A Yale University-led study suggests that abrupt shifts in climate caused by massive volcanic eruptions helped to trigger poorly understood revolts — such as the great 20-year Theban revolt — and other political upheaval in Ptolemaic Egypt.
Around 245 BC Ptolemy III, ruler of the Ptolemaic Kingdom in Egypt, made a decision that still puzzles many historians — after pursuing a successful military campaign against the kingdom’s nemesis, the Seleucid Empire, Ptolemy III suddenly decided to return home.
Yale historian Dr. Joseph Manning and co-authors identified a possible reason for Ptolemy III’s trek back to Egypt: volcanoes.
“Massive eruptions can disrupt the normal flow of the Nile River by cooling the planet’s atmosphere. In Ancient Times, that may have led to food shortages and heightened existing tensions in the region,” the researchers explained.
“Our study creates a strong case that sudden shifts in climate can have big impacts on human society,” Dr. Manning said.
“And it’s remarkable for doing so by drawing on a wide range of methods and evidence — from ice core records to Egyptian papyri.”
At the heart of that dynamic society was the Nile River, the lifeblood of the Ptolemaic Kingdom.
This empire arose in about 305 BC, not long after the death of Alexander the Great, and ended around 30 BC with the death of Cleopatra.
During this period, Egyptian farmers depended on the yearly flooding of the Nile in July through September to irrigate their grain fields — inventing systems of channels and dams to store the river’s overflow.
“When the Nile flood was good, the Nile valley was one of the most agriculturally-productive places in the Ancient World,” said co-author Dr. Francis Ludlow, a climate historian at Trinity College in Dublin, Ireland.
“But the river was famously prone to a high level of variation.”
In some years the Nile didn’t rise high enough to flood the land, and that could lead to trouble. Historical records suggest, for example, that a shortage of grain and the unrest that followed were behind Ptolemy III’s return to Egypt. And the authors had reason to think that volcanoes could be behind some of those bad years.
The reason comes down to a squiggly band of monsoon weather that circles the planet’s equator called the intertropical convergence zone (ITCZ).
Every year around summer in the northern hemisphere, this band moves up from the equator. That, in turn, soaks the headwaters of the Blue Nile River, a major tributary of the Nile.
But when volcanoes erupt, they blast out sulfurous gases that, through a chain of events, cool the atmosphere.
If that happens in the Northern Hemisphere, it can keep the monsoon rains from moving as far as they usually do.
“When the monsoon rains don’t move far enough north, you don’t have as much rain falling over Ethiopia. And that’s what feeds the summer flood of the Nile in Egypt that was so critical to agriculture,” Dr. Ludlow said.
But how often would eruptions diminish the river’s flooding?
To find out, the scientists turned to computer simulations and real-world measurements of the Nile River that date back to 622 CE.
They discovered that poor flood years on the Nile lined up over and over with a recently published timeline of major volcanic eruptions around the world. That evidence suggested that when volcanoes explode, the Nile tended to stay calm.
The team then dug further to see if that might have an impact on Egyptian society during the Ptolemaic era, which is rich in papyri and other written records. They include the trilingual Rosetta Stone.
Again, the timelines matched: volcanic eruptions preceded many major political and economic events that affected Egypt.
They included Ptolemy III’s exit from Syria and Iraq — just after a major eruption in 247 BC — and the Theban revolt.
“We then examined how likely it was that these events occurred so close in time to eruptions, finding it highly unlikely to have occurred by chance, such is the level of overlap,” Dr. Ludlow said.
“The volcanic eruptions didn’t cause these upheavals on their own. But they likely added fuel to existing economic, political and ethnic tensions.”
The research appears in the journal Nature Communications.