Estimating the Permafrost-Carbon Climate Response in the CMIP5 Climate Models Using a Simplified Approach

Under climate change, thawing permafrost may cause a release of carbon, which has a positive feedback on the climate. The permafrost-carbon climate response (gamma(PF)) is the additional permafrost-carbon made vulnerable to decomposition per degree of global temperature increase. A simple framework was adopted to estimate gamma(PF) using the database for phase 5 of the Coupled Model Intercomparison Project (CMIP5). The projected changes in the annual maximum active layer thicknesses (ALT(max)) over the twenty-first century were quantified using CMIP5 soil temperatures. These changes were combined with the observed distribution of soil organic carbon and its potential decomposability to give gamma(PF). This estimate of gamma(PF) is dependent on the biases in the simulated present-day permafrost. This dependency was reduced by combining a reference estimate of the present-day ALT(max) with an estimate of the sensitivity of ALT(max) to temperature from the CMIP5 models. In this case, gamma(PF) was from -6 to -66 PgC K-1 (5th-95th percentile) with a radiative forcing of 0.03-0.29 W m(-2) K-1. This range is mainly caused by uncertainties in the amount of soil carbon deeper in the soil profile and whether it thaws over the time scales under consideration. These results suggest that including permafrost-carbon within climate models will lead to an increase in the positive global carbon climate feedback. Under future climate change the northern high-latitude permafrost region is expected to be a small sink of carbon. Adding the permafrost-carbon response is likely to change this region to a source of carbon.