The climate impact of medieval volcanism is illuminated by lunar eclipses

The Lunar Eclipse of November 1258 ce: Refractive Reflection from the Atmosphere by Vulcanical Aerosols

“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.”

Dust produced by aerosols can change the atmosphere by altering Earth’s surface temperatures and atmospheric circulation.

They looked at the tree-ring records with respect to the temperatures in the Northern Hemisphere, and they found that there is a correlation between the temperature in the summer and reduced wood formation. By combining these records with climate simulations, the authors further refined the timing of five eruptions and showed that they had a pronounced impact on the climate. The eruptions seem to have had less impact on the situation.

When the moon is in Earth’s shadow, monks and other sholders make detailed descriptions of lunar eclipses. The events were thought to warn of future disasters.

Such studies certainly became more accurate when Latin translations of Islamic astronomical texts and planetary tables were made available in Europe from around 11507. However, these texts used an entirely different calendar and referenced dates that were meaningful only for their original compilers. They said that lunar observations were made far from northern Europe. Early translations were literal and offered little help to users unfamiliar with the Islamic calendar.

The solar year didn’t exactly align with a year in theJulian calendar. Sacrobosco argued that the accumulated error had reached some 11 days by around 1230 and that Easter was therefore being celebrated on the wrong day10. Church leaders were aware of the problem, but feared there would be serious resistance to changing the calendar11. The calendar of major religious institutions meant that the timing of the phases of the Moon were out of step.

There is a need for careful treatment and interpretation when looking at historical reports of lunar eclipses as some descriptions can be too brief or cryptic to provide useful information. One such example is the lunar eclipse of November 1258 ce recorded in the Azuma Kagami (吾妻鏡, vol. 5, pp. 625)66:

During a total lunar eclipse, as the Moon passes through the shadow of the Earth, it is partially illuminated by light refracted by the atmospheric limb. The spectrum of the refracted sunlight is influenced by scattering and absorption. Rayleigh (molecular) scattering is stronger at shorter wavelengths, least affecting orange or red-coloured light. When the stratosphere is little perturbed, the eclipsed Moon thus tends to appear copper to deep red. The scattering of visible light is intensified by a turbid stratosphere, which reduces transmission through the atmospheric limb and causes the Moon in an eclipse to appear dark. In extreme cases, it can appear to vanish almost completely16,17,18,36,56,57. The colour and luminosity (L) of the eclipsed Moon were rated according to the Danjon scale35, which runs from L = 0 to L = 4:

To determine the period (Tdark) when SAOD exceeded 0.1, that is, conditions for a dark total lunar eclipse (step 2.1), we used four (five for Samalas) SAOD time series. For UE1–UE6, we extracted SAOD time series around the 1883 ce Krakatau and 1991 ce Pinatubo eruptions from the Sato/GISS dataset37. This dataset, based on satellite observations, ground-based optical measurements and volcanological evidence, reports SAOD at 550 nm since 1850 ce. We also extracted SAOD time series for the Pinatubo eruption from the Global Space-based Stratospheric Aerosol Climatology (GloSSAC v2) dataset38, which spans the period 1979–2018 ce. As observational data are unavailable before the mid-nineteenth century, we estimated the residence time of volcanic stratospheric aerosols for each eruption (UE1–UE6) from the eVolv2k database4. In the case of the 1257 ce Samalas eruptions, we also relied on results of the IPSL climate model (IPSL-CM5A-LR)40, as it treats aerosol microphysics and has been validated for the well-observed case of the 1991 ce Pinatubo eruption58.

As with any other methods, the procedure presented in this study to constrain the timing of HMP eruptions comes with several limitations. We address these caveats in the following sections but also present several avenues of research to further refine our estimates.

Totality should preferably have been observed in good weather conditions (that is, clear, dark sky), not too near the horizon and not too close to dawn or dusk16.

The Moon Colors and Darkness of the Little Ice Age: Results from a Classical Source Identified as a Volcanic Miscellanies

Reports should be contemporary with the event and preferably by an eyewitness. These conditions are not always met for the medieval sources available (see Supplementary Dataset S1 for more information).

It’s clear in the morning. Heavy rain and flooding started after the Snake’s hour. People were killed when houses were swept away. At the hour of the Horse [11 am–1 pm] the weather began to clear. During the hour of the Rat [11 pm–1 am] the Moon was eclipsed; it was not properly visible” (see Supplementary Dataset S1).

The research, published in Nature on 5 April, corroborates data extracted from ice cores and could help scientists to understand the run-up to the cold period known as the Little Ice Age, as well as the effects of a controversial technique proposed for manipulating the climate.

The authors identified hundreds of accounts that documented the 99 lunar eclipses. The information about the moon colors and darkness can be used to determine how much volcanic haze was present at the time.

A Nod to Ancient Observers: The History of the December 2, 1229 Eclipse Revealed by Vulcanos in Antarctic Polar Ice Cores and the Indonesian Island of Lombok

Knowing more about what happened in the past could also help to clarify the potential impacts of a controversial proposal known as solar geoengineering, in which particles would be injected into the stratosphere to reflect sunlight back into space. Efforts to cool the planet could be an emergency backstop to stop the worst effects of global warming, according to some scientist.

For Guillet, who spent a decade poring over historical records in his free time before finishing the project during pandemic lockdowns, the study is also a nod to the medieval observers whose accounts historians have long used to document social and political trends. “If they are able to provide such an accurate record about social and political events, why not about natural events?” he says.

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 place of the disk of the Moon was not visible, just like during the eclipse, so the old folk saw it in a different way. It was truly something to fear,” wrote Japanese poet Fujiwara no Teika, of an unprecedented dark eclipse observed on December 2, 1229.

According to Sébastien Guillet, a senior research associate at the Institute for Environmental Sciences at the University of Geneva, there is a correlation between the presence of volcanic dust in the atmosphere and an extremely dark eclipse.

Guillet believes that medieval manuscripts contain information about large but little-understood volcanic eruptions on Earth.

These dates correspond to five volcanic eruptions that were identified by traces of volcanic ash found in polar ice cores. (Of these, only the location of the 1257 eruption is known, at the Samalas volcano on the Indonesian island of Lombok.)

The effect on lunar eclipses of eruptions in the past few months are what researchers think led to volcanic eruptions happening three to 20 months before the dark eclipses.

The study authors said in a news release that they knew about the eruptions due to the fact that they left marks in the ice of the two countries.

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.