Permafrost contains approximately 1,500 petagrams of carbon — roughly twice the amount currently in Earth's atmosphere. The standard concern: as temperatures rise, permafrost thaws, microbes decompose the exposed organic matter, and the released CO₂ and methane accelerate warming. The standard hope: if humanity reaches net-zero emissions, permafrost loss will stabilize.
Earth system model simulations show this hope is wrong on timing. Permafrost ecosystems flip from net carbon sink to net carbon source just before global emissions reach net-zero. The carbon loss then persists through the entire negative-emissions phase, reaching a cumulative net loss of approximately 14 petagrams of carbon — 14 billion metric tons — even under aggressive carbon removal scenarios.
The mechanism: landscape inundation. As permafrost thaws, the ground collapses, creating waterlogged depressions — thermokarst lakes and bogs. These saturated, anaerobic environments produce methane instead of CO₂. Critically, the landscape transformation is irreversible on policy-relevant timescales. You can cool the atmosphere, but you cannot un-collapse the ground or drain the thermokarst lakes.
Methane emissions from permafrost actually increase under net-zero and negative emissions because the landscape changes continue after the temperature signal stabilizes. The damage compounds fastest precisely when humanity thinks it has solved the problem. The carbon budget is smaller than currently modeled, because the models assume permafrost stops releasing carbon when warming stops. It doesn't. It releases carbon because of what warming already did to the landscape.