Net release of CO2 from thawing permafrost soil carbon predicted to occur earlier in this century

Accelerating permafrost thaw may mobilize vast stores of deep and frozen soil carbon (>3 m), releasing CO₂ into the atmosphere. Yet, the magnitude of this release remains uncertain due to the absent deep carbon processes in current Earth system models (ESMs). Here, we use an updated ORCHIDEE-MICT model that explicitly simulates Yedoma formation during the Pleistocene and the transient development of northern peatlands during the Holocene to project northern (>30°N) carbon responses under climate change. Incorporating these deep, frozen carbon pools improves agreement with carbon cycle observations and reduces previously projected net CO₂ uptake by 47–74 Pg C between 1900 and 2100 across three future scenarios. Under high-emission pathways, the northern soil carbon balance shifts from a net sink to a net source of up to 32 Pg C, advancing the reversal predicted by the original model earlier in the 21st century. This earlier reversal is primarily driven by accelerated deepening of the active layer after mid-century, exposing more previously frozen carbon, particularly from Yedoma. Consistent with field data, our model shows that colder soils retain more labile carbon—contrary to assumptions in many IPCC models, which helps explain their prediction of a continuous carbon sink. Our results highlight the need to represent both the quantity and quality of permafrost carbon in ESMs to improve projections of permafrost–climate feedbacks.