“There’s this series of factors that all lined up. And if you took any of them away, this wouldn’t have happened,” says Hannah Barker, a historian at Arizona State University in Tempe who wasn’t involved with this work. It took a confluence of climate change, animal interactions and human actions to produce medieval Europe’s plague pandemic.
(Fast and lethal, the Black Death spread more than a mile per day.)
Clues from tree rings and ice cores
Geographer Ulf Büntgen of the University of Cambridge found new clues about climate’s role in the pandemic while working on a climate archive based on tree rings. He and his colleagues use tree ring rata to reconstruct temperature and precipitation records for the past 2,000 years. “The dating is so precise,” he says.
Looking at the climate record compiled from trees across Europe, Büntgen noticed that temperatures across the Mediterranean were slightly cooler than average from 1345 to 1357. “Nothing super striking,” he says. But Büntgen wanted to understand why the cooling occurred. He suspected volcanic activity, which spews climate cooling aerosols. So he conferred with experts who study ice cores that preserve a chemical record of Earth’s atmospheric conditions. Cores from Greenland and Antarctica contained elevated sulfur, which is emitted by eruptions, in layers dated to around 1345. That suggested there may been one or a series of eruptions, likely in the tropics, at the time.
Wondering about potential societal connections, Büntgen teamed up with medieval historian Martin Bauch to piece together more of the story. Bauch found hints of volcanic activity in the historical record. People in China and Bohemia reported lunar eclipses when, based on calculations of orbits, they should not have occurred. It’s possible that particle-laden skies altered the appearance of the moon leading to the strange reports, says Bauch of the Leibniz Institute for the History and Culture of Eastern Europe in Leipzig, Germany.
