How do you pinpoint a historical volcano eruption? Look at medieval writings about the moon, new study says

By Alexandra Mae Jones, CTVNews.ca Writer

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    Toronto, Ontario (CTV Network) — When medieval monks were looking up at the night sky, writing down their observations of celestial objects, they had no idea that their words would be invaluable centuries later to a group of scientists in a completely different field: volcanology.

A new study published Wednesday in the peer-reviewed journal Nature explains how descriptions of lunar eclipses by monks and scribes were key in studying some of the largest volcanic eruptions on Earth.

Using a combination of these medieval writings and climate data stretching back centuries, researchers were able to clarify the date of around 10 volcanic eruptions that took place between the year 1100 and 1300.

“I was listening to Pink Floyd’s Dark Side of the Moon album when I realized that the darkest lunar eclipses all occurred within a year or so of major volcanic eruptions,” lead author Sébastien Guillet, senior research associate at the Institute for environmental sciences at the UNIGE, said in a press release. “Since we know the exact days of the eclipses, it opened the possibility of using the sightings to narrow down when the eruptions must have happened.”

Researchers say that getting a more accurate date for these Earth-shattering events could help paint a clearer picture of how volcanos contribute to extreme climate variability.

HOW LUNAR ECLIPSES CONNECT TO VOLCANOS Why look at writings about lunar eclipses to find out about volcanic eruptions? Well, many historical eruptions during that time period went unrecorded.

We know that they happened — prior to this study, scientists used sedimentary traces in ice shelves in Antarctica and Greenland to establish that the time period between 1100 and 1300 was one of the most volcanically active time periods of the past 2,500 years.

The lack of eyewitness accounts for the majority of these could be due to a combination of records not surviving, the Earth being less populated, and the vast dust clouds that spread for thousands of kilometres after a big enough eruption, perhaps registering only as poor weather, and not notable enough to be recorded.

However, dust in the atmosphere that wasn’t very noticeable during daytime would have shown up in writings about eclipses, researchers theorized.

Usually a total lunar eclipse leaves the moon appearing rust-red in the sky, still visible due to light from the sun bending around the Earth even though Earth’s shadow is blocking most of the light.

This meant that if any historical writings described darker eclipses than normal, they could be a sign of recent volcanic activity.

For five years, researchers scoured medieval records, searching for descriptions of eclipses that seemed out of the ordinary.

Monks and scribes from across Europe, the Middle East and East Asia kept detailed descriptions of lunar eclipses, down to the coloration during these spectacular events.

Researchers noted that it helped that many religious texts used a blood-red moon to prophesize coming calamities, which would’ve been on the mind of ancient scribes and monks when they took great pains to describe the moon’s hue.

Scientists have been able to predict lunar eclipses with mathematical precision for centuries based on the movements of the sun, the Earth and the moon. These patterns mean we also have pinpointed dates for eclipses that occurred long before anyone was carefully tracking them.

In Europe, we know there were 64 total lunar eclipses that occurred between 1100 and 1300. Of these 64 confirmed eclipses, 51 had been described in medieval writings in detail.

Five of these reported that the moon was very dark in the sky.

A Japanese scribe also wrote about an unusually dark eclipse in early December of 1229, writing that “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.”

CLIMATE DATA CONFIRMING VOLCANIC WINTERS To confirm the theory that the darker eclipses described in these centuries-old texts could have been affected by volcanic clouds, researchers cross-referenced them with existing climate data.

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

Huge volcanic eruptions can have wide-ranging impacts on the climate immediately after – impacts so huge they leave measurable traces on the Earth’s climate history. For instance, the famous 1815 eruption of Mount Tambora in what is now Indonesia was so massive it lowered global temperatures for a year, an event sometimes called the “Year Without a Summer.”

“We know from previous work that strong tropical eruptions can induce global cooling on the order of roughly 1 C over a few years,” Markus Stoffel, full professor at the Institute for environmental sciences at the UNIGE and last author of the study, said in the release. “They can also lead to rainfall anomalies with droughts in one place and floods in another.”

By looking for these “volcanic winters” in climate records, along with other climate markers associated with large eruptions, and cross-referencing them with the written records of anomalous eclipses, researchers were able to refine the date of five notable eruptions, along with several other volcanic events.

One of these was the estimated eruption time of the 1257 eruption of the Samalas volcano in present-day Indonesia, which researchers narrowed down to the spring/summer of that year, countering an argument presented by some experts that it could have happened in 1256.

Another eruption they were able to clarify the date for in this study was narrowed down to occurring between May and August 1171 — a very specific time window for an eruption that occurred more than 800 years ago.

Getting a better picture of volcanic eruptions that occurred in the past tells us more about how volcanic eruptions impact the climate, researchers explained.

“Knowing the season when the volcanoes erupted is essential, as it influences the spread of the volcanic dust and the cooling and other climate anomalies associated with these eruptions,” Guillet said.

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