When the bubonic plague reached England in the summer of 1348 – spread through fleas, lice or infected peopleaccording to the latest theories – it has reached a breeding ground for disease. Londoners’ immune systems had little defense against the new strains of plague circulating through Europe, and London’s streets were a cesspool, surrounded by overcrowded, poorly ventilated houses. The conditions high in the atmosphere were also conducive to an epidemic. The jet stream, the band of winds that sails above Europe, has shifted dramatically northward, bringing with it two years of cool, damp summers that have sent people indoors, where disease spreads easily. By 1350 the Black Death had killed about a third of England’s population, if not more.
The patterns of Earth’s high winds have surprisingly widespread effects on life on land. A recent study in the journal Nature show that when the summer jet stream over Europe veers north or south from its usual path, it again brings extremes that can exacerbate epidemics, ruin crops and fuel wildfires.
“The jet stream has caused these extreme conditions for 700 years in the past without greenhouse gases,” said Ellie Broadman, a co-author of the study and a researcher at the University of Arizona. “It’s a little scary to me, thinking about the compounding effects of just more heat at the atmosphere and imagining how those extremes could become more extreme in the future.”
Understanding how the jet stream behaved in the past is essential to figuring out how it might change as the Earth warms. Scientists believe that these fickle high winds move northwards and become “wobblier”, swing closer to the poles and then closer to the equator instead of going in a straight line. But it has been difficult to draw firm conclusions since actual measurements of the jet stream go back only 60 years, Broadman said. By then, greenhouse gas emissions spewed out during the Industrial Revolution had already begun to affect his patterns.
However, for the recent study, a team of researchers from the United States, China and several countries in Europe used data from tree rings to reconstruct the position of the jet stream over the past 700 years. Then they tried to understand how these shifts affect people, comparing the results with records on epidemics, crop yields and wildfires. According to Broadman, the years when the Black Death raged through England were some of the times when the jet stream was furthest north in the new records, which go back to the year 1300.
“The big challenge now is to work out how we can actually use this new information to test and improve our climate models, and to make more confident predictions about how the jet [stream] may differ in the future,” wrote Tim Woollings, a professor of climate science at the University of Oxford a book about the jet streamsaid in an email.
The jet stream’s vagaries can lead to what the study calls cascading effects. For example, bad harvests can lead to malnutrition, which can impair people’s immune systems and exacerbate epidemics. And when people are sick, they can’t work as much in the fields, further limiting harvests. The study points out what happened in Russia in 2010, when a “blocking” pattern in the jet stream — which deflects incoming weather — caused a prolonged heat wave, which exacerbated wildfires and killed an estimated 55,000 people. In the aftermath, the country’s wheat production fell by 25 percent.
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That same kind of stable pattern may have exacerbated the devastating floods in Central Europe in September, causing Storm Boris to latch on and dump rain over the same area for days, leading to some of the heaviest rainfall the region has ever seen. Across countries, including Poland, the Czech Republic and Romania, the storm led at least two dozen deaths and caused billions of euros worth of damage.
Tracing the jet stream’s movement back to the Middle Ages was not a simple process. The researchers knew that when the jet stream shifts north, it leads to cold, wet summers in the British Isles, and hot, dry ones in the Balkans and the Mediterranean. (When the jet stream turns south, those conditions are reversed.) They also knew that the density of the wood cells in tree rings says something about the type of weather the tree endured that year. During hot, dry weather, trees are stressed, and they start adding smaller and smaller wood cells, resulting in a thin, dense band of wood, Broadman said.
So researchers sampled very old trees in different parts of Europe to see if they could piece together the position of the jet stream based on that data. After showing that the method worked reasonably well for predicting the past 60 years of jet stream behavior, they used tree rings to estimate the jet stream’s position further back centuries.
Then they matched the data with what they knew about European history, examining historical records on diseases, grain prices and more. They found that the most extreme positions of the jet stream tended to create their own extremes on the ground. In the Mediterranean, for example, wildfires occurred mostly during the hot, dry years when the jet stream was further north, and grape harvests (and wine quality) were particularly poor during the cool, wet years when the jet stream turned south.
“The really wonderful, convenient thing about working in Europe is that people have been writing things down for a very long time,” Broadman said. “Like, monks in Ireland have been writing down things about famines and epidemics for centuries and centuries.”
