season, bacteria, archaea, and fungi decompose organic matter and recycle nutrients.

Changes in active-layer thickness and soil moisture can alter vegetation communities. In some regions, warming favors taller shrubs or trees. In others, abrupt thaw produces wetter conditions, erosion, or lake formation that replaces existing terrestrial habitat.

Animals are affected indirectly as vegetation, wetlands, snow conditions, insects, and food availability change.

Permafrost and the Carbon Cycle

One of the most important properties of permafrost is its role as a long-term carbon reservoir.

Plants remove CO2 from the atmosphere through photosynthesis. When plants die, microorganisms normally decompose their remains and return much of the carbon to the atmosphere.

Cold, waterlogged permafrost environments slow decomposition. Over thousands of years, partially decomposed plant and animal material accumulated in frozen soils.

When permafrost thaws, microorganisms gain access to this organic matter. Decomposition then releases carbon.

Under oxygen-rich conditions, much of the carbon leaves as carbon dioxide (CO2). Under oxygen-poor, waterlogged conditions, microbial processes also produce methane (CH4).

This creates a positive climate feedback:

Climate warming → permafrost warming and thaw → increased decomposition → CO2 and CH4 release → additional warming

This feedback does not mean that all permafrost will suddenly thaw or that all stored carbon will enter the atmosphere. The rate and form of carbon release depend on temperature, moisture, vegetation, microbial activity, fire, hydrology, and the physical characteristics of the permafrost.

Permafrost and Wildfire

Wildfire provides another connection between climate, ecosystems, and permafrost.

The layer of vegetation and organic soil above permafrost acts as insulation. Severe fires can remove some of this insulating material, allowing more summer heat to penetrate the ground.

As a result, wildfire can increase active-layer depth and accelerate permafrost thaw. Fire also releases carbon directly from vegetation and organic soils.

Recovery varies among ecosystems. In some places, vegetation regrowth eventually restores insulation, while repeated or severe fires may promote longer-lasting changes.

Effects of Climate Change on Permafrost

Climate change affects permafrost through both gradual warming and abrupt thaw.

Rising Ground Temperatures

As air temperatures rise, heat penetrates the ground. Permafrost temperatures increase, especially near the surface.

Permafrost does not necessarily thaw immediately when climate warms. Very cold or very thick deposits may take considerable time to respond.

A Deeper Active Layer

Warmer summers can thaw a greater depth of soil, increasing active-layer thickness. This exposes previously frozen organic material to decomposition and changes conditions for plant roots and soil organisms.

Thermokarst and Ground Collapse

When ice-rich permafrost thaws, melting ground ice leaves empty space. The land surface settles or collapses, producing thermokarst.

Abrupt thaw can transform a landscape much faster than gradual downward thaw of the permafrost table.

Changes in Water

Permafrost thaw alters drainage.

In some places, subsidence creates new ponds, wetlands, and lakes. Elsewhere, the development of drainage channels or taliks causes lakes and wetlands to drain.

The result can therefore be either wetter or drier conditions, depending on local geography.

Coastal Erosion

Many Arctic coastlines contain ice-rich permafrost. Warming, thawing, waves, and reduced protection from sea ice can make these coasts particularly vulnerable to erosion.

Ecosystem Changes

Thaw affects soil moisture, nutrient cycling, vegetation, microbial activity, and habitat. Changes in vegetation can themselves alter snow accumulation and surface temperatures, producing additional feedbacks.

Greenhouse Gas Emissions

Thaw exposes previously frozen organic carbon to microbial decomposition, increasing potential emissions of CO2 and CH4.

Recent research indicates that the northern permafrost region’s greenhouse-gas balance is changing as warming, wildfire, thaw, vegetation growth, lakes, and wetlands interact. The magnitude and timing of future emissions remain important areas of climate research.

Permafrost and Human Infrastructure

Building on permafrost requires special engineering because structures can change the thermal balance of the ground.

A heated building placed directly on frozen soil may warm the permafrost beneath it. If ice-rich ground thaws, the foundation may settle unevenly.

Engineers use several techniques to reduce these problems, including:

• Elevating buildings above the ground
• Using piles driven into stable frozen ground
• Providing ventilation beneath structures
• Insulating foundations
• Using passive cooling systems called thermosyphons
• Designing roads and embankments to minimize heat transfer

Climate-driven permafrost thaw adds another challenge because infrastructure designed for historical ground temperatures may experience warmer conditions during its service life.

