Medieval moon observations reveal volcanic eruptions

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Medieval observations of the moon are helping present-day researchers study a mysterious cluster of volcanic eruptions on Earth.

Monks, and other scribes from the era, made detailed descriptions of lunar eclipses, when the moon is fully in Earth’s shadow. At the time, the events were thought to foretell calamities.

Their writings often noted a reddish orb surrounding the eclipsed moon, as well as more unusual instances where the eclipsed moon seemed to disappear entirely from the sky.

“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… It was truly something to fear,” wrote Japanese poet Fujiwara no Teika, of an unprecedented dark eclipse observed on December 2, 1229.

What the chroniclers could not have known was this: An exceptionally dark eclipse is associated with the presence of a large amount of volcanic dust in the atmosphere, according to Sébastien Guillet, a senior research associate at the Institute for Environmental Sciences at the University of Geneva.

Guillet believes medieval manuscripts contain an important source of information about a string of large but little-understood volcanic eruptions on Earth.

“Improving our knowledge of these otherwise mysterious eruptions is crucial to understanding whether and how past volcanism affected not only climate but also society during the Middle Ages,” Guillet said in a news release.

Over a five-year period, Guillet and his colleagues scoured 12th and 13th-century European, Middle Eastern and East Asian sources for lunar descriptions, which — when combined with ice core and tree ring data — are allowing more accurate dating of what scientists think must have been some of the biggest volcanic eruptions the world has seen.

Volcanic dust

Of the 64 total lunar eclipses that occurred in Europe between 1100 and 1300, the study, published April 5 in the journal Nature, found documentation of 51. In six of these cases, these documents also reported that the moon was exceptionally dark — in May 1110, January 1172, December 1229, May 1258, November 1258 and November 1276.

These dates correspond with five major volcanic eruptions identified from traces of volcanic ash found in polar ice cores — in 1108, 1171, 1230, 1257 and 1276. (Of these, only the location of the 1257 eruption is known, at the Samalas volcano on the Indonesian island of Lombok.)

“These eruptions were significantly more powerful than some of the most well-known volcanic eruptions in recent history,” Guillet said. “One of these powerful eruptions, the 1257 Samalas eruption, stands as one of the largest volcanic eruptions of the past millennium.”

“The resulting volcanic aerosols blocked sunlight and caused widespread climate disruption. Historical records show that the following summer in Europe… was one of the coldest summers recorded over the past millennium.”

Researchers believe volcanic eruptions took place three to 20 months before the dark eclipses, based on observations of more recent eruptions and their effect on lunar eclipses.

“We only knew about these eruptions because they left traces in the ice of Antarctica and Greenland,” said study coauthor Clive Oppenheimer, a professor at the University of Cambridge, in a news release.

“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.”

Little Ice Age

Climate scientists usually identify past volcanic eruptions by measuring the amount — and acidity — of volcanic ash in cores drilled from polar ice, or by inferring abrupt temperature changes in tree ring records.

However, these sources sometimes conflict, because volcanic eruptions disrupt weather patterns in different ways depending on their location, intensity and timing, said Andrea Seim, chair of Forest Growth and Dendroecology at the University of Freiburg’s Institute of Forest Sciences in Germany, and Eduardo Zorita, a senior scientist at Helmholtz-Zentrum Hereon, a German research center, in a commentary that accompanied the study.

“The strength of Guillet and co-workers’ study lies in the precision with which the authors estimated the timing of volcanic eruptions — pinpointing the year, and even in some cases the month, of the event,” the pair noted. Seim and Zorita were not involved in the research.

The study said the new research would help to shed light on the onset of the Little Ice Age, a period of cold weather between 1280 and 1340 that disrupted harvests, saw the advance of European glaciers, and, according to some historians, led to a shift in the social and economic order.

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