Unlike Vesuvius, whose Pliny the Younger narrated the eruption that buried Pompeii in 79, most volcanoes are not lucky enough to have a chronicler at their disposal keeping a diary of their moods. Volcanologists as well as climatologists who seek to define the influence of eruptions on past climates manage differently, mainly by examining the sulfur deposits contained in the natural archives that are the ice cores taken from the polar ice caps, in Greenland and in Antarctica. However, there is another method, indirect but very elegant, that an international team has just exploited in a study published by Nature April 5to reconstruct the impact of medieval volcanism, a method which consists of finding and exploiting stories describing… total lunar eclipses.
But how can these astronomical phenomena tell us regarding the violence and climatic impact of volcanic eruptions? The cause and effect link turns out to be as hidden as it is cute. Who has already observed a lunar eclipse – during which this star passes through the shadow of the Earth – has found that our satellite remains visible all the same but takes on a tint which, in normal times, ranges from brilliant orange with the less bright color displayed by the clay of tennis courts. This is explained as follows: the rays of the Sun are deflected by atmospheric refraction while being “filtered”, and the red part of the light spectrum travels to the lunar disc. However, the brightness of the eclipsed Moon is highly dependent on the abundance of aerosols in the upper atmosphere.
Lune sang
This is where volcanoes come in. Indeed, a powerful eruption is capable of propelling volcanic material into the stratosphere, which will increase its turbidity and drastically reduce the transmission of sunlight into the Earth’s shadow. During an eclipse, the Moon can then appear very dark, or even not be visible at all. Lesser eruptions, which do not reach an altitude of ten kilometers, will have no effect on the color of our satellite.
To determine the intensity of the volcanism of the XIIe and 13e centuries, which the ice cores say were rich in eruption, the authors of the study of Nature therefore immersed themselves in various texts from the Middle Ages in search of descriptions of lunar eclipses. In Europe, explains Sébastien Guillet, a French paleoclimatologist working at the University of Geneva and the first signatory of the article, “The main sources are annals and chronicles written mostly in monasteries. The monks gave information on what they saw on a daily basis, important political facts, but they also evoked a certain number of natural phenomena such as lunar eclipses..
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