Estimating Potential Damping of Cryoturbation on Permafrost Carbon Emissions using A Perturbed Parameters Approach in A Land

Thursday, December 15, 2016 - 13:40
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Permafrost soils in the northern hemisphere contain about half of the world’s total soil organic carbon (SOC), which has the potential to be a large carbon source as a consequence of anticipated climate changes. However, divergent estimates of the magnitude and the trend of soil carbon emission between different global land surface models with simple representation of permafrost processes highlighted the demand to understand the influence of processes that are most likely to affect the permafrost carbon cycle feedbacks (PCF), including cryoturbation, oxygen limitation and microbial dynamics. Here we use a land surface model, the Integrated Science and Assessment Model with one-dimensional soil biogeochemistry (ISAM-1DSB), to examine how the response of cryoturbation to a changing thermal and hydrological regime will affect the PCF under the IPCC RCP8.5 climate scenario. ISAM-1DSB contains an extended permafrost representation, a 1-D frost heave model that resolves ice lens formation and growth to represent cryoturbation and a gas diffusion model to estimate oxygen availability in poorly-drained soil. To validate the model's ability to capture the vertical variability of SOC profiles, ISAM-1DSB has been forced with CRU-NCEP reanalysis to build up quasi-equilibrium contemporary SOC storage and compared to a set of permafrost soil profiles from three different permafrost soil suborders: Histel, Turbel and Orthel. For the first time, soil Δ14C profiles across the pan-arctic region has been utilized to constrain the key uncertain parameter linking ice lens velocity to the cryoturbation rate in the frost heave scheme by matching model estimated Δ14C profiles with observations. The estimated range of the cryoturbation rate has been utilized to represent the uncertainty of the cryoturbation under future climate change. Finally, ISAM-1DSB has been forced with future climate projections from CMIP5 model outputs for the entire permafrost region to perform three simulation cases with the lower, the median and the upper bound of the parameter. These experiments estimate the permafrost carbon emission till 2100 to test a hypothesis: the accumulated permafrost SOC emission will be smaller with consideration of cryoturbation, but this trend will be enhanced once the cryoturbation stalls.

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