A few years ago, Valerie Trouet, a tree scientist from Belgium, began to notice something curious. When her summer holidays at home were marred by cold rainy days, countries like Greece and Italy in the Eastern Mediterranean appeared to be battling unseasonal droughts and heatwaves. The reverse seemed to also be true—hot dry summers in Belgium and the British Isles coincided with cool wet weather in southeast Europe. Trouet started to wonder if the jet stream, that narrow current of air that encircles the globe, had anything to do with it.
Jet streams are concentrated bands of wind in the upper atmosphere that form at different latitudes at the boundaries of global air masses. The most powerful streams travel around the polar fronts, where warmer air meets colder air from the Arctic and Antarctic regions. In the north, the position and shape of the jet stream is determined by the intensity of high- and low-pressure weather systems that form seasonally over the European continent. Most of the time, the jet stream flows in more or less a straight line, keeping colder pressure systems to the north and warmer pressure systems to the south. But in some years, the shape can slacken or elongate into waves, forcing cold and wet conditions southeast into the Mediterranean and pushing warm weather to northwest. (A similar bipolar dynamic exists between Southern California and the Pacific Northwest.)
Plagues spread faster when summers were unseasonably warm.
As she dug into the research, Trouet learned that many climate models suggest this northern continental jet stream is becoming wavier with ongoing climate change, a trend that can increase the frequency of simultaneous extreme weather events at opposite ends of the continent, including heatwaves, droughts, floods, and wildfires. If the models are accurate, such extremes could result in frequent harvest failures in many parts of the continent at once. But many of these climate models rely on data collected after the 1980s, when satellite-based observations became more widely available.
To get a better sense of long-term jet stream and climate patterns, Trouet decided to team up with historians and climate scientists. In a recent publication for Nature, she and her team unveiled a model for jet stream variability that extends the record by 700 years. By studying tree rings, Trouet and a team of paleoclimate experts produced a reconstruction of summer weather extremes for the period from 1300 to 2004 A.D. They then examined historical records to see how these weather patterns were reflected in records of storms, plagues, and harvests dating back to the Middle Ages. The researchers believe that what they found suggests a causal link between jet stream anomalies and such seemingly disconnected events such as wildfires in Greece and Turkey, the spread of plagues in Ireland, and increases on wine taxes in Medieval Hungary.
“It was a non-stop treasure hunt,” says study co-author Ellie Broadman, a tree ring and paleo-climate scientist now working at the Western Ecological Research Center in California.
To reconstruct historical jet stream patterns, Trouet and her colleagues relied on trees from three locations: the Scottish Highlands in the northwest, the European alps, and Bosnian pine growing in the southeastern Mediterranean. Tree cores were studied for their “maximum late wood density,” which shows up as a dark line that separates tree rings. The color, shape, and chemical composition of this dark edge depends on late summer weather. Years heavy in drought and heatwaves result in darker and more irregular lines. “We also looked at fire scars” Broadman says, “where you can count on which years there was a wildfire.”
With the help of historians, Broadman and colleagues were also able to uncover data about the frequency of plagues, storms, and fluctuations in crop prices within their regions of interest in the southeastern and northwestern corners of Europe. Each time, they were chasing after simultaneous observations. “And it was either a null response or the expected response for everything we looked at,” says Broadman.
In Medieval Britain, they found that plagues spread faster when summers were, according to their models, unseasonably warm, when both fleas and humans were on the move. In those same years, vine crop failures and higher prices on wine were recorded in the Balkans, likely due to colder temperatures. Historic records of grain prices from the Netherlands, Belgium, and the British Isles for barley and wheat also suggested rates of crop failure were higher during years when tree ring reconstructions indicated unusually cool weather.
“While there is no super satisfying way of going back in time and saying, ‘Certainly I can tell that these things are causally linked,’” Broadman says, “I take confidence in the sheer number, the sheer variety of totally disparate records and totally different time periods and types of documents,” that confirm the correlations. “It seems too much to be a coincidence.”
For the moment, climate scientists remain in disagreement over whether the jet stream will get wavier or shift latitudinally with ongoing climate change. “There’s a broad range of opinions” says Rachel White, professor of atmospheric science at the University of British Columbia, who was not involved in the study. Much remains to be discovered about what drives the jet stream, White says.
Still, Trouet and her colleagues say their results suggest we should take fluctuations in the jet stream into account when considering future climate change-related risks, including global food security. Seven hundred years of data is worth considering. 
Lead art: Oana Liviu / Shutterstock
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