A DATA-BASED REAPPRAISAL OF THE TERRESTRIAL CARBON BUDGET AT THE LAST GLACIAL MAXIMUM

International audienceAn estimate of the glacial-interglacial change in terrestrial carbon storage was calculated from CLIMAP reconstructions of the earth's surface at the Last Glacial Maximum (LGM). It implies an increase of almost-equal-to 715 Pg C (430-930 Pg C) from the LGM to present, mainly due to the buildup of boreal and temperate forests. This new attempt to reconstruct the LGM ecosystems and carbon content offers an alternative to a previous data-interpretation-based estimate of almost-equal-to 1350 Pg C change. Our estimate is half-way between that 1350 Pg C value and the previous model-based estimate of no change. Furthermore it is consistent with the mean deltaC-13 shift of the ocean. The greatest biomass and soil carbon increases occurred around 60-degrees-N. Our results show that the interpretation of the data is crucial to the estimation of such a budget. It further demonstrates the need for a more complete knowledge of the complexity of the biogeophysical interactions

RECONSTRUCTION OF PAST TERRESTRIAL CARBON STORAGE IN THE NORTHERN-HEMISPHERE FROM THE OSNABRUCK-BIOSPHERE-MODEL AND PALAEODATA

International audienceUntil now the reconstruction of past carbon storage from data has often been done by using modern carbon databases. The results are Likely a rough approximation of the reality, and can be improved by the use of biosphere models. These models usually need to be parameterized by a large number of environmental inputs, which are often not available from palaeodata. The empirical Osnabruck Biosphere Model (OBM) needs as input only 3 environmental parameters, easily derivable from pollen data. We adapted it to reconstruct the past terrestrial carbon storage from palaeodata. Sensitivity experiments performed by uniformly decreasing the mean annual temperature, average annual precipitation and/or CO2 concentration suggest that temperature and CO2 concentration affect the carbon storage more than does precipitation. The use of the only palaeodata available at a global scale shows that only a weakening of CO2 fertilization must be invoked to reconstruct the Last Glacial Maximum (LGM) carbon storage, which is an intermediate situation between the no-fertilization effect assumed by previous studies and the fertilization effect based on modernist empirical equations. The terrestrial carbon storage in the Northern Hemisphere for this period ranged from 910 to 1270 Pg, which represents an increase of 330 to 710 Pg (a planetary increase of 470 to 1014 Pg from the LGM to the present). This result is similar to our previous reconstruction and agrees broadly with the values estimated on the basis of 0.32 parts per thousand change obtained for global deep ocean delta(13)C

The variation of terrestrial carbon storage at 6000 yr BP in Europe: Reconstruction from pollen data using two empirical biosphere models

International audienceTwo empirical biosphere models, the Osnabruck Biosphere Model (OBM) and a simple model based on evapotranspiration (AET model), were used to reconstruct the terrestrial carbon storage from pollen at 6000 yr sp in Europe. The pollen-based climatic reconstructions of Guiot et al. (1993) and the ecosystems reconstruction of Peng et al. (1994) provide the input data of the two models, i.e. annual temperature, annual precipitation, actual evapotranspiration, potential evapotranspiration and ecosystems with a spatial resolution of 0.50 longitude/latitude. For the region considered, our results suggest that there are no significant variations in the terrestrial carbon storage between 6000 yr sp and present due to a compensation of the forest extension towards the north by the replacement of conifer forest (with higher carbon) by deciduous forests towards the east. This is consistent with the recent results of Foley (1994) based on climate and biosphere models. The comparison of the two model results with those obtained from the carbon densities of Olson er al. (1985) and Zinke et al. (1986) shows a large similarity for vegetation but a higher value of the OBM for the soil

TEMPORAL AND SPATIAL VARIATIONS OF TERRESTRIAL BIOMES AND CARBON STORAGE SINCE 13,000 YR BP IN EUROPE - RECONSTRUCTION FROM POLLEN DATA AND STATISTICAL-MODELS

International-Boreal-Forest-Research-Association Conference on Boreal Forests and Global Change, SASKATOON, CANADA, SEP 25-30, 1994International audienceStatistical models calibrated from field measurement data are used to reconstruct the past terrestrial carbon (C) storage from pollen data for the last 13 000 yr BP in Europe. The pollen-based climatic and biome reconstructions provide the input data for these statistical models, i.e., annual mean temperature, total annual precipitation, annual actual evapotranspiration, annual potential evapotranspiration and biome type with a spatial resolution of 0.5 degrees x 0.5 degrees longitude/latitude. Our reconstructions indicate that the last 13 000 yr BP were characterized in Europe by variations of terrestrial biome and net primary productivity (NPP) at various temporal and spatial scales. For the considered region, our results also suggest that changes in climate have significantly altered the distribution of terrestrial biomes and affected the uptake of CO2 for NPP. However, these changes did not translate into significant C storage change in potential terrestrial biosphere during the Holocene. The largest decrease of terrestrial C storage (compared to modern levels) is found during the late-Glacial period mainly due to the persistence of ice sheets and the small extension of Forest

Holocene environmental changes in Bangong Co basin (western Tibet) .4. Discussion and conclusions

A 12.4 m core taken in Lake Bangong provides a continuous Holocene climatic record. We summarize information on changes in stable isotope and radiocarbon balances in the lake, hydrobiology and vegetal cover in the catchment, deduced from detailed analytical results given in the three preceding papers. The Bangong record is then compared with the environmental history of the neighbouring Lake Sumxi also constructed from multidisciplinary analyses. The two records show a major environmental change at approximate to 10-9.5 ka B.P., attributed to a rapid strengthening of the summer monsoonal circulation which led to wet-warm conditions. This event was followed by a long-term trend toward aridity which culminated around 4-3 ka B.P.. In Western Tibet, maximum monsoon rainfall seems to have occurred from approximate to 9.5 to 8.7 ka B.P, and from approximate to 7.2 to 6.3 ka B.P., as two wet pulses separated by a reversal event centered on 8.0-7.7 ka B.P. Our results broadly agree with the records from Lake Seling in Central Tibet, and Lake Qinghai at the plateau's northeastern margin, and with palaeoclimatic studies in western China which document conditions wetter and warmer than those of today during the early-middle Holocene. The environmental fluctuations recorded in western Tibet appear in phase with climatic changes recognized in tropical North Africa, suggesting that the 8.0-7.7 ka B.P. and the 4.0-3.0 ka B.P. dry events were caused by abrupt disequilibrium in the climatic system