THE eruption of Toba in Sumatra 73,500 years ago was the largest known explosive volcanic event in the late Quaternary1. It could have lofted about 1015 g each of fine ash and sulphur gases to heights of 27-37 km, creating dense stratospheric dust and aerosol clouds. Here we present model calculations that investigate the possible climatic effects of the volcanic cloud. The increase in atmospheric opacity might have produced a 'volcanic winter'2 - a brief, pronounced regional and perhaps hemispheric cooling caused by the volcanic dust-followed by a few years with maximum estimated annual hemispheric surface-temperature decreases of 3-5°C. The eruption occurred during the stage 5a-4 transition of the oxygen isotope record, a time of rapid ice growth and falling sea level3. We suggest that the Toba eruption may have greatly accelerated the shift to glacial conditions that was already underway, by inducing perennial snow cover and increased sea-ice extent at sensitive northern latitudes. As the onset of climate change may have helped to trigger the eruption itself4, we propose that the Toba event may exemplify a more general climate-volcano feedback mechanism.
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