365 Days of Climate Awareness 57 - Tundra and Permafrost

Tundra (transliterated from Russian) is land with cold temperatures and a limited growing season, and it can occur in the plains or on mountainsides. Permafrost is ground continually frozen for two years or more. It can occur at or below the ground surface, on land or beneath the ocean. The two are closely related but not identical in extent.

Canadian tundra.

Tundra does not support trees, as the soil is generally too thin, and temperatures too cold. It can be barren, or feature grasses, low scrub, lichens or moss. When, as usual, underlain by permafrost, when there is a summer melting season the result is a cold bog called muskeg, where the melted ice does not drain off, because of the impermeable frost underneath. Muskeg is common in western Canada and portions of Siberia during the brief growing season.

Northern hemisphere tundra.

Permafrost can be of varying thickness, from less than a meter to several kilometers. In areas with a freeze/thaw cycle, the zone which thaws is called the active layer. At its base is the permafrost table, the depth at which the ground never comes unfrozen. Below this is a zone in which ground temperatures fluctuate but never rise above freezing; below this is the isothermal layer which is of a consistent temperature. Where the soil is not frozen to bedrock, there might be unfrozen soil beneath, due to the geothermal heat gradient.

Permafrost, global extent.

Permafrost covers approximately 22% of the northern hemisphere, under both land and sea. Not all is continuous, meaning, some zones cyclically thaw, on a multiyear scale. Subsea permafrost in particular is known to hold large amounts of methane hydrate, a solid form of methane (CH₄) gas. Methane is produced by bacterial processes, and 1.4 billion tons--roughly twice the estimated content of the atmosphere--is estimated to be held within the permafrost. Permafrost loss has accelerated in recent decades, increasing the rate of its release.

Schematic of permafrost.

The melting of permafrost removes its structural stability, leading to subsidence of land and slumps. This has affected many villages in alpine regions and the far north. The destabilizing effects of thaw led to the creation of many specialized construction methods, such as surrounding foundations with insulative jackets, in the Trans-Alaskan Pipeline.

Tundra in the Kerguelen Islands.

Tomorrow: seasonally snow-covered regions.

Be well!