Imagine it’s 245 BCE. You’re Ptolemy III, a Macedonian king ruling Egypt, and you’re leading an army against your biggest rival, the Seleucid Empire. But there’s a revolt back home, and you must return to the people that your dynasty would rule from 305 BCE to 30 CE.
Sometime in the 3rd century CE, the Roman historian Justin would write that if you had “not been recalled to Egypt by disturbances at home, [you] would have made [yourself] master of all Seleucus’s dominions.” Other writings declare that you did what you had to do to relieve a famine in Egypt. The summer was unusually dry that year, wrote a priest, and the Nile didn’t flood like it usually did. You imported grain from rainier territories—gaining you a priestly commendation in 238 BCE about the sacrifices you made for the “salvation for the population.”
Fast-forward to the present day: A team of 21st century CE scientists and historians is offering a new look at something that might have indirectly caused the upheaval that changed Ptolemy III’s fortunes as well other unrest that beset the Ptolemaic empire—large volcanic eruptions.
In a new paper in Nature Communications, the researchers present a raft of evidence—from climate modeling and ice core and Nile River hydrological records to ancient Egyptian chronicles—indicating that a series of eruptions may have caused sharp drop-offs in the summer rainfall usually brought by the African monsoon.
The loss of monsoon precipitation would have depleted the headwaters of the Nile River and deprived Egyptian civilization of the annual Nile flooding that it depended on to sustain its agriculture. As food became scarce, insurrections may have followed, including one known as the Theban revolt, which rose against the ruling Macedonian Ptolemies starting in 207 BCE.
“The Ptolemies lost control over huge areas of Egypt for almost 2 decades” during the Theban revolt, said Francis Ludlow, a coauthor of the study and a climate historian at Trinity College in Dublin, Ireland. It “was a very destructive revolt, with damage to temples and huge losses of tax revenues for the state.”
Volcanoes and the Nile
Researchers know well the climatic effects of volcanic eruptions. Major eruptions spew sulfur dioxide, among other substances, into the atmosphere, which reacts to form aerosol particles. These particles reflect sunlight back into space, cooling the atmosphere.
Generally, this cooling decreases evaporation, and less evaporation means less water in the atmosphere for rain. The summer floods along the 6,800-kilometer-long Nile got their water mainly from rainfall in the Ethiopian highlands, and that rainfall comes from the African monsoon, which is driven by summer heating of the atmosphere. A cooler atmosphere supplies less water for the monsoon and thus less water for the Nile.
This effect has been documented. Texts from 1788, for example, describe a low-flowing Nile in the summers of 1783 and 1784—during and after Iceland’s Laki eruption—which caused a food shortage that killed many people. Recently, other scientists have used models to show how the Laki eruption and its 80 megatons of sulfur aerosols could have indirectly wiped out those monsoons so integral to the Egyptian population’s well-being.
Some possible factors didn’t make it into this research, however, according to Kevin Anchukaitis, a paleoclimatologist at the University of Arizona in Tucson. He told the New York Times that the study didn’t account for weather influences on the Nile like El Niño.
To investigate a possible connection between volcanic eruptions and a Nile flood failure, the researchers on the new paper turned to one of the longest hydrological records on Earth: the Islamic Nilometer, an ancient structure built in 622 CE that people used to track water levels on the Nile. The researchers studied eruptions that occurred from 622 CE to 1902 CE and compared the dates of those eruptions—like Laki in 1783 and Tambora in 1815—with dates of Nile failures recorded by operators of the Nilometer. In summers during eruption years, the researchers found, floodwaters averaged 22 centimeters lower than in noneruption years.
Although these eruptions occurred long after the end of the Egyptian empire, the researchers assume that they would have affected the African monsoon similarly. “It would be surprising if the climate system operated completely differently in the Ptolemaic era,” Ludlow said.
The researchers then looked back in time with ice cores from Greenland and Antarctica to find evidence, like the presence of sulfur, that would point to volcanic eruptions. They then counted the layers in the ice cores like tree rings to date those eruptions and compared those dates with dates of major societal challenges like famine, disease, and land abandonment.
It turns out there was a major eruption in 209 BCE, just 2 years before the Theban revolt began. Other major eruptions occurred in 247 and 244 BCE, just as Ptolemy III clashed with Seleucus II in the east. “[We observed] in historical records a dynamic societal response” to the climate changes, said Joseph Manning, a historian at Yale and lead author of the new paper.
The mix of different records “is an important part of putting together the effects of the eruptions,” Heli Huhtamaa, a climate historian at Utrecht University in the Netherlands, told Science. She said that she found the team’s analysis very convincing.
Climate and Society
The researchers stress that their claim is not that volcanic eruptions caused social unrest or that regional climate change alone brought down an empire.
“Environmental pressures don’t act in a vacuum,” Ludlow said. More likely, “pressures from poor flooding coalesced at certain points of time with political and economic factors” like tensions between warring regions and high state-levied taxes. Food shortages resulting from lack of Nile flooding could have added the heat needed to boil an already simmering pot of unrest.
—JoAnna Wendel (@JoAnnaScience), Staff Writer