Medieval moon observations illuminate the greatest volcanoes in history

However, following a powerful volcanic eruption, the amount of dust in the stratosphere—the region of the atmosphere that begins roughly where commercial airplanes fly—can be so great that the obscured moon almost completely vanishes.

When prompted with the question of what made him link the monks' records of the brightness and color of the eclipsed moon with volcanic activity, lead author Sébastien Guillet from the University of Geneva, Geneva, Switzerland said in a press release: "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."

"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."

Guillet and colleagues looked back on historical records to find total lunar eclipses with apparent volcanic dust "veils" that could block out sun energy. Using this data, they ranked the hue and brightness of the veils and calculated the amount of aerosol in the stratosphere.  

Following this, they then reverse-engineered the dates of the Medieval eruptions, separating eruptions that influenced the stratosphere, which would produce climate changes, from those that only affected the troposphere. 

This reversal was aided by current knowledge of the duration between eruption and stratospheric aerosols, combined with historical tree-ring records.

A volcanic rival to the well-known Tambora eruption of 1815

Significantly, among the 15 eruptions analyzed in the new study, one from the middle of the 13th century was found to rival the well-known Tambora eruption from 1815, which triggered "the year without a summer" of 1816. 

The researchers argued that the Little Ice Age may have resulted from the cumulative impact of the medieval eruptions on Earth's climate. 

"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," concluded Guillet.

The full study was published in Nature on April 5 and can be found here.

Study abstract:

Explosive volcanism is a key contributor to climate variability on interannual to centennial timescales1. Understanding the far-field societal impacts of eruption-forced climatic changes requires firm event chronologies and reliable estimates of both the burden and altitude (that is, tropospheric versus stratospheric) of volcanic sulfate aerosol2,3. However, despite progress in ice-core dating, uncertainties remain in these key factors4. This particularly hinders investigation of the role of large, temporally clustered eruptions during the High Medieval Period (HMP, 1100–1300 CE), which have been implicated in the transition from the warm Medieval Climate Anomaly to the Little Ice Age5. Here we shed new light on explosive volcanism during the HMP, drawing on analysis of contemporary reports of total lunar eclipses, from which we derive a time series of stratospheric turbidity. By combining this new record with aerosol model simulations and tree-ring-based climate proxies, we refine the estimated dates of five notable eruptions and associate each with stratospheric aerosol veils. Five further eruptions, including one responsible for high sulfur deposition over Greenland circa 1182 CE, affected only the troposphere and had muted climatic consequences. Our findings offer support for further investigation of the decadal-scale to centennial-scale climate response to volcanic eruptions.