The rule of Egypt's Ptolemaic Pharaohs ran from 305 to 30 BCE, bookended by the conquests of Alexander the Great at its rise and the suicide of Cleopatra at its downfall. The larger-than-life characters produced by this Hellenistic dynasty, combined with the period's spectacular cultural achievements—including the flourishing of the Library of Alexandria—have distinguished the Ptolemaic Kingdom as one of the most consequential reigns in history.
The world of the Ptolemies was not only shaped by political intrigues and intellectual enrichment, but by volcano-induced climate changes, according to a paper published Tuesday in Nature Communications. Led by Yale historian Joseph Manning, the research charts out the intricate relationship between volcanic eruptions and the summer monsoon that drives the annual flooding of the Nile river, an event upon which Egyptian agricultural production was totally dependent in ancient times (and remains so to this day).
When these environmental factors are overlaid on the social history of the period, a pattern emerges between volcanic eruptions and bouts of social unrest in the Ptolemaic Kingdom, suggesting that climate shocks may have had a role in triggering periods of upheaval. These expansive studies combining historical and scientific information are shedding new light on the mysteries of the past, but they are also a useful guide to preparing for climate-related challenges of the future, especially in the wake of increasingly intense natural disasters.
To reach its findings, Manning's team, which included historical climatologist Francis Ludlow of Trinity College Dublin, combined scientific datasets like ice core records and climate models with ancient Egyptian writings, including priestly decrees and land sale records, to paint an interdisciplinary portrait of the era.
"There are not that many climate historians, or historical climatologists, that fully specialize in trying to act as a bridge between historians on the one hand and climatologists or paleoclimatologists on the other hand, to help them communicate with each other," Ludlow told me over Skype. "They have so much to offer each other," he said, but the academic language and methodologies are so different that that it requires specialists to effectively connect the fields.
In this instance, an integrated approach of social history and climate modeling revealed that volcanic eruptions as distant from Egypt as Alaska or Iceland can suppress the flooding of the Nile. Volcanoes have this impact because they spew aerosol-rich ashes into the sky, which reflect sunlight back into space, thus cooling the planet slightly. This has climatic ripple effects across the globe, and is known to disrupt monsoons, including the one that drives Nile River patterns.
The team studied 20th century volcanic eruptions to gauge recent climatic impacts on the Nile, but also relied heavily on the Islamic Nilometer, which is the longest-known record of environmental variability. Compiled from numerous columned instruments that note the flood watermarks of the river (the most famous being the Roda Island Nilometer in Cairo), the surviving record runs from 622 to 1902, though these measurements would have been made for centuries before the 7th century start date.
"I often hear the argument that in the past, people were not really conscious of longer-term environmental or climatic changes happening on anything greater than a human lifespan," Ludlow, who has previously integrated Irish historical documents with evidence of extreme weather patterns, told me.
"But surely, an institution like this would have been well aware," he continued. "Even if they weren't able to visualize it with fancy graphics, they'd be reading that the average levels of flood were quite different in some periods relative to others. I think it's an early reflection of a quite long-term environmental awareness."
Though the Ptolemaic Kingdom was long gone by 622, the Nilometer outlines the general pattern of volcanoes suppressing Nile flooding, over the timescale of a full millennium. "It really does capture, across multiple centuries, the fingerprint of these volcanic eruptions in terms of what they do to the Nile," Ludlow told me. "The Nilometer is a real rarity, and it's amazing that it's been overlooked to a degree."
Historical documents produced during the Ptolemaic reign hint at the civil distress that might have been caused, or exacerbated, by volcano-induced flood suppression. Flood deficits influenced by major eruptions in 247 and 244 BCE contributed to Ptolemy III's decision to curb his ambitions of military conquest in order to buy grain to compensate for his own empire's crop failures, and stave off a revolt stoked by famine.
Perhaps most dramatically, the reign of the charismatic leader Cleopatra from 51 to 30 BCE as the last of the Ptolemies, was rocked by "famine, plague, inflation, administrative corruption, rural depopulation, migration, and land abandonment," according to the paper. Some of this unrest was likely intensified by major eruptions in 46 and 44 BCE, which means those natural disasters may have directly contributed to the fall of the empire, though the authors warn against leaning on "environmental determinism" too heavily.
"A lot of people have clearly been reading the story we're presenting in the paper as the story of how volcanic eruptions caused the collapse of the [Ptolemaic] kingdom, which is not exactly what we're going for," Ludlow said, though he noted that "it's hard to avoid the suspicion that the kingdom collapses in 30 BCE, and in the previous decade, there's the third largest eruption in the last two-and-a-half thousand years."
However, there were numerous other contributing factors to Cleopatra's downfall, including ethnic divisions and her ill-fated partnerships with Roman leaders. Ludlow notes that she was relatively responsive to victims of the disasters during her reign.
"She did manage to avert revolt against her rule despite all of the stresses that occurred following the Nile failures and eruptions, in contrast to many of her male predecessors as Pharaohs," Ludlow said. "Cleopatra seems to have been much better at disaster management than some of our modern politicians."
Speaking of modern times, Ludlow said that we are currently in a "volcanically quiescent" period, meaning that we haven't suffered the same frequency of climate-shifting eruptions that helped send the Ptolemaic Kingdom reeling toward doom. That doesn't mean we should relax and enjoy the silence while it lasts, but rather learn to anticipate the best responses to these inevitable future eruptions, to prevent the kind of social strife that has brought down past empires.
"It's nice that you can be a historian and to work collaboratively on something that has such contemporary relevance at the moment," Ludlow said.
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