High-latitude eruptions cast shadow over the African monsoon and the flow of the Nile

Nile River records indicate very low flow following the 1783 - 1784 Laki volcanic eruption, as well as after other high- latitude volcanic eruptions. As shown by climate model simulations of the Laki eruption, significant cooling ( -1 degrees to - 3 degrees C) of the Northern Hemisphere land masses during the boreal summer of 1783 resulted in a strong dynamical effect of weakening the African and Indian monsoon circulations, with precipitation anomalies of - 1 to - 3 mm/ day over the Sahel of Africa, thus producing the low Nile flow. Future high- latitude eruptions would significantly impact the food and water supplies in these areas. Using observations of the flow of the Nile River, this new understanding is used to support a date of 939 for the beginning of the eruption of the Eldgja volcano in Iceland, the largest high- latitude eruption of the past 1500 years.