THE ROLE OF MICROPHYSICAL AND CHEMICAL PROCESSES IN PROLONGING THE CLIMATE FORCING OF THE TOBA ERUPTION

The mega-eruption of Toba, Sumatra, occurred around 73 Ka ago, during the onset of a glaciation of the Late Quaternary. This coincidence com bined with the unprecedented amount of sulphur released by this volcan o has led to the hypothesis that Toba sulphate aerosols caused a trans ient surface cooling which may have contributed to a shift of the clim ate system. Because of tile self limiting effect of gravitational sedi mentation, the climatic impact of extremely large sulphur injections i nto the stratosphere are thought to be rather limited. Here we present model calculations combining microphysical and chemical feedbacks whi ch show that the eruption could instead have led to the formation of a long-lasting volcanic aerosol layer. Although the concentrations of r adiatively active species such as O-3 or SO2 could also have been cons iderably perturbed, the resulting forcings should have only slightly m oderated the aerosol cooling effect during the first few years followi ng tile eruption. According to our results, extremely high stratospher ic sulphur loading could lead to a more prolonged effect on the climat e than previously assumed.