Extreme weather caused by shifts in a jet stream seems to have brought summers of fires, failed harvests and rampant disease across Europe over the past 700 years. That is the conclusion of an analysis of tree ring data, which also provides worrying clues for what the future holds under a changing climate.
Jet streams are bands of fast-moving winds high in the atmosphere, which blow around the world from west to east. The northern hemisphere polar jet influences weather conditions over Europe by dictating where events such as storms and heatwaves strike.
Observational records using satellite data only began in the past 40 years, while non-satellite data goes back to the late 1940s, so scientists have struggled to piece together how the influence of this jet stream on European weather has changed over longer periods.
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at the University of Arizona and her colleagues set out to extend this historical record by analysing tree ring data from samples taken in Scotland, the Alps and the Balkans to understand the position of the jet during European summers since 1300.
The team analysed the wood structure to identify the weather conditions at various times, and by comparing the samples, the position of the jet stream. 鈥淚nstead of 60 to 70 years of jet stream variability, we now have 700 years of jet stream variability, thanks to the tree ring data that we used,鈥� says Trouet.
The researchers then cross-referenced the information with other historical records of grape harvests, wildfires, grain prices and mortality rates, to understand the 鈥渃ascade鈥� impacts of the jet stream鈥檚 shifting position during summer.
When it is in a more northerly position, cooler and wetter weather tends to prevail over the British Isles, while hotter, drier conditions linger over southern and eastern Europe. Historically, this has brought more wildfires to eastern Europe and lower agricultural yields, leading to higher grain prices, they found. In the British Isles, by contrast, cold, wet summer conditions probably meant people spent more time inside and disease such as plague spread more easily, leading to higher levels of mortality.
Conversely, when the jet stream was in a southerly position, wet weather ruined grape harvests in the Balkans and prompted the spread of diseases through much of continental Europe.
鈥淲e understand the link between these societal factors and weather, we know that cold and wet weather is bad for your grape harvest, we know it鈥檚 bad for diseases, and so now we show that the jet stream controls that kind of weather over Europe,鈥� says Trouet.
at the University of Oxford says the study could be really valuable, particularly as satellite records for the jet stream are relatively short. 鈥淚n terms of the direct measurements of the atmosphere, we can only go back so far,鈥� he says. 鈥淭hat makes it really hard to unpick what鈥檚 going on, because we have such a short observational record. So having these studies piece together how the jet behaved is really valuable.鈥�
鈥淭he challenge for us now is to work out how to use this information practically to test and improve our models and our predictions,鈥� he adds.
Climate scientists believe jet streams are shifting polewards as the climate warms, as increasing heat in the tropics pushes the storms that help fuel the jets further from the equator.
A jet stream positioned further north than normal over Europe during summer has the most extreme impacts, the paper says, increasing the risk of hotter, drier conditions for most of the continent. Climate change will amplify the impacts of these events, says Trouet, increasing, for example, the risk of catastrophic wildfires, the spread of disease and food scarcity.
鈥淓ven under natural conditions the jet stream has a very important role in creating really extreme weather conditions and extreme societal conditions. Now if you add anthropogenic climate change to that natural variability, it amplifies the pattern,鈥� she says.
Nature
