Enlarge / This Nilometer at Cairo is an ancient device that Egyptians used to measure Nile flooding, to predict the harvest and set tax levels.
Scientists used historical data from Nileometers to see how volcanic eruptions affected Nile floods (and by extension, the health of the harvest). (credit: Berthold Werner / Wikimedia Commons)
From 305-30 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.
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.
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