Coupled Hydrological and Thermal Modeling of Permafrost and Active Layer Dynamics: Implications to Permafrost Carbon Pool in Northern Eurasia


Recent observations indicate a warming of permafrost in many northern regions with the resulting degradation of ice-rich and carbon-rich permafrost. Permafrost temperature has increased by 1 to 3 deg C in the Northern Hemisphere during the last 30-40 years. To assess possible changes in the permafrost and the active layer dynamics we developed a robust coupling of a GIPL (Geophysical Institute Permafrost Lab) transient model and modified version of the pan-Arctic Water Balance Model (P/WBM) developed at the University of New Hampshire. Through explicit coupling of the Permafrost Model with the Water Balance Model we are able to simulate the hydrological budgets, temporal and spatial variability in soil water/ice content, active layer thickness, and associated large-scale hydrology that are driven by contemporary and future climate variability and change. Coupling of the GIPL model with a suitably-scaled hydrological model captures thresholds and highly non-linear feedback processes induced by changes in hydrology and the temperature regime over the panArctic. Input parameters to the model are spatial datasets of mean monthly air temperature, snow properties or SWE (Snow Water Equivalent), prescribed vegetation and thermal properties of the multilayer soil column, and water content. The climate scenario was derived from an ensemble of five IPCC Global Circulation Models (GCM) ECHAM5, GFDL21, CCSM, HADcm3 and CCCMA. The outputs from these five models have been scaled down to 25 km spatial resolution with monthly temporal resolution, based on the composite (mean) output of the five models, using the IPCC SRES A1B CO2 emission scenario through the end of current century. The model takes into account the geographic distribution of organic soils and peatlands, vegetation cover and soil properties, and is tested against a number of permafrost temperature records for the last century.

We estimated dynamics of the seasonally thawed volume of soils within the two upper meters for the entire North Eurasia using a coupled, large scale, grid-based water balance/permafrost model. The model results indicate 1,200 km3 of seasonally unfrozen soils within the two upper meters across 10,800,000 km2 of northern Eurasian permafrost domain during the last two decades of the 20th century. Our projections have shown that unfrozen volume of soil within two upper meters increases to 3,500 km3 by 2050 and to 9,500 km3 by the last decade of the 21st century due to active layer deepening. According to this specific climate scenario, the area of permafrost with active layer shallower than 2 m in depth could decrease from 10,800,000 km2 in 2000 to 9,000,000 km2 by 2050 and to 6,000,000 km2 by the end of current century. Despite the slower rate of soil warming in peatland areas and a slower degradation of permafrost under peat soils, a considerable volume of peat (approximately 20% of the total volume of peat in Northern Eurasia) could be thawed by the end of the current century. The potential release of carbon and the net effect of this thawing will depend on the balance between increased productivity and respiration, and will be mitigated by peat moisture.

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