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Biospheric carbon stocks reconstructed at the Last Glacial Maximum: comparison between general circulation models using prescribed and computed sea surface temperatures

Biospheric carbon stocks reconstructed at the Last Glacial Maximum: comparison between general circulation models using prescribed and computed sea surface temperatures
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  Biospheric carbon stocks reconstructed at the Last Glacial Maximum: comparison between generalcirculation models using prescribed andcomputed sea surface temperatures Dominique Otto*, Daniel Rasse, Jed Kaplan, Pierre Warnant, Louis Franc  ois  Laboratoire de Physique Atmosphe´ rique et Plane´ taire, Institut d’Astrophysique et de Ge´ ophysique, Universite´  de Lie`  ge,alle´ e du Six Aouˆt B5c, B-4000 Lie`  ge, Belgium Received 20 March 2001; received in revised form 27 August 2001; accepted 7 September 2001 Abstract The terrestrial biosphere model Carbon Assimilation in the Biosphere (CARAIB) was improved by introducing twovegetation storeys and implementing a new module which simulates the equilibrium distribution of the vegetation inferredfrom physiological processes and climatic constraints. In this fourth version of CARAIB, we differentiate ground-levelgrasses from tree canopies, which allows us to determine the light available to grasses as a direct function of the leaf areaindex (LAI) of the forest canopy. Both of these storeys are potentially composed of several plant functional types (PFT).The cover fraction of each PFT within each storey is estimated according to its respective net primary productivity (NPP).A biome is assigned to each grid cell on the basis of three physiological criteria: (1) the cover fraction, (2) the NPP, and(3) the LAI; and two climatic constraints: (1) the growing degree-days (GDD) and (2) the lowest temperature reachedduring the cold season ( T  min ), which are well-known indices of vegetation expansion boundaries. Total biospheric carbonstocks (vegetation+soil) are reconstructed by forcing the model with eight climatic scenarios of the Last Glacial Maximum(LGM, 21 ka BP), which were obtained from the Paleo-Modelling Intercomparison Project (PMIP) from four generalcirculation models (MRI2, UGAMP, LMD4, and GEN2) using prescribed and computed sea surface temperatures (SSTs).The model was also forced with a current climate together with a preindustrial atmospheric CO 2  level of 280 ppm asreference simulation. To validate the model, current biome distribution is reconstructed and compared, for the modernclimate, with two distributions of potential vegetation and, for the LGM, with pollen data. The model simulations are ingood agreement with broad-scale patterns of vegetation distribution. The results indicate an increase in the total biosphericcarbon stock of 827.8–1106.1 Gt C since the LGM. Sensitivity analyses were performed to discriminate the relative effectsof the atmospheric CO 2  level (‘‘fertilization effect’’), the climate (present or LGM), and the sea level. Our results suggest that the CO 2  fertilization effect is mostly responsible for the total increase in vegetation and soil carbon stocks. The four GCMs diverged in their predicted responses of continental climate to calculated SSTs. Only one of them, i.e., MRI2, predicted a marked decline of the continental temperatures in response to lower calculated SSTs. For this GCM, the effect of reduced SSTs on continental biospheric carbon stocks was a decrease of 544.1 Gt for the soil carbon stock and of 283.7 0921-8181/02/$ - see front matter   D  2002 Elsevier Science B.V. All rights reserved.PII: S0921-8181(02)00066-8 * Corresponding author. Tel.: +32-4-366-9780; fax: +32-4-366-9729.  E-mail address: (D. Otto) and Planetary Change 33 (2002) 117–138  for the vegetation carbon stock, which means a decrease in the total biopsheric carbon stock of 827.8 Gt.  D  2002 Elsevier Science B.V. All rights reserved.  