Permafrost is defined as ground—soil, rock, or sediment—that remains at or below 0°C for two or more consecutive years. This permanently frozen ground underlies approximately 15% of the Northern Hemisphere, spanning vast regions of Alaska, Siberia, and Canada. Permafrost is a major geological feature of the global cryosphere, but its thickness is highly variable across the Arctic and high-mountain landscapes. The depth of this frozen layer is determined by a balance of surface cooling and heat rising from the Earth’s interior. This variability means that while some permafrost is only a few meters thick, the deepest examples extend over 1,500 meters beneath the surface.
Understanding the Structure of Frozen Ground
The total depth of permafrost is measured from the base of a transitional layer known as the active layer. The active layer is the uppermost section of ground that undergoes seasonal changes, thawing completely each summer and refreezing each winter. This seasonal thaw zone can range from less than a meter to several meters deep, depending on local conditions.
The permafrost table marks the boundary where the active layer meets the continuously frozen ground below. The overall depth of permafrost is the vertical distance from this table down to the point where the ground temperature finally rises above 0°C. Below this lower boundary, the earth material is unfrozen, primarily due to the heat generated deep within the planet.
The Measured Range of Permafrost Depth
The thickness of permafrost varies dramatically based on its location and the mean annual surface temperature. In the sub-Arctic, where temperatures hover near freezing, permafrost is often discontinuous and relatively shallow, sometimes only a few meters thick. This shallow depth reflects the limited duration of sustained cold necessary to freeze the ground permanently.
Moving into the high Arctic, where mean annual temperatures are much colder, the permafrost becomes continuous and substantially deeper. Along the Arctic Slope of Alaska, continuous permafrost commonly ranges from 200 to 400 meters thick, with some localized areas reaching depths of 600 meters. The deepest known permafrost is found in the Siberian interior, particularly in the Vilyui River basin of Russia, where the ground remains frozen to a depth exceeding 1,500 meters.
Environmental Controls on Maximum Depth
The maximum depth of permafrost is governed by the balance between the cold surface climate and the heat flowing upward from Earth’s interior, known as the geothermal gradient. This gradient describes the rate at which temperature increases with depth beneath the Earth’s surface. For permafrost to exist, the cold temperatures penetrating from the surface must overcome this rising geothermal heat.
Permafrost depth is limited to the point where the geothermal heat raises the temperature of the rock and soil above the 0°C freezing point. The thermal conductivity of the subsurface materials is a significant factor, often proving more important than the local surface temperature. Where ground material, such as highly porous, ice-rich sediments, has high thermal conductivity, heat transfers more efficiently. This results in a lower geothermal gradient and allows the permafrost to extend much deeper.
Other factors, like surface insulation and ground ice content, influence the upper layers and, indirectly, the overall depth. A thick layer of snow cover or dense vegetation can insulate the ground, preventing deep winter cooling and restricting the depth of the permafrost. Conversely, extremely cold surface temperatures allow the thermal zero-point to be pushed hundreds of meters down until the geothermal heat flow finally thaws the material.
Implications of Deep Permafrost
The extreme depth of permafrost has two major implications related to global climate and human infrastructure. Deep permafrost is a significant reservoir for the permafrost carbon pool, storing massive amounts of ancient, frozen organic material. The northern permafrost region holds nearly twice the amount of carbon currently in the atmosphere, locked away by the permanent freezing conditions.
The depth of this frozen ground also provides a stable, rigid foundation for human development in Arctic regions. Deep permafrost supports roads, pipelines, buildings, and airstrips across the Arctic. When the upper layers of this deep, ice-rich ground begin to thaw, the resulting ground subsidence, or thermokarst, destabilizes infrastructure.