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The Climate Induced Variation of the Continental Biosphere a Model

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The Climate Induced Variation of the Continental Biosphere a Model GEOPHYSICAL RF.,SEARCHLETTERS, VOL. 19, NO. 9, PAGES 89%900, MAY 4, 1992 THE CLIMATE INDUCED VARIATION OF THE CONTINENTAL BIOSPHERE ' A MODEL SIMULATION OF THE LAST GLACIAL MAXIMUM P. Friedlingstein•,a,C. Delire 2, J.F.Mtiller a andJ.C. G•rard2 Abstract.A simplifiedthree-dimensional global climate model was madeby Adams et al. Discrepanciesbetween this work and that of usedto simulate the surface temperature and precipitation distributions Prenticeand Fung are alsodiscussed. forthe Last Glacial Maximum (LGM), 18 000 years ago. These fields wereapplied to a bioclimaticscheme wict• parameterizes thedistri- Theatmospheric model and the bioclimatic scheme butionof eightvegetation types as a functionof biotemperatureand Themodel used for this work is the quasi-three dimensional global annualprecipitation. The model predicts a decrease, forLGM com- climatemodel developed bySellers (1983, 1985). Description ofthis paredtopresent, inlorested area balanced byan increase indesert and modeland discussion of its abilityto reproducethe present climate tundraextent, in agreementwith a reconstructionof the distribution canbe found in theoriginal papers. Most of themajor features of the ofvegetation based on palcodata. However, the estimated biospheric sealevel pressure and temperature fields are quite well simulated. As carboncontent (phytomass and soil carbon) at LGM is lessreduced it is thecase for most GCMs, the simulated precipitation field presents thanin thereconstructed one. Possiblereasons for thisdiscrepancy somediscrepancies withthe observed field. For present conditions, arediscussed. it yieldsa globalaverage sea level temperature of 289.6 K andtotal Introduction precipitationof 2.3 mm/day. Anempirical model of thebiosphere has been developed. It pre- Analysisof air bubblesin icecores has clearly established that dicksthe global distribution ofthe vegetation types and the main car- theatmospheric concentration of carbon dioxide during the Last bonpools and fluxes from two climatic variables: theannual precipi- GlacialMaximum, 18000 years ago, wasabout 75 ppmless than tationand the biotemperature (thebiotemperature is the mean annual thepreindustrial (interglacial) level. A largercontent of carbonin temperature(inøC) considering onlypositive monthly temperatures). theterrestrial biosphere during glacial periods could be a possible Startingfrom the World Ecosystem Database of Olson et al. (1985) causeof thislow atmosphericCO2 concentration. Transfer of carbon whichincludes 52 ecosystemsona 0.5øx0.5 ø resolution grid, we de- fromthe biosphere to the atmosphere could thus explain the rise in fineeight broad vegetation types (numbers inparenthesis refer to Ol- atmosphericCO,.,concentration between LGM and the preindustrial son's classification): perennial ice (17,69-71), desert and semi-desert times. (49-51),tundra (53,63), coniferous forest (20-23,57,60-62), deciduous Usinga generalcirculation model and a bioclimaticscheme, Pren- forest(24-26, 46,48,56), grassland and shrubland (4042,47,52,59,64), ticeand Fung (1990) estimated that the amount of carbonin theveg- seasonaltropical forest (27,28,32,43) and evergreen tropical forest etationand soils at LGM wasapproximately equivalent to thepresent (29,33).Present-day 5øx 5* annualprecipitation and biotemperature interglacial.They found that 200 Gt of carbonwere stored by veg- distributionswere derived from monthlymean climatological fields etationand soils on landexposed by thelow sealevel 18 kyr ago. ofprecipitation (Shea, 1986) and surface temperature (Trenberth et Thisadditional carbon pool at LGMhas almost e•actly counterbal- al., 1988).The relationship between the spatial distribution of each ancedthe lower amount ot' carbon stored by vegetationand soils on vegetationtype and the two climatic variables was then determined thepresent land area. They concluded that carbon transfer between empirically.Each vegetation type is allowed to exist within a defined thebiosphere and the atmosphere should not have been a dominant domainof precipitationandbiotemperature (Table 1). Thevegetation factorin the large atmospheric CO,• level change between glacial and distributionpredicted by thisbioclimatic scheme, for present-day pre- interglacialevents. In contrast,Adams et al. (1990),basing their cipitationand temperature fields, agree with Olson's database. The estimatesonpalynological, pedo!ogical and sedimentological data, limitedresolution of the modelgenerate some discrepancies when foundthat the amountof carbonstored in the biospheremay have comparedwith the data, particularly in mountainous regions. Net beensmaller by morethan 1000 Gt of carbonat LGM thanduring primaryproductivity (NPP), phytomass and soil organic carbon pool theHolocene, corresponding to a doublingin theterrestrial carbon are calculatedas follows.The Miami