Study reshapes understanding of climate change’s impact on early societies

October 17, 2017
A new study linking paleoclimatology — the reconstruction of past global climates — with historical analysis by researchers at Yale and other institutions shows a link between environmental stress and its impact on the economy, political stability, and war-fighting capacity of ancient Egypt.
 
The team of researchers examined the hydroclimatic and societal impacts in Egypt of a sequence of tropical and high-latitude volcanic eruptions spanning the past 2,500 years, as known from modern ice-core records. The team focused on the Ptolemaic dynasty of ancient Egypt (305-30 B.C.E.) — a state formed in the aftermath of the campaigns of Alexander the Great, and famed for rulers such as Cleopatra — as well as material and cultural achievements including the great Library and Lighthouse of Alexandria.
 
Using an interdisciplinary approach that combined evidence from climate modelling of large 20th-century eruptions, annual measurements of Nile summer flood heights from the Islamic Nilometer — the longest-known human record of environmental variability — between 622 and 1902, as well as descriptions of Nile flood quality in ancient papyri and inscriptions from the Ptolemaic era, the authors show how large volcanic eruptions impacted on Nile river flow, reducing the height of the agriculturally-critical summer flood.
 
The findings, published in the journal Nature Communications, show that integrating evidence from historical writings with paleoclimate data can advance both our understanding of how the climate system functions, and how climatic changes impacted past human societies.
 
“Ancient Egyptians depended almost exclusively on Nile summer flooding brought by the summer monsoon in east Africa to grow their crops. In years influenced by volcanic eruptions, Nile flooding was generally diminished, leading to social stress that could trigger unrest and have other political and economic consequences,” says Joseph Manning, lead author on the paper and the William K. & Marilyn Milton Simpson Professor of History and Classics at Yale.
 
The reason for reduced flooding of the Nile is because volcanic eruptions can disrupt the climate by injecting sulfurous gases into the stratosphere, says Francis Ludlow, the study’s corresponding author. Ludlow is a climate historian who began collaborating with Manning as a postdoctoral fellow at Yale, and is now based in history in Trinity College, Dublin. These gases react to form aerosols that remain in the atmosphere in decreasing concentrations for one or two years, reflecting incoming solar radiation back to space. These volcanic aerosols can influence global hydroclimate. The reduction in surface temperatures can lead to reduced evaporation over waterbodies, and hence lessen rainfall. If the aerosols are dispersed primarily in the Northern Hemisphere, the greater cooling in this hemisphere can also diminish the summertime heating that drives the northward migration of monsoon winds over Africa up to the Ethiopian highlands where the Blue Nile is supplied with its summer floodwaters.
 
Read the full story at Yale News and read the article at Nature.com.