Modelling Uncertainties in the Climate of the Last Millennium: the ASTER Project

The LOVECLIM model (Driesschaert et al., 2007; Goosse et al., 2007) is used to simulate the climate of the last millennium with several 'climate' parameter sets yielding different sensitivities of the climate and the carbon cycle model. The purpose of these simulations is twofold. We intend to assess first the role of the carbon cycle on the climate, and second, the ability of the different selected parameter sets to drive the model within the range of the observed climate, and further to assess the uncertainty related to these parameters. The high frequency variability of the forcings is taken into account. For each set of parameters, LOVECLIM is driven by the natural evolution of insolation, solar irradiance and stratospheric aerosol concentrations due to volcanic activity as well as by changes caused by human activities such as deforestation, CO2 emission or concentration changes, changes in concentrations of greenhouse gases other than CO2 (including ozone) and in sulphate aerosol load. Several transient experiments are conducted for each parameter set. A first transient simulation (CONC) is forced with reconstructed atmospheric CO2 concentration. In the next two simulations, the emissions of carbon are taken into account, the model computing the corresponding atmospheric CO2 concentration. First (EMIS), the emissions due both to the land use changes and the fossil fuel burning are provided. Second (EFOR), only the emissions from fossil fuel burning are provided in addition to the vegetation change related to deforestation. The Northern Hemisphere annual mean temperatures simulated by the model according to the different parameter sets and carbon cycle sensitivities do not show striking differences. The general pattern shows slightly warmer conditions in the early part of the simulation and cooler ones during the Little Ice Age. At last, the global warming of the last century is also clearly simulated. The response of the carbon cycle to the evolution of forcings over the last millennium does not differ much among experiments although there is a much larger spread when considering different emission scenarios (e.g. EFOR and EMIS). Sensitivity tests to the amplitude of the variation of the total solar irradiance (TSI) are performed; a very first quick look at the simulations does not show significant differences in the pattern of the simulated climate in response to modification in the TSI amplitude. Further analysis must be conducted. Climate response to different schemes of deforestation will also be presented. Driesschaert E., Fichefet T., Goosse H., Huybrechts P., Janssens I., Mouchet A., Munhoven G., Brovkin V., and Weber S. L., 2007. Modelling the influence of Greenland ice sheet melting on the Atlantic meridional overturning circulation during the next millennia. Geophys. Res. Lett., 34:L1070, 2007. Goosse H., Driesschaert E., Fichefet T., and Loutre M.F., 2007. Information on the early Holocene climate constrains the summer sea ice projections for the 21st century Clim. Past 3, 683-692.