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Climate chaos in ancient Egypt

Posted September 15, 2015

A study involving Yale historians and climate scientists is re-writing the history of the ancient world, says Joe Manning, William K. and Marilyn M. Simpson Professor of History & Classics.  But more importantly, he says, collaboration with climate scientists is fundamentally changing how history is done, with an impact he likens to discovery of the Rosetta Stone.

Because historians and climatologists have always worked on such different time scales, Manning says, he never expected to collaborate on a study of ancient Egypt with a modeler of clouds like Associate Professor Trude Storelvmo or a monsoon specialist like Associate Professor Bill Boos, both with the Yale Department of Geology and Geophysics. But that changed when Francis Ludlow, a geographer and postdoctoral researcher with the Yale Climate and Energy Institute, suggested an explanation for something that had bothered Manning for years.  Why had life under the Ptolemies, Egypt’s last pharaohs, been so chaotic?  Is there evidence to suggest that interruptions of annual Nile flooding played a role?

Ludlow, co-author of a recent study that resolved a 7-year discrepancy in ice core records from the first millennium, showed Manning how sulfate levels in ice cores recorded some of the largest volcanic eruptions in human history.  About 30 of them, Manning says, aligned “perfectly” with Egypt’s years of greatest hardship, and complemented historical references to failures of Nile flooding that he had collected in a shoebox over his career.

Although the Ptolemies introduced free-threshing wheat, a labor saving grain that should have significantly improved the quality of life, history has not been kind to them. “History had judged the Ptolemies to be bad rulers,” says Manning.  Overall, their three centuries of rule were characterized by social unrest and warfare, culminating with the death of Cleopatra VII and conquest by Rome.  Until now, says Manning, those events were almost wholly explained in terms of muddled politics, nationalism, resentment of Greek rule and Roman military power.

Ludlow’s new chronology flips that.  Understanding that sulfate-spewing eruptions blanketed the globe and disrupted the Indian Ocean monsoon cycle and thus the annual flooding of the Nile, Manning crafted a new story of the Ptolemies.  The massive shift to  free-threshing wheat, whose germ separates more easily from the chaff but is less hearty than traditional Emmer wheat, turns out to have been less a case of beneficial innovation and more of a case of introducing risk to the system (free threshing wheat is more sensitive to cool weather and drought) at a time of inherent political instability and military mobilization.

“Climate alone doesn’t explain everything,” says Manning, mindful that historians who invoke climate have historically been branded “environmental determinists.”  Factors such as property rights, governance, food substitution and irrigation management are no less important, he says, but advances in climate science, such as Ludlow’s new chronology of volcanic eruptions reveals humans battling with environmental events that overwhelmed human institutions.  After decades of steering clear of the question of climate’s role in history, the pendulum swings back.

The scientists benefit too.  Trude Storelvmo studies how sulfate particles and other atmospheric aerosols affect climate by reflecting incoming solar radiation. Together with Boos, who studies relationships between climate change and the Asian monsoon, she is testing to what extent Intergovernmental Panel on Climate Change (IPCC) climate models support the newly discovered relationship.  Determining a link between high latitude volcanic eruptions and cessation of the Nile flood has enormous relevance to current discussions about cooling the earth by modifying the atmosphere, says Storelvmo. The very goal of the type of climate modification that has received the most attention so far is to mimic the effect of large and repeated volcanic eruptions.

“Here’s a natural experiment, a case in history where nature briefly loaded the atmosphere with sulfate,” she says.  “Could it be responsible for the climate shocks that shook the Ptolemies?”

Source: Yale University

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