環境：中世の月の観測記録から浮かび上がった当時の火山活動

Historical observations of lunar eclipses in the High Medieval Period (1100–1300 CE), many of which were recorded by European monks, may aid our understanding of volcanism in the period. The research, published in Nature, also offers insights into the effects of volcanic eruptions on the climate.

The dating of historical volcanic activity is necessary for understanding the climatic and social impacts of these events, but current methods, such as ice core dating, are subject to uncertainties. Such uncertainties have hindered the investigation of volcanism in the High Medieval Period, which is important as this activity may have played a role in the transition to the Little Ice Age. Historical recordings of lunar eclipses can aid the identification and dating of historical eruptions as the release of aerosols can affect the brightness of the moon.

Sébastien Guillet and colleagues analysed historical records — predominantly collected by European monks and clerics — from the 12th and 13th centuries, to identify total lunar eclipses that produced visible ‘veils’ of volcanic dust that can reduce incoming solar radiation. The researchers used this documentary evidence to rank the colour and brightness of the veils, from which they estimated stratospheric aerosol content. From modern understanding of the duration between eruption and stratospheric aerosols, combined with historical tree-ring records, the authors then reverse-engineered the timing of the Medieval eruptions and separated eruptions that affected the stratosphere — which would generate climate effects — from those limited to the troposphere. The findings help to improve estimates of the timings of volcanic eruptions and determine where in the atmosphere the aerosols were released. The authors hope that their dataset might help future studies to understand the roles of these eruptions in the start of the Little Ice Age.