The permafrost that underlies roughly a quarter of the Northern Hemisphere's land surface stores an estimated 1,500 billion metric tons of organic carbon—roughly twice the amount currently present in the atmosphere. This carbon has been accumulating for millennia as plant and animal matter froze before it could fully decompose. As global temperatures rise, however, the active layer of soil above the permafrost deepens seasonally, exposing previously frozen organic matter to microbial decomposition. Microbes metabolize this material and release carbon dioxide and methane—potent greenhouse gases—creating a feedback loop: warming thaws permafrost, which releases gases, which warm the climate further.

The rate at which this feedback operates depends on several interacting factors. Soil moisture is critical: in waterlogged anaerobic conditions, decomposition produces relatively more methane, which has a warming potential roughly 80 times that of carbon dioxide over a 20-year horizon. In drier, aerobic conditions, decomposition favors carbon dioxide release. Because permafrost landscapes are hydrologically heterogeneous—containing both peatlands and well-drained upland soils—projecting the net greenhouse-gas output requires integrating these contrasting processes across spatially variable terrain.

A complicating factor is thermokarst formation. When ice-rich permafrost thaws abruptly, the ground can collapse, forming depressions that fill with water. These thermokarst lakes dramatically accelerate methane production from sediments. Recent remote-sensing surveys have revealed that thermokarst lake formation is expanding faster than models anticipated, suggesting that abrupt thaw mechanisms may be under-represented in current climate projections. Researchers are now working to incorporate abrupt thaw dynamics into Earth system models, though significant uncertainty remains about the spatial extent and timing of such events at the continental scale.