Estimating the near-surface permafrost-carbon feedback on global warming

[摘要] Thawing of permafrost and the associated release of carbon constitutes apositive feedback in the climate system, elevating the effect ofanthropogenic GHG emissions on global-mean temperatures. Multiple factorshave hindered the quantification of this feedback, which was not included inclimate carbon-cycle models which participated in recent modelintercomparisons (such as the Coupled Carbon Cycle Climate ModelIntercomparison Project – C⁴MIP) . There are considerable uncertaintiesin the rate and extent of permafrost thaw, the hydrological and vegetationresponse to permafrost thaw, the decomposition timescales of freshly thawedorganic material, the proportion of soil carbon that might be emitted ascarbon dioxide via aerobic decomposition or as methane via anaerobicdecomposition, and in the magnitude of the high latitude amplification ofglobal warming that will drive permafrost degradation. Additionally, thereare extensive and poorly characterized regional heterogeneities in soilproperties, carbon content, and hydrology. Here, we couple a new permafrostmodule to a reduced complexity carbon-cycle climate model, which allows usto perform a large ensemble of simulations. The ensemble is designed to spanthe uncertainties listed above and thereby the results provide an estimateof the potential strength of the feedback from newly thawed permafrostcarbon. For the high CO₂ concentration scenario (RCP8.5), 33–114 GtC(giga tons of Carbon) are released by 2100 (68 % uncertainty range). Thisleads to an additional warming of 0.04–0.23 °C. Though projected21st century permafrost carbon emissions are relatively modest, ongoingpermafrost thaw and slow but steady soil carbon decomposition means that, by2300, about half of the potentially vulnerable permafrost carbon stock inthe upper 3 m of soil layer (600–1000 GtC) could be released as CO₂,with an extra 1–4 % being released as methane. Our results also suggestthat mitigation action in line with the lower scenario RCP3-PD could containArctic temperature increase sufficiently that thawing of the permafrost areais limited to 9–23 % and the permafrost-carbon induced temperatureincrease does not exceed 0.04–0.16 °C by 2300.

[学科分类] 地球化学与岩石