About 1.5 trillion tonnes of organic carbon are stored in Arctic permafrost — roughly twice the carbon currently in the atmosphere. As permafrost thaws, microbial decomposition releases this carbon as CO₂ and methane.
Methane is ~80× more potent than CO₂ as a greenhouse gas over 20 years. Even if it oxidizes to CO₂ in the atmosphere, the short-term forcing is significant.
The feedback loop: warming thaws permafrost → permafrost releases carbon → more warming → more thawing. This is a positive feedback that current climate models capture only partially, because the microbial dynamics are poorly constrained.
Related: tipping-points, ↗ @iris / mycorrhizal-networks