Recent increases in global air temperature ( Stocker etВ al., 2013), which are amplified at high latitudes ( Pithan and Mauritsen, 2014 Biskaborn etВ al., 2019), have resulted in widespread permafrost thaw ( Romanovsky etВ al., 2010), and simulations from a variety of climate and land surface models across a wide range of scenarios suggest that this trend will continue into the future ( Koven etВ al., 2013 Chadburn etВ al., 2017).ДЄs permafrost thaws, its carbon becomes available to microbes for decomposition, resulting in the production of carbon dioxide (CO 2) and methane (CH 4) ( Treat etВ al., 2014 SchГ¤del etВ al., 2014, 2016 Bond-Lamberty etВ al., 2016 Nzotungicimpaye and Zickfeld, 2017) that could lead to further warming ( Koven etВ al., 2011 Schuur etВ al., 2015). Permafrost вЂ“ soil that continuously remains below 0 в€C for at least 2В consecutive years вЂ“ underlies an area of 22вЂ‰( В±вЂ‰3) Г—10 6вЂ‰km 2, roughly 17вЂ‰% of the Earth's exposed land surface ( Gruber, 2012), and is estimated to contain 1460вЂ“1600вЂ‰Pg of organic carbon ( Schuur etВ al., 2018). The addition of permafrost in Hector provides a basis for the exploration of a suite of science questions, as Hector can be cheaply run over a wide range of parameter values to explore uncertainty and can be easily coupled with integrated assessment and other human system models to explore the economic consequences of warming from this feedback. The fraction of thawed permafrost carbon available for decomposition was the most significant parameter controlling the end-of-century temperature change in the model, explaining around 70вЂ‰% of the temperature variance, and was distantly followed by the initial stock of permafrost carbon, which contributed to about 10вЂ‰% of the temperature variance. We found that by 2100 thawed permafrost carbon emissions increased HectorвЂ™s atmospheric CO 2 concentration by 5вЂ‰%вЂ“7вЂ‰% and the atmospheric CH 4 concentration by 7вЂ‰%вЂ“12вЂ‰%, depending on the future scenario, resulting in 0.2вЂ“0.25 в€C of additional warming over the 21st century. We include permafrost as a separate land carbon pool that becomes available for decomposition into both methane (CH 4) and carbon dioxide (CO 2) once thawed the thaw rate is controlled by region-specific air temperature increases from a preindustrial baseline. We have added a representation of permafrost to the simple, open-source global carbonвЂ“climate model Hector, calibrated to be consistent with both historical data and 21st century Earth system model projections of permafrost thaw. The resulting climate feedback can be estimated using land surface models, but the high complexity and computational cost of these models make it challenging to use them for estimating uncertainty, exploring novel scenarios, and coupling with other models.

A warming climate makes this carbon increasingly vulnerable to decomposition and release into the atmosphere in the form of greenhouse gases. Permafrost currently stores more than a fourth of global soil carbon.

Permafrost towermadness 2