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Particularly cool periods are known to have played an important role in socioeconomic changes during the past 2500 years. That they were caused or boosted by the eruptions of large volcanos had been assumed, but never accurately quantified. This has been achieved now by group of scientists including Ulf Büntgen from the Swiss Federal Research Institute WSL, as reported in “Nature”.


As earlier studies have determined, particularly cool periods triggered crop failures and famines in the past. These events also may have contributed to pandemics and, social upheavals and migrations. Those atmospheric cool downs are widely known to be caused or strongly influenced by large volcanic eruptions: Large amounts of volcanic sulfate particles injected into the upper atmosphere, shielding the Earth’s surface from incoming solar radiation. However, until now there was no reliable method to accurately date them and thus measure their impact.

Tree rings and ice cores tell it: Volcanoes caused abrupt summer cool downs in the past

An international consortium of scientists from the Desert Research Institute (DRI), the Swiss Federal Research Institute for Forest, Snow and Landscape (WSL), the Oeschger Centre at University Bern (OCCR) and many other institutions have published today in Nature a reconstruction of the timing and associated radiative forcing of nearly 300 volcanic eruptions that occurred since the early Roman period.

Analyzing ice cores from Greenland and Antarctica for volcanic sulfate provided a year-by-year history of atmospheric sulfate levels. But only comparing the results with annually resolved and absolutely dated tree rings allowed the researchers to get a fairly accurate picture of the climatic development.

Mystery cloud and other catastrophes explained

According to the study, tropical volcanoes and large eruptions in the Northern Hemisphere high latitudes (such as Iceland and North America) – in 536, 626, and 939 CE, for example – often caused severe and widespread summer cooling in the Northern Hemisphere by injecting sulfate and ash into the high atmosphere. These particles also dimmed the atmosphere over Europe to such an extent that the effect was noted and recorded in independent archives by numerous historical eyewitnesses.

The findings resolve a long-standing debate regarding the causes of one of the most severe climate crises in recent human history. Starting in March 536, a “mystery cloud” was observed in the Mediterranean region for 18 months. It was the product of large eruption in the high-latitudes of the Northern Hemisphere. The initial cooling was intensified when a second volcano located somewhere in the tropics erupted only four years later. In the aftermath, exceptionally cold summers were observed throughout the Northern Hemisphere. This thermal shock persisted for several years, with subsequent crop failures and famines – likely contributing to the outbreak of the Justinian plague that spread throughout the Eastern Roman Empire from 541 to 543 CE, and which ultimately decimated the human population across Eurasia.

Interdisciplinary work lead to success

24 scientists from 18 universities and research institutes in the United States, United Kingdom, Switzerland, Germany, Denmark, and Sweden contributed to this work – including specialists from the solar, space, climate, and geological sciences, as well as historians.  Bringing together independent information from ice-core records, tree-ring chronologies and documentary evidence is the key to assessing possible links between climate variability and human history.” Says Ulf Büntgen, Head of the Dendroecology Group at WSL. This reconciliation of ice-core records and other records of past environmental change will help define the role that large climatic perturbations may have had in the rise and fall of civilizations throughout human history. “With new high-resolution records emerging from ice cores in Greenland and Antarctica, it will be possible to extend this reconstruction of global volcanic activity and climate impact probably all the way back into the last Ice Age,” adds Michael Sigl, formerly from DRI and now at Paul Scherrer Institute.

Link to the original article in Nature

This research was led by DRI’s Dr. Michael Sigl (now at the Paul Scherrer Institut, Switzerland) and the overall project by DRI’s Dr. Joe McConnell. It was largely funded by the U.S. National Science Foundation’s Polar Programs; with contributions from additional funding agencies and institutions in Belgium, Canada, China, Denmark, France, Germany, Iceland, Japan, Korea, The Netherlands, Sweden, Switzerland, and the United Kingdom.