VOLCANIC CLASSIC: VOLCANIC eruptions and CLimAte response - Stratospheric Sulfate isotopes in Ice Cores, data assimilation, and climate sensitivity

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How does Earth's climate respond to perturbation? Despite decades of work, this fundamental question remains difficult to answer, with a wide range of climate sensitivities to external forcing persisting in state-of-the-art models. To improve understanding of the climate system and narrow the range of uncertainties in future climate projections, innovative new approaches are required. Here I propose a novel strategy that will provide unique new tests of climate models and emergent constraints on climate sensitivity, by harnessing the record of major volcanic eruptions.Volcanic eruptions exert an enormous influence on climate, as their sulfate aerosols reflect incoming sunlight, driving abrupt cooling on timescales of 1-5 years. However our ability to read this record is currently limited by uncertainties in volcanic forcing (the impact of eruptions on incoming radiation - primarily a function of stratospheric sulfate) and global climate response. By using cutting edge new technology to measure sulfur isotopes in ice cores, I will uniquely constrain stratospheric sulfate and eruption latitude and season, transforming knowledge of past volcanic forcing. I will compare this to new and improved reconstructions of global climate response, achieved by incorporating new, globally-distributed paleoclimate records into model-data assimilation products. By examining the climate response to each of the 233 major eruptions of the last 2000 years, I will provide robust observational constraints on volcanic climate sensitivity. I will use these to test sensitivity and feedbacks in state-of-the-art climate models. These tests will inform both understanding of the wide range of sensitivity to aerosols in current models and the debate on controversial geoengineering schemes. As model response to volcanoes and CO2 are linked, this work will ultimately be used to refine the range of sensitivity to CO2 rise and improve projections of future climate.