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Beware the Ides of March, especially in years preceding cataclysmic volcanic eruptions in Alaska. Julius Caesar’s assassination in March 44 BCE set off a seventeen-year power struggle for control of the Mediterranean amidst a climate crisis.

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The conflicts between Octavian and Mark Anthony versus Brutus and Cassius, then Octavian versus Mark Antony and Cleopatra of Egypt, took place during some of the coldest and wettest years experienced in the Mediterranean region in the last 2,500 years. The resulting famines exacerbated the political instability and vice versa. By the time Octavian proclaimed himself Augustus and launched the Roman Empire in 27 BCE, the Roman Republic was finished, and the last kingdom of Egypt’s long reign was reduced to being just another Roman province.

It had been suspected, as Joseph R. McConnell and co-authors write, that a volcanic eruption inaugurated the destabilizing cold wave that saw 43 and 42 BCE as two of the coldest Mediterranean years, the start of one of the coldest decades, in the last two and a half millennia. But which volcano?

McConnell and team pinpointed the cold, with its resulting famines and political instability, to the effects of the early 43 BCE explosion of Alaska’s Okmok II. (Some volcanic eruptions throw up so many tons of material into the atmosphere, particularly sulfur-based aerosols, that these clouds act as a solar blockers, cooling the planet below.) The scientists delved into ice cores for tephra (volcanic ash deposits), climate proxies (e.g., tree rings), historical records, and other sources to pinpoint the explosion and its effects on the other side of the world. Through its chemical composition, tephra can be chemically traced to individual eruptions.

Mount Okmok is one of numerous volcanoes making up the Aleutian Arc on the Pacific Ring of Fire. That’s a long way from the Mediterranean. But the 43 BCE eruption was one of the biggest volcanic eruptions on Earth: its effects were global, notable especially in the northern hemisphere where records across Eurasia and North Africa attest to the resulting cold, crop failures, and political chaos. In 43 and 42 BCE, for instance, there was famine in Italy (exacerbated by blockade), Greece, and Egypt (where the Nile failed to flood for two years in a row): the “breadbasket” of the eastern Mediterranean failed.

“The effect of climate shocks on ancient societies most usually and directly occurred through diminished agricultural yields,” write McConnell et al., “with crop failures occurring because of insufficient or excessive rainfall […] or because of unusual growing season temperatures.”

The interpretation of “anomalous weather and other volcanically induced phenomena (e.g., visually spectacular dimming or discoloration of the solar disk) as portents also endowed these events with a significance that made their appearance politically influential.”

In this case, Virgil, Plutarch, Seneca, Suetonius, Cicero, and other Romans noted the dimming of the Sun after Caesar’s assassination, the appearance of multiple Suns (probably sun dogs), and halos around the Sun. These observations would echo in Shakespeare’s Julius Caesar (1599), where Titinius merges the metaphorical and climatological: “The sun of Rome is set! Our day is gone; Clouds, dews, and dangers have come; our deeds are done!”

Vulcanologist Clive Oppenheimer quotes this line of Shakespeare in his commentary on the McConnell et al. paper. The Okmok-Mediterranean connection is but one of the examples in what Oppenheimer calls “the entangled biographies of volcanoes and humankind.”

The scale of the Okmok eruption was akin to the much better known Tambora eruption in Indonesia in 1815, which resulted in 1816 being called “the year without a summer” in Europe and “1800 and Froze to Death” in New England. That terrible weather was what kept Mary Shelley indoors writing Frankenstein.

Okmok is still an active volcano, one of many in Alaska. In 2008, with little warning, Okmok erupted again, though to a much lesser extent than in 43 BCE. Nonetheless, Okmok and its 1,550 potentially active volcanic siblings around the world remind us that volcanoes remain what Oppenheimer calls “low probability/high-consequence” affairs of global reach. They’ve had civilization-level effects in the past and presumably will do so again—but these are just the sort of natural disaster that humans seem to be unable to prepare for.

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Proceedings of the National Academy of Sciences of the United States of America, Vol. 117, No. 27 (July 7, 2020), pp. 15443–15449
National Academy of Sciences
Proceedings of the National Academy of Sciences of the United States of America, Vol. 117, No. 30 (July 28, 2020), pp. 17470–17472
National Academy of Sciences