Pierre Warnant

CARAIB: A global model of terrestrial biological productivity

CARAIB, a mechanistic model of carbon assimilation in the biosphere estimates the net primary productivity (NPP) of the continental vegetation on a grid of 1° × 1° in latitude and longitude. The model considers the annual and diurnal cycles. It is based on the coupling of the three following submodels; a leaf assimilation model including estimates of stomatal conductance and leaf respiration, a canopy model describing principally the radiative transfer through the foliage, and a wood respiration model. Present-day climate and vegetation characteristics allow the discrimination between ecotypes. In particular, specific information on vegetation distribution and properties is successfully used at four levels; the leaf physiological level, the plant level, the ecosystem level, and the global level. The productivity determined by the CARAIB model is compared with local measurements and empirical estimates showing a good agreement with a global value of 65 Gt C yr−1. The sensitivity of the model to the diurnal cycle and to the abundance of C4 species is also tested. The productivity slightly decreases (10%) when the diurnal cycle of the temperature is neglected. By contrast, neglecting the diurnal cycle of solar irradiance produces unrealistically high values of NPP. Even if the importance of this increase would presumably be reduced by the coupling of CARAIB with a nutrient cycle model, this test emphasizes the key role of the diurnal cycle in a mechanistic model of the NPP. Uncertainties on the abundance and spatial distribution of C4 plants may cause errors in the NPP estimates, however, as demonstrated by two sensitivity tests, these errors are certainly lower than 10% at the global scale as shown by two tests.

Modelling the glacial-interglacial changes in the continental biosphere

Abstract A new estimate of the glacial–interglacial variations of the terrestrial carbon storage was obtained with the CARAIB biosphere model. The climatic data for the Last Glacial Maximum (LGM) necessary to drive the biosphere model are derived from results of the ECHAM2 General Circulation Model (GCM). Six model simulations (four under typical interglacial and two under typical glacial climatic conditions) were performed to analyse the roles of different environmental changes influencing the biospheric net primary productivity (NPP) and carbon stocks. The main differences between these simulations come from the adopted CO 2 levels in the atmosphere, the presence or absence of crops and from changing continental boundaries. The variation of the terrestrial carbon stocks since the LGM are estimated by comparing the pre-agricultural (280 ppm of CO 2 , no crops, modern climate) and the full glacial simulations (200 ppm of CO 2 , LGM climate reconstruction). Our model predicts a global NPP increase from 38 Gt C year −1 to 53 Gt C year −1 during the deglaciation, a substantial part of that change being due to CO 2 fertilization. At the same time, the terrestrial biosphere would have fixed between 134 (neglecting CO 2 fertilization effects) and 606 Gt C. The treatment of both the C 3 and C 4 photosynthetic pathways in the CARAIB model enabled us further to reconstruct the partitioning between C 4 and C 3 plants. Following our experiments, 29.7% of the total biospheric carbon stock at the LGM was C 4 material, compared to an interglacial fraction of only 19.8%. The average biospheric fractionation factor was ∼1.5‰ less negative at LGM than it is today. Considering an atmospheric δ 13 C 0.5±0.2‰ lower at LGM than at pre-industrial times, the 606 Gt C transfer would lead to a global ocean δ 13 C shift of roughly −0.41‰, fully consistent with currently available data. For the smaller change of 134 Gt C obtained without the CO 2 fertilization effect, this shift would only be on the order of −0.10‰.

Carbon stocks and isotopic budgets of the terrestrial biosphere at mid-Holocene and last glacial maximum times

Abstract The carbon fluxes, stocks and isotopic budgets of the land biosphere at mid-Holocene (6 ka BP) and last glacial maximum (21 ka BP) times are reconstructed with the CARbon Assimilation In the Biosphere (CARAIB) model forced with two different sets of climates simulated by the European Centre-HAMburg (ECHAM) and LMD general circulation models. It is found that the trends predicted on the basis of both sets of GCM climatic fields are generally consistent with each other, although substantial discrepancies in the magnitude of the changes may be observed. Actually, these discrepancies in the biospheric results associated with the use of different GCM climatic fields are usually smaller than the differences between biospheric runs performed while considering or neglecting the CO2 fertilization effect (which might, however, be overestimated by the model due to uncertainties concerning changes in nutrient availability). The calculated changes with respect to the present of the biosphere carbon stock range from −132 to +92 Gt C for the mid-Holocene and from −710 to +70 Gt C for the last glacial maximum. It is also shown that the relative contribution of the material synthesized by C4 plants to the total biomass of vegetation, litter and soils was substantially larger at mid-Holocene and last glacial maximum times than today. This change in the relative importance of the C3 and C4 photosynthetic pathways induced changes in the 13 C fractionation of the land biosphere. These changes in the average biospheric fractionation resulting from the redistribution of C3 and C4 plants were partly compensated for by changes of opposite sign in the fractionation of C3 plants due to the modification of the intercellular CO2 pressure within their leaves. With respect to present times, the combination of both processes reduced the 13 C discrimination (i.e., less negative fractionation) of the land biosphere by 0.03 to 0.32‰ during the mid-Holocene and by 0.30 to 1.86‰ at the last glacial maximum.

