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.
Support for climate models
“Previous reconstructions are based on the width or density of the tree's annual rings. Both are very dependent on temperature, but sometimes other factors play a role in how wide or dense an annual ring becomes in the tree”, says Kristina Seftigen, researcher in dendrochronology at the University of Gothenburg.
Together with researchers from the Swiss Federal Research Institute, WSL, Kristina Seftigen has created a new reconstruction using the trees' annual rings, based on a particularly precise method for deciphering temperature information from trees. The new results agree better with climate models and show that the Medieval Warm Period was cooler than previously thought. At least in Scandinavia, where the studied trees have grown.
The researchers therefore conclude that today's warming is likely outside the range of natural temperature fluctuations over the past 1,200 years.
50 million cells measured
In the study, the researchers used a new method to measure the cell wall thickness of the cells in the annual rings.
“Each individual cell in each annual ring records climate information from the time it was formed. By analyzing hundreds, sometimes thousands of cells per ring, we can get very good information about the climate”, says WSL researcher Jesper Björklund.
For the new time series, the researchers measured the cell walls of 50 million cells. These come from 188 living and preserved dead pine trees (Pinus sylvestris) from Sweden and Finland, whose annual rings together cover a period of 1170 years. Based on these measurements, the researchers then reconstructed the summer temperatures in Scandinavia and compared them both with model simulations of the regional climate and with previous reconstructions based on the density of the annual rings.
Unparalleled heating
The study shows that the temperatures in the climate models and the new time series from the annual rings align.
“This means that there are now two independent accounts of the regional climate that provide evidence that the medieval phase was not as warm as we previously thought. Instead, both show that the current warming is unprecedented, at least for the last millennium. It clearly shows the important role of greenhouse gas emissions in the Scandinavian temperature variations”, says Jesper Björklund.
Dendrochronology: Scientific method for dating wood based on the trunk's annual rings.
Scientific article in Nature: Fennoscandian tree-ring anatomy shows modern climate warmer than Medieval
Contact: Kristina Seftigen, researcher in dendrochronology at the Department of Earth Sciences at the University of Gothenburg, e-mail: kristina.seftigen@gvc.gu.se
Jesper Björklund, researcher in dendrochronology at the Swiss Federal Research Institute, WSL, phone +41 44 739 28 16, e-mail: jesper.bjoerklund@wsl.ch
