(CNN)In the years after the assassination of Julius Caesar in ancient Rome, historical accounts paint a picture of unusual cold, food shortages, disease and famine that accompanied a pivotal moment in Western history.
Scientists identify Alaskan volcano that may have helped the rise of the Roman Empire
Historians had long suspected that this unexplained extreme weather could be linked to a volcanic eruption, but they had been unable to pinpoint where or when such an eruption had occurred or how severe it was. Candidates had included volcanoes in Nicaragua, Sicily and Kamchatka in Russia's Far East.
After analyzing ash trapped in ice and other records, an international group of scientists and historians now think that an eruption of Alaska's Okmok volcano more than 2,000 years ago was responsible. The massive explosion created a 10-kilometer-wide crater that is still visible today.
"To find evidence that a volcano on the other side of the Earth erupted and effectively contributed to the demise of the Romans and the (ancient) Egyptians and the rise of the Roman Empire is fascinating," said Joe McConnell, a research professor of hydrology at the Desert Research Institute in Reno, Nevada, and an author of the study, in a news statement.
"People have been speculating about this for many years, so it's exciting to be able to provide some answers," McConnell said.
Caesar's stabbing by Rome's senators triggered a power struggle that ultimately ended the Roman Republic, leading to a shift from a more democratic governance to the dictatorship of the the Roman Empire. It also ultimately led to Egypt coming under Roman rule.
The study said that crop failures, famine and disease resulting from the eruption likely exacerbated social unrest and contributed to political realignments at this "critical juncture of Western civilization."
"While we can't actually prove how the extreme weather and resulting crop failures, food shortages and epidemic disease contributed to the downfall of the Republic 2,000 years ago, it seems only logical that it must have played a significant role," McConnell said.
The team analyzed volcanic ash, known as tephra, found trapped in Arctic ice cores taken from Greenland and Russia to link the period of unexplained extreme climate in the Mediterranean with the massive eruption of Okmok volcano on Umnak Island in the Aleutian Islands chain.
"The tephra match doesn't get any better," said Gill Plunkett, co-author and a reader at the School of Natural and Built Environment at Queen's University Belfast. The study published Monday in the journal Proceedings of the National Academy of Sciences.
"We compared the chemical fingerprint of the tephra found in the ice with tephra from volcanoes thought to have erupted about that time and it was very clear that the source of the 43 BCE fallout in the ice was the Okmok II eruption."
The eruption produced volcanic fallout that lasted two years, the study said, lowering temperatures in the Northern Hemisphere by up to 7 C (13 F).
The change in temperature can be seen in tree ring records in Scandinavia, Austria and California, the study found, with one bristlecone pine, in California's White Mountains, showing a frost ring that suggested temperatures below freezing in early September 43 BCE.
Similarly, climate records from caves in China also showed temperature drops in the three years after the eruption. The researchers' models suggested it would have been much wetter than normal during the summer and autumn that followed the 43 BCE eruption of Okmok, which is still active today and last erupted in 2008.
"In the Mediterranean region, these wet and extremely cold conditions during the agriculturally important spring through autumn seasons probably reduced crop yields and compounded supply problems during the ongoing political upheavals of the period," co-author Andrew Wilson, a classical archaeologist at the University of Oxford, said in the statement.
"These findings lend credibility to reports of cold, famine, food shortage and disease described by ancient sources."
The study noted that natural disasters like a volcanic eruption are known 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."
The researchers also identified a smaller and more limited volcanic eruption in 44 BCE at Mount Etna in Italy.
They said this eruption could help explain unusual phenomena described around the immediate time of Caesar's death by writers like Virgil — solar halos, the sun darkening in the sky or three suns appearing in the sky that at the time were interpreted as omens.