Jet stream controls on European climate and agriculture since 1300 CE

The jet stream, a high-altitude air current, is a significant driver of climate variability in the Northern Hemisphere mid-latitudes. Its latitudinal position in the North Atlantic-European sector (EU JSL) influences pressure, temperature, precipitation, and drought patterns across the region, creating a dipole effect between northwestern and southeastern Europe. However, our understanding of EU JSL variability and its historical impacts, particularly before the era of anthropogenic warming, remains limited. This study addresses this gap by using temperature-sensitive tree-ring records to reconstruct EU JSL variability over the past seven centuries (1300–2004 CE). By comparing this reconstruction to independent historical records, we aim to understand the long-term relationship between EU JSL variability and various societal impacts. This is particularly pertinent given projections of altered jet stream behavior and intensified climate extremes under climate change, which may lead to more frequent and severe heatwaves, droughts, floods, and wildfires, impacting agricultural yields and potentially threatening global food security. The European continent, with its rich historical documentation of climate and societal events, provides an ideal setting to investigate the influence of pre-industrial EU JSL variability on these extreme events and their cascading consequences.

Previous research has established the importance of the jet stream in driving climate variability, particularly in the context of modern climate change. Studies have projected a poleward shift and increased sinuosity of the jet stream under anthropogenic warming, leading to more persistent and extreme weather events. The literature highlights the amplified meridional configuration of the jet stream resulting in more frequent and intense heatwaves, droughts, floods, and wildfires. However, the long-term historical context and societal impacts of jet stream variability are less understood. While studies have utilized tree-ring data to reconstruct past climate dynamics, including storm tracks and jet stream variability, the long-term societal impacts associated with regional-scale jet stream variability in Europe have been largely undocumented. This study builds upon existing research by extending the temporal scope of jet stream variability reconstructions and exploring its linkages with historical societal events in Europe.

The study uses three temperature-sensitive tree-ring width and maximum latewood density (MXD) records from different European locations: British Isles (BRIT), the Alps (ALP), and northeastern Mediterranean (NEMED). These records, extending back to at least 1300 CE, were selected for their robust temperature signals and their contrasting responses to EU JSL positions. A new MXD record was developed for NEMED. The tree-ring chronologies exhibit significant positive correlations with regional instrumental July-August temperatures, confirming their temperature sensitivity across different frequency domains. A regression model was developed to reconstruct the summer EU JSL variability using these tree-ring data. The model’s skill was assessed using standard statistical measures, including R-squared, reduction of error (RE), and coefficient of efficiency (CE). The reconstruction was validated using instrumental EU JSL data (1948–2004 CE), showing a satisfactory fit and preservation of high-and low-frequency variability. Independent historical documentary data, including records of grape harvests, grain prices, plagues, and human mortality, were used to analyze the societal impacts of past EU JSL extremes. These historical data were complemented by gridded climate reconstructions of temperature, precipitation, and drought indices for the period 1300–2004 CE to confirm the climate dipole patterns between northwestern and southeastern Europe associated with EU JSL variability.

The 705-year reconstruction of summer EU JSL variability (1300–2004 CE) explains 38.5% of the observed variance. The reconstruction reveals a consistent dipole pattern in summer climate between northwestern (BRIT) and southeastern Europe (NEMED) driven by the EU JSL. Northern EU JSL positions coincide with cool, wet conditions in BRIT and warm, dry conditions in NEMED, and vice-versa for southern EU JSL positions. This dipole pattern is reflected in various climate variables (temperature, precipitation, drought indices) and vegetation productivity, as observed in both the instrumental period and in historical reconstructions. The impact of EU JSL variability on crop yields is more pronounced in NEMED than in BRIT. Modern maize and wheat yields in NEMED are negatively correlated with northern EU JSL positions (hot, dry summers) and positively correlated with southern EU JSL positions (cool, wet summers). Historical climate extremes corroborate the EU JSL-driven dipole pattern. Independent documentary data show that hot, dry summers in NEMED occurred mainly during northern EU JSL anomalies, while wet, cool summers were associated with southern EU JSL anomalies, with the opposite pattern being observed in BRIT. Historical records of natural hazards, such as floods and wildfires, were also linked to specific EU JSL anomalies. Significant EU JSL anomalies were also associated with extreme agricultural events (e.g., poor grape harvests, low grain prices), epidemics, and human mortality. The study further shows an increased frequency of EU JSL extremes in the 19th and 20th centuries compared to earlier periods. Recent EU JSL values, except for the exceptionally northern position in 2010, fall within the historical range, indicating that the current situation is not unprecedented but underscores the increasing frequency of these events.

The findings strongly support the long-term association between EU JSL variability and extreme climate events across Europe, impacting agricultural productivity, human health, and economic stability. This relationship is consistent across multiple independent datasets and reflects first-to-third-order impacts in a climate–society interaction model. The study highlights how EU JSL-induced climate extremes cascade through multiple societal sectors. Failed harvests lead to food scarcity and compromised immune function, increasing susceptibility to epidemics. The observed consistency of the relationship between EU JSL extremes and various societal stressors is striking, demonstrating the complex interplay between climate variability and societal impacts. The study's findings have significant implications for assessing future climate risks. Projections of northward jet stream shifts and enhanced waviness under continued warming suggest increased risk of heatwaves, droughts, wildfires, and crop failures. The historical record emphasizes the importance of considering EU JSL variability in risk assessments of compound climate extremes, especially in Europe, where this variability has been a consistent driver of climate and societal changes for centuries.

This study provides a long-term perspective (1300–2004 CE) on the influence of the North Atlantic-European jet stream on European climate and society. Using tree-ring data and historical records, it reveals a consistent relationship between jet stream position and climate extremes, impacting agriculture and human populations. The findings emphasize the importance of considering jet stream variability in assessing future climate risks, particularly concerning heatwaves, droughts, wildfires, and crop failures. Future research should focus on improving model projections of jet stream behavior under climate change and on refining the understanding of the complex interactions between climate variability and societal responses.

The tree-ring-based reconstruction of EU JSL variability focuses primarily on interannual to multi-decadal variability. The reconstruction may not fully capture the lower-frequency variability associated with longer-term climate shifts, like the Little Ice Age. The study also acknowledges limitations in the historical documentary data, including potential biases in data collection and incomplete coverage of certain regions and crops. The interpretation of historical agricultural data is further complicated by evolving farming practices and seed types across centuries. While the study successfully highlights the linkage between EU JSL positions and various societal impacts, a more precise quantification of these impacts might require more granular data and advanced analytical techniques.