Reconstructing Stratospheric Aerosol Loadings from Extratropical Eruptions

Abstract

Extratropical eruptions have been associated with major cooling in the Northern Hemisphere over the last 2000 years (Toohey et al., 2019; Burke et al., 2023).  However, our ability to reconstruct the stratospheric aerosol loading from these events is limited by uncertainties in the conversion of sulfate flux to the polar ice sheets into a stratospheric sulfur loading or aerosol optical depth. Current methods (e.g. Gao et al. 2007; Toohey and Sigl, 2017) assume that all of the ice core sulfate from these eruptions is deposited via the stratospheric overworld, but sulfur isotope evidence shows that a substantial proportion of ice core sulfate comes via lower altitudes. Here we evaluate the magnitude of this complication across many of the major extratropical eruptions over the last several thousand years, including eruptions from the Katmai/Novarupta, Aniakchak, and Okmok volcanoes, and investigate the implications of these findings for stratospheric aerosol forcing records currently used in climate models.     Burke, A., Innes, H.M., Crick, L., Anchukaitis, K.J., Byrne, M.P., Hutchison, W., McConnell, J.R., Moore, K.A., Rae, J.W., Sigl, M. and Wilson, R., 2023. High sensitivity of summer temperatures to stratospheric sulfur loading from volcanoes in the Northern Hemisphere. Proceedings of the National Academy of Sciences, 120(47), p.e2221810120. Gao, C., Oman, L., Robock, A. and Stenchikov, G.L., 2007. Atmospheric volcanic loading derived from bipolar ice cores: Accounting for the spatial distribution of volcanic deposition. Journal of Geophysical Research: Atmospheres, 112(D9). Toohey, M. and Sigl, M., 2017. Volcanic stratospheric sulfur injections and aerosol optical depth from 500 BCE to 1900 CE. Earth System Science Data, 9(2), pp.809-831. Toohey, M., Krüger, K., Schmidt, H., Timmreck, C., Sigl, M., Stoffel, M. and Wilson, R., 2019. Disproportionately strong climate forcing from extratropical explosive volcanic eruptions. Nature Geoscience, 12(2), pp.100-107.