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Freie Universitaet Berlin Researchers Develop Tree-Ring Data To Indicate Unusual Summer Droughts

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Using data taken from tree-ring isotopes, researchers have proven that the summer droughts that have swept parts of Europe in recent years were actually very unusual in comparison to droughts experienced during the past centuries. In their study, the group of researchers – several of whom have ties to Freie Universität Berlin – applied a novel method of analysis using isotope data. The results of their investigation have been published in the renowned scientific journal Nature. Communications Earth and Environment under the title “European Tree-Ring Isotopes Indicate Unusual Recent Hydroclimate.”

Recent decades have seen an increase in the number of floods and droughts experienced across Europe. However, little was known about the long-term, hydroclimatic changes taking place across the continent until now. The research team behind this study created a climate field reconstruction based on tree-ring stable isotopes that spans the entire European continent. This method allows for reconstructions of the European summer hydroclimate to be carried out for as far back as 1600 C.E. Among other things, the method also makes it possible to deduce information on rainfall in Europe over the past centuries. The researchers were able to determine three distinct phases of European hydroclimate variability and established that a long-term trend toward dryness had begun taking place from the mid-20th century onward.

“We found that the stable oxygen isotope ratios and carbon isotope ratios from our network of twenty-six individual well-distributed tree sites across Europe showed a pronounced seasonal consistency in climate response. This contrasts with the variety in seasonal reactions that can be observed in traditional tree-ring records such as tree-ring widths,” says Dr. Mandy Freund, principal author of the article “European Tree-Ring Isotopes Indicate Unusual Recent Hydroclimate.” Moreover, the isotope records for all tree-rings recorded as part of the study demonstrated a certain level of sensitivity to summer drought conditions – regardless of the type, height, and geographical location of the tree. By comparing the seasonal consistency and increased climate sensitivity of tree-ring isotope records with traditional tree-ring records, the researchers were able to create a coherent seasonal drought signal for the entire European continent.

“We show that the recent European summer drought (2015–2018) is highly unusual in a multi-century context and unprecedented for large parts of central and western Europe,” says Freund. By using tree-ring isotope data, the method used in the study was able to provide additional evidence for the recent summer droughts in Europe very likely being the result of human activity. The new method of analysis also made it possible to accentuate regional differences.

Dr. Freund laid the foundation for this study in her master’s thesis during her time as a meteorology student at Freie Universität Berlin, during which time she also worked closely together with the GFZ German Research Center for Geosciences in Potsdam. She went on to receive her doctorate from the University of Melbourne and is currently working as a post-doctoral fellow at the CSIRO research center in Australia.