Large carbon cycle climate sensitivities across a permafrost thaw gradient in subarctic Sweden

Permafrost stores large amounts of carbon potentially vulnerable to decomposition and its return to the atmosphere. However, permafrost responses to a changing climate remain uncertain in models due to complex interactions among hydrological, biogeochemical, microbial, and plant processes. In this study, we estimated effects of climate forcing biases present in global reanalysis products on carbon cycle predictions at a thawing permafrost peatland in subarctic Sweden. The analysis was conducted with a comprehensive biogeochemical model (ecosys) across a permafrost thaw gradient encompassing intact permafrost palsa, partly thawed bog, and fully thawed fen. Reanalysis data from the Global Soil Wetness Project Phase 3 (GSWP3) was bias corrected by site observations taken from 1913 to 2010. The simulations driven by the bias-corrected climate suggest that the three peatland types are currently annual carbon dioxide (CO₂) sinks from the atmosphere, although the bog and fen sites can have annual positive radiative forcing impacts due to their higher methane (CH₄) emissions. Our results indicate that projected precipitation increases could accelerate CH₄ emissions from the palsa area, even without further degradation of palsa permafrost. The differences in simulated CO₂ and CH₄ fluxes driven by uncertainty from climate forcing biases (i.e., with and without proper bias corrections) are as large as those from landscape heterogeneity across the examined permafrost thaw gradient. Future studies should thus not only focus on the transition of net carbon balance proportional to the morphological changes in thawing permafrost, but also incorporate the dynamic effects of climate sensitivity on carbon cycling.