As less water is absorbed into the clouds, less rain falls into seas and lakes. And if a volcano erupts in the Northern Hemisphere, especially at high latitudes, the cooling effect may tamper with the summertime heating that controls monsoon winds over Africa.
When rainfall is reduced and monsoons are suppressed, the Nile fails to flood as usual, starving the crops that depend on its water.
“We guess there was a lot of fear when the Egyptians see the Nile not flooding that year,” said Joseph Manning, a historian from Yale and co-author on the study. “There was fear about what’s going to happen. ‘Are we going to starve like last time when there was no flood three years in a row?’”
That fear could have fueled riots. But to establish a connection between volcanoes and revolts in ancient Egypt, the team first had to determine the dates for when the volcanoes erupted.
They did that by looking at ice core data from Greenland and Antarctica, which contain trapped sulfur from ancient volcanic eruptions. The scientists then turned to papyrus records to figure out when the Nile River failed to flood as usual.
But the records from the Ptolemaic period were all qualitative, not quantitative. So the team turned to the Nilometer record, which contains measurements taken by large instruments built during Egypt’s early Islamic period to monitor the Nile River’s annual flood level.
The researchers used the data from the Nilometer record to gain measurements from 622 A.D. to 1902 A.D., and identified 60 eruptions between those years. On average the Nile flood level was nearly nine inches lower during eruption years, the team discovered.
This suggested a pattern that may have existed during the Ptolemaic Period, as well.
After confirming a link between volcanic eruptions and poor Nile flooding, the team then matched the dates of Ptolemaic eruptions with papyrus records of well-known rebellions. They found that eight of ten large uprisings happened within two years of a volcanic eruption.
The biggest of these, the 20-year Theban revolt, began in 207 B.C., followed a large tropical eruption two years earlier. A papyrus report from this time indicated that most of the farmers were killed and the land had gone dry.
In their paper, the researchers were careful to clarify that volcanoes alone were not the cause of Egyptian revolts. Rather, the natural disasters set off a reaction that mixed other ingredients — like heavy taxation, ethnic conflict and disease — to incite social unrest.
“You have all of these things coalescing at a time, and you can image it’s a powder keg,” said Dr. Ludlow. “All of it puts a strain on the social system and can just ignite into revolt against the Ptolemaic Greek elites.”
Not every eruption in that period was linked to a revolt. The river failed to flood in the years following massive eruptions in 46 and 44 B.C. during Cleopatra’s reign, but her food allocation policies may have helped avert uprisings.
Kyle Harper, a professor of classics at the University of Oklahoma who has studied environmental change and the fall of the Roman Empire, said the new paper was compelling and that it showed a strong link between volcanic forces and their effects on the Nile River. He added that he would like to see if the analysis could be extended to Egypt’s Roman and early Islamic periods.
But Kevin Anchukaitis, a paleoclimatologist at the University of Arizona, said that the study had some shortcomings. For instance, he called the variability in the Nile River flooding during eruption years relatively small. The study also did not account for weather effects like El Niño during that period, he said.
Dr. Ludlow responded by saying that the study shows that the flood levels are consistently lower following eruptions, and that larger eruptions produced a correspondingly greater drop in flood levels.
To Michael McCormick, a professor of history at Harvard, the study offers a note of caution as we face climatic changes in the future.
“It really gives us pause for the future because volcanic eruptions will continue, and they will come at unpredictable times,” Dr. McCormick said. “It is sobering to see how this may have had an effect on a very productive economy in the ancient world, and we need to reflect on how it may affect us.”
Continue reading the main story