Changes in climate affected human societies throughout the last millennium. While European cold periods in the 17th and 18th century have been assessed in detail, earlier cold periods received much less attention due to sparse information available. New evidence from proxy archives, historical documentary sources and climate model simulations permit us to provide an interdisciplinary, systematic assessment of an exceptionally cold period in the 15th century. Our assessment includes the role of internal, unforced climate variability and external forcing in shaping extreme climatic conditions and the impacts on and responses of the medieval society in north-western and central Europe. Climate reconstructions from a multitude of natural and anthropogenic archives indicate that the 1430s were the coldest decade in north-western and central Europe in the 15th century. This decade is characterised by cold winters and average to warm summers resulting in a strong seasonal cycle in temperature. Results from comprehensive climate models indicate consistently that these conditions occurred by chance due to the partly chaotic internal variability within the climate system. External forcing like volcanic eruptions tends to reduce simulated temperature seasonality and cannot explain the reconstructions. The strong seasonal cycle in temperature reduced food production and led to increasing food prices, a subsistence crisis and a famine in parts of Europe. Societies were not prepared to cope with failing markets and interrupted trade routes. In response to the crisis, authorities implemented numerous measures of supply policy and adaptation such as the installation of grain storage capacities to be prepared for future food production shortfalls

Tropical cyclones undergoing extratropical transition (ET) are thought to cause high‐impact weather (HIW) close to the transitioning tropical cyclone and in remote regions. However, no study so far clearly attributed European HIW to the downstream impact of North Atlantic ET. When Hurricane Katia underwent ET in September 2011, severe thunderstorms occurred downstream in Central Europe. We quantify the role of Katia in the European HIW, using numerical sensitivity experiments. Results show that Katia was crucial for the evolution of a narrow downstream trough. Large‐scale forcing for ascent ahead of this trough triggered deep convection. In the absence of ET, no trough was present over Europe and no HIW occurred. This study is the first unambiguous documentation that European HIW is caused by the downstream impact of North Atlantic ET and would not occur otherwise. It likewise corroborates the crucial role of ET in altering the large‐scale midlatitude flow in downstream regions.