CO2 loss by permafrost thawing implies additional emissions reductions to limit warming to 1.5 or 2 °C

Large amounts of carbon are stored in the permafrost of the northern high latitude land. As permafrost degrades under a warming climate, some of this carbon will decompose and be released to the atmosphere. This positive climate-carbon feedback will reduce the natural carbon sinks and thus lower anthropogenic CO2 emissions compatible with the goals of the Paris Agreement. Simulations using an ensemble of the JULES-IMOGEN intermediate complexity climate model (including climate response and process uncertainty) and a stabilization target of 2 degrees C, show that including the permafrost carbon pool in the model increases the land carbon emissions at stabilization by between 0.09 and 0.19 Gt C year-1 (10th to 90th percentile). These emissions are only slightly reduced to between 0.08 and 0.16 Gt Cyear(-1) (10th to 90th percentile) when considering 1.5 degrees C stabilization targets. This suggests that uncertainties caused by the differences in stabilization target are small compared with those associated with model parameterisation uncertainty. Inertia means that permafrost carbon loss may continue for many years after anthropogenic emissions have stabilized. Simulations suggest that between 225 and 345GtC (10th to 90th percentile) are in thawed permafrost and may eventually be released to the atmosphere for stabilization target of 2 degrees C. This value is 60-100 GtC less for a 1.5 degrees C target. The inclusion of permafrost carbon will add to the demands on negative emission technologies which are already present in most low emissions scenarios.