Medieval Monks as Accidental Geologists
Written by: Elise Hebert '24
Edited by: Wonjin Ko '25
The year was 1258 CE, and something was wrong in the sky. An unseasonable chill plagued the European summer, bringing poor harvests and destructive flooding. Famine and pestilence followed, and all the while, a strange veil of clouds hung on the horizon . On May 18, in Wiltshire, England, perched in the cloisters of the Abbey of Stanley, a monastic scribe had noted a disturbing omen: during a lunar eclipse, the shadowy moon, usually still visible as a red disk, disappeared completely into the night. “The Moon was imbued with a blood-red color for about half an hour,” the writer observed, “then darkness followed for about an hour until its wonted light was restored.” 
Thanks to ice cores, carbon dating, and a well-studied record of rock strata, geologists now know what happened in 1258 to squelch the summer and extinguish the blood moon. It wasn’t divine punishment or demonic influence, but the Samalas volcano in Indonesia, which had erupted with fire and fury sometime the year prior, propelling ash and dust 43 kilometers into the atmosphere .
Volcanoes are well known for causing local destruction, but thanks to all that ejected material, they also influence climate on a global scale. Sulfur gasses, released into the atmosphere by eruptions, can convert into fine sulfate aerosols that remain suspended above Earth, absorbing and blocking the sunlight. As the surface below cools down, we experience chills like the people of Europe in 1258. Meanwhile, the trapped sunlight heats up the cloud itself, disturbing air circulation in the high atmosphere and causing warmer Northern winters . None of this, of course, has ever escaped the notice of the human population. After the Samalas eruption, German townspeople described the conditions with a new proper name: the munkeliar, or dark year. The Norman scribe of the Notes of Coutances lamented, “There was no summer during summer” .
Such records guided the effort behind a new study, led by an international research team working from the Université de Genève, and published in Nature this April. This project took the scientists from the field to the library, where they spent five years combing through the astronomical journals of medieval monks. Their results shed new light on a mysterious spurt of volcanism that may have left a centuries-long legacy in Earth’s climate.
Volcanic eruptions provide well-defined case studies for scientists interested in the mechanisms of climate change, both over eons and within lifetimes, and for historians curious about how nature drives social change. But glimpsing the past is no easy task. Climate scientists use everything from frozen gas bubbles to dead plankton shells to pinpoint significant events and track the planet’s response, but every method comes with uncertainty.
For eruption dating, geologists turn to sulfur-rich deposits buried in ice sheets - that’s what allowed us to link the anomalies of 1258 CE to a volcanic cause . Researchers in this field can run into trouble when the natural motion of wind and ice scatter the settling dust into a patchy distribution. The ice core method also fails to capture where the dust had been before it settled, and for sulfur aerosols, that’s a critical distinction: only the dust clouds that reach the stratosphere have a noticeable effect on climate .
The Université de Genève team focused on a cluster of eruptions that occurred between 1100 and 1300 CE, an era known as the High Medieval Period. This spate of volcanism interests geologists for its potential role in causing the Little Ice Age, a cold snap that covered the Northern Hemisphere from the late 1200s to the mid-1400s . To pin down the timing and influence of the eruptions, the researchers left behind the field for the library and spent five years combing through the journals of medieval chroniclers, astronomers, and monks. Among all the notes of dark haze and cold summers, they searched for one kind of observation in particular: records of a vanishing lunar eclipse, like the one that occurred in Wiltshire, in 1258.
Dark eclipses make a surprisingly sound indicator for past volcanic events. They’re caused by the same sulfate clouds that trigger climate shifts and become noticeable only when those clouds reach the stratosphere - the crucial difference that ice cores fail to capture . Additionally, we know exactly when they happened. Scientists today can use mathematical models to calculate the dates of lunar eclipses in the past.
At the Université de Genève, the team drew on 389 eclipse records from Europe, Asia, and the Middle East. Of the 37 lunar eclipses mentioned across all the sources, only 6 were dark.
For the observers, a dark eclipse was a significant occurrence. Some of the testimonies describe awe and fear in onlookers. In Japan, in December 1229, one Fujiwara no Teika penned a dramatic paragraph: “Regarding the recent total lunar eclipse, although on previous occasions there have been totality, the old folk had never seen it like this time, with the location of the disk of the Moon not visible, just as if it had disappeared during the eclipse. Moreover, the duration was very long, and the change was extreme. It was truly something to fear.”
Across the medieval Eurasian world, astronomers interpreted these “extreme changes” with both natural and supernatural causes. The researchers at the Université de Genève noted that for Christian sources in particular, lunar eclipses came charged with a special meaning since one is mentioned as a sign of the end times in the Book of Revelation.
All six of the dark medieval eclipses lined up in time with known volcanic eruptions. With a computer simulation of global aerosol movement and settling, the researchers predicted how much dust each volcano would have to eject for the cloud to stick around until its associated eclipse record. Combined with previously existing data from ice cores and tree rings, the human observations could pin down eruptions to a range of only a few months.
Additionally, several ordinary red eclipses also lined up with known eruptions, indicating a dust-free stratosphere and weaker climate influence for these events .
The authors hope that a better timeline of eruptions and cloud movement will help future researchers dig into the causes of climate variations such as the Little Ice Age, which followed the High Medieval Period. Today, of course, we need this understanding more than ever. Like the monks of 1258 CE, we’re witnessing something significant.
Maybe seven hundred years from now, creative climatologists will comb through our own records, seeking answers in our incidental notes and announcements, our observations of a changing world.