A Theory of the Linkage between Volcanoes, Climate Cooling, Malnutrition and Epidemics: Case Study of Justinianic Plague

Abstract. Major volcanic eruptions have long been proposed to have triggered and amplified historical plague pandemics through climatic shocks and harvest failures, but the mechanisms linking volcanic forcing, nutrition and disease remain poorly quantified. We develop a theoretical model of a single region with zero spatial dimensions ("0-D") that couples volcanic forcing, climate, agricultural productivity, food storage, malnutrition and plague epidemiology. The epidemiological component includes reservoir hosts, vectors, human-to-human transmission, disease-driven migration and waning immunity, with transmission and mortality rates dependent on nutritional status.

We first evaluate the model against agricultural and archaeological evidence from England following the 1345 CE eruption, shortly before the Second Plague Epidemic. The simulations reproduce the observed sudden drop in crop output and prolonged deterioration in nutritional conditions, as well as the observed relationship between plague mortality and an isotopic proxy for malnutrition derived from human skeletons. We then apply the model to the Justinianic Plague following the 536 and 540 CE eruptions. Representing three population groups with different vulnerability to harvest failure and malnutrition, the model predicts eventual plague mortality of about 39 %, consistent with historical estimates. Sensitivity experiments show that removing the effect of volcanic forcing on malnutrition substantially reduces predicted plague mortality, whereas excluding disease-driven migration increases it. Doubling eruption intensity produces only a modest increase in total mortality, revealing nonlinear interactions among climate, nutrition and epidemic dynamics.

A reduced phase-space analysis further shows that epidemic feedbacks change from net positive to net negative as susceptible individuals are depleted, with the system transitioning from an initially unstable state towards a quasi-equilibrium attractor. Together, the simulations suggest that volcanic eruptions could have substantially amplified historical plague epidemics through their effects on failure of harvests and food production, malnutrition and human behaviour. The results provide a mechanistic framework linking volcanic climate forcing to epidemic severity and highlight nutrition as a potentially important mediator of climate–disease interactions.