E7 Permafrost

Topic

Permafrost is soil or rock that remains at or below zero degrees Celsius for at least two consecutive years, covering approximately twenty-four percent of the Northern Hemisphere's land surface. This permafrost stores nearly 1,500 gigatonnes of organic carbon—almost double the amount currently circulating in the entire atmosphere. The thermal structure of permafrost is determined by the balance between two opposing heat flows: the heat flux arriving from the surface, governed by the climate, and the geothermal heat flux rising slowly from the base of the permafrost itself.

The uppermost layer of permafrost—known as the active layer—thaws each summer and refreezes each winter; the thickness of this layer varies widely by region, ranging from a mere twenty centimeters in the coldest Arctic tundra to between two and three meters in subarctic zones, where summer heat warms the ground more intensely.

The way heat diffuses through permafrost changes drastically whenever the soil temperature crosses the point at which the water within it freezes or thaws—a threshold determined by the soil's texture, salinity, and the pressure to which it is subjected. At this critical point, the permafrost releases or absorbs a vast amount of latent heat; this massive exchange acts as a natural thermostat, slowing both the warming and cooling of the permafrost while the water within it undergoes a phase change. Permafrost thaw driven by global warming releases carbon dioxide and methane from the organic matter being thawed; this release of gases further warms the climate, which in turn intensifies permafrost thaw and releases even more carbon, creating a positive feedback loop between the released carbon and the warming climate. Methane is a particular concern within this feedback loop because each released molecule traps significantly more heat than a molecule of carbon dioxide.

Permafrost thaw also creates thermokarst features—ground depressions that occur when the subsurface ice supporting the soil melts. These features irreversibly alter the terrain's topography, surface water drainage, and the ecosystems that relied on the original permafrost structure, causing ground subsidence that can reach several meters in areas where the permafrost contained massive ice deposits.

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