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Abstract
Supervolcano eruptions have occurred throughout Earth’s history and have major environmental impacts. These impacts are mostly associated with the attenuation of visible sunlight by stratospheric sulfate aerosols, which causes cooling and deceleration of the water cycle. Supereruptions have been assumed to cause so-called volcanic winters that act as primary evolutionary factors through ecosystem disruption and famine, however, winter conditions alone may not be sufficient to cause such disruption. Here we use Earth system model simulations to show that stratospheric sulfur emissions from the Toba supereruption 74,000 years ago caused severe stratospheric ozone loss through a radiation attenuation mechanism that only moderately depends on the emission magnitude. The Toba plume strongly inhibited oxygen photolysis, suppressing ozone formation in the tropics, where exceptionally depleted ozone conditions persisted for over a year. This effect, when combined with volcanic winter in the extra-tropics, can account for the impacts of supereruptions on ecosystems and humanity.
Highlights
Supervolcano eruptions have occurred throughout Earth’s history and have major environmental impacts
In 1991 the Mount Pinatubo eruption in the Philippines released nearly 20 Mt of sulfur dioxide (SO2) into the stratosphere which oxidized into sulfate aerosols, which caused a global cooling of the oceans of about 0.3 °C and prolonged an El Nino event[1,2,3,4]
We show that the Toba volcanic plume caused massive ozone (O3) loss and produced hostile UV radiation conditions at the Earth’s surface, in the tropics
Summary
In view of the fundamental life-protecting role of stratospheric ozone on our planet, several environmental disaster options and catastrophic ozone loss mechanisms have been considered. Conditions 6–12 months after the eruption roughly represent the apogee of the Toba environmental impacts, i.e., in terms of UV exposure and cooling at the surface At this stage of the volcanic plume evolution the SO2 was mostly converted into sulfate, and the aerosol optical depth at middle latitudes reached a maximum of >5 (Fig. 2, Fig. 3 and “Methods”). Afterward, during the peak AOD stage of the volcanic plume evolution, SO2 was converted into the less efficiently UV-absorbing sulfate aerosols, leading to a partial offset of the ozone depletion and the associated UVI increase This explains the non-linear behavior as a function of plume size, and the moderate dependence upon the assumed amount of SO2 emitted by the Toba eruption. The tropics were most strongly affected by highly detrimental UV levels, in part related to the low solar zenith angles, while volcanic winter (cooling and drying), with an intensity much beyond that of Tambora in 1816, caused extreme weather conditions at extra-tropical latitudes
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