SF State professor helps link volcanoes and ancient unrest

If a volcano erupts in Iceland and no one is around to see it, did it really happen?

San Francisco State University Professor of Climate Sciences Zan Stine would say yes. Last month, he and a team of scientists and historians discovered that the world’s largest volcanic eruptions can influence the flooding of rivers — and even the stability of societies — thousands of miles away. Along with what it reveals about the world’s climate, the study could shed light on how natural disasters influenced the world’s great civilizations.

“Human institutions matter,” Stine explained. “But natural stresses also matter.”

Normally, the gases and tiny particles from a volcanic eruption get washed out of the sky by rain in a matter of days or weeks. The biggest volcanoes, though, can shoot the stuff more than six miles into the stratosphere, where it hovers for as long as two years. Eruptions of this size don’t happen often, so finding consistent patterns of how they influence far-flung environments has always been difficult. 

Stine and his collaborators figured out a novel way to spot these impacts. They matched ice cores in Greenland and Antarctica, which preserve traces of the ejecta of far-off volcanoes, with a unique record of Nile River floods extending all the way back to the seventh century.

“Pretty consistently, the Nile flood is failing when volcanic eruptions go off,” said Stine. Floods in post-eruption years, he and his colleagues found, were about nine inches lower than normal. That’s likely because such large eruptions, by shooting up particles that block out some sunlight, shift weather bands in a way that gives less rain to the Ethiopian highlands that feed the Nile. That alone would have interested climatologists — but the team wanted to go one step further, to look at how these shifts affected ancient societies.

Rippling impacts

A dive into the rich papyrus records of the Ptolemaic period in Egypt (from 305 BCE to 30 BCE) showed that riots tended to well up in the two years after large eruptions. The success of crops in Egypt is tied to the Nile’s regular floods — and those nine inches of water, apparently, could make the difference between a successful year and one that would sow the seeds of unrest. Landowners were also more likely to sell their property during these periods, and the resulting tension even caused military leaders to withdraw their armies from the field. “It looks like when the Nile fails, there’s a crisis at home, so they make peace,” explained Stine. The team published its results on Oct. 17 in the journal Nature Communications.

The study can’t say anything about whether volcanoes contributed to the ultimate fall of the Ptolemaic dynasty, ending with Cleopatra in 30 BCE. It does, however, support the idea that big trends in civilizations are products not just of the way societies are structured, but also of events in the natural world.

An artisanal analysis

Stine’s collaborators included both scientists and historians — and although his love is climate science, he served as the team’s resident statistics guru. For climate scientists working with historical records, “all of our data is really messy,” he said. “You have to rescue it and turn it into a climate record. You have to become a statistician.”

And as a statistician, Stine is like a farm-to-table chef. Rather than picking from a book of statistical recipes, he uses a technique called Monte Carlo analysis, which is specially crafted for the data available (what a chef might call “seasonal offerings”). Stine developed some of these techniques while teasing out climate clues from 1,200 years of Irish monastery records.

He and his collaborators next plan to look for yet another such archive — this time, in Babylonian cuneiform tablets. There, they hope to dig up another link between the natural world and our constructed one. Stine is optimistic about their chances. “I think there’s a lot more to unearth,” he said.

Other authors on the study are Joseph Manning and Jennifer Marlon (Yale University), Francis Ludlow (Trinity College), William Boos (University of California, Berkeley) and Michael Sigl (Paul Scherrer Institute).