The 1820s Marks a Shift to Hotter‐Drier Summers in Western Europe Since 1360

Recent soil moisture (SM) reconstructions revealed plunging trends and enhanced SM‐temperature couplings over the last two decades in dry regions. However, how SM changed and whether the land‐atmosphere interaction was intensified over time in humid regions remained unknown. Here we reported the first six‐century‐long regional summer SM reconstruction (1360–2000 CE) in western Europe (WE) using three individual tree‐ring δ18O chronologies in England and France. A sharp wet‐to‐dry change occurred around 1820, earlier than 1850–1900 CE, the commonly used historical baseline of anthropogenic climate changes. Enhanced coupling of SM‐temperature followed, with stronger summer sea level pressure anomalies in dry years after the 1820s. Our results reveal that the hotter‐drier regime has also become more frequent in humid WE under global warming. Plain Language Summary Soil moisture (SM) is a primary indicator for drought conditions, and its relationship with temperature can greatly impact regional climate conditions. So far, the existing studies on SM‐temperature comparisons have been focused on arid or semi‐arid regions. Historical SM variations and their relationship with temperature over time in humid regions remain largely unknown. Here we demonstrated a six‐century regional summer SM timeseries in humid western Europe, using a paleoclimate proxy tree‐ring δ18O. We found an abrupt wet‐to‐dry change occurring around 1820. Since the 1820s, the relationship between SM and temperature was enhanced, indicating that western Europe experienced more hotter‐drier summer more frequently. Our study implies that western Europe are likely to be affected more by hot droughts. Key Points Regional six‐century‐long summer soil moisture was reconstructed in western Europe An abrupt wet‐to‐dry change of soil moisture occurred around 1820 Soil moisture‐temperature coupling was strengthened since the 1820s, indicating hotter‐drier climate in western Europe