Chaos and conflict roiled the Mediterranean in the first century B.C. Against a backdrop of famine, disease and the assassinations of Julius Caesar and other political leaders, the Roman Republic collapsed, and the Roman Empire rose in its place. Tumultuous social unrest no doubt contributed to that transition — politics can unhinge a society. But so can something arguably more powerful.

Scientists on Monday announced evidence that a volcanic eruption in the remote Aleutian Islands, 6,000 miles away from the Italian peninsula, contributed to the demise of the Roman Republic. That eruption — and others before it and since — played a role in changing the course of history.

In recent years, geoscientists, historians and archaeologists have joined forces to investigate the societal impacts of large volcanic eruptions. They rely on an amalgam of records — including ice cores, historical chronicles and climate modeling — to pinpoint how volcanism affected civilizations ranging from the Roman Republic to Ptolemaic Egypt to pre-Columbian Mesoamerica.

There’s nuance to this kind of work, said Joseph Manning, a historian at Yale University who has studied the falls of Egyptian dynasties. “It’s not ‘a volcano erupts and a society goes to hell.’” But the challenge is worth it, he said. “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.”

Sleuthing in Ice

Image
Credit…Joseph R. McConnell

Roughly 1,500 volcanoes are potentially active right now, meaning that they’ve erupted at some point in the last 10,000 years. While scientists today have sophisticated tools to monitor volcanoes, the vast majority of historical eruptions have gone unrecorded, at least by modern scientific instruments. Sussing out those eruptions requires patience and ingenuity, and a willingness to manage a lot of ice.

At the Desert Research Institute in Reno, Nev., it’s not unusual to find researchers in puffy parkas and wool hats handling chunks of ice in a minus 4 Fahrenheit “cold room.” Ice cores, typically drilled vertically from glaciers, hide bits of volcanic material that rained down from long-ago eruptions within their layers.

Joseph McConnell, a climate scientist at the institute, and his collaborators are in the business of looking for that debris. Using an instrument they designed and built, they melt the ice and pipe the water into an array of sensors. With hundreds of feet of tubing, the setup looks downright chaotic, but it’s exquisitely sensitive. The sensors pinpoint many substances, including about 30 different elements, and they do so by catching just tiny whiffs.

“They have sensitivities of parts per quadrillion,” Dr. McConnell said.

Volcanic ash, more generally known as tephra, sometimes hides in ice. It’s a special find because it can be geochemically tied to a specific volcano. “The tephra comes from the magma itself,” said Michael Sigl, a chemist at the University of Bern in Switzerland who collaborates with Dr. McConnell. “It carries the composition of the rocks.”

Sulfur is also indicative of a past eruption. Sulfur dioxide, a gas commonly belched by erupting volcanoes, reacts with water in the atmosphere to create sulfate aerosols. These tiny particles can linger in the stratosphere for years, riding wind currents, but they, like tephra, eventually fall back to Earth.

The ice also carries a time stamp. Dr. McConnell and his colleagues look for variations in elements like sodium, which is found in sea spray that’s seasonally blown inland. By simply counting annual variations in these elements, it’s possible to trace the passage of time, Dr. McConnell said. “It’s like a tree-ring record.”

Dr. McConnell and his collaborators recently analyzed six ice cores drilled in the Arctic. In layers of ice corresponding to the early months of 43 B.C., they spotted large upticks in sulfur and, crucially, bits of material that were probably tephra. The timing caught the scientists’ attention.

Researchers have previously hypothesized that an environmental trigger may have helped set in motion the crop failures, famines and social unrest that plagued the Mediterranean region at that time. But until now, “There hasn’t been the kind of data that these scholars brought forth to really get those theories into the mainstream,” said Jessica Clark, a historian of the Roman Republic at Florida State University who was not involved in the research.

Gill Plunkett, a paleoecologist at Queen’s University Belfast, set out sleuthing. After extracting 35 pieces of tephra from the ice, she pored over the rock chemistry of likely volcanic suspects. Nicaragua’s Apoyeque. Italy’s Mount Etna. Russia’s Shiveluch.

But it was Okmok, a volcano in Alaska’s Aleutian Islands, that turned out to be the best match, at least on paper. Sealing the deal would require testing two tephra samples — one from the ice and one from Okmok — on the same instrument.

Dr. Plunkett arranged for a tephra handoff at a conference in Dublin. A colleague from the Alaska Volcano Observatory, Kristi Wallace, packed four bags of Okmok tephra in her carry-on luggage. The match was spot on, Dr. Plunkett said. “There are some events that are tricky. With Okmok, there’s nothing else that looks like it.”

Cold and Wet, a World Away

Image

Credit…C.A. Neal/Alaska Volcano Observatory/United States Geological Survey

This eruption was one of the largest of the last few millenniums, Dr. McConnell and his collaborators concluded, and the sulfate aerosols it created remained in the stratosphere for several years. These tiny particles are particularly good at reflecting sunlight, which means they can temporarily alter Earth’s climate.

“They’ve created, for a short term, global cooling events,” said Jessica Ball, a volcanologist at the California Volcano Observatory, who was not involved in the research.

