Investigating hydroclimatic impacts of the 168–158 BCE volcanic quartet and their relevance to the Nile River basin and Egyptian history

Abstract. The Ptolemaic era (305–30 BCE) represents an important period of Ancient Egyptian history known for its major material and scientific advances, but also ongoing episodes of political and social unrest in the form of (sometimes widespread) revolts against the Ptolemaic elites. While the role of environmental pressures has long been overlooked in this period of Egyptian history, ice-core-based volcanic histories have identified the period as experiencing multiple notable eruptions, and a repeated temporal association between explosive volcanism and revolt has recently been noted. Here we analyze the global and regional (Nile River Basin) climate response to a unique historical case of 4 consecutive and closely timed eruptions (first a tropical one, closely followed by 3 extratropical northern hemispheric events) between 168 and 158 BCE, a particularly troubled period in Ptolemaic history for which we now provide a more detailed hydroclimatic context. The NASA GISS ModelE Earth system model simulates a strong radiative response with a radiative forcing (Top of atmosphere) of -7.5 W/m2 (following the first eruption) and -4.0 w/m2 (after each of the 3 remaining eruptions) at a global scale. Associated with this, we observe a global cooling of the order of 1.5°C at the surface following the first (tropical) eruption, with the following three extratropical eruptions extending the cooling period for more than 15 years. Consequently, this series of eruptions constrained the northward migration of the inter-tropical convergence zone (ITCZ) during the northern hemisphere summer monsoon season, and major monsoon zones (African, South and East Asian) experienced suppression of rainfall >1 mm/day during the monsoon (JJAS) season averaged for 2 years after each eruption. A substantial suppression of north African and Indian summer monsoon over the Nile River headwater region vigorously affects the river flow in the catchment and river discharge. River mass flow consecutively decreases by up to more than 30 % relative to an unperturbed from volcanoes annual mean flow for 2 years after the tropical eruption. A moderate decrease of up to 15–20 % is produced after each of the remaining eruptions. These results show that the first eruption produces a strong hydroclimate response, and the following 3 eruptions prolonged the drying conditions. These results also support the contention that the observed association between ice-core-based signals of explosive volcanism and the hydroclimatic impact of these eruptions during the Ptolemy era, including the suppression of the critical for agriculture Nile summer flooding.