Abstract
Volcanic eruptions provide tests of human and natural system sensitivity to abrupt shocks because their repeated occurrence allows the identification of systematic relationships in the presence of random variability. Here we show a suppression of Nile summer flooding via the radiative and dynamical impacts of explosive volcanism on the African monsoon, using climate model output, ice-core-based volcanic forcing data, Nilometer measurements, and ancient Egyptian writings. We then examine the response of Ptolemaic Egypt (305–30 BCE), one of the best-documented ancient superpowers, to volcanically induced Nile suppression. Eruptions are associated with revolt onset against elite rule, and the cessation of Ptolemaic state warfare with their great rival, the Seleukid Empire. Eruptions are also followed by socioeconomic stress with increased hereditary land sales, and the issuance of priestly decrees to reinforce elite authority. Ptolemaic vulnerability to volcanic eruptions offers a caution for all monsoon-dependent agricultural regions, presently including 70% of world population.
Highlights
Volcanic eruptions provide tests of human and natural system sensitivity to abrupt shocks because their repeated occurrence allows the identification of systematic relationships in the presence of random variability
The hypothesis that a decrease in land-ocean temperature contrast will cause monsoon rainfall to weaken has been disproven by the observation that continental monsoon regions are cooler during years of enhanced monsoon precipitation[25], and by the fact that monsoon winds weaken as land-ocean temperature contrast strengthens in projections of next-century warming[26]
Induced drying occurs primarily in the southern Nile watershed, and the precipitation minus surface evaporation (P-E) signal indicates that volcanically induced cooling did not sufficiently inhibit evaporation in the watershed to compensate for decreased rainfall (Fig. 2b)
Summary
Volcanic eruptions provide tests of human and natural system sensitivity to abrupt shocks because their repeated occurrence allows the identification of systematic relationships in the presence of random variability. Ptolemaic Egypt featured one of the largest cities of the Ancient Mediterranean (Alexandria), including the Great Library and Lighthouse, and was a hub for invention, boasting minds such as Euclid and Archimedes Technological advances such as the saqiya[4], a rotary-wheel water-lifting machine documented by the mid-third century BCE, maslin (mixed wheat-barley) cropping, as well as grain storage, acted to mitigate the impacts of the mercurial Nile flood (Supplementary Note 1)[5]. Aerosol cooling of northern high latitudes can force a southward shift of northern hemisphere (NH) summer monsoon precipitation, promoting drought in the northern parts of monsoon regions[16,17,18, 23] These energy-budget arguments provide a more fundamental perspective on the controls on tropical rainfall than arguments based on land-ocean temperature contrast because large-scale tropical circulations are driven by horizontal gradients in the total (sensible plus latent) energy input to the atmosphere[24]. The hypothesis that a decrease in land-ocean temperature contrast will cause monsoon rainfall to weaken has been disproven by the observation that continental monsoon regions are cooler during years of enhanced monsoon precipitation[25], and by the fact that monsoon winds weaken as land-ocean temperature contrast strengthens in projections of next-century warming[26]
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