Atlantic dynamo turned up the heat over Medieval Europe

Stalagmites in a stalactite cave in Scotland Foto: Andy Baker

Taking of tree-ring cores on a Cedar tree (Cedrus atlantica) in Maroc. Photo: Jan Esper, WSL

In the latest edition of Science magazine, scientists from the WSL in Birmensdorf (Switzerland) report that medieval climate over Europe was heated by the North Atlantic Oscillation (NAO). European climate during medieval times was similar to modern-day climate and this study clarifies natural versus anthropogenic climate variability.

The NAO, defined as the pressure difference between the Icelandic Low and the Azores High, has a strong influence on European climate and results in either warm and wet (positive NAO index) or cold and dry (negative NAO index) winters. Scientists from the Dendro Sciences Unit at the WSL have teamed up with experts from England, Wales, and the USA to compare tree rings from 1000-year-old trees in Morocco with growth layers in a stalagmite from a cave in Scotland and to develop the first NAO reconstruction to extend back to the 11th century. By comparing these proxy archives to modelled climate simulations, the scientists were able to analyse temperature, precipitation, and wind conditions across Europe over the last millennium and to test the reliability of their results.

This is the first reconstruction to extend back to the "medieval climate anomaly" and to reveal the mechanism behind it. During this period of global warmth between 1000 and 1400 AD, the pressure difference between the Azores High and the Icelandic Low was historically large and, by driving warm Atlantic winds over the cold European continent in winter time, was heating the European mainland.

This investigation of medieval climate, an analogue to modern-day climate, clarifies natural versus anthropogenic climate variability. Valérie Trouet, WSL research scientist, points out that the modern-day effects of the NAO are relatively small and short-lived compared to those during the Middle Ages. This study demonstrates that climate has undergone large changes long before humans started releasing large amounts of greenhouse gases into the atmosphere, indicating that natural forcings should be taken into account when trying to understand the climate of the future.

Links

• Science magazine
  
• North Atlantic Oscillation
• European Climate of the last Millenium

Photo

• Stalagmites Scotland (Photo: Andy Baker; 408 KB)
  

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