D. Jolly

Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP)

Amplification of the northern hemisphere seasonal cycle of insolation during the mid-Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimates of biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.

SIMULATED CLIMATE AND BIOMES OF AFRICA DURING THE LATE QUATERNARY: COMPARISON WITH POLLEN AND LAKE STATUS DATA

New compilations of African pollen and lake data are compared with climate (CCM1, NCAR, Boulder) and vegetation (BIOME 1.2, GSG, Lund) simulations for the last glacial maximum (LGM) and early to mid-Holocene (EMH). The simulated LGM climate was ca 4°C colder and drier than present, with maximum reduction in precipitation in semi-arid regions. Biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations, but no change in the distribution of deserts and tropical rain forests. The lakes show LGM conditions similar or drier than present throughout northern and tropical Africa. Pollen data indicate lowering of montane vegetation belts, the stability of the Sahara, and a reduction of rain forest. The paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets, although they suggest the climate model underestimates equatorial aridity. EMH simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern Sahara and East Africa. Biome simulations show an upward shift of montane vegetation belts, fragmentation of xerophytic vegetation in southern Africa, and a major northward shift of the southern margin of the eastern Sahara. The lakes indicate conditions wetter than present across northern Africa. Pollen data show an upward shift of the montane forests, the northward shift of the southern margin of the Sahara, and a major extension of tropical rain forest. The lake and pollen data confirm monsoon expansion in eastern Africa, but the climate model fails to simulate the wet conditions in western Africa.

Intercomparison of Simulated Global Vegetation Distributions in Response to 6 kyr BP Orbital Forcing

Abstract The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated sh...

Study of the modern pollen rain in Western Uganda with a numerical approach

Modern soil samples from Western Uganda, from a range of ten plant communities belonging to five African phytogeographical regions and distributed along an altitudinal gradient from 650 m (grass savannas) to 4400 m (Afroalpine moorland) were analyzed for pollen content to define modern pollen/vegetation relationships. Correspondence analysis applied to the pollen counts (100 sites and 167 taxa) indicates four distinctive vegetation types arranged along an altitudinal gradient and thus a temperature one with respect to axis 1(contrast between montane and lowland vegetations), and along a physiognomical gradient (from densely structured to open vegetations) defined by axis 3. These results confirm the empirical interpretation proposed on the initial pollen data set and are in agreement with those previously obtained on modern or fossil pollen spectra from other African regions.

Glacialanterglacial record from intertropical Africa, high resolution pollen and carbon data at Rusaka, Burundi

Abstract In northern intertropical Africa, many geological data have documented several abrupt climatic fluctuations taking place during the time of the last deglaciation. One of them is an aridity event, generally placed in the interval 11,000–10,000 BP, and synchronous with the Younger Dryas cold episode of the temperate regions, that has been explained by the influence of the Atlantic Ocean circulation on global climate. In this paper we present high resolution (50 to 150 years) studies of organic matter and pollen undertaken on a new core from the Rusaka peat bog (3°26′ South, 29°37′ East, 2070 m altitude), dated by the conventional (l2 dates) and the AMS methods (18 dates obtained on bulk sediment and macroremains). Although the sediment at the bottom of the core was deposited during the last glacial period prior to 21,000 BP, the dating provides a reliable chronology for the last 12,000 BP only. Several vegetational changes are documented in the interval 12,000–9000 BP, but their chronological placement can only be achieved with a few years of uncertainties. A great dispersion of AMS dates is attributed to contamination by charcoal dated between 11,700 and 11,500 BP, and to a possible age plateau at ca. 10,000 BP. Therefore the results are preferably presented versus depth to keep synchronism in the stratigraphic placement of the successive events. At Rusaka, increase in carbon storage, C/N ratio for terrestrial plants, and tree cover began at ca. 12,000 BP, soon after the swamp had been established. This date is in good agreement with a major increased hydrological balance well-documented throughout the intertropical region. The forest installation is progressive, rather complex and seen as a two step process. The first forest phase (11,700 BP to 10,600 BP) is dominated by Hagenia, a montane taxon, in good correspondance with previous records from other authors in East African mountains. The second forest regeneration took place post 10.000 BP, after a clearly reversed trend of tree cover from 10,600 to 10,000 BP, ending with a short episode of minimum trees lasting no more than about 200 years. But there is no indication of cooling whatsoever, and the opening of the forest is better explained by increased aridity, rather than normal pattern of forest succession. This will remain a controversal interpretation among palynologists until high resolution pollen data can be obtained for the 13,000 BP and earlier period. Nevertheless, the timing of the arid episode at Rusaka is in good agreement with other evidence mentioned from highland and lowland pollen data. A detailed comparison with 25 available pollen diagrams is limited by poor or inconsistent dating or low resolution of the pollen data. Nevertheless pollen from four of them document an arid event in good agreement with its timing at Rusaka and high resolution isotopic results from two sites in equatorial lowlands and from the Sahel. This work shows that the arid Younger Dryas occurred at the same time both in lowlands and highlands, and therefore was most likely a precipitation signal rather than a response to temperature decrease.

Reconstructing and modeling past changes in terrestrial primary productivity

Reconstruction of net primary productivity (NPP) from pollen data by statistical methods is a useful tool for assuming a relationship between NPP and taxonomic composition. This approach, practicable in regions with a lot of data, has some limitations too. Several parallel approaches have attempted to estimate the global terrestrial carbon storage of the Last Glacial Maximum: the large differences between these approaches prove the difficulty of the task. To mitigate this lack of data, this chapter proposes an approach based on model simulations, controlled by data. Waiting for more sophisticated earth system models, BIOME3 is used asynchronously coupled with a climate model to simulate final vegetation distributions and NPP. Evidently, there is yet no reliable way to reconstruct past global patterns of NPP. Data-based reconstruction is possible in regions where the data network is exceptionally dense. Biogeochemical models can be used to convert empirical regional climate and biome reconstructions to NPP and carbon storage, taking into account external factors including atmospheric carbon dioxide content. Global analysis, however, requires a complete “forward modeling” approach, based on climate model simulations coupled to an equilibrium biogeography/biogeochemical model. Nevertheless, past data can be used to test the climate and vegetation models by experimentally assigning to them conditions drastically different from the present one. If the models are reasonably able to simulate these paleoenvironments, they prove to be robust and one can have some confidence in them for predicting the future.