In the bleak midwinter Frosty wind made moan, Earth stood hard as iron, Water like a stone —Christina Rossetti, "In the Bleak Midwinter"

Permafrost is hidden beneath the surface and less in the public consciousness than sea ice, glaciers, and ice sheets, but it directly affects human and biological systems that live with it. Permafrost can be hundreds of meters thick and tens to hundreds of millennia in age, with a geologic intransience but recent changes that remind us that it is not so permanent a feature of the landscape as it seems. Timescales of thermal diffusion in permafrost mean that surface temperature changes take a long time to propagate to depth, similar to glaciers. Seasonally frozen ground and the upper layers of permafrost do respond to seasonal and short-term climate and land-use fluctuations, however, so permafrost processes have major effects on the infrastructure, hydrology, ecology, and carbon cycle of northern high latitudes. This chapter gives a brief overview of permafrost.

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