Is permafrost thaw reversible in policy-relevant overshoot scenarios?

The northern permafrost regions contain significant amounts of carbon and are warming at approximately 3-4 times the global rate. Understanding the response of these carbon stocks under policy-relevant overshoot scenarios is a priority for climate policy. The Illustrative Mitigation Pathways (IMPs) were policy relevant pathways in AR6 designed to limit warming to 2°C. ESM simulations are not available for these scenarios, so regional information is unavailable for these mitigation pathways.

Here, we use output from a simple climate model that has run a selection of IMPs to drive the UK land surface model JULES, with an improved and explicit representation of permafrost processes compared to the standard version used in CMIP6. Our simulations include probabilistic estimates of uncertainty in future projections derived from climate sensitivity and the spatial patterns of CMIP6 ESMs.  

With the CMIP6 version of JULES, permafrost extent is reversible when global warming is reduced, even under high warming levels. However, the updated version of JULES shows a delayed recovery of permafrost extent beyond 2300 (i.e. no recovery had begun) when warming levels are reduced to 2°C. In addition, a sink-to-source transition in the northern high latitudes is more likely with explicit permafrost, and despite the temperature falling again remains a source until 2300 in many of the simulations, i.e. largely an irreversible change.