Permafrost Thaw and Methane Emissions Under Warming

Abstract

Northern circumpolar permafrost stores an estimated 1,460 Pg of organic carbon, nearly twice the quantity currently held in the atmosphere. As Arctic temperatures rise at two to three times the global average rate, progressive thaw threatens to mobilize this vast carbon reservoir through both gradual active-layer deepening and abrupt thermokarst processes. This review evaluates cumulative carbon release projections from five CMIP6 Earth system models under the high-emission SSP5-8.5 scenario, incorporating an abrupt thaw parameterization that adds approximately 40 percent to gradual-only estimates. Multi-model mean cumulative release reaches 112 Pg C by 2100 under gradual thaw alone, increasing to 156 Pg C when abrupt pathways are included. We further analyze the partitioning of released carbon between CO₂ and CH₄ across four thaw types, demonstrating that thermokarst lake environments produce the highest methane fractions (52 percent by mass, 82 percent by GWP-weighted radiative impact). These findings indicate that the permafrost carbon feedback represents a significant and potentially underestimated amplifier of anthropogenic warming, with policy implications for remaining carbon budgets compatible with the Paris Agreement temperature targets.