In the two years following the assassination of Julius Caesar in 44 BCE, written sources speak of famine, disease, and a freakishly cold burst of weather across the Mediterranean. A new study shows that these swift environment changes were forged by a massive volcanic upset on the other side of the planet, Alaska's Okmok eruption in early 43 BCE. The researchers go on to suggest that the cataclysmic eruption played a significant role in a number of big historical changes in the Mediterranean, such as the downfall of the Roman Republic and the end of the Ptolemaic Kingdom of Egypt.

Reported today in the journal Proceedings of the National Academy of Sciences, an international team of climate scientists and historians looked at ice cores from across the world, which act like records of atmospheric levels and climates over the centuries. They then compared them with tree-ring-based climate records from Scandinavia, Austria, and California's White Mountains, along with climate records from a cave formation in northeast China. 

Together, they reveal the presence of major atmospheric changes that occurred in 43 BCE and continued to rumble over the following two years. Further geochemical analysis of the volcanic ash found in ice in Greenland suggests the source of the drama was an eruption of Okmok II that occurred in early 43 BCE, an event that was so catastrophically powerful it blew a 10-kilometer-wide (6.2 miles) crater around the volcano.

“We geochemically fingerprinted the ash and compared the fingerprint to a library of ashes from volcanoes thought to have erupted about this time in the first century BCE. The match to the Okmok II ash was excellent, so there is no doubt that it came from that volcano,” Joe McConnell, lead study author from the Desert Research Institute in Nevada, told IFLScience.

“There was speculation based primarily on the approximate timing that it could have been the historically documented eruption of Mount Etna or the massive eruption of Apoyeque in Nicaragua. The early 44 BCE eruption of Mt Etna was quite a small eruption by comparison, however, and the tree-ring-based records of summertime temperature show extreme cooling in 43 and 42 BCE, not 44 BCE,” McConnell added. 

It’s well-known that major volcanic eruptions can have profound effects on Earth’s climate by spewing ash and gases into the atmosphere, which influence the greenhouse effect. In the case of this eruption, the effects were short but sharp. The two years following the Okmok II eruption were some of the coldest in the Northern Hemisphere in the past 2,500 years, with seasonal average temperatures being up to 7°C (13°F) cooler than normal. Rainfall also dramatically increased, with summer precipitation levels between 50-120 percent above normal throughout southern Europe. In the autumn months, rainfall was perhaps as high as 400 percent above typical.

It's not the first time climatic changes have been linked to the eventual downfall of civilization. Of course, the downfall of major world powers is almost always a complex combination of interwoven factors; there’s never a single smoking gun. With that in mind, the researchers argue that the brief climatic changes around this particular time were likely to be another nail in the coffins of both the Ptolemaic Kingdom of Egypt and the Roman Republic, which gave rise to the Roman Empire.

"These findings lend credibility to reports of cold, famine, food shortage and disease described by ancient sources,” Andrew Wilson, a classical archaeologist from the University of Oxford, UK, said in a statement. “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.”

"Particularly striking was the severity of the Nile flood failure at the time of the Okmok eruption, and the famine and disease that was reported in Egyptian sources," added Yale University historian Joe Manning, PhD. "The climate effects were a severe shock to an already stressed society at a pivotal moment in history."

Edited 23/06/2020: This article originally stated “In 44 BCE, the year of Julius Caesar's’ assassination…” when it intended to say “In the two years following the assassination of Julius Caesar in 44 BCE...” This has been corrected since publishing.