THE CONTRIBUTION OF EXPLOSIVE VOLCANISM TO GLOBAL ATMOSPHERIC SULFUR-DIOXIDE CONCENTRATIONS

SULPHUR dioxide from volcanic eruptions may have a significant effect on the Earth's climate and atmospheric chemistry, and it is therefore important to quantify outgassing rates for all types of volcanic activ ity. Non-explosive volcanoes (for example, Mount Etna) outgas at relat ively constant rates, providing an annual flux of about 9 million tons (Mt) SO2 (ref. 1). By contrast, the outgassing from volcanoes prone t o explosive eruptions (such as Mount Pinatubo) is sporadic and much mo re difficult to quantify. The total annual volcanic SO2 flux is theref ore poorly constrained, with ground-based estimates1-8 ranging from 1. 5 to 50 Mt-up to one-quarter of the estimated current anthropogenic co ntribution. The Total Ozone Mapping Spectrometer aboard the NASA satel lite Nimbus 7 recorded SO2 emissions from explosive eruptions from Nov ember 1978 to May 1993. We use these data to show that the annual flux from explosive volcanism is of the order of 4 Mt SO2, less than half of the non-explosive output. Thus it seems that the total volcanic emi ssion of SO2 to the Earth's atmosphere is about 13 Mt yr-1, which is o nly 5-10% of the current anthropogenic flux.