Correlations between eruption magnitude, SO2 yield, and surface cooling
Sulphurous gases from explosive eruptions have the potential to form stratospheric aerosols and so produce surface cooling on a hemispheric to global scale. However, testing for any correlation between SO2 yield and surface cooling is hampered by instrumental SO2 and temperature measurements being available for time periods that include only a few large eruptions. To overcome this, published dendroclimatological data, satellite (Total Ozone Mapping Spectrometer) data on SO2 emissions, stratospheric optical depth data, and volcanological observations are integrated, revealing several relevant new correlations. First, the efficient conversion of SO2 into stratospheric aerosols occurs when the ratio of plume height to tropopause height is greater than about 1.5. Second, the mass of emitted SO2 correlates well with the mass of erupted magma. The SO2 yield is 0.1 to 1% by mass of magma, irrespective of composition. The best-fit power law (r2=0.67) is mass of SO2 in Mt=1.77(mass of magma in Gt)0-64. Third, of the eruption clouds that are believed to have entered the stratosphere in the period 1400-1994, those with masses <5 Gt magma (DRE <2 km3) appear to have had insignificant effects on Northern Hemisphere summer temperature. The scattered data for eruptions of >10 Gt (>4 km3) magma suggest a mean cooling effect of about 0.35 °C.
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Humans have long marvelled at (and feared) the odorous and colourful manifestations of volcanic emissions, and, in some cases, have harnessed them for their economic value. The degassing process responsible for these phenomena is now understood to be one of the key factors influencing the timing and nature of volcanic eruptions. Moreover the surface emissions of these volatiles can have profound effects on the atmospheric and terrestrial environment, and climate. Even more fundamental are the relationships between the history of planetary outgassing, differentiation of the Earth’s interior, chemistry of the atmosphere and hydrosphere, and the origin and evolution of life. This book provides a compilation of 23 papers that investigate the behaviour of volatiles in magma, the feedbacks between degassing and magma dynamics, and the composition, flux, and environmental, atmospheric and climatic impacts of volcanic gas emissions.