Tree rings allow precise dating centuries into the past: The isotope ratios of carbon 13C to 12C and oxygen 18O to 16O in the cellulose of the tree trunk are indicators of drought or moisture at the time of tree growth.
Credit: G. Helle/GFZ
The 2015–2018 summer droughts have been exceptional in large parts of Western and Central Europe over the last 400 years, in terms of the magnitude of drought conditions. This indicates an influence of man-made global warming. However, multi-year droughts have occurred frequently in the 17th and 18th century, although not as severe.
This is the result of a new study in the journal Communications Earth & Environment. It was conducted by a research team led by Mandy Freund from the University of Melbourne and Gerhard Helle from the GFZ German Research Centre for Geosciences. They reconstructed European summer hydroclimate by analyzing the isotopic ratios of carbon (13C/12C) and oxygen (18O/16O) in tree-rings from a European network of forest sites.
It is the first spatial field reconstruction based on highly climate-sensitive tree-ring isotopes. This provides researchers with a unique tool for investigating climatic developments over the past centuries, both in a global overview and in a regionally differentiated manner.
European hydroclimate development
Recently, Europe has seen an increase in floods and droughts. These extreme events are part of the complex dynamics of European hydroclimate. Obtaining a precise spatially-resolved picture of the dynamics in frequency and intensity of extremes at regional to local scale is a challenge, especially in the context of longer-term climatic variability. Little is known about the long-term, spatiotemporal hydroclimatic variability across Europe due to the rather sparse availability of spatially explicit data series that properly reflect regional differences.
The European summer drought from 2015 to 2018 has particularly caused discussions about whether it falls within the normal range of climate fluctuations or is a result of anthropogenic warming.
New approach to reconstructing the hydro-climate of the last 400 years
To resolve this issue a spatially resolved reconstruction of European hydroclimate over the last 400 years was developed and published in the journal Communications Earth & Environment. It is based on the analysis of stable carbon and oxygen isotopes, i.e. variants of these elements with different masses, in old tree stands from 26 forest sites across Europe. The isotope ratios of carbon 13C to 12C and oxygen 18O to 16O are indicators of drought and moisture, respectively, at the time of tree growth.
In comparison to other natural environmental or climate archives like sediments in lakes or glacial ice, trees are widely distributed and their tree rings provide annually-resolved and absolutely-dated information about hydroclimatic changes over long periods of time and at relevant temporal frequencies from annual to multi-centennial.
The study was largely designed and led by Mandy Freund, former climate scientist at GFZ German Research Centre for Geosciences and now at CSIRO in Melbourne, Australia. It originates from an initiative of GFZ section 3.4 Climate Dynamics and Landscape Evolution, especially by Gerhard Helle, head of the Tree Ring Laboratory, and of Ulrich Cubasch, professor at the Institute for Meteorology, Freie Universität Berlin, as part of the GEO.X Research Network for Geosciences in Berlin and Potsdam. Further contributions come from colleagues of other Helmholtz-Centers (AWI, FZJ) and the University of Melbourne.
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