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Carbon 13 exchanges between the atmosphere and biosphere



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GLOBAL BIOGEOCHEMICAL CYCLES VOL 11 NO 4 PAGES 507 533 DECEMBER 1997 Carbon 13 exchangesbetween the atmosphere and biosphere I Fung C B Field 3 J A Berry 3 M V Thompson 3 4J T Randerson 3 4 C M Malmstr6m 3 4P M Vitousek 4 G JamesCollatz P J Sellers 6 D A Randall z A S Denning z F Badeck 8 andJ John 9 Abstract Wepresent a detailed investigation of thegross 2Cand13Cexchanges between theatmosphere andbiosphere andtheirinfluence onthe6 3Cvariations in theatmosphere Thephotosynthetic discrimination A against 3C is derived froma biophysical model coupledto a generalcirculationmodel Sellerset al 1996a where stomatalconductance and carbonassimilationare determinedsimultaneouslywith the ambient climate The 6 3Cof therespired carbonis calculated by a biogeochemical model Potteret al 1993 Randersonet al 1996 as the sumof the contributions from compartments with varying ages Theglobalflux weighted meanphotosynthetic discrimination is 12 16 0 whichis lower than previousestimates Factorsthat lower the discriminationare reducedstomatal conductance andC4photosynthesis Thedecreasing atmospheric 6 3Ccauses anisotopic disequilibrium betweentheoutgoingandincomingfluxes thedisequilibrium is 0 33 o for 1988 The disequilibriumis higherthan previousestimatesbecauseit accountsfor the lifetime of treesand for the agesrather than turnovertimes of the biosphericpools The atmospheric 6 3Csignature resulting fromthebiospheric fluxesis investigated usinga three dimensional atmospherictracermodel The isotopicdisequilibriumaloneproduces a hemispheric difference of 0 02 o in atmospheric 13C comparable tothesignalfrom a hypothetical carbonsinkof 0 5 Gt C yr intothemidlatitude northern hemisphere biosphere However the rectifier effect due to the seasonalcovariationof CO2 fluxes andheightof theatmospheric boundary layer yieldsa background 6 3Cgradient of the oppositesign Theseeffectsnearly cancelthusfavoringa strongernet biosphericuptake than withoutthe backgroundCO2 gradient Our analysisof the globally averagedcarbon budgetfor the decadeof the 1980sindicatesthat the biosphericuptakeof fossilfuel CO2 is likely to be greaterthanthe oceanicuptake the relativeproportionsof the sinkscannotbe uniquely determined using 2Cand 3Calone Theland ocean sinkpartitioning requires in addition informationaboutthe land usesource isotopicdisequilibriumassociatedwith grossoceanicexchanges as well as the fractionsof C3 and C4 vegetationinvolvedin the biosphericuptake 1 NASA GoddardInstitutefor SpaceStudies NewYork 2Schoolof EarthandOceanSciences Universityof Victoria Victoria British Columbia Canada Introduction Sincethe beginningof the industrialera the combustion a totalof 250 Gt 1 Gt 1012 3Department of PlantBiology Carnegie Institution of Washing of fossilfuelshasreleased ton Stanford California kg of carboninto the atmosphere Over the sameperiod 4Department of BiologicalSciences Stanford University Stan land use modification in the middle latitudes and more reford California cently in the tropics hasreleasedanother 100 Gt C The sNASAGoddardSpaceFlightCenter Greenbelt Maryland 150 Gt C increasein CO2 in the atmosphere from 280 6NASAJohnson SpaceFlightCenter Houston Texas parts per million by volume ppmv in 1800 to 351 ppmv 7Department of Atmospheric Sciences ColoradoStateUniverin 1988 is equivalent to 60 of the fossil fuel releaseor sity Fort Collins 40 of the total anthropogenic CO2 source Balancing the 8Laboratoire d EcologieVegetale Universitede ParrisSudXI Orsay France carbonbudgetrequiresboth the oceansand terrestrialbio9Department of AppliedPhysics ColumbiaUniversity New sphereto haveactedasrepositories or sinks for the anthroYork pogenicCO2 Direct measurementsof CO2 reservoirsizes and of CO2 fluxes into and out of the oceans and terrestrial Copyright1997 by the AmericanGeophysicalUnion biosphereare sparse andthe methodologies for extrapolat Papernumber97GB01751 0886 6236 97 97GB 01751 ing laboratoryor site measurementsto the globe are under considerable debate Henceestimates of the globalstrengths 12 00 507 508 FUNG ET AL CARBON 13 EXCHANGES BETWEEN THE ATMOSPHERE AND BIOSPHERE of the terrestrial and oceanic sinks have not been established hasa 5 3C of 24 to 28 0 Andres et al 1996 typi with certainty CO2 is nearly but not completelymixed in the atmo cal of the plant materialfrom which fossilfuels are derived The anthropogenic emissions if all airborne would produce sphere The mixingtime of CO2 andotherinerttracegases in the atmosphereis about3 monthswithin a hemisphere and about 1 year betweenthe hemispheres Thereforeinformationaboutbroadpatternsof CO2 exchangesbetween the atmosphereand differentcarbonreservoirscan be extractedfrom the geographicandtemporalvariationsof carbon dioxidein the atmosphere with the aid of atmospheric transportmodels e g Keelinget al 1989a Tanset al 1990 Entinget al 1993 1995 Ciais et al 1995a b Over anatmospheric 5 3C gradient thatmirrorstheCO2gradient with isotopicratios0 3 o lowerin thenorthernthanin the southernhemisphere Figure lb This compareswith the observed gradientof 0 2 o Keelinget al 1989b Ciais et al 1995a Trolier et al 1996 Retrieving information about carbon sourcesand sinks from the atmosphericCO gradientsrequiresancillary data Tanset al 1990 didnothaveaccess to 5 3C informationat the time and useda compilationof availablemarinepCO2 90 of the fossil fuel emission is from the northern hemimeasurementsin their atmospherictracer transportmodel sphere and most of the emissionfrom land use modifica calculation They inferred that midlatitude northernhemition is from the tropics If all the anthropogenicCO2 re sphere land surfacemay have acted as a significantsink mained airborne this would yield a hemisphericgradient for anthropogenicCO2 Keeling et al 1989a usedtheir measurements in theiratmospheric tracermodel that is largerthanthatobserved Figure 1 Fossilfuel CO2 own 5 3C and deducedfor the early 1980s an oceanic sink that has approximatelythe same magnitudeas the terrestrialsink Ciaiset al 1995a b combined theatmospheric 5 3C data for 1992 1993withtheoceanic pCO2and 5 3C measure o mentsin their two dimensionalatmospherictransportmodel and deducedthat the northernmiddle to high latitudeter restrialbiosphere wasa sinkaslargeas 3 5 Gt C yr for those 2 years Sinks of this magnitudemay however be unusual becausethese2 yearshad significantclimatic per turbation Whentheatmospheric 5 3C variations areusedto 0 0 0 5 0 0 0 5 1 0 sin latitude i 0 10 i 1 constraintheestimatesof land


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