You won’t find it in history textbooks, but the Roman Empire’s rise to dominance in Egypt and the Middle East may have been influenced by a series of volcanic eruptions that reduced rainfall.

These eruptions could have contributed to the sabotage and destruction of the Ptolemaic Kingdom on the Nile, paving the way for the rise of Cleopatra and the Roman Empire – and, ultimately, the modern Western world.

The Ptolemaic Kingdom ruled Egypt and North Africa in the final three centuries BC, known as the Hellenistic period. Now, teamwork between volcanologists and historians has revealed a close match in timing between volcanic eruptions and domestic unrest, revolts and uprisings that led to the kingdom’s downfall.

“So far, Hellenistic history has never had any climate component,” says Joseph Manning, a historian at Yale University. Bringing in the impact of climate shocks on the unfolding of history is important, he says.

“There were revolts and social unrest from 245 BC onwards, down to the mid-first century BC,” Manning told the meeting of the European Geosciences Union in Vienna, Austria, last week. “One involved the entire river valley along the Nile for 20 years.”

At the heart of the unrest were starvation and famine in the Ptolemaic Kingdom, where grain harvests were critically dependent on annual flooding of fertile plains by the East African monsoon.

Rain drops

Fallout from major eruptions that affected global climate would have cut the annual rains in the highlands of Ethiopia that drained into the Blue Nile and ultimately irrigated the kingdom’s crops.

“Aerosols from volcanoes reduce evaporation and cool the temperature, leading to fewer clouds,” says Francis Ludlow of Trinity College Dublin in Ireland, and the team’s climate historian.

Volcanic fallout is known from more recent times to interfere with an equatorial belt of air called the Intertropical Convergence Zone that seasonally shifts up and down around the world, bringing monsoon rains either north or south depending on the time of year.

“You must have rain in the Ethiopian highlands to irrigate the Nile valley, so if monsoon rains are disrupted, the usual floods would have been lost,” says Ludlow.

Matching times

Ludlow, Manning and their colleagues have now found almost exact matches between the timing of uprisings in the kingdom and new eruptions they identified. The latter were deduced from spikes in sulphate contamination in ice cores taken from Greenland and Antarctica, while dating of the uprisings came from historical accounts.

The researchers found that eight out of nine documented revolts against the Ptolemaic rulers began within two years of eruption dates.

Further historical investigations showed that the revolts severely disrupted attempts by Ptolemaic armies to seize new territory in Mesopotamia through at least nine major wars with their main rivals, the Seleucid Empire that straddled parts of the Middle East and Central Asia, between 274 and 96 BC.

Manning has now tied previously unexplained retreats from battle by the Ptolemaic rulers to the need to deal with insurrections on home territory. Unable to retain new territory on the battlefield, the kingdom eventually shrank, giving way to rivals.

The misery caused by eruptions was also linked to significant decrees issued by Ptolemaic rulers. For instance, the priests’ decree of Canopus in 238 BC reports moves by Egypt to import grain in huge amounts from overseas, underscoring the severity of the domestic famine.

“Now, we can see it’s all lining up,” says Manning. “The last four centuries BC were very active volcanically and as a result saw this incredible instability, which ultimately paved the way for the kingdom to fall and for the rise of the Roman Empire.

Modern origins

In the grand scheme of things, Manning says, the weakening of the Ptolemaic Kingdom led to the forces of Cleopatra and Antony being defeated by Octavian’s Roman army. And without Rome as we know it, Europe would have looked very different, too.

“This is the beginning of the modern world, around the second century BC,” he says, and had the Ptolemaic kingdom not fallen, the world could have looked very different.

“I find it to be a very intriguing study,” says Matthew Toohey, of the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany. He cites more recent eruptions, particularly those of Laki, Iceland, in 1783 and Novarupta, Alaska, in 1912, that we know affected the levels of the Nile river.

“There is an emerging understanding of the physical mechanisms behind changes in the monsoons and tropical rainfall after such eruptions,” says Toohey. “It’s not hard to imagine that eruptions of the more distant past had similar effects on the Nile.”

The authors have presented “fascinating” correlations between well-dated records of climate change, Nile-flood reduction and societal unrest, says Brian Dermody, who studies the impact of climate on the Roman Empire at Utrecht University in the Netherlands. “The coincidence in dating indicates that there is likely a link between these environmental changes and societal unrest within the Ptolemaic Kingdom.”

However, he says that Nile floods were highly variable throughout history – so it would be interesting to explore why some societies, such as those of the Ptolemaic period, seemingly had lower resilience to fluctuations in Nile floods than those at other times.