How Scientists Study Permafrost

Scientists monitor permafrost using both field measurements and remote sensing.

Boreholes: Temperature sensors placed at different depths reveal how ground temperatures change over time.

Active-layer measurements: Researchers measure the maximum annual depth of thaw.

Soil and ice cores: Cores reveal sediment composition, ice content, organic matter, and the history of frozen deposits.

Radiocarbon dating: Organic material provides information about the age of carbon preserved in permafrost.

Satellite observations: Satellites monitor changes in surface temperature, vegetation, lakes, snow cover, ground deformation, and landscape disturbance.

Airborne surveys: Radar and other instruments provide additional information about terrain, hydrology, and greenhouse-gas emissions.

Combining these methods helps researchers distinguish seasonal variations from long-term changes.

Permafrost vs. Similar Terms

Common Misconceptions About Permafrost

Permafrost Is Permanently Frozen

Not necessarily. The name is misleading. By definition, ground needs to remain frozen for only two consecutive years to qualify as permafrost. It can later thaw.

Permafrost Is Frozen Water

Permafrost is ground, not simply ice. It may contain soil, rock, sediment, organic matter, and varying quantities of ice.

All Permafrost Contains Lots of Ice

Some does, but some permafrost consists primarily of relatively dry rock or sediment. Ice content strongly influences what happens when permafrost thaws.

Permafrost Is the Same as a Glacier

No. A glacier is a large mass of ice formed primarily from compacted snow. Permafrost is ground defined by its temperature.

Permafrost Exists Only in the Arctic

Most permafrost occurs at high northern latitudes, but it also occurs in Antarctica and in high mountains at lower latitudes.

Permafrost Never Thaws During Summer

The active layer above permafrost normally thaws each summer. The underlying permafrost remains frozen.

Thawing Permafrost Always Makes the Landscape Wetter

Not necessarily. Thaw can create ponds and wetlands, but it can also improve drainage or cause existing lakes to drain.

Permafrost Thaw Releases Methane That Was Simply Trapped in the Ice

Some methane may already exist in frozen sediments, but much of the climate concern involves new microbial production of CO₂ and CH₄ after previously frozen organic matter thaws and begins decomposing.

Permafrost Thaw Is a Single Sudden “Tipping Point”

Permafrost response is more complicated. Thaw varies by location, depth, ice content, vegetation, hydrology, and climate. Some change occurs gradually, while ice-rich landscapes can undergo rapid thermokarst collapse.

Frequently Asked Questions

What is the simple definition of permafrost?

Permafrost is ground that remains at or below 0 °C (32 °F) for at least two consecutive years.

Why is it called permafrost?

The word combines the ideas of permanent and frost. American geologist Siemon W. Muller introduced the term permafrost in the 1940s as a shortened form of “permanently frozen ground.”

How old is permafrost?

Its age varies widely. Some permafrost is only years or centuries old, while ancient deposits have remained frozen for tens of thousands to hundreds of thousands of years.

How deep is permafrost?

Its thickness ranges from less than 1 meter (3.3 feet) near its climatic limits to more than 1,500 meters (4,900 feet) in exceptionally cold regions.

Is permafrost always covered with snow?

No. Snow may cover permafrost during winter, but many permafrost landscapes become snow-free during summer.

Is tundra the same thing as permafrost?

No. Tundra is an ecosystem or biome, while permafrost describes the thermal condition of the ground. They commonly occur together, but the terms are not interchangeable.

Are glaciers permafrost?

No. Glaciers are masses of ice formed from accumulated snow. Permafrost is soil, sediment, rock, or other ground that remains at or below freezing for at least two years.

Can trees grow over permafrost?

Yes. Forests grow over extensive areas of discontinuous permafrost. However, frozen ground restricts rooting depth and affects soil moisture, which influences tree growth and distribution.

Why does permafrost contain so much carbon?

Cold and often waterlogged conditions slow microbial decomposition. Dead plant material therefore accumulated faster than it completely decomposed and became incorporated into frozen soils over thousands of years.

Why does thawing permafrost release methane?

