Old Climate Clues Shed New Light on History

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This story originally appeared on Yale Environment 360 and is part of the Climate Desk collaboration.

Joseph Manning, a Yale University professor of ancient history, likes to recall the moment when he was shown an advance copy of a scholarly paper that pinpointed the timing of major volcanic eruptions over the last 2,500 years. As he read the paper, “I literally fell off my chair,” he said recently.

Relying on new geochemical techniques for analyzing ice core sediment to determine the dates of ancient volcanic activity down to the year or even season, the paper, published in Nature in 2015, showed that major eruptions worldwide caused precipitous, up-to-a-decade-long drops in global temperatures. Later research pegged those drops at as much as 13 degrees F.

What stunned Manning, an Egyptologist, was that the paper recalibrated earlier chronologies by seven to eight years, so that dates of the eruptions neatly coincided with the timing of well-documented political, social, and military upheavals over three centuries of ancient Egyptian history. The paper also correlated volcanic eruptions with major 6th century A.D. pandemics, famines, and socioeconomic turmoil in Europe, Asia, and Central America. The inescapable conclusion, the paper argued, was that volcanic soot—which cools the earth by shielding its surface from sunlight, adversely affecting growing seasons and causing crop failures — helped drive those crises.

Since then, other scholarly papers relying on paleoclimatic data—most of it drawing on state-of-the-art technologies originally designed to understand climate change—have found innumerable instances when shifts in climate helped trigger social and political tumult and, often, collapses. The latest is a paper published last month in Communications Earth and Environment that posited “a systematic association between volcanic eruptions and dynastic collapse across two millennia of Chinese history.”

The study found that 62 of 68 dynastic collapses occurred soon after Northern Hemisphere volcanic eruptions, an outcome that had only a one-in-2,000 chance of happening if the eruptions and collapses were unrelated. Chinese have traditionally cited the withdrawal of the “mandate of heaven” to explain the cold weather, droughts, floods, and agricultural failures that seemed to accompany the fall of dynasties. The paper contends that those phenomena have a climatic explanation.

All these papers are propelled by a nearly-decade-long revolution in climate science technology. A blizzard of quantitative data from “climate proxies”—ice cores, tree rings, cave stalagmites and stalactites, and lake, bog, and seabed sediments—has upended the way some historians do their work.

Joe McConnell, who runs a pathbreaking ice core analytical laboratory at the Desert Research Institute in Reno, Nevada, believes that climate data offers historians what DNA evidence provides the judicial system: an incontrovertible, objective source of crucially important information. Like DNA evidence that overturns a guilty verdict, McConnell said, the climate data is information that historians “have to take in.”

To tap that data, some historians are crossing extensive barriers within their discipline to work with biologists, geologists, geographers, paleoclimatologists, climate modelers, anthropologists, and others. These mold-breaking historians are learning geochemistry and climatology; the scientists they work with are reading history.

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