Tree rings reveal: It has never been this warm in the past 1200 years

The Middle Ages and the centuries that followed were turbulent, also climatically: not only was there a "Little Ice Age", but also its opposite: the "Medieval Climate Anomaly", during which it may have been unusually warm. The latter can clearly be seen in reconstructed temperatures from annual tree rings. In fact, reconstructed Medieval temperatures are often portrayed as higher than today’s temperatures. This has long been a puzzle because there is no known physical explanation for such exceptional Medieval warmth. Climate models are therefore unable to simulate it and instead show only moderately warm temperatures for the Medieval Climate Anomaly.

"Previous reconstructions are based on the width or density of the annual tree rings," explains Georg von Arx from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. "Both are very much dependent on temperature, but sometimes other factors play a role in how wide or dense a tree ring gets". Together with other researchers, the head of the Dendrosciences research group has created a new reconstruction based on a particularly precise method to extract temperature information from trees. In contrast to previous work, the new results lead to the same conclusion as the climate models: the Medieval Climate Anomaly was cooler than previously thought, at least in Scandinavia, where the wood studied originated. Today's warming is thus likely outside the range of natural fluctuations in temperatures over the past 1200 years, the researchers conclude.