model(Lieth, 1975) is modi- contentfrom LGM to presentinterglacial. This result has important fiedby expressing NPP as the minimum value of twofunctions, the implicationson the abilityof the oceanto storethe largeamounts firstdepending onmean annual precipitation and latitude, the second ofcarbon that may have been transfered to theatmosphere-biosphere depending only on biotemperature. systemsince the LGM. The contrastbetween the two sets of results NPP = min wasalready present in thereconstruction theymade of thevegetation distributionat the LGM. Accordingto Adamset al. (1990),drier with vegetationtypes having a lowcarbon storage per unit area, such as f/3• = 69.1875xBT if BT < 8 øC desertand arid scrub,were muchmore extensive18 kyr agothan = 1350.x(1. + exp(•a•r,_t•• err)) if BT > 8 øC theyare currently. Moist climate types (with high carbon storage) werealmost absent from the large areas which they occupy today. In fe = 1125.x(1.- exp(-6.64x10-axP)) in thetropics contrast,following the simulation made by Prentice and Fung (1990), = 1350.x(1. - exp(-6.64x10-4xP)) in otherregions thedriest vegetation types would have covered a smaller area than BT = today. biotemperatureexpressed in øC Theaim of this study is to simulate the climate and the distribution P = annualprecipitation expressed in mm/y ofvegetation types at LGMand to compareit to thereconstruction NPP = netprimary production expressed in gC/m2/y. Theparameters were adjusted to fit theglobal NPP distribution givenby Funget aI. (1983).The global annual NPP predicted by • Universit•Libre de Bruxelles, 2 Institut d'Astrophysique- Universit• the model is 53 gtC/yr.The livingphytomass (Table 2, Column4) deLiege, :• Institut d'AGronomie Spatiale - Bruxel!es is calculatedas the product of NPPand the mean residence time for carbonin herbaceousand woodybiomass in the differentvegetation typesderived from Goudriaan et al. (1984). Valuesadopted are Copyright1992 by theAmerican Geophysical Union. listedin Table 3. Finally, soil carbonis inl'erredfrom Postet al's Papernumber 92GL00546 study(1982) which relates this variable to biotemperature andannual 0094-8534/92/92GL-005 4 6503,00 precipitation. 897 898 Fricdlingsteinetal.' Continental Biosphere atLGM Table1. Minimumbiotemperature (BTm)and maximum biotemperature (BTam)(in øC) for different annual precipitation levels(in mm/yr)allowing the existence of eachvegetation type (refer to Table2, Column1). Vegetationtype 1 2 3 4 5 6 7 8 Precipitations BTm BTM BTmBT.sl BTm BT.4• BTm BT:u BT,, BT•uBT,, BT.wBTm BT:xjBT,n BT,w 10 o. 5.0 4.0 - 50 o. 5.0 4.0 - 0.6 4.0 2.5 4.0 60 o. 0.6 4.0 - 0.6 4.0 2.5 4.0 80 0. 0.6 4.0 - 0.6 4.0 2.5 4.0 100 0. 0.8 4.0 - 0.6 4.0 2.5 4.1 4.0 4.0 130 0. 1.0 4.0 - 0.6 4.0 2.5 4.5 4.3 6.0 160 0. 1.0 7.0 - 0.6 4.0 2.5 4.7 4.4 28.0 200 0. 1.0 10.0 - 0.6 4.0 2.5 5.0 4.5 230 0. 1.0 15.0 - 0.6 4.1 2.5 5.5 4.7 - 300 0. 1.0 0.6 4.2 2.5 6.5 5.0 - 400 0. 0.8 0.6 4.4 2.7 7.2 6.2 - 500 0. 0.7 0.6 4.6 3.0 8.0 8.5 8.5 7.2 - 600 0. 0.7 0.6 4.7 3.2 8.7 8.5 17.6 8.8 700 0. 0.7 0.6 4.7 3.4 9.2 9.1 18.7 17.4 800 0. 0.7 0.6 5.2 3.5 10.0 9.5 !9.5 19.1 20.8 20.8 900 0. 0.7 0.6 5.2 3.8 11.0 9.9 20.0 20.2 26.6 20.0 35.0 1000 0. 0.7 0.6 5.2 4.0 11.5 10.0 20.7 21.4 25.7 20.0 - 1100 0.6 5.3 4.2 11.5 10.0 20.8 23.4 23.4 20.2 - ! 300 0.6 5.5 4.6 11.5 10.0 21.1 20.7 - 1500 0.6 5.5 4.7 ! 1.5 10.0 21.2 20.8 - 23.0 23.0 2000 0.6 5.5 4.8 11.5 10.0 2!.3 21.0 - 23.0 35.0 3000 0.6 5.5 5.0 11.5 10.0 21.3 21.1 23.0 22.7 - Glacial climate simulation fieldswere thenused to calculatethe biotemperatureand annual precipitationdistributions. These two fields, simulatedat a 10øx 10o In orderto simulatethe climate of theEarth during the Last Glacial resolution,were linearly interpolated on a 5øx 5ø gridand applied to Maximum,18000 years ago, someinput parameters and boundary the bioclimatic scheme described before. conditionsof the modelwere changed. The orbitalparameters were takenfrom Berger(1978). The atmosphericCO• concentrationwas Simulatedglacial biosphere decreasedto 200 ppm,in agreementwith theCO-., measurements from the Vostok ice core (Bamola et al., 1987). Coastlineswere modified Globalmaps of the simulateddistribution o1' vegetation are pre- assuminga sea level drop of 130m (Climap,1976). Seaice and snow sentedin Figure 1. Followingthis simulation,the borealvegetation coverwere taken from Climap for Februaryand August. Their annual typeswere shiftedtowards lower latitudes with tundraoccupying ar- cyclewas approximated by a sinefunction with February and August easpresently covered by coniferous,and to a lesserextent, deciduous valuestaken as extrema.The modelwas run to steadystate for the forests.This is basicallya resultof the reducedtemperatures over presentday and the LGM conditions. The differencebetween the theseregions. Lower precipitation rates 18 kyr yearsago in Australia simulatedtemperature field for thepresent and for theLGM wasthen andSahel explain why deserts were more extensive and grasslands and addedto the observedtemperature field abovementioned.
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