Medieval writings link volcanic eruptions, cold weather

Medieval chronicles offer a look from the past to assess how volcanic eruptions affected the weather in Ireland for more than 1,200 years, scientists say. An international group of researchers assessed more than 40,000 written entries in the Irish Annals and compared them with measurements taken from ice cores to link the climatic consequences of volcanic eruptions to extreme cold weather events in Ireland for a 1200-year period from 431 to 1649, a study published in the journal Environmental Research Letters reported. Over this timescale as many as 48 explosive volcanic eruptions could be identified in Greenland ice cores that record the deposition of volcanic ash in annual layers of ice. Of those, 38 were associated closely in time with extreme cold events -- identified by systematically examining written entries in the Irish Annals and then picking out directly observed meteorological phenomena and conditions, such as heavy snowfall and frost, prolonged ice covering lakes and rivers, and contemporary descriptions of abnormally cold weather, researchers said. "It's clear that the scribes of the Irish Annals were diligent reporters of severe cold weather, most probably because of the negative impacts this had on society and the biosphere," lead study author Francis Ludlow from the Harvard University Center for the Environment and Department of History said. Volcanic eruptions can play a significant role in the regulation of Earth's climate through the injection into the stratosphere of sulphur dioxide gas, which is converted into sulphate aerosol particles that reflect incoming sunlight and result in an overall temporary cooling of the planet's surface. "Our major result is that explosive volcanic eruptions are strongly, and persistently, implicated in the occurrence of cold weather events over this long timescale in Ireland," Ludlow said. "In their severity, these events are quite rare for the country's mild maritime climate." The Irish Annals contain more than 1 million written words and about 40,000 distinct written entries, detailing major historical events on an annual basis.