Et tu Etna: Volcanism, Climate, and Death of Caesar

Explosive volcanic eruptions affect climate: lowering global temperatures, changing rainfall patterns, and diminishing crop yields. These eruptions are important case studies in abrupt climate change and the complex impacts on the Earth system and human society that persist years after an eruption. Our understanding of these feedbacks has been driven by a few known eruptions (e.g. 1991 Pinatubo, 1982 El Chichon, 1815 Tambora, 1783 Laki, and 1883 Krakatau.) However, ice core sulfate records indicate several large volcanic eruptions with uncertain provenance and environmental impacts. One of the largest sulfate anomalies of the past 2,500 years (∼ 40 Tg sulfur, twice the volume erupted in the 1991 Mt. Pinatubo eruption) occurred in 44 BCE. In this study, we propose that an eruption at Mt. Etna, Sicily is consistent with this sulfate signal and the climatic effects documented. Contemporary accounts describe a large volcanic eruption in that year, potentially linked to the assassination of Julius Caesar. We compile a global set of proxy records such as tree rings, speleothems, and isotopic records for this time period to compare with model predictions. We find that multiple records show a synchronous drop in global temperature of ~ 2 ºC with the largest changes in Europe. Additionally, this time period corresponds to some of the largest documented crop failures in Egypt, India, and China. We find that the distribution of the effects is consistent with a large Mt. Etna eruption by modeling the dispersal of ash and aerosols using a suite HySplit aerosol dispersal model for 10 different climatologies to account for climate variability. We suggest that the 44 BCE eruption has been underestimated, and that the extremely high sulfur content of Etna's lavas and its proximity to major agricultural centers may have allowed a vigorous smaller eruption to have outsized climatic and societal impacts.