Permafrost covers approximately 25% of the Northern Hemisphere's land surface (Siberia, Alaska, Canada, Greenland). It can extend hundreds of metres deep. The active layer (0.5–3 m) thaws in summer and refreezes in winter.
Carbon bomb
Permafrost contains roughly 1,500 gigatonnes of organic carbon — twice as much as the entire current atmosphere. As it thaws due to global warming, microorganisms decompose the organic matter, releasing CO₂ and methane, creating a positive feedback loop that accelerates warming. Permafrost thaw rates have doubled since the 1970s.