How Centuries-Old Records Reveal Nature's Dramatic Ecological Shifts

Field report

By carefully observing the natural world, medieval chroniclers accidentally recorded some of history's most dramatic ecological upheavals.

Melissa Breyer was Treehugger’s senior editorial director before moving to Martha Stewart. Her writing and photography have been featured in The New York Times, The Guardian, National Geographic, Audubon Magazine, and elsewhere.

British Library Board / The New York Public Library

One day, researcher Sébastien Guillet was listening to Pink Floyd's album, Dark Side of the Moon, as one does. But rather than a trippy reverie, Guillet, a senior research associate at the Institute for environmental sciences at the University of Geneva (UNIGE), had an epiphany. He connected medieval monks' careful records of unusual weather and harvest failures with the lingering ecological aftermath of massive volcanic eruptions.

"I realized that the most severe crop failures and strange atmospheric observations all occurred within a year or so of major volcanic eruptions," he recounts. "Since we know the exact dates of the chroniclers' entries, it opened the possibility of using their observations to narrow down when the eruptions must have happened."

Which is how an international team of researchers ended up drawing on readings of 12th and 13th-century European and Middle Eastern chronicles to date some of the biggest environmental disruptions known to humankind.

Led by UNIGE, the findings reveal new information about one of the most ecologically turbulent periods in Earth's history.

As explained by the university in apress releasefor the research:

"Medieval chroniclers recorded and described all kinds of historical events, including the deeds of kings and popes, important battles, and natural disasters and famines. Just as noteworthy were the unusual signs in the natural world that might foretell such calamities—failed harvests, dead livestock, ash-darkened skies, and blood-red sunsets. Of the 64 total years between 1100 and 1300 with notable ecological disturbances, the chroniclers had faithfully documented 51. In five of these cases, they also reported that the sky was exceptionally dark and the air choked with haze."

The team spent five years combing through texts from Europe and the Middle East, looking for references to crop failures, unusual cold, and atmospheric dimming. After a very large volcanic eruption, dust in the stratosphere can be so excessive that sunlight is dramatically reduced, leading to failed harvests and dying forests. As the text in the illustration above notes: "hic sol obscurabitur et luna in sanguine versa est" ("and the Sun was obscured and the Moon turned into blood").

The team also looked at the work of scribes in Japan, who likewise meticulously noted observations of ecological distress. UNIGE explains that Fujiwara no Teika wrote of an unprecedented environmental catastrophe observed on December 2, 1229:

"The old folk had never seen it like this time, with the fields barren and the rivers running thin ... It was truly something to fear."

The researchers compared what they found from the texts withice core and tree ring data.

"We know from previous work that strong tropical eruptions can induce global cooling on the order of roughly 1°C over a few years," said Markus Stoffel, UNIGE professor and last author of the study, who is also a specialist in converting measurements of tree rings into climate data. "They can also lead to rainfall anomalies with droughts in one place and floods in another."

At the time, people would not have understood that withered crops and blood-red sunsets were connected to distant volcano eruptions and the ensuing stratospheric dust. And the eruptions themselves were mostly left undocumented, making the modern detective work all the more challenging.

"We only knew about these eruptions because they left traces in the ice of Antarctica and Greenland," said co-author Clive Oppenheimer, professor at the Department of Geography at the University of Cambridge. "By putting together the information from ice cores and the descriptions from medieval texts we can now make better estimates of when and where some of the biggest eruptions of this period occurred."

The collective effect of the medieval eruptions on Earth's climate may have led to the Little Ice Age, notes UNIGE. The spate of volcanic eruptions might have helped usher in this long period of cooling. While not a true "ice age" in scientific terms, the enduring cold spell reshaped forests, shifted animal migration routes, and shortened growing seasons across Europe. The stratospheric dust from large eruptions could be so vast that it cooled temperatures by limiting the sunlight reaching the planet's surface.

As the study notes, "We hope that our new dataset will help to inform the extent of their [volcanic eruptions] role in the onset of the Little Ice Age."

"Improving our knowledge of these otherwise mysterious eruptions is crucial to understanding whether and how past volcanism affected not only the climate but also society during the Middle Ages," conclude the researchers.

Illustration: (LEFT) "Commentary on the Apocalypse by Beatus of Liébana," from the monastery of Santo Domingo de Silos, near Burgos, Spain, 1090–1109 CE. The text at the bottom of the miniature, between the dark circle on the left representing a total solar eclipse and the red circle on the right representing a total lunar eclipse, reads: "hic sol obscurabitur et luna in sanguine versa est" ("and the Sun was obscured and the Moon turned into blood"). The blood-red eclipsed Moon was seen as one possible sign of the Apocalypse. Lunar occultation descriptions from the Middle Ages often follow the Book of Revelation, suggesting that the Bible provided justification and inspiration for recording lunar eclipses and their color. (RIGHT) Thirteenth-century depiction of a lunar eclipse by Johannes de Sacrobosco.