Keywords:  prescribed and computed sea surface temperatures; Last Glacial Maximum; vegetation distribution; continental carbon stocks;climate and vegetation models; asynchronous coupling 1. Introduction The relationship between biospheric carbon stocksand atmospheric CO 2  concentrations during the gla-cial–interglacial cycles, i.e., the Pleistocene epochduring the past 2 million years, may help us under-stand the mechanisms which will drive climatechange during the 21st century and beyond. Measure-ments in ice and deep-sea sediment cores revealedthat the atmospheric CO 2  concentration has fluctuatedwidely in concert with temperature variations duringthis period. From the Last Glacial Maximum (LGM,21,000 years BP) to preindustrial time, atmosphericCO 2  concentration increased from 200 to 280 ppmv(Petit et al., 1999). Earlier research hypothesised that the reservoir size and turnover time of the ocean werethe sole responsible factors for this increase andinvoked a redistribution of carbon in the ocean– atmosphere system attributable to changes in theoceanic circulation (Broecker and Takahashi, 1984)and nutrient cycles (Broecker and Peng, 1987). How-ever, foraminifera data suggest that the  d 13 C value of oceanic carbon at the LGM was from 0.3 x  to0.7 x lower than at present (Shackleton, 1977;Duplessy et al., 1988; Curry et al., 1988). This changeof the oceanic carbon isotopic composition impliesthat a transfer of 470–1100 Gt of carbon from theocean to the biosphere occurred during deglaciation.This calculation rests on the hypothesis that theterrestrial biosphere is the only reservoir havingexchanged carbon with the ocean during the deglaci-ation, at least at an isotopic signature different fromthat of the ocean. It also assumes that the average d 13 C fractionation of photosynthesis at the LGM wasthe same as today. However, taking into account  possible variations of this photosynthetic fractionationwould only slightly modify this range probablytowards lower values, since C 4  species (exhibiting alower fractionation factor) are thought to have beenmore widespread at the LGM than today (Bird et al.,1994; Franc  ois et al., 1998). Two additional methodssupport the concept that massive amounts of carbonwere removed from the ocean to the land biosphereduring deglaciation. First, reconstructions of paleove-getation from palynological and sedimentological proxy data suggest that the increase in the biosphericcarbon stock from the LGM to the present rangesfrom 700 to 1600 Gt C (Adams et al., 1990; VanCampo et al., 1993; Crowley, 1995; Adams andFaure, 1998). Second, biospheric models forced withoutputs of general circulation models (GCM) estimatethis change to range from 0 to 700 Gt C (Prentice andFung, 1990; Friedlingstein et al., 1992, 1995; Prenticeet al., 1993; Esser and Lautenschlager, 1994; Franc  oiset al., 1998, 1999). In addition to increased carbonstocks in the land biosphere, the dissolution of ter-restrial calcrete, i.e., soil carbonate, during deglacia-tion appears as another potential sink of atmosphericCO 2  (Adams and Post, 1999). Both estimates of change in biospheric carbon stock and calcrete implyan increased efficiency of the oceanic mechanismsincreasing the atmospheric CO 2  level during deglaci-ation for the net effect to explain the observed 80 ppmv shift between the LGM and the pre-industrial period.Sea surface temperatures (SSTs) are pivotal to thesimulations of atmospheric global circulation models(AGCM), which drive the biospheric models used for estimating the evolution of LGM carbon stocks.Tropical SST values during the LGM remain uncer-tain, which propagates uncertainties to biosphericmodel estimates of LGM carbon stocks. The first global SST reconstructions for the LGM were gen-erated by CLIMAP Project Members (1981). Theywere prescribed based on the abundances of plank-tonic fossils in deep-sea sediment cores (‘‘fixedSST’’). CLIMAP reconstructions of tropical SSTsare on average 1  j C cooler than present-day tropicalSSTs. A considerable controversy has arisen about thevalidity of CLIMAP SST estimates. Numerous geo-  D. Otto et al. / Global and Planetary Change 33 (2002) 117–138 118  chemical studies indicate that the tropics were 4–6  j Ccooler at the LGM than under present conditions and provide some support for the hypothesis that glacia-tions