Fire, brimstone and cooling

Large volcanic eruptions contribute significantly to global cooling

Published: Saturday 04 July 2015

volcanoes eject large amounts of sulphur into the atmosphere when they erupt, and actually contribute significantly to global cooling. The eruption of Mount Pinatubo, Philippines, on June 15, 1991, discharged huge amounts of ash and other debris into the atmosphere, creating the largest stratospheric cloud of sulphur dioxide ( so 2 ) ever measured -- at least 17 megatonnes of the gas was released. According to a study, this had a significant effect on the global climate and ozone layer ( Nature , Vol 389, No 6651).

The potential influence of volcanic eruptions on the Earth's climate has long been recognised, but quantifying the effects has not been easy. Compared to factors such as dust, which permits cooling by blocking solar radiation, sulphur emissions produce sulphuric acid 'aerosols'. These stay longer in the atmosphere than dust. Studies of polar ice cores show that these aerosols are dispersed globally. The implication is that the effects of volcanic activity on climate are mainly controlled by the amount of sulphur emitted, and not simply related to the size of the eruption or the amount of volcanic dust produced.

Though global estimates suggest that they are much smaller than human-induced emissions, large explosive volcanic eruptions can inject sulphur directly into the atmosphere, whereas anthropogenic sulphur is mainly limited to low altitudes and is rapidly removed by precipitation.

Estimation of the amount of sulphur injected into the atmosphere during eruptions relies on studies of debris or, in case of recent events, on remote-sensing measurements. These measurements make use of satellite-based spectroscopy designed to map atmospheric ozone, and can provide reasonable estimates. Studies of erupted materials involve analysis of melt inclusions -- elements trapped within crystals of the erupted material. This provides a measure of the sulphur content of the magma before eruption -- the crystals act like sealed containers for samples of high-pressure melts. Comparison of the sulphur content of the inclusions with that of the surrounding degassed matrix glass provides a measure of how much sulphur was lost during an eruption. The difference for the total volume of material erupted gives an estimate of the sulphur injected into the atmosphere.

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