From 305 BCE to 30 BCE, ancient Egypt was ruled by the Ptolemaic Dynasty, a Greek family put in place after Alexander the Great conquered Egypt. Though Egypt's wealth and importance were legendary during this time, it was also a period of great civil unrest. Perhaps because they were being ruled by foreigners, the Egyptian people revolted against their leadership several times during the 200s, sparking civil wars. But now scientists believe these revolts may have broken out in part because of a chain reaction started by volcanoes halfway across the world.

There's no doubt that the Egyptians were chafing under the yoke of their foreign monarchs. The days of the great pharaohs were over, and leaders from the north were replacing Egyptian culture with Greek gods and architecture. But why did the Egyptians' resentment boil over into open revolt sometimes and remain at a steady simmer otherwise? Historian Francis Ludlow of Trinity College, Ireland, and his colleagues believe that ash, dust, and other particles released by volcanoes during the 200s BCE caused temperatures to cool around the globe. Cooling resulted in less water evaporation, which meant less rain for northern Africa and, therefore, less flooding of the life-giving Nile River.

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Because the ancient Egyptians were a farming culture that lived and died by the harvest, the annual Nile flood was key to survival. Floods meant nutrient-rich waters fed the fields and everyone could eat. Nile levels were so important to the Egyptian economy that the government based tax amounts on readings from "Nilometers," stone wells fed by the river where they could measure its height in cubits. If the levels were trending too high (destructive flooding) or too low (drought), taxes were scaled back to account for people's diminished fortunes.

Years with low rainfall inevitably meant people wanted for more and had less to lose. Gripes with the government became full-scale rebellions, like the 20-year "Theban revolt" that started in 207 BCE and the "Egyptian revolt" against Ptolemy III between 245-238 BCE. Both came after periods of increased volcanic activity. Though many other factors were in play, there is an undeniable correlation between eruptions and rebellion against the Ptolemaic regime.

The volcanic house of cards

Ludlow and his colleagues gathered data for their hypothesis from a range of sources. They consulted the historical record to figure out when rebellions and unrest took place in Egypt. In addition, they measured social unrest by looking at when the Ptolemaic government issued new edicts, such as the famous Rosetta Stone (erected during the Great Revolt of 206-186 BCE), affirming the king's power over a restless population. The researchers did this under the assumption that the king would only need to issue such edicts if he felt his regime was threatened.

Having established a chronology of uprisings, the researchers went volcano hunting. Volcanoes large enough to affect the climate also leave behind a telltale debris signature of volcanic sulfates in the ice of Greenland and Antarctica. By drilling out cores of ice from these areas, scientists can reconstruct when an eruption took place in the geological record.


But could they be certain that these eruptions affected Nile flooding? To answer this question, the researchers returned again to the historical record, observing that eruptions in the 20th century had reduced Nile floods. Another treasure trove of data came from careful records of flood levels measured by keepers of the Islamic Nilometer between 622–1902 CE. By analyzing this data, Ludlow and his colleagues were able to model how much a given eruption would affect the Nile's annual flood cycle. The researchers found a clear connection between volcanic aerosol levels in the atmosphere and weak Nile flooding.

University of Hawaii classicist Robert Littman, who was not involved in the study, urged caution when looking at flood levels. He's studied Nilometers extensively and noted that fluctuations in the level of the Nile would have varied greatly from region to region. "We have texts that say 16 cubits is the ideal level in Luxor. But in the north, in the delta, it's seven cubits." So there's no guarantee that an eruption would have affected all areas in Egypt the same way. Littman added that we also can't discount political pressures, such as ongoing wars in Syria, as catalysts for revolt.

Ludlow agreed with Littman that poor flood seasons were just one cause for social unrest. He acknowledged the revolts were "the result of many conditions that happened to coalesce at the right time—these include ethnic tensions, with some resentment bubbling away among the native Egyptian populace against the new Greek elites ruling Egypt after Alexander the Great's conquest, combined with... economic policies enacted by the Ptolemies, such as the decision to move to a great focus on free-threshing wheat—as opposed to other grains that were less vulnerable to drought—because this was the favorite bread grain in the Greek world and could be exported for greater profit."


But, he added, the low flood levels could be a tipping point. "We frame the volcanic eruptions and the associated failures of the Nile summer floodwaters as triggers because their impact occurs suddenly without warning. There may be decades without a major volcanic eruption, but then one or more suddenly occurs," he explained via e-mail. "Another way of putting it is that you could consider ethic tensions and economic stress as a chronic contribution or cause, while the sudden and short-term shock from a volcanic eruption is an acute contribution of cause."

In other words, the already-troubled Egyptian polity was teetering on the brink. The abrupt reversal of fortunes caused by bad flood seasons pushed them over into armed rebellion.

Climate and future social problems

There are interesting exceptions. During the 40s and 30s BCE, Egypt suffered greatly from reduced flooding and its attendant perils like mass starvation, plague, and inflation. But Queen Cleopatra, the last of the Ptolemies, managed to quell revolt. She did it, Ludlow said, by "offering grain relief from state granaries and banning exports of grain." Volcano-induced revolts were not inevitable. Good public policies could make up for environmental disasters.

Ludlow and his colleagues say the climate history of the Nile offers a lesson to cities coping with disaster today. Yale environmental scientist and study co-author Jennifer Marlon noted that "it is very rare in science and history to have such strong and detailed evidence documenting how societies responded to climatic shocks in the past."


UC Berkeley public policy professor Solomon Hsiang, who was not involved in the study, has spent most of his career showing correlations between natural disasters and long-term socio-economic problems. Over the past several years, he has worked with the Global Policy Lab to show that disasters caused by climate change, such as typhoons and wildfires, can lead to economic downturns from which nations may never recover.

Still, speaking at a recent conference, Hsiang said these studies should make us hopeful. "This is the first time in human history when we have the technology to look ahead at what might come down the road," he mused. "In past societies, leaders relied on fortune tellers to make decisions. We have data and science and analytics that allow us to see what's coming more clearly. So we can make informed decisions."

Nature Communications, 2017. DOI: 10.1038/s41467-017-00957-y (About DOIs.)