Kiona N. Smith

The 10km-wide caldera on Alaska's Unmak Island formed during the 43 BCE Okmok II eruption.
Enlarge / The 10km-wide caldera on Alaska’s Unmak Island formed during the 43 BCE Okmok II eruption.

Kerry Key (Columbia University, New York, NY)

Roman writers described unusual weather and famines in the years following Julius Caesar’s assassination in 44 BCE, adding to the turbulence of the civil war that marked the transition from Republic to Empire. A recent study has now identified the culprit: a volcano 9,000km (6,000 miles) away in Alaska.

“Madness of wolves in winter”

“Madness of wolves in winter; in summer, no grain is harvested,” cried a voice from the Oracle of Apollo, in Delphi, in the months following Julius Caesar’s death. Ancient writers who survived the period describe cold weather, short growing seasons, and widespread famine around the Mediterranean, from Rome to Egypt. Throughout the empire, starvation led to disease and fueled growing civil unrest in an already turbulent time.

For years, modern historians have speculated that a major volcanic eruption might have been the culprit. An erupting volcano blasts sulfur dioxide high into the atmosphere. As the sulfur dioxide spreads out in the stratosphere, chemical reactions turn it into other sulfur compounds that reflect solar radiation, blocking the Sun’s light and warmth. Huge swaths of the planet turn colder, often continents away from the eruption.

But no one has been sure which volcano to blame for Rome’s troubles. It could have been as far away as Shiveluch in Kamchatka or Apoyeque in Nicaragua. Then again, it could have been as close as Mt. Etna, which famously exploded in 44 BCE—not long before Caesar’s death—and probably caused some of the strange omens and portents described in that year.

A recent study of Greenland ice cores now suggests the culprit is Mt. Okmok, in what is now Alaska, half a world away from Rome. Every year in the Arctic, new snow piles up and gets compacted into ice layers, which preserve a record of whatever fell with the snow. In north-central Greenland, environmental researcher Joe McConnell (of, ironically, the Desert Research Institute) and his colleagues found volcanic ash and sulfuric acid in ice layers laid down in the early winter of 43 BCE and continuing for the next two years.

Blaming Okmok

“We found the Okmok II sulfur fallout in every Arctic ice core record that we had access to, including five from all over Greenland and one from Akademii Nauk in the Russian Arctic,” McConnell told Ars. “The layer of increased sulfur concentration is about 30 to 50cm thick depending on the ice core, and we would expect that the sulfur was deposited all over the Northern Hemisphere—but not uniformly.”

The geochemistry of that frozen ash matched layers of rock fragments, called tephra, associated with an ancient eruption of Mt. Okmok. Known as Okmok II, this was one of the largest eruptions of the last few thousand years, comparable to Mt. Pinatubo in 1991, Krakatoa in 1883, or Tambora in 1815. But volcanologists haven’t been sure precisely when the eruption happened; radiocarbon dates on organic material buried beneath the tephra only narrowed it down to a window between 190 BCE and 50 CE.

“For volcanic eruptions that are not historically documented (and most are not), absolute dates come from ice core studies such as this,” explained McConnell in an email to Ars.

Arctic ice cores hold records of atmospheric events going back millennia.
Enlarge / Arctic ice cores hold records of atmospheric events going back millennia.

Joseph R. McConnell

Et tu, volcano?

McConnell and his colleagues used a computer simulation to model how the Okmok eruption might have impacted the ancient world. The models predicted that summer temperatures in the years after the eruption would be 2⁰C to 3⁰C cooler than usual across most of the Northern Hemisphere. That matches what ancient writers described, and it’s backed up by climate clues such as tree rings.

“Geochemically fingerprinting the ash was the key to identifying the volcanic source, and that knowledge underpinned the detailed climate modeling,” McConnell told Ars.

The Okmok II eruption lasted from 43 BCE to 41 BCE, but its effects on the other side of the world lasted more than a decade. Spring came later, fall came earlier, and the summer growing season fell short. It was also much too rainy for healthy crops. Ancient writers describe crop failures and famine in northern Italy and northern Greece from April 43 BCE through 36 BCE. With famine came disease and a swelling of civil unrest.

With the Republic already shaken by power struggles in the wake of Julius Caesar’s death, years of famine probably helped undermine what little stability was left. “Natural disasters are known historically to create a ‘state of exception’ in which business as usual becomes unfeasible, and political and cultural norms are suspended, thereby providing room for rapid social and political change,” wrote McConnell and his colleagues.

Things like volcanic eruptions and pandemics don’t happen in a vacuum. They influence—and are influenced by—political and social events. For example, when Augustus Caesar’s political opponent, the military leader Sextus Pompeius (whose father, Pompey the Great, had been an important opponent of Augustus’ adopted father, Julius Caesar) set up a naval blockade of Italian ports, he worsened the food shortages in Rome’s heartland. But he may have gained some extra leverage from the fact that people in Italy were already dealing with a famine, so they were even more vulnerable to having outside supplies cut off.

Denial River

While farmers along the Mediterranean coast were dealing with too much rain, farmers along the Nile in Egypt faced the opposite problem. Egypt’s survival has always depended on the Nile’s summer floods, which bring water and fertile soil to the narrow strip of land along the river’s banks. But for two years in a row, 43 and 42 BCE, the floods didn’t come. Documents from the period describe a terrible famine in those years.

The climate models run by McConnell and his colleagues suggest that when Mt. Okmok cooled the Northern Hemisphere, it also short-circuited the weather systems that produce the Nile’s annual flood. “The differential cooling moves the equatorial rain belts north or south,” explained McConnell. “High northern latitude eruptions like Okmok cause a failure of the East African Summer Monsoon, which results in drought in the headwaters of the Nile and so a failure in the all-important summer flood.”

Meanwhile, Octavian (who would become Augustus Caesar, Rome’s first emperor) and his ally Marcus Antonius were still fighting Brutus (the one with the knife) and Cassius for power, and neither side could get grain from Egypt to feed their armies. Cassius reportedly asked Cleopatra for military aid, and she turned him down because her country was already struggling with famine and disease.

But the volcanic eruption in Alaska may have ultimately helped make Cleopatra the last Ptolemaic ruler of Egypt. Military conflicts raged, and nearly every corner of the old Roman Republic was desperately short of food, which likely made Egypt look very appealing as a potential “breadbasket.” At the same time, the failure of the Nile floods in 43 and 42 BCE probably made Cleopatra’s Egypt much more vulnerable to outside attack.

History on ice

Some of the political and economic impacts of Mt. Okmok’s 43 BCE eruption can be seen in the same ice cores that helped identify it. Lead pollution from mining, smelting, and other industrial work across Europe also tends to end up in the Greenland ice, thanks to atmospheric circulation. During years when ancient Europe faced plague, war, or famine—and therefore mined and smelted less silver—less lead from Europe ended up in the snow falling over Greenland.

So the ice on another continent records the economic history of ancient Rome. That’s actually what McConnell and his colleagues were studying when they found the evidence of Okmok II. As McConnell told Ars, “We will be extending these studies relating detailed ice core measurements to ancient history back into the Bronze Age and even the Neolithic–should be fun!”

PNAS, 2020 DOI: 10.1073/pnas.2002722117  (About DOIs).



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