Vulnerability of permafrost carbon to global warming. Part II: sensitivity of permafrost carbon stock to global warming

In the companion paper (Part I), we presented a model of permafrost carbon cycle to study the sensitivity of frozen carbon stocks to future climate warming. The mobilization of deep carbon stock of the frozen Pleistocene soil in the case of rapid stepwise increase of atmospheric temperature was considered. In this work, we adapted the model to be used also for floodplain tundra sites and to account for the processes in the soil active layer. The new processes taken into account are litter input and decomposition, plant-mediated transport of methane, and leaching of exudates from plant roots. The SRES-A2 transient climate warming scenario of the IPSL CM4 climate model is used to study the carbon fluxes from the carbon-rich Pleistocene soil with seasonal active-layer carbon cycling on top of it. For a point to the southwest from the western branch of Yedoma Ice Complex, where the climate warming is strong enough to trigger self-sustainable decomposition processes, about 256 kgC m−2, or 70% of the initial soil carbon stock under present-day climate conditions, are emitted to the atmosphere in about 120 yr, including 20 kgC m−2 released as methane. The total average flux of CO2 and methane emissions to the atmosphere during this time is of 2.1 kgC m−2 yr−1. Within the Yedoma, whose most part of the territory remains relatively cold, the emissions are much smaller: 0.2 kgC m−2 yr−1 between 2050 and 2100 for Yakutsk area. In a test case with saturated upper-soil meter, when the runoff is insufficient to evacuate the meltwater, 0.05 kgCH4 m−2 yr−1 on average are emitted as methane during 250 yr starting from 2050. The latter can translate to the upper bound of 1 GtC yr−1 in CO2 equivalent from the 1 million km2 area of the Yedoma. 1. Introduction A large fraction of the frozen soil carbon stocks of northern latitudes are prone to disappear in a future warmer world, following permafrost thawing (e.g. Tarnocai, 1999; IPCC, 2001). Given the huge size of the frozen Arctic carbon pools, equalling roughly half that of the atmospheric carbon pool, their thawingwould imply massive losses of CO2 to the atmosphere, acting as a strong positive feedback on climate change in the next centuries. The uncertainties on the vulnerability of frozen soil carbon pools remain however very large. In particular, information is needed to estimate the rates of thawing and the threshold points above ∗Corresponding author. e-mail: Dimitry.Khvorostiyanov@lsce.ipsl.fr DOI: 10.1111/j.1600-0889.2007.00336.x which the frozen soil carbon stores could start to decompose