Borehole temperature measurements indicate that permafrost and ground temperatures have warmed in the past century. There is also widespread evidence of increasing active layer thickness, melting at the base of permafrost in regions where it is less than ~100 m thick, northward migration of the permafrost zone, and the development of thermokarst terrain and thaw slumps. Because the timescale for surface temperature signals to penetrate to the base of the permafrost is long (ca. 1 year to reach 20 m and 100 years to reach 150 m depth), permafrost thickness and the geographic distribution of permafrost are primarily responding to century- and millennial-scale temperature shifts. Changes in active layer depth, in contrast, can be interpreted as a short-term response to changing temperature and snowpack patterns. Where near-surface permafrost contains massive ice, this surface degradation leads to geomorphologic effects such as thaw slumps and coastal erosion. These and other effects are touched on in the next section.
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