The interannual change of atmospheric CO2: Contribution of subtropical ecosystems?

The global terrestrial carbon cycle model CARAIB (CARbon Assimilation In the Biosphere) is used to study the response of the terrestrial ecosystems to the large scale climate variations over the period 1980–1993. The global net carbon exchange flux with the atmosphere is calculated and compared with the terrestrial contribution derived from the deconvolution of the atmospheric CO2 and δ13C measurements. A fairly large CO2 biospheric source is predicted during the strong El Nino events of 1982–83 and 1986–87 as a consequence of the induced global warming. The direct and indirect temperature controls of the primary production and respiration dominate the CO2 anomaly. An analysis of the relative contribution by latitudinal bands and ecosystems shows that low-latitude vegetation dominates the variability at the El Nino time scale. In savannas, the model indicates that the interannual changes result, to a large extent, from the control of soil water content on gross primary production (GPP). In the tropical rain forests, both respiration and GPP contribute to the response of the net biospheric flux.

The seasonality of the CO2 exchange between the atmosphere and the land biosphere: A study with a global mechanistic vegetation model

Two simulations of the seasonal variation of the global atmospheric CO2 distribution are obtained by combining an atmospheric transport model, two parameterizations of soil heterotrophic respiration (SHR), and a mechanistic model of carbon assimilation in the biosphere (CARAIB) that estimates the net primary production (NPP) of continental vegetation. The steady state hypothesis of the biosphere allows the spatial distribution and the global content of the soil carbon to be expressed as a function of the root fractions of soil respiration under forested and herbaceous vegetation covers. The sensitivity of the modeled CO2 signal to the wind field does not exceed the observed interannual variability. The influence of the various vegetation zones is quantified by the Fourier analysis of the modeled atmospheric signal. In the northern hemisphere, the temperate ecosystems dominate the seasonal atmospheric signal of the extratropical latitudes. The ecosystems of the tropical northern zone determine the local signal, while the southern tropical ecosystems influence largely the signal in the whole southern hemisphere. The results give credence to the mechanistic modeling of NPP since the simulated atmospheric signal is comparable with that obtained with normalized difference vegetation index (NDVI) based diagnostic models coupled with a parameterization of SHR fitted to optimize the atmospheric signal.

Stochastic generation of meteorological variables and effects on global models of water and carbon cycles in vegetation and soils

Abstract Global models of water and carbon cycles in continental vegetation and soils are usually forced with monthly mean climatic data-sets and thus neglect day to day variations of the weather. This treatment may be justified for empirical models based on parametrizations validated at a monthly timescale. Mechanistic models handling hydrological and biological processes at much shorter timescales might, however, be largely affected by such an approximation, since the various processes described are highly nonlinear. A random generator of daily precipitations and temperatures applicable at the global scale has thus been developed from worldwide meteorological data covering 6 years of observations. The probability of a wet day is correlated to the weather encountered the previous day. The amount of precipitation, the daily mean temperature and the diurnal range of temperature are described from the statistical point of view by the cumulative distribution functions (CDF) of three random variables. The CDFs relative to temperatures are different for rainy and dry days. This stochastically generated weather field is used as input to IBM (Improved Bucket Model) and CARAIB (CARbon Assimilation In the Biosphere), two global models of respectively soil hydrology and vegetation productivity. Large differences in both the geographical distribution and the global value of soil water, vegetation productivity and carbon stocks are obtained between the model runs using monthly uniform weather on one side and randomly generated weather on the other. The main contribution to this difference at the global scale arises from the precipitation generation occurring as a result of high degree of nonlinearity of the interception scheme used in IBM.

Seasonal and interannual influences of the terrestrial ecosystems on atmospheric CO2: a model study

Abstract The prognostic CARAIB (Carbon Assimilation In the Biosphere) model has been used in conjunction with the Max-Planck Institut TM2 atmospheric transport model to calculate the atmospheric CO 2 fluctuations at the global scale. Two applications are briefly described. In the first one, the seasonal CO 2 variation is calculated and a Fourier analysis is performed to determine the relative contributions of the various vegetation types. It is found that the seasonal signal is dominated by the grasslands and needle leaf forests in the northern boreal and temperate zones. In the southern hemisphere, tropical deciduous forests and grasslands make the primary contribution. In the second application, the net primary productivity (NPP), soil heterotrophic respiration (SHR) and net ecosystem productivity (NEP) are calculated for years 1987 and 1988 with the model driven by observed climatic variables. Preliminary results indicate that the NEP variations between these two years are strongly dominated by tropical ecosystems. However, it is shown that the results are strongly dependent on the dataset used for the 1987-88 temperature record, raising the question of reliability of such modelling studies of the interannual variability of the biosphere.