There’s good evidence that the Northern Hemisphere was colder than normal around 43 B.C. Trees across Europe grew more slowly that year, and a pine forest in North America experienced an unusually early autumn freeze. Using climate models to simulate the impact of an Okmok eruption, Dr. McConnell and his collaborators estimated that parts of the Mediterranean, roughly 6,000 miles away, would have cooled by as much as 13.3 degrees Fahrenheit.

“It was bloody cold,” Dr. McConnell said.

Rain patterns changed as well — some regions would have been drenched by 400 percent more precipitation than normal, the modeling revealed.

That climate shock came at precisely the wrong time, Dr. Clark said. “This was a period of Mediterranean-wide political, social and economic upheaval.”

These cold, wet conditions would have almost certainly decimated crops, Dr. McConnell and his colleagues said. Historical records compiled by Roman writers and philosophers note food shortages and famines. In 43 B.C., Mark Antony, the Roman military leader, and his army had to subsist on wild fruit, roots, bark and “animals never tasted before,” the philosopher Plutarch wrote.

For a society already reeling from the assassination of Julius Caesar the year before, such trying conditions might have exacerbated social unrest, the researchers concluded. They might even have kick-started transfers of political power that led to the rise of the Roman Empire.

“It’s an incredible coincidence that it happened exactly in the waning years of the Roman Republic when things were falling apart,” said Dr. McConnell, who published the team’s results in Proceedings of the National Academy of Sciences.

Karen Holmberg, an archaeologist at New York University who studies volcanic events and was not involved in the research, said she found the study “compelling and persuasive.”

Unfortunately, the archaeological record doesn’t often record volcanic eruptions, she said, except in cases of very nearby eruptions when there’s an obvious layer of tephra. The links in the study are probable, but not definite. “They’re not being heavy handed and saying this is absolutely it,” Dr. Holmberg said.

Nile Interrupted

Image

Credit…Prisma by Dukas Presseagentur GmbH, via Alamy

Around the same time, another great civilization was also feeling the effects of volcanism, including the Okmok eruption. But rather than reeling from climatic variability, this society was being rattled by changes in a critical resource: water.

Egyptian society, before the installation of the Aswan Low Dam in the early 20th century, was anchored by the annual summer flooding of the Nile River. These summer floods, sustained by monsoon rains in the highlands of Ethiopia, delivered irrigation and silt, both critical to Egypt’s agrarian society. “The whole rhythm of the year was built around responding to the flood,” Dr. Manning of Yale said.

But volcanic eruptions, even those on the other side of the world, could have disrupted that flooding, Dr. Manning and his colleagues recently showed. Using records from Cairo’s Nilometer — an octagonal marble column that was used for recording Nile flood height from 622 to 1902 A.D., the team found that flooding tended to be weaker, or entirely absent, during years when there was a large volcanic eruption somewhere in the world.

Dr. Manning and his collaborators next mined roughly 100 papyrus records to qualitatively estimate flooding during the Ptolemaic dynasty, which lasted from 305 to 30 B.C. Again, they found that the Nile typically flooded weakly, if at all, at the time of large eruptions.

The culprit, the team reasoned in a paper published a few years ago, was cooling caused by sulfate aerosols. When Earth cools after a large eruption, its atmospheric circulation patterns change. That can shift the invisible meeting point of Northern and Southern Hemisphere trade winds — the Intertropical Convergence Zone — that affects where monsoon rains tend to fall. When less precipitation falls over Ethiopia, home to a major tributary of the Nile, there’s less water available for flooding that year.

Ptolemaic-era records revealed that this reduced flooding had socioeconomic and political consequences. Revolts increased in the years following “Nile failure,” Dr. Manning and his colleagues found. Priestly decrees — intended to establish the political legitimacy of Greek rulers — also became more commonplace.

Reinforcing elite authority during times of turmoil makes sense, Dr. Manning said. “Bad flooding is interpreted as having a bad king in office.”

Mystery Eruptions

Volcanic eruptions have left fingerprints on other societies, too. They’ve been tied to economic decline in sixth-century pre-Columbian Mesoamerica and famine-induced migration in eastern China in the 10th century. Eruptions have potentially helped trigger widespread outbreaks of disease, such as the Justinian plague around 540 A.D. Just over a decade ago, an eruption of Iceland’s Eyjafjallajökull led to the costly closure of a wide swath of European air space.

Questions persist, however, for geoscientists, historians and archaeologists alike. Having more ice core records might reveal the identities of orphan eruptions, their sulfur signatures in ice cores still unpinned to a parent volcano, said Robert Dull, a paleoecologist at California Lutheran University who studies volcanic eruptions.

“There are still large unsourced mystery eruptions up until the early 19th century.”

A better understanding of how societies were structured is also important. Increased knowledge of trade patterns, for instance, would shed light on how a crop failure in one geographic area would have trickle-down effects throughout a wider region, Dr. Dull said. “You need to understand how human beings were connected.”

Right now, roughly a dozen volcanoes are erupting. In all likelihood, they’re nothing to worry about — it’s doubtful you’ve even heard of them. But every once in a while, there’s bound to be a really big eruption. How its effects ripple around the world awaits to be seen.

“Okmok volcano is not exactly a commonly known threat,” Dr. Holmberg said. “But then neither was Eyjafjallajökull.”