Lunar eclipses recorded by monks of the Middle Ages occurred about a year after major volcanic eruptions. Scientists cross-checked the exact days of these events with ice core information, and compared these results with contemporary data.
They deduced the date of the explosions that occurred a few months earlier - of which the monks were not aware because they were too far away. Strong tropical eruptions can cause global cooling of about 1 degree in a few years, says geomorphologist Markus Stoffel, one of the authors of the study.