A collaborative research effort involving scientists from the University of Geneva and several international institutions has shown that medieval monks, while observing the night sky, documented some of the era’s most powerful volcanic eruptions. These events are linked to the onset of the Little Ice Age, a prolonged phase of cooler global temperatures. The findings appear in a study published in Nature.
The Middle Ages stand out as a time of unusually intense volcanic activity. These eruptions are believed to have contributed to a long period of global cooling that altered climate patterns across continents and impacted agricultural practices, weather, and daily life.
Researchers spent nearly five years examining hundreds of records from Europe, the Middle East, and Japan to search for references to total lunar eclipses and the way the Moon appeared during those events. The color and darkness of the eclipsed Moon can be influenced by fine volcanic ash suspended in the atmosphere, providing a clue to past volcanic activity.
Historian records show a reliable count of 51 of 64 total lunar eclipses documented in Europe between 1100 and 1300, with five of those cases noting a moon that appeared unusually dark. This pattern aligns with cycles of atmospheric dust following major eruptions, suggesting a direct link between volcanic activity and observed celestial phenomena.
Geologists note that such darkened lunar displays occur within roughly a year of large eruptions, underscoring a temporal connection that helps scientists date ancient eruptions. By integrating ice core data from Antarctica and Greenland with medieval descriptions, researchers were able to refine the timelines of the most significant volcanic events, offering a clearer picture of their climatic impact.
The eruptions would have lowered summer temperatures and reduced the amount of sunlight reaching the Earth’s surface, creating stress for crops and altering growing seasons. In medieval culture, eclipses were often interpreted as ominous signs, yet people of the era did not automatically attribute these celestial events to volcanic activity; instead, they appeared as independent indicators of troubling times (attribution: Nature).