National Aeronautics and Space Administration

Abstract. 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 activity. Non-explosive volcanoes (for example, Mount Etna) outgas at relatively constant rates, providing an annual flux of about 9 million tons (Mt) SO₂ (ref. 1). By contrast, the outgassing from volcanoes prone to explosive eruptions (such as Mount Pinatubo) is sporadic and much more difficult to quantify. The total annual volcanic SO₂ flux is therefore poorly constrained, with ground-based estimates ranging from 1.5 to 50 Mt - up to one quarter of the estimated current anthropogenic contributions. The Total Ozone Mapping Spectrometer aboard the NASA satellite Nimbus 7 recorded SO₂ emissions from explosive eruptions from November 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 emission of SO₂ to the Earth's atmosphere is about 13 Mt yr^-1, which is only 5-10% of the current anthropogenic flux.