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While explosive volcanic eruptions cause ozone loss in the current atmosphere due to an enhancement in the availability of reactive chlorine following the stratospheric injection of sulfur, future eruptions are expected to increase total column ozone as halogen loading approaches preindustrial levels. The timing of this shift in the impact of major volcanic eruptions on the thickness of the ozone layer is poorly known. Modeling four possible climate futures, we show that scenarios with the smallest increase in greenhouse gas concentrations lead to the greatest risk to ozone from heterogeneous chemical processing following future eruptions. We also show that the presence in the stratosphere of bromine from natural, very short-lived biogenic compounds is critically important for determining whether future eruptions will lead to ozone depletion. If volcanic eruptions inject hydrogen halides into the stratosphere, an effect not considered in current ozone assessments, potentially profound reductions in column ozone would result.

Plain Language Summary The ozone layer is essential for the existence of life on Earth. Due to the human release of chlorine-containing chemicals such as chlorofluorocarbons into the atmosphere in the twentieth century, a large volcanic eruption occurring today would initiate chemical reactions that reduce the thickness of the ozone layer. In the future, when atmospheric levels of chlorine have fallen, large volcanic eruptions are instead expected to increase the thickness of the ozone layer, but important details relevant to this shift in volcanic impact are poorly known. We use a computer model to simulate a large volcanic eruption in four different climate change futures, finding that optimistic scenarios with lower greenhouse gas (GHG) emissions result in greater potential losses of ozone following an explosive volcanic eruption than scenarios with more GHG emissions. In the coming decades, the stratospheric presence of bromine supplied by natural, very short-lived compounds makes the ozone layer more susceptible to loss following volcanic eruptions than if this halogen source were not present. If HCl, a chlorine-containing compound often present in large quantities in volcanoes, were to reach the stratosphere following a future explosive eruption, substantial ozone loss could result, regardless of the year in which the eruption occurred.