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The year 43 BCE was, by all accounts, a tumultuous time for the Mediterranean region. The assassination of Julius Caesar one year earlier had sparked a seventeen-year power struggle that would lead to the demise of the Roman Republic and the Ptolemaic dynasty that governed ancient Egypt and the dawn of the Roman Empire. It was also a time of extremely weird weather; the years 43 and 42 BCE were marked by bitter cold and excessive rains that led to widespread famines and disease.

Historians and scientists had previously suspected that a cataclysmic volcanic eruption may have been responsible for these miserable conditions. Now, an international team of researchers has identified when and where this event likely took place. After examining six ice cores taken from sites across the Arctic, the researchers concluded that Alaska’s Okmok volcano blew its top in 43 BCE and the fallout from the event lasted for two years. This would have altered the climate across the Northern Hemisphere and resulted in chilly, wet conditions that match descriptions in the historical record, the team reported on June 22 in the journal Proceedings of the National Academy of Sciences.

“The idea that a volcano on the other side of the Earth could blow and result in such extreme cooling at exactly this time of political and social unrest at such an important time in the history of Western civilization…is fascinating,” says Joseph R. McConnell, a climate and environmental scientist at the Desert Research Institute in Reno, Nevada and a coauthor of the new findings. “We’re not trying to say this caused the downfall of the Roman Republic or the Egyptian Ptolemaic Kingdom, but we’re saying that it’s a new piece of information, and historians need to consider whether it may have contributed to the situation.”

During an eruption, a volcano launches debris into the atmosphere that prevents incoming solar radiation from reaching the planet’s surface. This can cause abnormally cool temperatures that may last for months to years and extend far from the site of the initial eruption. When this fallout eventually falls back to Earth, some of it—including tiny particles of rock called tephra and some sulfur-containing compounds—becomes trapped and buried in ice sheets at northern latitudes. This debris is how researchers are able to look back in time and figure out when eruptions occurred.

McConnell and his colleagues noticed two spikes in volcanic debris dating back to 45 BCE and 43 BCE in ice cores extracted from Greenland and Russia. They estimated that the earlier of these two eruptions took place in a high-latitude area such as Iceland in a “powerful but short-lived event,” producing fallout that was mainly confined to northern Greenland and waned after a few months.

By contrast the eruption that occurred in early 43 BCE was massive enough to impact the atmosphere for two years—the amount of sulfur preserved in the ice only returned to typical levels around the spring of 41 BCE. The chemical composition of the tephra shards in the ice cores matched that of material from Okmok volcano in Alaska.

Based on evidence from both mineral deposits in caves in China and tree rings in Europe that are sensitive to shifts in temperature, researchers know that 43 and 42 BCE were among the coldest of the past 2,500 years for the Northern Hemisphere. In documents preserved from the Mediterranean, writers lamented the cold, storms, crop failures and food shortages, and disease; the historian Appian described Rome as being “devastated by famine,” while the Greek writer Plutarch recounted the army of Mark Antony being forced to subsist on wild fruits, roots, bark and animals “never tasted before by men” while fleeing from their defeat at the Battle of Mutina in 43 BCE.

ice core analysis apparatus
The apparatus used to analyze ice cores for volcanic fallout Joseph R. McConnell

When McConnell and his colleagues used computer models to estimate the impacts of an eruption at Okmok, they found that this event would have created extremely cold and wet conditions, with parts of southern Europe and northern Africa experiencing temperatures up to 7.4 degrees Celsius below normal in the summer of 43 BCE. McConnell says the effect would have been similar to the infamous “year without a summer” in 1816. The previous spring, Mount Tambora in Indonesia had erupted and caused Earth’s global temperature to drop by 3 degrees Celsius, leading to summer snowstorms and frosts as far away as New England.

In the ancient Mediterranean, it’s very likely that these conditions would have exacerbated social and political upheavals across the region. Still, McConnell says, any connections between the eruption and historical events are “highly speculative.”

“There’s a lot of spin about the meaning of these natural phenomena that were interpreted as portents or things like that to try to deify Julius Caesar,” he says. “So separating what was a factual objective observation from something that had been [mythologized]…was an interesting challenge.”

While explosive volcanic eruptions can profoundly alter conditions like temperature and rainfall, it’s difficult to pin down how either the climate or society of the Mediterranean of two millennia ago would have been affected by the eruption in Alaska, says Christopher Colose, a climate scientist at the NASA Goddard Institute for Space Studies in New York who was not involved in the research.

“For any given eruption, even a large one, there is going to be a large range of possible climate responses, simply due to natural weather variability that occurs at the time of the event,” he said in an email. Additionally, he said, “there would be a rich history of famine, revolt, bad leadership, war, Nile River flooding or suppression, etcetera even in a world without volcanic eruptions.”

Still, Colose added, the new paper convincingly dates the eruption of Okmok volcano to a period that coincided with several exceptionally cold years. “They did make a compelling case for adding volcanic eruptions to the list of things historians should be thinking about.”

For McConnell, the findings are a reminder that we are—and always have been—vulnerable to the whims of nature. “If nature suddenly decided to throw a two-year cooling period like this, or maybe even a pandemic like we’re currently experiencing, it’s quite an external shock to society.”

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