In a warming Arctic, frozen soil organic carbon (SOC) stored in permafrost will increasingly become vulnerable to thaw and mobilization. Over millennia, permafrost soils accumulated about 1672 Petagram of SOC, about twice the carbon currently in the atmosphere. Rapid permafrost thaw (thermokarst) releases fossil SOC as greenhouse gases, constituting a positive feedback to global warming. However, complex landscape, hydrological, and ecological feedbacks necessitate quantification of landscape scale carbon pools and fluxes in Arctic permafrost regions. A globally important question is whether permafrost soils will turn from a natural carbon sink into a source.The project combines remote sensing based change detection, mapping, and spatial data analysis for permafrost landscapes, quantitative field studies, and modelling of thermokarst processes to quantify the size and vulnerability of deep permafrost SOC pools to rapid permafrost thaw and resulting impacts. The three research topics are: (1) Systematic measurement of rapid permafrost thaw, (2) Determining deep permafrost SOC stocks and carbon accumulation rates, and (3) Quantification of deep permafrost SOC pools and vulnerability assessment.The project will provide for the first time quantitative data on rapid permafrost thaw over large regions, provide first-time data on the size of SOC pool components related to thermokarst, substantially enhance previous SOC pool estimates for Yedoma deposits and arctic river deltas, and characterize overall permafrost SOC distribution and vulnerability to thaw. It will answer the question of how climate change affects permafrost SOC pools and how permafrost thaw feeds back to climate.