Permafrost carbon cycle

The movement of carbon into, through, and out of permafrost-affected soils. It is distinctive because thaw can expose ancient organic matter to decomposition, releasing carbon dioxide and methane.

Permafrost thaw: The warming and degradation of ground that remains at or below freezing for at least two consecutive years. Thaw exposes previously frozen organic matter to liquid water and microbial decomposition.
Climate feedback: A process in which a climate change triggers effects that amplify or dampen that change. Carbon emissions from thaw can amplify the warming that caused the thaw.
Permafrost carbon pool: The quantity of organic carbon stored in permafrost-affected soils and sediments. Estimating this reservoir sets the starting point for projections of thaw-driven emissions.
Permafrost carbon feedback: The increase in greenhouse-gas emissions caused when warming thaws permafrost and releases stored carbon. Its strength depends on how much thawed carbon escapes and how quickly.
Gradual permafrost thaw: The progressive deepening or warming of permafrost without sudden ground collapse. It contrasts with abrupt thaw, which can rapidly expose ice-rich soil and alter drainage.
Soil organic carbon: Carbon contained in organic compounds within soil, including dead plant material and microbial products. This is the main carbon pool accumulated and transformed in permafrost soils.
Carbon dioxide: A colorless gas composed of one carbon atom and two oxygen atoms, released by respiration and combustion. Aerobic decomposition of thawed organic matter releases this long-lived greenhouse gas.
Active layer: The surface layer of permafrost terrain that thaws during summer and refreezes in winter. Its thickness indicates how deeply seasonal thaw reaches stored soil carbon.
Permafrost carbon estimate: A scientific estimate of the amount of carbon stored in permafrost-affected ground. Estimates vary because carbon is unevenly distributed and deep deposits remain poorly sampled.
Aerobic respiration: Cellular energy production that uses oxygen and releases carbon dioxide from organic molecules. Oxygen-rich thawed soil tends to favor carbon dioxide production over methane formation.

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