Incorporating the Permafrost Carbon Feedback into a Coupled Earth System Climate Model of Intermediate Complexity

Northern high latitudes sequester an estimated 1700 Gt of carbon held in perennially frozen ground. Much of this permafrost soil will warm and thaw as a consequence of anthropogenic climate warming, liberating the previously sequestered carbon and allowing this material to undergo decay. Here the permafrost carbon feedback is incorporated into the UVic Earth System Climate Model (ESCM),  a model of intermediate complexity that presently includes a fully coupled terrestrial and oceanic carbon-cycle in addition to soil freeze-thaw physics.  Sequestered permafrost carbon is prescribed into the top 3m of perennially frozen soils. Carbon in the active layer of permafrost soils is created and administered by the existing soil carbon and vegetation model components. When a soil layer thaws for the first time the permafrost carbon within the layer is moved from the sequestered carbon pool to the active soil carbon pool and the formerly sequester carbon is allowed to decay via heterotrophic respiration. The modified UVic ESCM is spun-up to preindustrial conditions and model runs are performed with and without permafrost carbon. The model is forced with Representative Concentration Pathways (RCPs) 2.6, 4.5, 6.0, and 8.5. Fossil fuel emission pathways to achieve each RCP are back-diagnosed from the simulated coupled carbon-cycle.  Under each RCP allowable emissions with and without permafrost carbon diverge in the first quarter of the 21st century. Despite the wide range in atmospheric CO₂ concentration and corresponding surface temperature changes between the RCPs, total carbon released from the permafrost during the 21st century ranges only from 160 Gt C  (RCP 2.6) to 250 Gt C  (RCP 8.5). Forced with RCP 2.6 release of permafrost carbon is halted by the mid 22nd century after 43% of the original stock of permafrost carbon is released into the active soil carbon pool.  Forced with RCPs 4.5 and 6.0 release of permafrost carbon continues until the end of the simulated period in 2300 CE, decades after CO₂ concentrations in the atmosphere have stabilized. Forced with RCP 8.5 nearly the entire original pool of carbon sequestered in the permafrost is respired by the mid 23rd century.  These results indicate that failing to include permafrost carbon in coupled carbon-cycle simulations will result in significant over estimation of the allowable carbon emissions to achieve each RCP.