Why Thawing Permafrost Releases Carbon
Permafrost is ground that remains frozen for at least two consecutive years. Thawing reactivates microbes that convert some organic matter into carbon dioxide and methane.
Soil, rock or sediment whose temperature remains at or below 0°C for at least two consecutive years. It contains ice, but also gravel, roots and large quantities of ancient organic matter.
Permafrost is therefore not a layer of pure ice. At the surface, an active layer thaws each summer and freezes again in winter. It is found mainly in the Arctic, but also in mountain regions.
The IPCC estimates that it underlies about 15% of the Northern Hemisphere's exposed land area. Arctic frozen soils store nearly 1.7 trillion tonnes of carbon.
How cold locks carbon away
In unfrozen soil, bacteria and fungi decompose plant and animal remains. This microbial respiration releases carbon dioxide. When the ground stays frozen, activity slows sharply and organic matter accumulates for centuries without decomposing.
Thawing does not therefore “melt carbon”. It makes material previously protected by cold available to microbes again.
- Warming pushes the thaw front deeper each summer.
- Microbes regain access to ancient organic matter.
- In the presence of oxygen, decomposition mainly produces carbon dioxide.
- In waterlogged, oxygen-poor soils, it produces more methane.
Methane remains in the atmosphere for less time than carbon dioxide, but causes much stronger warming per unit over a period of several decades.
Gradual thaw, sometimes abrupt collapse
The common pattern is a slow thickening of the active layer. But ice-rich ground can collapse rapidly when the ice disappears. This process, known as thermokarst, creates depressions, landslides and lakes and can expose deep, carbon-rich layers within days.
Roads, buildings and pipelines then lose stability, producing climatic, ecological and social consequences.
No, according to available assessments. The IPCC concludes with high confidence that this is a positive feedback: warming causes additional emissions, which in turn amplify warming. But these emissions reduce the remaining carbon budget without outweighing the influence of fossil fuels.
Permafrost could release between 3 and 41 petagrams of carbon for each degree of global warming by 2100, equivalent to 3 to 41 billion tonnes. This wide range reflects real uncertainties: the rate of thaw, the share of abrupt collapse, soil moisture, fire frequency and vegetation growth.
Permafrost is not a switch that flips everywhere at once. Limiting warming reduces the area affected, slows the depth of thaw and prevents some future emissions.