were global. These studies include: (1) loweredtropical snowlines (Rind and Peteet, 1985); (2) fora-miniferal records (Curry and Oppo, 1997); (3) Sr/Caratios from Barbados corals (Guilderson et al., 1994);(4) tropical ice cores (Thompson et al., 1995); (5)noble gases in Brazil aquifer (Stute et al., 1995). Thisnew evidence of sensitivity of the tropics to climatechange could have dramatic implications for forecastsof the future global warming. The CLIMAP resultsimplie that the tropics, which represents 40% of Earth’s surface, might not be affected by the futurewarming. Opposite to these results, geochemical stud-ies described above suggest that the tropics might experience similar warming to that of higher latituderegions. On the other hand, analysis of the temper-ature-dependent production of alkenone molecules bymarine organisms, which responds to changes inwater temperature by altering the molecular compo-sition of their cell membranes (Eglinton et al., 1992;Herbert and Schuffert, 1998), yields small temperaturechanges of about 2  j C, closer to the CLIMAPestimates. Recently, slab ocean models, i.e., atmos- phere-mixed layer ocean models, (Broccoli, in press),have simulated air–sea interactions and computed theSST distribution at the LGM (‘‘calculated SST’’). Thecomputed tropical cooling is comparable to recon-structions based on alkenones, but smaller than thecooling inferred from other geochemical studies.The objective of this research is to analyse thesensitivity of LGM biospheric carbon stocks andvegetation distribution to SSTs. For this purpose, weused the biospheric model CARAIB (Warnant et al.,1994; Warnant, 1999) forced with four sets of twoAGCM scenarios, which differed only on their SSTinputs: (1) SST are prescribed based on the CLIMAP(CLIMAP Project Members, 1981) reconstructions,and (2) SST are computed with a thermodynamic slabocean model integrated as a submodule in the AGCM.Since most recent geochemical studies indicate that tropical SST during LGM were cooler than SST prescribed by CLIMAP, we aim at providing a morerealistic estimate of the change in biospheric carbonstocks during the last deglaciation. For this study, wehypothesized that colder SSTs resulted in decreasedcontinental temperatures. 2. The model 2.1. General structure CARAIB is a global model of the carbon cycle inthe continental biosphere (Warnant et al., 1994; Nemry et al., 1996). It calculates carbon fluxes between the atmosphere and the terrestrial biosphere,and estimates the evolution of carbon pools resultingfrom these fluxes. Five pools are considered: (1) theleaves (GC, for ‘‘green carbon’’); (2) the rest of the plant, i.e., branches, stems and roots (Lu¨deke et al.,1994) (RC, for ‘‘remaining carbon’’); (3) the litter from GC (GL, for ‘‘green litter’’); (4) the litter fromRC (RL, for ‘‘remaining litter’’); (5) the humus, i.e.,the product of litter decomposition (SC, for ‘‘soilcarbon’’). Eight plant functional types (PFTs) areconsidered: (1) C 3  grasses; (2) C 4  grasses; (3) needle-leaved evergreen trees; (4) needleleaved deciduoustrees; (5) temperate broadleaved evergreen trees; (6)tropical broadleaved evergreen trees; (7) temperate broadleaved deciduous trees; (8) tropical broadleaveddeciduous trees. Carbon contents and fluxes in andout of each pool are estimated daily for each grid celland each PFT. The model contains no nutrient cycle. It should be included in future. The different carbon andwater fluxes are described in the following subsec-tions. 2.1.1. Soil water budget  The Improved Bucket Model (IBM) developed byHubert et al. (1998) is used to calculate the soil water  budget of a soil layer of a given thickness with a timestep of 1 day. This budget is described by the follow-ing equationd w d t   ¼  P    E    D  SR   ð 1 Þ where  w  is the water content in mm of the soil layer and the water fluxes  P  ,  E  ,  D , and SR are respectively precipitation, actual evapotranspiration, deep drain-age, and surface runoff in units of mm year   1 .Precipitation is an input of the model. Actual evapo-transpiration is estimated as a fraction of the potentialevapotranspiration rate. This fraction depends on soilwetness, while the potential evapotranspiration rate iscalculated from Penman’s equation (Mintz andWalker, 1993). Deep drainage, i.e., the downward  D. Otto et al. / Global and Planetary Change 33 (2002) 117–138  119  water flux at the bottom of the soil layer, is estimatedfrom the soil hydraulic conductivity parameterized asa function of soil texture, i.e., % sand, % silt, and %clay, and soil wetness, according to Saxton et al.