Link Found Between Volcanic Eruptions and Regional Rainfall

Volcanic eruptions in the tropics may cause cold, rainy summers in Europe. According to researchers from the University of Bern, volcanic emissions into the atmosphere block sunlight and can thereby affect the amount of precipitation in other parts of the world.

"Historical records provide evidence that strong volcanic eruptions in the tropics are often followed by a cold and rainy summer in Central Europe. These "years without a summer" often lead to catastrophic famines; the last time in 1816 after the eruption of the Tambora volcano in Indonesia when even in Switzerland many people starved to death. Although it is known that volcanic eruptions lead to a cooling of the climate, it has until now been unclear where the additional rain comes from," according to a press release announcing the findings.

"We believe that oscillations in the African monsoon may be responsible for the rainy European summers," said Stefan Brönnimann, principal investigator of the study from the Oeschger Center.

Brönnimann and his team researched the effects of 14 "intense tropical eruptions" during the last 400 years on the climate of Europe and monsoon likely regions. Aerosols put into the stratosphere by the eruptions reflect incoming sunlight. These reflections cause a cooling effect that is more pronounced over continents than over oceans. "As a result, the summer monsoons in Africa and Asia are weakened," said the study.

These shifts lead to droughts in the Sahel, the biogeographic zone in Africa between the Sahara to the north and the Sudanian savanah to the south, and also to a southward shift of the Atlantic low-pressure systems as well as enhanced thunderstorm formation. This explains the increased precipitation over southern Central Europe and the northern Mediterranean.

"The study shows, once again, how regions separated by great distances can affect each other climatically," said Stefan Brönnimann. The findings of the study are very practical today.

The researchers stated, "Cyclic rainfall regimes like the monsoon are important for food security in many parts of the world. Although large volcanic eruptions are quite rare, they help us better understand the monsoon systems."

Understanding the connection between volcanic eruptions and climate change can prove useful. For instance, "the suggestion of slowing down warming using so-called geoengineering techniques - for example by artificially injecting particles into the stratosphere that reflect sunlight - can be seen in a new light, as even this could influence monsoon systems," according to the statement.

The study is available in the Journal of Climate.