Distant volcanic eruptions may have indirectly triggered a series of revolts by the people of ancient Egypt against their despised Ptolemaic overlords.

Key points: The Ptolemaic Dynasty, which ruled Egypt between 305-30BC, was the last ancient Egyptian empire

The Ptolemaic Dynasty, which ruled Egypt between 305-30BC, was the last ancient Egyptian empire Analysis of ice core and historical data indicated many local uprisings occurred around volcanic eruptions in the last 260 years of the dynasty

Analysis of ice core and historical data indicated many local uprisings occurred around volcanic eruptions in the last 260 years of the dynasty This also coincided with reduced flooding of the Nile river

The eruptions, which took place between 305-30BC far from Egypt itself, may have altered the climate enough to reduce the annual Nile flooding.

The resulting crop failures may have further inflamed tensions between the Egyptians and their tax-loving Greek conquerors leading to uprisings, according to new research by a team of historians and climate experts.

It may have even forced the Ptolemaic dynasty to withdraw from a long-running intermittent war with their great rival the Seleucid Empire, they report today in Nature Communications.

To understand what happened during this turbulent period of history, which ended with Cleopatra, the researchers combined ice-core data on ancient volcanic eruptions with historical Nile river data and papyrus records.

This revealed an astonishing match-up between the dates of ancient volcanic eruptions, the dates of lower-than-usual Nile flows, and periods of civil unrest in Egypt.

Ancient Egyptians relied on the annual Nile flooding to grow crops. ( Flickkr: Michael Gwyther-Jones )

Researcher and climate historian Dr Francis Ludlow, from Trinity College Dublin, said Egypt's climate is so dry that the annual Nile flooding, which is caused by monsoonal rain in the Ethiopian highlands, was essential for agriculture.

"If that doesn't happen it's almost automatically catastrophic because there's no rainfall — you're not going to get your crops happening any other way."

Dr Ludlow said volcanic eruptions influence global climate in a number of ways, but chiefly by spewing sulphur dioxide gas into the upper atmosphere forming tiny particles. These particles reflect ultraviolet energy from the Sun and contribute to a slight cooling.

This cooling not only means less water is evaporated from oceans, but it also affects the formation of monsoon weather systems.

Volcanoes and war

The study looked at a period of 260 years — from 297 BC to 38BC — during which there were 16 "eruption years". This showed that three of 10 local revolts started in eruption years, and another five happened within two years of an eruption year.

Similarly, the on-again off-again war between the Ptolemaics and their Syrian rivals showed a pattern linked to eruptions; three of the nine times the war ceased happened in an eruption year, two occurred within two years of an eruption year, and one occurred three years after an eruption year.

The Ptolemaic Dynasty had a long-running war with the Seleukid Empire to the north. ( Wikimedia commons: Thomas Lessman )

As well as looking at records of Nile river heights from the Islamic Nilometer, researchers looked at other historical records for any references to Nile river floods.

Professor Joseph Manning, researcher and professor of history and classics at Yale University, gives the example of one text called The Prophecy of the Potter, which was originally thought to be a religious document with anti-Greek overtones.

"It says the sun shall not appear, seed will not sprout forth in the land, the land will be scorched, there'll be no crops but the tax man will still come," Professor Manning said.

"It's always been read metaphorically, but if you read it literally... that could describe a volcanic dust veil really well, and in fact if you have 'the Sun is obscured, the seed does not grow' juxtaposed in the same sentence, that's exactly a volcanic response."

Professor Manning said the eruptions were clusters rather than single events, which would have compounded the agricultural and social stress.

"If you have Nile failures — and some of the texts say this — of three years or more of no Nile flood, now we're talking some pretty serious vulnerabilities."

Vulnerability of monsoon-dependent regions

Commenting on the study, volcanologist Richard Arculus said awareness is growing of the potential climatic effects of large volcanic eruptions.

For example, he said, there was evidence that global temperatures declined — albeit by only a fraction of a degree — following the eruption of Mt Pinatubo in 1993 in the Philippines.

"As our records become better in terms of tie-in with societies and whether they were collapsing or succeeding or failing or dispersing, we'll see more of this," Professor Arculus said.

Impacts of large volcanic eruptions on rainfall patterns exposed a vulnerability for monsoon-dependent regions around the planet, he added.

"If you understand that if there is an eruption somewhere in the world and it will lead to a diminution of the monsoon effect in the Ethiopian highlands and that the amount of water in the Nile is going to be reduced, you might want to go short on whatever rice crop or everything else that is growing in the Nile valley."