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. Social unrest no doubt contributed to that transition — politics can unhinge a society. But so can something arguably more powerful.

Scientists 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 used an amalgam of records — ice cores, historical chronicles and climate modeling — to pinpoint how volcanism affected civilizations like the Roman Republic, Ptolemaic Egypt and pre-Columbian Mesoamerica.

Most historical eruptions have gone unrecorded. But climate scientists, such as Joseph McConnell of the Desert Research Institute in Reno, Nev., are harnessing modern technology to look into the past. Volcanic ash, known as tephra, sometimes hides in ice, and it can be geochemically tied to a specific volcano.

McConnell and his collaborators designed and built an instrument that melts ice drilled from glaciers and pipes the water into sensors. The sensors pinpoint many substances, including about 30 elements.

In layers of ice corresponding to the early months of 43 B.C., the team spotted large upticks in sulfur — indicative of a past eruption — and, crucially, bits of material that were probably tephra. The timing caught the scientists' attention.

Gill Plunkett, a paleoecologist at Queen's University Belfast, extracted 35 pieces of tephra from the ice and pored over the rock chemistry of likely volcanic suspects. It was Okmok, a volcano in Alaska's Aleutian Islands, that turned out to be the best match. "There are some events that are tricky. With Okmok, there's nothing else that looks like it," Plunkett said.

This eruption was one of the largest of the last few millenniums, McConnell and his team concluded, and the sulfate aerosols it created can temporarily alter Earth's climate.

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. "It was bloody cold," he said.

Rain patterns changed as well — some regions would have been drenched by 400% more precipitation than normal, the modeling revealed. That climate shock came at precisely the wrong time, Clark said. "This was a period of Mediterranean-wide political, social and economic upheaval," she said.

These cold, wet conditions would have almost certainly decimated crops, they said. Historical records compiled by Roman writers and philosophers noted 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," philosopher Plutarch wrote.

For a society reeling from the assassination of 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," said McConnell, who published the results in Proceedings of the National Academy of Sciences.