Jet Streams Behind Medieval 'Extreme Weather'
Published 1 October 2024 at 19.08
Domestic. Scientists have used annual rings in trees to reconstruct the jet stream's variations in Europe over the past 700 years. It turns out that jet streams may have contributed to the spread of diseases such as the plague in the Middle Ages - which could also improve the ability to predict future climates.

Jet streams travel around the globe in the northern and southern hemispheres. Their exact locations are not fixed but shift in response to changes in the location and intensity of high and low pressure systems. Sometimes jet streams act like a fast flowing stream and sometimes like a slowly meandering river. The jet stream largely determines the weather during the summer in Europe.

- When the jet stream is in an extreme northerly position we get cooler and wetter conditions over the British Isles and warmer and drier conditions over the Mediterranean and the Balkans. This is related to the climate conditions we are witnessing right now, such as catastrophic floods in Central Europe, says one of the study's co-authors, Björn Gunnarson, researcher at the Department of Natural Geography at Stockholm University, in a dispatch.

Direct measurements of the jet stream have only existed since the late 1940s. But using long series of annual rings from trees from different parts of Europe that capture temperature changes, the research team was able to reconstruct jet stream variations back to the year 1300.

By studying existing series of historical data on church taxes, grain prices, wine harvests and epidemics, the research team was able to compare these with the jet stream reconstruction they produced themselves. The researchers found, among other things, that when the Black Death plague pandemic hit Europe in 1346–1353, the jet stream was in an extreme northern position over Europe.

The study shows that epidemics occurred more frequently in the British Isles when the jet stream was further north. Because the summers were wet and cold, growth was often stunted and conditions were more favorable for the spread of disease among both animals and humans.

- There were many other, more well-known, climate factors than the jet stream that also greatly affected the harvests and the spread of disease. But it is the first time that it has been proven that the position of the jet stream affected living conditions in Europe already in the Middle Ages, says one of the study's co-authors, Fredrik Charpentier Ljungqvist, professor at the Department of History at Stockholm University.