The unexpected contribution of medieval monks to volcanology

By observing the night sky, medieval monks unwittingly recorded some of history's largest volcanic eruptions. An international team of researchers, led by the University of Geneva (UNIGE), drew on readings of 12th and 13th century European and Middle Eastern chronicles, along with ice core and tree ring data, to accurately date some of the biggest volcanic eruptions the world has ever seen. Their results, reported in the journal Nature, uncover new information about one of the most volcanically active periods in Earth's history, which some think helped to trigger the Little Ice Age, a long interval of cooling that saw the advance of European glaciers.

It took the researchers almost five years to examine hundreds of annals and chronicles from across Europe and the Middle East, in search of references to total lunar eclipses and their colouration. Total lunar eclipses occur when the moon passes into the Earth's shadow. Typically, the moon remains visible as a reddish orb because it is still bathed in sunlight bent round the Earth by its atmosphere. But after a very large volcanic eruption, there can be so much dust in the stratosphere -- the middle part of the atmosphere starting roughly where commercial aircraft fly -- that the eclipsed moon almost disappears.

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 celestial phenomena that might foretell such calamities. Mindful of the Book of Revelation, a vision of the end times that speaks of a blood-red moon, the monks were especially careful to take note of the moon's coloration. Of the 64 total lunar eclipses that occurred in Europe between 1100 and 1300, the chroniclers had faithfully documented 51. In five of these cases, they also reported that the moon was exceptionally dark.

The contribution of Japanese scribes

Asked what made him connect the monks' records of the brightness and colour of the eclipsed moon with volcanic gloom, the lead author of the work, Sébastien Guillet, senior research associate at the Institute for environmental sciences at the UNIGE, said: "I was listening to Pink Floyd's Dark Side of the Moon album when I realised that the darkest lunar eclipses all occurred within a year or so of major volcanic eruptions. 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."

The researchers found that scribes in Japan took equal note of lunar eclipses. One of the best known, Fujiwara no Teika, wrote of an unprecedented dark eclipse observed on 2 December 1229: '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.' The stratospheric dust from large volcanic eruptions was not only responsible for the vanishing moon. It also cooled summer temperatures by limiting the sunlight reaching the Earth's surface. This in turn could bring ruin to agricultural crops.

Cross-checking text and 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, full professor at the Institute for environmental sciences at the UNIGE and last author of the study, a specialist in converting measurements of tree rings into climate data, who co-designed the study. "They can also lead to rainfall anomalies with droughts in one place and floods in another."

Despite these effects, people at the time could not have imagined that the poor harvests or the unusual lunar eclipses had anything to do with volcanoes -- the eruptions themselves were all but one undocumented. "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."

Climate and society affected

To make the most of this integration, Sébastien Guillet worked with climate modellers to compute the most likely timing of the eruptions. "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," he said.

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Wave created by Tonga volcano eruption reached 90 meters -- nine times taller than 2011 Japan tsunami

The initial tsunami wave created by the eruption of the underwater Hunga Tonga Ha'apai volcano in Tonga in January 2022 reached 90 metres in height, around nine times taller than that from the highly destructive 2011 Japan tsunami, new research has found.

An international research team says the eruption should serve as a wake-up call for international groups looking to protect people from similar events in future, claiming that detection and monitoring systems for volcano-based tsunamis are '30 years behind' comparable tools used to detect earthquake-based events.

Dr Mohammad Heidarzadeh, Secretary-General of the International Tsunami Commission and a senior lecturer in the University of Bath's Department of Architecture & Civil Engineering, authored the research alongside colleagues based in Japan, New Zealand, the UK and Croatia.

By comparison, the largest tsunami waves due to earthquakes before the Tonga event were recorded following the Tōhoku earthquake near Japan in 2011 and the 1960 Chilean earthquake, reached 10 metres in initial height. Those were more destructive as they happened closer to land, with waves that were wider.

Dr Heidarzadeh says the Tonga tsunami should serve as a wake-up call for more preparedness and understanding of the causes and signs of tsunamis cause by volcanic eruptions. He says: "The Tongan tsunami tragically killed five people and caused large scale destruction, but its effects could have been even greater had the volcano been located closer to human communities. The volcano is located approximately 70 km from the Tongan capital Nuku'alofa -- this distance significantly minimized its destructive power.

"This was a gigantic, unique event and one that highlights that internationally we must invest in improving systems to detect volcanic tsunamis as these are currently around 30 years behind the systems we used to monitor for earthquakes. We are under-prepared for volcanic tsunamis."

The research was carried out by analysing ocean observation data recordings of atmospheric pressure changes and sea level oscillations, in combination with computer simulations validated with real-world data.

The research team found that the tsunami was unique as the waves were created not only by the water displaced by the volcano's eruption, but also by huge atmospheric pressure waves, which circled around the globe multiple times. This 'dual mechanism' created a two-part tsunami -- where initial ocean waves created by the atmospheric pressure waves were followed more than one hour later by a second surge created by the eruption's water displacement.

This combination meant tsunami warning centres did not detect the initial wave as they are programmed to detect tsunamis based on water displacements rather than atmospheric pressure waves.

The research team also found that the January event was among very few tsunamis powerful enough to travel around the globe -- it was recorded in all world's oceans and large seas from Japan and the United States' western seaboard in the North Pacific Ocean to the coasts within the Mediterranean Sea.

The paper, co-authored by colleagues from New Zealand's GNS Science, the Association for the Development of Earthquake Prediction in Japan, the University of Split in Croatia and at London's Brunel University, was published this week in Ocean Engineering.

Dr Aditya Gusman, Tsunami Modeller at the New Zealand-based geoscience service, says: "The 2018 Anak Krakatau volcano and 2022 Hunga Tonga-Hunga Ha'apai volcano eruptions clearly showed us that coastal areas surrounding volcano islands are at risk of being hit by destructive tsunamis. Although it may be preferable to have low-lying coastal areas completely clear from residential buildings, such a policy may not be practical for some places as volcanic tsunamis can be considered infrequent events."

Co-author Dr Jadranka Šepić, from the University of Split, Croatia, adds: "What is important is to have efficient warning systems, which include both real-time warnings and education on what to do in a case of a tsunami or warning -- such systems save lives. In addition, at volcanic areas, monitoring of volcanic activity should be organized, and more high-quality research into volcanic eruptions and areas at hazard is always a good idea."

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