Uncertainties in the atmospheric loading to ice-sheet deposition for volcanic aerosols and implications for forcing reconstruction

Volcanic radiative forcing reconstruction is an important part of paleoclimate simulation and attribution efforts, and the conversion factor used to transfer ice core-based sulfate observation into stratospheric volcanic aerosol loading (LTD factor) is critical for such reconstruction. A Pinatubo-based LTD combing observations of nuclear tomb test debris in Greenland and volcanic sulfate aerosols in Antarctic was derived and widely applied in the CMIP5 and CMIP6 simulations. This study revisits the LTD factor, by using 58 polar ice core records of volcanic depositions and a Monte Carlo sampling model. A set of Tambora-based LTDs with associated uncertainties are obtained, which corrects the bias of over-representing the west Antarctic. New LTDs for Pinatubo and Agung are calculated using 18 and 24 Antarctic ice core observations, respectively, and the uncertainties are evaluated against the Monte Carlo characterization with varying ice core numbers. The comparison of Southern Hemispheric LTD among Tambora, Pinatubo and Agung suggests that, the conversion factor may vary significantly among different eruptions. Even larger uncertainty is revealed when compare the ice-core-based conversion factor with the model results. Both results suggest systematic and stochastic causes that are difficult to anticipate, and call for precaution when single conversion factor is used for reconstruction.