(CN) – Environmental stress often sparked economic, political and social instability in early societies, according to a study that examines the impact of climate change on ancient Egypt.
Using an interdisciplinary approach that combined evidence from historical records, first-hand accounts and climate modeling of large 20th-century eruptions, a team of researchers shows how volcanic events affected Nile river flow, reducing agriculturally critical summer floods.
The findings, published Tuesday in the journal Nature Communications, demonstrate how integrating historical descriptions with reconstructions of past global climates can develop both our understanding of how the climate system functions, and how climatic changes have influenced bygone societies.
“The study is of particular importance for the current debate about climate change,” said lead author Joseph Manning, a historian at Yale University.
Examining a sequence of tropical and high-latitude volcanic eruptions, the researchers focused on summer flood levels during Egypt’s Ptolemaic dynasty (305-30 B.C.), which formed after the campaigns of Alexander the Great and featured rulers such as Cleopatra.
“Ancient Egyptians depended almost exclusively on Nile summer flooding brought by the summer monsoon in East Africa to grow their crops,” Manning said.
“In years influenced by volcanic eruptions, Nile flooding was generally diminished, leading to social stress that could trigger unrest and have other political and economic consequences.”
The Nile’s flooding variations stem from volcanic eruptions, which can disrupt the climate by discharging sulfurous gases into the stratosphere, according to co-author Francis Ludlow, a climate historian at Trinity College in Ireland. These gases form aerosols that remain in the atmosphere for one or two years, reflecting solar radiation back into space.
Volcanic aerosols can also influence global hydroclimate – the varied physical and chemical factors that characterize aquatic habitats. Decreases in surface temperatures can lead to reduced evaporation over bodies of water, resulting in less rainfall. If the aerosols are distributed primarily in the Northern Hemisphere, the increased cooling in this portion of the world can also reduce the summertime heating that spurs the northward migration of monsoon winds over Africa up to the Ethiopian highlands, where the Blue Nile receives its summer floodwaters.
Since the Ptolemaic dynasty is one the most well-documented periods of ancient Egypt, researchers are confident in the timeline of major political events, the team notes, adding that the specific factors that sparked events like revolts are generally less clear. Nonetheless, the scientists were able to identify a recurring proximity between such events and the timing of major volcanic eruptions.
Understanding historical context is essential to determining how shocks from diminished Nile flooding triggered revolts and constrained Ptolemaic war-making. The team explains that disturbances stemming from poor Nile flooding would have occurred against a backdrop of various socioeconomic and political difficulties, which would have compounded the effects of the river’s variability.
“Egypt and the Nile are very sensitive instruments for climate change, and Egypt provides a unique historical laboratory in which to study social vulnerability and response to abrupt volcanic shocks,” Manning said. “Nile flood suppression from historical eruptions has been little studied, despite well documented Nile failures with severe social impacts coinciding with eruptions in 939, in 1783-1784 in Iceland, and 1912 in Alaska.
“With volcanic eruption dates fixed precisely in time, we can see society in motion around them. This is the first time for ancient history that we can begin to talk about a dynamic understanding of society.”
According to Manning, the findings alter the perception of climate changes of various scales and revolutionize our understanding of human societies and how nature shaped them in the past.
“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,” said co-author Jennifer Marlon, a research scientist at Yale.
The study provides historical context for today’s events, and what may happen going forward, and demonstrates the need for additional research into the effects of climate change on modern societies, according to the team.
“There hasn’t been a large eruption affecting the global climate system since Mount Pinatubo in the Philippines in 1991,” says Manning. “We are living in a period where we are fairly quiescent in terms of large volcanic eruptions that are affecting climate. A lot of volcanoes erupt each year but they are not affecting the climate system on the scale of some past eruptions.
“Sooner or later we will experience a large volcanic eruption, and perhaps a cluster of them, that will act to exacerbate drought in sensitive parts of the world.”