E11 Frozen Soils and Surface Permafrost

Topic

Permafrost is soil or rock that remains at or below zero degrees Celsius for at least two consecutive years, regardless of whether it contains ice; it covers approximately twenty-five percent of the Northern Hemisphere's land surface.

The active layer is the uppermost zone of this permafrost, thawing each summer and refreezing each winter; its thickness varies widely, ranging from just a few centimeters in high Arctic mountains to several meters in the discontinuous permafrost of boreal zones.

The freezing and thawing of this soil profoundly alter its hydrological properties: frozen soil acts as an impermeable barrier because ice effectively blocks water passage, and this impermeability causes a dramatic increase in surface runoff during the spring thaw.

The formation of the frost front follows the Stefan equation, whereby the depth of the freezing front scales with the square root of accumulated freezing degree-days; the latent heat of fusion of the soil's water content—334,000 joules per kilogram—is the dominant term in the thermal balance of this frozen soil.

Climate warming is accelerating the thawing of this permafrost, leading to critical, simultaneous consequences: the decomposition of previously frozen organic matter releases carbon dioxide and methane (a particularly potent greenhouse gas); ground subsidence—of the type described elsewhere—destroys infrastructure built upon the permafrost; and the hydrological changes accompanying the thaw alter Arctic river and lake regimes.

Under current warming scenarios, the system is expected to exhibit a thawing front advancing at rates of between 0.5 and 2 meters per decade. Thus, from the thermal definition that opens this document to the advancing thaw front that concludes it—and encompassing the active layer, the impermeability of frozen ground, and the Stefan equation—permafrost is causing soil that remained frozen and stable for millennia to now begin releasing the carbon, water, and structural stability it had held in reserve all that time.

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