Recent hot and dry summers in Europe – most likely brought on by the climate crisis – have led to major droughts on a scale “far more severe” than at any other time over the past 2,100 years, a new study warns.
Recent heatwaves across the continent have had major ecological and economic consequences and scientists have warned that, as the planet continues to warm, the situation will worsen.
The findings are based on natural records going back more than two millennia.
An international team led by the University of Cambridge studied what it described as the “chemical fingerprints” of European oak trees, in order to examine environmental conditions over 2,110 years.
They said the results revealed a “long-term drying trend” and found that drought conditions since 2015 had suddenly intensified beyond any other series of events over the period studied.
“This anomaly is likely the result of human-caused climate change and associated shifts in the jet stream,” the team said.
The study’s lead author, Professor Ulf Büntgen from Cambridge’s Department of Geography, who is also affiliated with the CzechGlobe Centre in Brno, in the Czech Republic, said: “We’re all aware of the cluster of exceptionally hot and dry summers we’ve had over the past few years, but we needed precise reconstructions of historical conditions to see how these recent extremes compare to previous years.
“Our results show that what we have experienced over the past five summers is extraordinary for central Europe, in terms of how dry it has been consecutively.”
Previous studies attempting to reconstruct past climates are largely restricted to temperature, the team said, but stable isotopes in tree rings can provide a greater level of detail, revealing detailed annual hydro-climatic changes.
Prof Büntgen and his colleagues from the Czech Republic, Germany and Switzerland studied more than 27,000 measurements taken from carbon and oxygen isotopes from 147 living and dead European oak trees.
The samples came from archaeological remains, subfossil materials, historical constructions and living trees from what is now the Czech Republic and parts of southeastern Bavaria in Germany.
“Generally, our understanding is worse the further back we go back in time, as datasets looking at past drought conditions are rare,” said Professor Büntgen, who is a specialist in dendrochronology, the study of tree-ring growth.
“However, insights before medieval times are particularly vital, because they enable us to get a more complete picture of past drought variations, which were essential for the functioning and productivity of ecosystems and societies.”
For each ring in each tree, researchers extracted and analysed carbon and oxygen isotopes independently, enabling them to build the largest and most detailed dataset of summer hydroclimate conditions in central Europe from Roman times to the present.
Co-author Prof Jan Esper, from the University of Mainz, in Germany said: “These tree-ring stable isotopes give us a far more accurate archive to reconstruct hydroclimate conditions in temperate areas, where conventional tree-ring studies often fail.”
Stable tree-ring isotopes differ from the usual tree-ring measures of ring width and wood density, as they reflect physical conditions and tree responses rather than net stem growth.
Over the 2,110-year period, the tree-ring isotope data showed there were very wet summers, such as 200, 720 and 1100 CE, and very dry summers, such as 40, 590, 950 and 1510 CE. Despite these “out-of-the-ordinary years”, the results showed that for the past two millennia, Europe has been slowly getting drier.
The samples from 2015 to 2018, however, showed that drought conditions in recent summers far exceeded anything prior.
“We’ve seen a sharp drop following centuries of a slow, significant decline, which is particularly alarming for agriculture and forestry,” said co-author Prof Mirek Trnka, also from the CzechGlobe Research Centrec.
“Unprecedented forest dieback across much of central Europe corroborates our results.”
Prof Büntgen said the findings did not mean all of Europe was necessarily on course for worsening summer heat waves, but rather greater unpredictability in the weather systems affecting the continent.
He said: “Climate change does not mean that it will get drier everywhere: some places may get wetter or colder, but extreme conditions will become more frequent, which could be devastating for agriculture, ecosystems and societies as a whole.”
The research is published in the journal Nature Geoscience.
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