Ninth International Conference on Permafrost ... - IARC Research

Interactions Between Permafrost and the Carbon CycleKevin SchaeferNational Snow and Ice Data Center, University of ColoradoTingjun ZhangNational Snow and Ice Data Center, University of ColoradoLixin LuCooperative Institute for Research in Environmental Sciences, University of ColoradoIntroductionIan BakerDepartment of Atmospheric Science, Colorado State UniversityThe net terrestrial carbon flux or Net Ecosystem Exchange(NEE) is the small difference between two large gross fluxes:R – GPP, where R is ecosystem respiration and GPP is GrossPrimary Productivity or photosynthesis. GPP for plant growthremoves CO 2from the atmosphere; R, due to microbialdecay of dead plant material, returns CO 2to the atmosphere.A positive NEE indicates a net flux into the atmosphere, anda negative NEE indicates a net biological uptake of CO 2.Microbial decay is slow in permafrost regions due to lowtemperatures, resulting in a large buildup of organic materialboth in the active layer and the underlying permafrost. About950 Gt of carbon (equivalent to current atmospheric CO 2)was frozen into permafrost during the last ice age, protectedfrom decay and effectively removed from the active carboncycle (Zimov et al. 2006).Climate warming across the high northern latitudes hasresulted in widespread permafrost degradation (Zhang et al.2005). Future projections indicate a loss of ~90% of nearsurface permafrost by 2100 (Lawrence & Slater 2005).As the permafrost thaws, the frozen organic matter willdecay, rapidly increasing atmospheric CO 2in addition toanthropogenic emissions. To predict the fate of this frozencarbon, we must understand how snow cover, soil thermalregime, and soil freeze-thaw processes influence the carboncycle in regions of permafrost.To assess these interactions, we simulated permafrost andcarbon cycle dynamics across the Northern Hemisphere usingthe Simple Biosphere Carnegie-Ames-Stanford Approach(SiBCASA) driven by the NCEP reanalysis at 2x2 degrees.Correlations between output model variables identifiedkey relationships between simulated soil temperatures,active layers, photosynthetic uptake, respiration fluxes, andbiomass.SiBCASA computes surface energy and carbon fluxesat 15-minute time steps using the Community Land Modelsnow and soil models and the CASA biogeochemical model(Schaefer et al. 2008). To improve soil thermodynamics,we (1) added the Sturm et al. (1995) snow classifications toaccount for depth hoar effects on snow thermal properties;(2) added the effects of organic matter to soil properties toaccount for the insolating effect of peat; and (3) extended thesoil model depth from 3.4 m to 15 m. To reach steady state,we ran three simulations from 1982 to 2007 (75 years total)using the final soil temperatures and soil moistures fromFigure 1. Simulated permafrost in Northern Hemisphere (black).Figure 2. Seasonal cycle in simulated NEE at 70N and 120E.one simulation as initial values to the next. We algebraicallycalculated steady state carbon pool sizes using time averagedecay rate constants.ResultsSiBCASA produced a fairly realistic permafrostdistribution for the Northern Hemisphere (Fig. 1). Thewave-like pattern in Siberia results from a similar patternin NCEP precipitation, which is an artifact of the spectralrepresentation of wind fields (Schaefer et al. 2004). Bandsof heavy precipitation strongly insulate the soil in winter,resulting in warmer soil temperatures and preventingpermafrost formation.Figure 2 shows a typical seasonal cycle in NEE for arepresentative point in Siberia (70N, 120E). The NEEshown in Figure 2 is typical in both magnitude and seasonalvariation for permafrost regions across both North Americaand Eurasia. Most permafrost areas are near steady state andthe carbon fluxes balanced, meaning that NEE ~ 0 whenaveraged over several years. All GPP and nearly all R occurin the short arctic summer, peaking at about 6 µmol C m -2s -1 in July. GPP is stronger than R in summer due to nearly271