Permafrost is ground that remains at or below its freezing threshold for at least two consecutive years. Thaw can expose previously frozen organic material to decomposition, with methanogenesis where conditions are anaerobic.
Thawing permafrost makes previously frozen organic matter available for decomposition, releasing carbon dioxide and, in waterlogged oxygen-poor conditions, methane. These greenhouse gases amplify warming, which can cause further thaw. The amount reaching the atmosphere depends on decomposition, transport and oxidation; this feedback does not imply an inevitable abrupt release of an entire carbon reservoir.
Thermokarst is subsidence and related landscape change caused by thaw of ice-rich permafrost. Resulting lakes and saturated sediments can promote methanogenesis.
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