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The most significant global climatic effects of volcanic eruptions are related to the stratospheric injection of sulfuric-acid aerosols. In addition, large halogen emissions during some explosive eruptions may affect atmospheric chemistry and lead to ozone depletion, but the proportion of halogen adsorbed on tephra and incorporated in proximal fallout versus halogen transported to the stratosphere is unknown. The role of volcanic ash in modification of the Earth's thermal radiation budget is short lived and minor, because of particle aggregation and consequently short atmospheric residence time. Tephra transport in the atmosphere is thus less than 3,000 km for 100-μm silicate particles, but has varied markedly in geologic time, presumably reflecting variations in the vigor of atmospheric circulation. Thus, Cenozoic fluctuations in the frequency of volcanic ash layers in deep-sea sediments are likely to be a transport function and unrelated to source variations. Source parameters required to evaluate global environmental impact of volcanic eruptions include mass-eruption rate, column height, and sulfur and halogen yield to the atmosphere. Based on petrologic studies, the yield of sulfur and halogens is strongly dependent on magma type, reflecting volatile solubility in magmas. Flood-basalt eruptions have the highest potential yield, but their potential climatic impact is diminished by a low column height and therefore dominantly tropospheric residence of the emitted volcanic aerosol. A correlation between sulfur-mass yield and observed surface-temperature decrease following several historic eruptions established the importance of sulfuric-acid aerosol as the climate-modifying volcanic effect. The observed relation suggests that a 1012 kg sulfuric acid aerosol could bring about 3° to 4°C surface cooling and a major global climatic event. This is one order of magnitude larger mass than was produced during the 1815 Tambora and 1783 Laki events, and no Holocene eruptions are known of this magnitude. The only Quaternary event of this magnitude is the 75-ka Toba eruption, whose climatic effects are masked by the glacial stage.

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