Tree-ring analysis indicates that changes in strong upper-atmosphere winds have caused varying climates across different regions of Europe for the last 700 years, possibly even longer. This phenomenon has led to distinct weather patterns, agricultural impacts, and societal changes.
While visiting her homeland of Belgium during summer, Valerie Trouet, a professor at the University of Arizona, made an intriguing observation that turned into a significant scientific insight: On days when the sun was obscured by clouds, prompting locals to wear sweaters rather than summer attire, areas like Italy, Greece, and the Balkans were experiencing warm and dry conditions. These regions are favored holiday spots for visitors from the cooler central and northern parts of Europe.
At the Laboratory of Tree-Ring Research at the University of Arizona, Trouet examines tree rings to extract information about historical climate patterns, interpreting the wavy lines in wood much like a linguist deciphering a long-forgotten language. Her curiosity led her to wonder if these silent trees might hold critical insights into Europe’s unpredictable summers, which have witnessed centuries of fluctuating warmth and cold, sunlight and precipitation.
Trouet established a global team to gather tree-ring samples from across Europe. Their findings, presenting the first reconstruction of jet stream behavior over the last 700 years, were published on Tuesday in the journal Nature.
The Jet Stream and the Black Death
Jet streams are fast-flowing air currents found in the upper atmosphere, circulating around the globe in both northern and southern hemispheres. Their positions are not stationary; they can shift north or south or change direction depending on the varying strengths of high- and low-pressure weather systems, sometimes flowing briskly and at other times meandering slowly.
Research indicates that the jet stream plays a crucial role in determining Europe’s summer weather, functioning in a back-and-forth pattern referred to as a “dipole.”
“When the jet stream shifts to a far northern position, we observe cooler, wetter conditions over the British Isles while the Mediterranean and the Balkans experience warmer, drier weather,” explained Ellie Broadman, co-author of the study and a former postdoctoral research fellow at the Laboratory of Tree-Ring Research, now a biologist at the Sequoia-Kings Canyon Field Station of the U.S. Geological Survey. “This aligns with the severe flooding events we are currently seeing in central Europe.”
Warmer conditions in the Balkans lead to increased evaporation from the Mediterranean Sea, resulting in precipitation further north. In contrast, when the jet stream moves southward, it brings in warmer, drier air to the British Isles while sending cooler, moister air towards southeastern Europe.
Since the measurement of the jet stream only began in the late 1940s, Trouet mentioned that the research team utilized tree-ring samples across Europe to estimate temperature variations over the past seven centuries.
Every growing season, trees form rings composed of less dense wood in spring and denser wood in summer. By examining these rings under a microscope, dendrochronologists can create a detailed archive of historical climate conditions.
“We connect tiny structural features in the wood to atmospheric winds that flow miles above the Earth, which is incredibly intriguing,” Trouet noted.
Interestingly, the team observed that past jet stream patterns correlated with societal conditions, as documented in historical records.
“Europe has kept extensive written records,” said Trouet. “For instance, monks in Ireland began documenting storms as early as the 600s, and there are records spanning centuries related to grape harvest timings, grain prices, and epidemic outbreaks.”
By aligning historical data with the reconstructed jet stream patterns, Trouet’s team discovered that the climate variations attributed to the jet stream have significantly influenced European history for at least the last 700 years, possibly longer.
“Epidemics occurred more often in the British Isles when the jet stream was positioned northward,” Trouet explained. “The cooler, wetter summers caused people to stay indoors, fostering conditions that facilitated disease spread.”
The Black Death decimated Ireland from 1348 to 1350, coinciding with the jet stream’s extreme northward position over Europe.
These discoveries provide essential data that can enhance climate models used by researchers to forecast future climate scenarios, according to Broadman. Much existing research has examined how global warming impacts the jet stream.
“It’s challenging to predict trends with only 60 years of data, so a 700-year reconstruction is invaluable,” she remarked. “It allows for a comparison between historical climate fluctuations and recent events tied to greenhouse gas emissions.”
Crop Failures, Wildfires, and Extreme Weather
There is a noticeable trend indicating that the jet stream is gradually shifting northward, independent of seasonal or short-term changes.
“By correlating our reconstruction with crop failures, we see this trend likely contributes to challenges with major cereal crops and other severe weather patterns,” Trouet said. “This insight provides a glimpse into the extreme events and social consequences we might face if this trend continues.”
The findings also lay the groundwork for understanding how jet stream variations could lead to extreme weather incidents, including wildfires, Trouet added.
“Our research indicates that wildfires in the Balkans historically occurred more frequently when the jet stream was positioned northward, creating hot and dry conditions,” she explained. “This pattern mirrors the events we are witnessing this summer. The scenarios reflected in our reconstruction are playing out in real time.”
“Observing how the natural variability of the jet stream has affected societies gives us insights into potential outcomes if additional atmospheric heat and instability are introduced,” Broadman commented. “Identifying specific jet stream configurations can significantly enhance our ability to predict climate-related extremes.”
