Permafrost as a tipping element in the Earth System: scales matter

The permafrost soils in the northern high latitudes contain about twice as much carbon as the atmosphere. This organic soil matter has accumulated over many thousands of years and is now exposed to anthropogenic warming, which is amplified by a factor of three to four in the Arctic compared to global warming.  The thawed organic matter is mineralized and released into the atmosphere as CO₂ or CH₄, which amplifies ongoing warming (permafrost carbon feedback). In addition, the thawing of permafrost soils leads to changes in land surface hydrology and potential drainage, which could also amplify global warming due to the decrease in summer cloud cover (permafrost cloud feedback). The permafrost changes impact other regions and Earth tipping elements, including tropical forests.

Are permafrost feedbacks nonlinear, is there a threshold for global warming above which the feedbacks lead to disproportional increase in carbon thaw? Future projections using Earth system and land surface models suggest a rather linear permafrost response to global warming, but they are mostly based on gradual thawing processes and do not take into account abrupt thawing and extreme events. Numerous processes that lead to abrupt thawing at the local level, such as thermokarst, lake formation and drainage, or surface subsidence, have been neglected in large-scale models to date. We will present the results of model experiments and discuss the potential impact of these missing processes on the nonlinear response, as well as indicators of multistability of carbon and hydrology at different scales. We will also discuss irreversibility of permafrost changes and their response timescales as supported by paleo evidence.