In waterlogged, oxygen-poor environments, microorganisms known as methanogens break down organic material and produce CH4. Better-drained, oxygen-rich soils tend to favor CO2 production instead.

Can thawed permafrost freeze again?

Yes. Ground can refreeze if climatic and local conditions become sufficiently cold. However, refreezing does not necessarily restore the original landscape, particularly if ground ice has melted, the surface has collapsed, or stored organic carbon has decomposed.

What happens to buildings when permafrost thaws?

Ice-rich ground can lose volume and strength when it thaws. Uneven settlement may damage foundations, roads, pipelines, runways, and other infrastructure.

Does melting permafrost raise sea level?

Not directly in the way melting land glaciers do. Ground ice is already contained within the landscape rather than forming a large above-ground ice mass whose meltwater simply enters the ocean. However, permafrost thaw can indirectly contribute to future sea-level rise by releasing greenhouse gases that amplify warming.

Glossary

Active layer: The surface layer above permafrost that thaws during the warm season and refreezes during the cold season.

Alpine permafrost: Permafrost occurring at high elevations, including mountainous regions outside the polar zones.

Continuous permafrost: A region where permafrost underlies approximately 90–100% of the landscape.

Cryosphere: All parts of Earth where water occurs in solid form, including glaciers, ice sheets, snow, sea ice, and frozen ground.

Discontinuous permafrost: A region where permafrost generally underlies about 50–90% of the landscape.

Ground ice: Ice occurring within soil, sediment, or rock.

Ice wedge: A wedge-shaped mass of ground ice formed as water repeatedly enters and freezes within thermal-contraction cracks.

Isolated permafrost: Permafrost occurring beneath less than about 10% of a region.

Methanogen: A microorganism that produces methane under oxygen-poor conditions.

Palsa: A mound containing permanently frozen peat or mineral soil and ice, commonly found in peatlands.

Patterned ground: Repeating surface patterns produced by freezing, thawing, and movement of soil and stones.

Periglacial: Describing cold-climate environments and processes associated with repeated freezing and thawing, whether or not glaciers are present.

Permafrost: Ground that remains at or below 0 °C (32 °F) for at least two consecutive years.

Permafrost table: The upper surface of permafrost.

Pingo: A hill containing a core of massive ground ice.

Seasonally frozen ground: Ground that freezes during part of the year but does not remain frozen long enough to qualify as permafrost.

Solifluction: Slow downslope movement of water-saturated soil, particularly common where frozen ground restricts drainage.

Sporadic permafrost: A region where permafrost generally underlies about 10–50% of the landscape.

Subsea permafrost: Permafrost beneath the seafloor, particularly beneath Arctic continental shelves that were exposed during periods of lower sea level.

Talik: A body or layer of unfrozen ground occurring within or beneath a permafrost region.

Thermokarst: Irregular terrain formed when thawing of ice-rich permafrost causes ground subsidence or collapse.

Thermokarst lake: A lake occupying a depression created by thaw and subsidence of ice-rich permafrost.

References and Further Reading

• Cooper, M. G.; Zhou, T.; et al. (2023). “Detecting Permafrost Active Layer Thickness Change From Nonlinear Baseflow Recession”. Water Resources Research. 57 (1) e2022WR033154. doi:10.1029/2022WR033154
• Huggel, C.; Allen, S.; Deline, P. (2012). “Ice thawing, mountains falling; are alpine rock slope failures increasing?”. Geology Today. 28 (3): 98–104. doi:10.1111/j.1365-2451.2012.00836.x
• Langer, Moritz; von Deimling, Thomas Schneider; et al. (2023). “Thawing permafrost poses environmental threat to thousands of sites with legacy industrial contamination”. Nature Communications. 14 (1): 1721. doi:10.1038/s41467-023-37276-4
• Obu, J. (2021). “How Much of the Earth’s Surface is Underlain by Permafrost?”. Journal of Geophysical Research: Earth Surface. 126 (5) e2021JF006123. doi:10.1029/2021JF006123
• Sayedi, Sayedeh Sara; Abbott, Benjamin W.; et al. (2020). “Subsea permafrost carbon stocks and climate change sensitivity estimated by expert assessment”. Environmental Research Letters. 15 (12): B027-08. doi:10.1088/1748-9326/abcc29