Ninth International Conference on Permafrost ... - IARC Research

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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
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tcontinuous daylight, resulting in net CO 2uptake (negativeNEE). In spring and fall, cooler temperatures and shorterdays limit GPP, but not R, resulting in a net CO 2release intothe atmosphere (positive NEE). GPP shuts down in winter,but R can continue even in partially frozen soils; so we seepositive, but weak NEE in winter.Winter snow depths modulate the annual NEE cycle byinfluencing soil temperature and active layer depth. Deepersnows in winter insulate the soil, resulting in warmer soilsin spring, which in turn results in deeper active layers thefollowing summer. The timing of snowfall in fall is asimportant as snow depth: early snows in fall also result inwarmer soils in winter and spring and a deeper active layerthe following summer.Winter snow depths affect GPP and R at different timesof the year, producing a lopsided, time-delayed effect onthe NEE seasonal cycle. Warmer soils and deeper activelayers due to deeper winter snow increases R all year, butincrease GPP only in late summer, when the active layer isdeepest. Increased R due to warmer soil temperatures leadto increased NEE in spring and fall. In summer, increases inGPP overpower increases in R, resulting in increased CO 2uptake (decrease in NEE).Our results indicate a strong, time lagged response by thebiosphere to changes in winter snow depth. Variations insnow depth in fall and early winter are effectively saved asvariations in soil temperature, which in turn influences activelayer depth the following summer. Through this memory ofthe soil temperature and active layer depth, winter snowdepths effectively modulate biological fluxes throughout thefollowing summer and fall.Zhang, T.J., Frauenfeld, O.W., Serreze, M.C., Etringer, A.,Oelke, C., McCreight, J, Barry, R.G., Gilichinsky,D., Yang, D.Q., Ye, H.C., Ling, F. & Chudinova, S.2005. Spatial and temporal variability in active layerthickness over the Russian Arctic drainage basin. J.Geophys. Res.: Atmos. 110(D16): Art. No. D16101.Zimov, S.A., Schuur, E.A.G. & Chapin, F.S. 2006. Permafrostand the global carbon budget. Science 312(5780):1612-1613.AcknowledgmentsThis study was supported by the U.S. National Aeronauticsand Space Administration (NASA) grant NNX06AE65G tothe University of Colorado at Boulder.ReferencesLawrence, D.M. & Slater, A.G. 2005. A projection ofsevere near-surface permafrost degradation duringthe 21st century, Geophys. Res. Lett. 32(24):doi:10.1029/2005GL025080.Schaefer, K., Denning, A.S. & Leonard, O. 2004. Thewinter Arctic Oscillation and the timing of snowmeltin Europe. Geophys. Res. Lett. 31(22): Art. No.L22205.Schaefer, K., Collatz, G.J., Tans, P., Denning, A.S., Baker, I.,Berry, J., Prihodko, L., Suits, N. & Philpott, A 2008.The combined Simple Biosphere/Carnegie-Ames-Stanford Approach (SiBCASA) terrestrial carboncycle model, J. Geophys. Res. (in press).Sturm, M., Holgrem, J. & Liston, G.E. 1995. A seasonalsnow cover classification system for local to globalapplications. J. Clim. 8(5): 1261-1283.272
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Account of Long-Term Air Temper
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Rock Glaciers in the Kåfjord Area,
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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Pleistocene Sand-Wedge, Composite-W
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