Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified data gaps

Soils and other unconsolidated deposits in the northern circumpolar permafrost region store large amounts of soil organic carbon (SOC). This SOC is potentially vulnerable to remobilization following soil warming and permafrost thaw, but SOC stock estimates were poorly constrained and quantitative error estimates were lacking. This study presents revised estimates of permafrost SOC stocks, including quantitative uncertainty estimates, in the 0–3 m depth range in soils as well as for sediments deeper than 3 m in deltaic deposits of major rivers and in the Yedoma region of Siberia and Alaska. Revised estimates are based on significantly larger databases compared to previous studies. Despite this there is evidence of significant remaining regional data gaps. Estimates remain particularly poorly constrained for soils in the High Arctic region and physiographic regions with thin sedimentary overburden (mountains, highlands and plateaus) as well as for deposits below 3 m depth in deltas and the Yedoma region. While some components of the revised SOC stocks are similar in magnitude to those previously reported for this region, there are substantial differences in other components, including the fraction of perennially frozen SOC. Upscaled based on regional soil maps, estimated permafrost region SOC stocks are 217± 12 and 472± 27 Pg for the 0–0.3 and 0–1 m soil depths, respectively (±95 % confidence intervals). Storage of SOC in 0–3 m of soils is estimated to 1035± 150 Pg. Of this, 34± 16 PgC is stored in poorly developed soils of the High Arctic. Based on generalized calculations, storage of SOC below 3 m of surface Published by Copernicus Publications on behalf of the European Geosciences Union. 6574 G. Hugelius et al.: Circumpolar stocks of permafrost carbon soils in deltaic alluvium of major Arctic rivers is estimated as 91± 52 Pg. In the Yedoma region, estimated SOC stocks below 3 m depth are 181±54 Pg, of which 74±20 Pg is stored in intact Yedoma (late Pleistocene iceand organic-rich silty sediments) with the remainder in refrozen thermokarst deposits. Total estimated SOC storage for the permafrost region is ∼ 1300 Pg with an uncertainty range of ∼ 1100 to 1500 Pg. Of this, ∼ 500 Pg is in non-permafrost soils, seasonally thawed in the active layer or in deeper taliks, while ∼ 800 Pg is perennially frozen. This represents a substantial ∼ 300 Pg lowering of the estimated perennially frozen SOC stock compared to previous estimates.