Last Millennium Volcanic Radiative Forcing and Climate Effects Simulated in an Interactive Aerosol-Climate Model

Last millennium climate variability is dominated by the effects of large-magnitude volcanic eruptions, however a long-standing mismatch exists between model-simulated and tree-ring derived cooling. Accounting for the self-limiting effects of large sulfur dioxide (SO2) injections and the limitations in tree-ring records such as lagged responses due to biological memory can reconcile some of the discrepancy, although differences still exist for the largest tropical eruptions. Although the representation of volcanic forcing in climate models has improved, uncertainties remain. Here, using the UK Earth System Model (UKESM) we have simulated the last millennium using volcanic SO2 emissions, accounting for the evolution of volcanic sulfate aerosol online. Across all eruptions, we find similar Northern Hemisphere (NH) summer cooling compared with PMIP4 models run with the latest prescribed volcanic forcing dataset. However, for the eruptions of 1257 Samalas and 1815 Tambora, our new simulations suggest a much smaller NH cooling that is in better agreement with tree-ring records. For these eruptions our simulated volcanic forcing differs considerably from that in the PMIP4 dataset, with large differences in the hemispheric transport of the sulfate aerosol. Our results therefore suggest that for the largest eruptions, the spatial distribution of aerosol can resolve remaining discrepancies between model-simulated and tree-ring derived cooling.