An international team of scientists and historians, including members of the School, has found evidence connecting an unexplained period of extreme cold in ancient Rome with an unlikely source: a massive eruption of Alaska’s Okmok volcano, located on the opposite side of the Earth.
Around the time of Julius Caesar’s death in 44 BCE, written sources describe a period of unusually cold climate, crop failures, famine, disease, and unrest in the Mediterranean Region – impacts that ultimately contributed to the downfall of the Roman Republic and Ptolemaic Kingdom of Egypt. Historians have long suspected a volcano to be the cause, but have been unable to pinpoint where or when such an eruption had occurred, or how severe it was.
In a new study published this week in Proceedings of the National Academy of Sciences (PNAS), a research team led by Professor Joe McConnell, of the Desert Research Institute in Reno, Nevada, uses an analysis of tephra (volcanic ash) found in Arctic ice cores to link the period of unexplained extreme climate in the Mediterranean with the caldera-forming eruption of Alaska’s Okmok volcano in 43 BCE.
“The ice core science corroborates the ancient sources in an unexpected way,” said Professor Andrew Wilson, Professor of the Archaeology of the Roman Empire, a co-author on the study. “People had long tried to attribute the climatic changes to an eruption of Etna known to have occurred in 44 BC. But it seemed unlikely that Etna could have caused such Mediterranean-wide climate change without having left other traces of devastating effects on Sicily. Now we know that while Etna may have caused the strange atmospheric phenomena observed around the time of the assassination of Julius Caesar, and interpreted at the time as signs and portents, the severe climate cooling of the following year was caused by an eruption in Alaska.”
The discovery was initially made last year in DRI’s Ice Core Laboratory, when McConnell and Swiss researcher Professor Michael Sigl, from the Oeschger Centre for Climate Change Research at the University of Bern happened upon an unusually well preserved layer of tephra in an ice core sample and decided to investigate.
New, state-of-the-art measurements were made on ice cores drilled in Greenland and Russia in the 1990s and archived in the U.S., Denmark, and Germany. Using these and earlier measurements from six ice cores, they were able to clearly delineate two distinct eruptions – a powerful but short-lived, relatively localized event in early 45 BCE, and a much larger and more widespread event in early 43 BCE with volcanic fallout that lasted more than two years in all the ice core records.
“The six ice core records clearly show that 43 BCE event was one of the largest eruptions of the past 2500 years,” Sigl said.
They then conducted a geochemical analysis of the tephra samples from the second eruption, matching the tiny shards with those of the Okmok II eruption in Alaska.
“The tephra match doesn’t get any better,” said tephra specialist Dr Gill Plunkett from Queen’s University Belfast. “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.
Working with colleagues from the U.K., Ireland, Germany, Denmark, Alaska, and Yale University in Connecticut, the team of historians and scientists gathered supporting evidence from around the globe, including tree-ring-based climate records from Scandinavia, Austria and California’s White Mountains, and climate records from cave speleothem (cave formations) from Shihua Cave in northeast China. They then used Earth system modeling to develop a more complete understanding of the timing and magnitude of volcanism during this period and its effects on climate and history.
According to their findings, the two years following the Okmok eruption were some of the coldest in the Northern Hemisphere in the past 2,500 years, and the decade that followed was the fourth coldest. Climate models suggest that temperatures may have dropped up to 7oC below normal during the summer and autumn that followed the 43 BCE eruption of Okmok II, with summer precipitation of 50 to 120 percent above normal throughout Southern Europe, and autumn precipitation reaching as high as 400 percent of normal.
“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,” Andrew Wilson. “These findings lend credibility to reports of cold, famine, food shortage and disease described by ancient sources. Climate-induced grain shortages would have made Sextus Pompeius’ naval blockade of Italy, cutting off grain supplies from Sicily and North Africa from 43 to 36 BC, all the more effective.”
Volcanic activity also helps to explain certain unusual atmospheric phenomena that were described by ancient Mediterranean sources around the time of Caesar’s assassination – things like solar halos, the sun darkening in the sky, or three suns appearing in the sky (a phenomenon now known as a parahelia, or ‘sun dog’). However, many of these observations took place prior to the eruption of Okmok II in 43 BCE, and are likely related to a smaller eruption of Mt. Etna in 44 BCE.
“At the time, many of these atmospheric phenomena were interpreted as signs or omens – but today, we know that they were mostly likely caused by volcanic ash in the atmosphere,” said Yale University historian Professor Joe Manning.
Although the study authors acknowledge that many different factors contributed to the fall of the Roman Republic and Ptolemaic Kingdom, they believe that the impacts of the Okmok II eruption played an undeniably large role – and that their discovery helps to fill a knowledge gap about this period of history that has long puzzled archaeologists and ancient historians.
“People have been speculating about this for many years, so it’s exciting to be able to provide an answer,” McConnell said.