(1986). Surface runoff occurs when precipitation istoo high and exceeds maximum infiltration into thesoil. The model requires as inputs daily mean valuesof air temperature, precipitation, cloudiness, relativeair humidity, and wind speed. The last variable is usedto calculate the aerodynamic resistance needed inPenman’s equation. The balance between the twowater fluxes, i.e., precipitation, which is imposed bythe GCM data, and evapotranspiration, which isdriven by wind data, is critical for determining thevegetation distribution and the extent of deserticareas. 2.1.2. Photosynthesis Photosynthesis of C 3  and C 4  plants is computedaccording to the methodologies of Farquhar et al.(1980) and Collatz et al. (1992), respectively. Thecanopy is divided in several layers for computing theabsorption of the photosyntheticaly active radiation(PAR). PAR is estimated for each layer following themethod of Goudriaan and van Laar (1994), whichseparates the effects of direct and diffuse light. Photo-synthetic fluxes are estimated on a 2-h time step, totake into account the diurnal cycle of the solar insolation. The stomatal conductance is from Leu-ning’s (1995) model modified by Wijk et al. (2000) asfollows: the empirical coefficients of the originalstomatal conductance model from Leuning were opti-mized by comparing modeled and measured transpi-ration fluxes, and the stomatal response to soil water content was incorporated in this formulation by multi- plying the stomatal conductance by the standardresponse function proposed by Jarvis (1976) andStewart (1988). 2.1.3. Autotrophic respiration The autotrophic respiration is divided between twofluxes: maintenance and growth respiration. Mainte-nance respiration was parametrized as an exponentialfunction of temperature and a linear function of carbon content of the GC or RC pools (Warnant,1999). The growth respiration is assumed proportionalto the biomass increase (Raich et al., 1991; Parton et al., 1993; Ruimy et al., 1996). For tree woody tissues,only sapwood is respiring. The sapwood fraction wasestimated from data reported by Duvigneaud et al.(1971), Dubroca (1983), Ryan and Waring (1992) andcollected by Ruimy et al. (1996). 2.1.4. Allocation of photosynthates and reserve use The carbon assimilated during photosynthesis is partitioned between the GC and the RC pools accord-ing to the simulated environmental conditions. Whentemperature falls between a minimum and a maximumvalue and soil water content is above a critical value(Table 1), which have been calibrated to reproduce areasonable vegetation distribution, one half of theassimilated carbon is allocated to each pool. Whenthese conditions are not met, the assimilated carbon isentirely allocated to the RC pool. The GC pool islimited to a maximum value, defined by an allometricrelationship with the RC content (Lu¨deke et al., 1994).A new leaf layer is created only if its productivity ishigher than its mortality rate, which prevents CAR-AIB from generating leaf layers with a negative car- bon budget. Budburst is simulated by a transfer of carbon from the RC pool to the GC pool. The carbon Table 1Tresholds used for determination of stress conditionsPFTs  T  min1  T  min2  T  max1  T  max2  SW min1  SW min2 (1) C 3  grass   55   55 40 40 0.1 0.0(2) C 4  grass 0 0 50 50 0.1 0.0(3) Needle-leavedevergreen boreal/temperate  40   40 30 30 0.3 0.3(4) Needle-leavedsummergreen boreal/temperate  50   55 30 35 0.3 0.0(5) Broad-leavedevergreentemperate  10   10 35 35 0.3 0.3(6) Broad-leavedevergreen tropical0 0 45 45 0.2 0.2(7) Broad-leavedsummergreen boreal/temperate0   25 40 40 0.3 0.0(8) Broad-leavedraingreen tropical0 0 40 45 0.2 0.0 T  min  is the absolute minimum temperature ( j C),  T  max  is the absolutemaximum temperature ( j C), SW min  is the minimum soil water content, expressed as a fraction of the field capacity and limited tothe wilting point. Index 1 refers to leaves and index 2 refers to therest of the plant.  