The two years after Julius Caesar’s assassination in 44 B.C. were rife with bad luck. The sky turned dark, the weather grew cold, and Mediterranean civilizations experienced drought and famine.
Now, a multidisciplinary team of researchers has pinned down an explosive explanation for these strange occurrences: As detailed in the Proceedings of the National Academy of Sciences, a volcanic eruption some 6,000 miles away from Rome may have thrown off the region’s weather patterns—and perhaps even contributed to the rise of the Roman Empire.
The new study combines evidence from ice cores, tree rings and historical records to identify an eruption of Alaska’s Okmok volcano in 43 B.C. as the cause of unusual weather following the Ides of March, reports Paul Voosen for Science magazine. The enormous eruption triggered an average 13-degree Fahrenheit drop in temperature across southern Europe and northern Africa.
“This is the second coldest year in the last 2,500 years—I mean, that’s not a small thing,” lead author Joe McConnell, a snow hydrologist at the Desert Research Institute in Nevada, tells Atlas Obscura’s Isaac Schultz. “And when you’re talking about an agrarian society that’s living close to the edge as it is, it had to have had a big impact.”
By all accounts, 43 B.C. was a tumultuous year in Roman history. Most of the senators who’d conspired to assassinate Caesar had fled the city. The dead dictator’s grand-nephew and adopted heir, Octavian, had seized power with money and military force. At just 19 years old, he joined the Second Triumvirate, a trio of consuls with dictatorial power. That same year, another member of the Triumvirate, Mark Antony, murdered Cicero, one of the republic’s last defenders.
Cicero’s death is considered the symbolic end of the Roman Republic, according to Science. Letters from the ancient statesman mention the unusually cold weather occurring around the time of the Okmok eruption. In April, northern Italy was struck by famine; Roman biographer Plutarch wrote that Antony’s army was forced to eat wild fruit, roots, bark and animals “never tasted before by men.” In 42 B.C., both northern Greece and Rome suffered similar shortages.
Experts have long suspected that a volcanic eruption was responsible for the extreme weather. But until now, writes Katie Hunt for CNN, researchers had “been unable to pinpoint where or when such an eruption had occurred or how severe it was.”
Arctic ice cores proved key to unraveling the mystery. As Katherine Kornei explains for the New York Times, samples gathered in northern Greenland had high concentrations of sulfur and sulfuric acid in layers corresponding to early 43 B.C. And shards of tephra, or glassy volcanic material, found in the cores matched the chemical makeup of Okmok, allowing the scientists to pinpoint the geological culprit.
“The tiny glass particles in the ice are a clinching piece of evidence,” Siwan Davies, a geographer at Swansea University who wasn’t involved in the study, tells Science.
The Okmok eruption was about the same size as the 1815 eruption of Mount Tambora in Indonesia. This blast sparked the so-called “year without a summer” in Western Europe, according to Atlas Obscura.
Tree ring records in Scandinavia and North America show that 43 and 42 B.C. were cooler than other years. But in the Alps, the cooling trend began ten years prior to the eruption and was actually strongest in 45 B.C.
“If we’re connecting Roman Republican history to climate and volcanoes, we need more of these records,” Kevin Anchukaitis, a paleoclimatologist at the University of Arizona who also wasn’t involved in the study, tells Science.
Other experts point out that the climate model used in the research paper can exaggerate cooling effects from eruptions. Additionally, they argue, extreme weather can’t account for all of the political turmoil that took place as Rome transitioned from a republic to an empire.
“The problems with the republic were political, deep in origin, fought out between members of the elite, not a popular revolution or a subsistence crisis,” Charles University archaeologist Guy Middleton, who was not involved in the study, tells Science.
Unrest persisted long after the physical effects of the volcanic eruption faded. Only after more than a decade of civil war did Octavian emerge as Augustus, emperor of the newly unified Roman Empire. Still, the new paper presents compelling evidence that natural disasters can affect the course of history in unexpected ways.
“It’s not ‘a volcano erupts and a society goes to hell,’” study co-author Joseph Manning, a Yale University historian who studies the fall of Egyptian dynasties, tells the Times. But by unraveling the nuances of past collapses, he says, “We hope in the end that we get better history out of it, but also a better understanding of what’s happening to the Earth right now.”