D. Otto et al. / Global and Planetary Change 33 (2002) 117–138 120  available for buds is computed as a fraction of themaximum value of the GC pool of the previous year. 2.1.5. Litter production and mortality Litter is producted by the GC and RC mortality,resulting from three contributions: (1) seasonal leaf fallfor deciduous species controlled by three external parameters, i.e., temperature, soil water content, andPAR; (2) plant death due to the natural regeneration of thecanopy,withamortalitycharacteristictimedepend-ing on the type of plant, i.e., grass or tree, evergreen or deciduous, and the reservoir, i.e., GC or RC; and (3) plant death due to unfavourable climatic conditions(see Table 1) with a characteristic time of 1 week. 2.1.6. Heterotrophic respiration Heterotrophic respiration is due to organic matter decomposition by soil bacteria. Litter decompositionis computed as a function of temperature, soil water content, and litter carbon content (GL or RL). Theadopted temperature dependence has been fitted by Nemry et al. (1996) on the data reported by Raich andSchlesinger (1992) for all major world’s ecosystems.A fixed fraction (20%) of this flux contributes to theSC formation (Johnson et al., 1987) and the rest isreturned to the atmosphere as CO 2 . The SC mineral-ization is computed using an equation similar to that for the litter decomposition, but with a proportionalitycoefficient reduced by a factor of 100 (Esser, 1984). 2.2. The new biome prediction module A biome prediction module was developed, usingthe NPP and LAI outputs of the CARAIB model,which allows us to determine the cover fractions of the different PFTs and to assign a biome to each gridcell. This assignment is made in three steps: (1) theapplication of climatic constraints; (2) the simulationof competition between PFTs; and (3) the assignment of a biome. 2.2.1. Application of climatic constraints Two climatic constraints, which are indices of thegeographical extent of vegetation, have been chosenwith the aim of selecting the species potentially present in each grid cell (Table 2): the absoluteminimum temperature ( T  min ), i.e., the average temper-ature of the coldest day of the year, and the growingdegree-days based on a threshold temperature of 5  j C(GDD 5 ), defined byGDD 5  ¼ X 365 i ¼ 1 ð b T  i > 5 Þ ð T  i  5 Þ ð 2 Þ where  T  i  is the mean temperature of day  i  ( j C).According to Woodward (1987), there is a strongcorrelation between the distribution of dominant spe-cies of trees and their winter freezing resistance.Tropical species die as soon as temperature falls below 0  j C, while boreal and temperate species must endure freezing temperatures. Broadleaved evergreenspecies resist to   10  j C. All plants require a periodwith temperatures warm enough for growth. At highlatitudes, it is a severe limitation to the growth of leaves, and therefore establishment of LAI. Twovalues of GDD 5  have been adjusted: (1) 50 simulates Table 2Climatic constraints applied to PFTsPFTs  T  min  GDD 5  new PFTs(1) C 3  grass / / (1) C 3  grass(2) C 4  grass / / (2) C 4  grass(3) Needle-leavedevergreen boreal/temp<0 [50, 1350 z 1350(3) Needle-leavedevergreen boreal(4) Needle-leavedevergreen temperate(4) Needle-leavedsummergreen boreal/temp<0 [50, 1350] z 1350(5) needle-leavedsummergreen boreal(6) needle-leavedsummergreentemperate(5) Broad-leavedevergreentemperate[  10,0] >50 (7) broad-leavedevergreen temperate(6) Broad-leavedevergreentropical>0 >50 (8) broad-leavedevergreen tropical(7) Broad-leavedsummergreen boreal/temp<0 [50, 1350] z 1350(9) broad-leavedsummergreen boreal(10) broad-leavedsummergreentemperate(8) Broad-leavedraingreentropical>0 >50 (11) broad-leavedraingreen tropical T  min  is the absolute minimum temperature and GDD 5  is the growingdegree-days based on a threshold temperature of 5  j C.  D. Otto et al. / Global and Planetary Change 33 (2002) 117–138  121
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