European summer droughts extreme since 2015

Recent summer droughts in Europe are far more severe than anything in the past 2,100 years, according to a new study. The results are reported in the journal Nature Geoscience.

An international team studied the chemical fingerprints in European oak trees to reconstruct summer climate over 2,110 years. They found that after a long-term drying trend, drought conditions since 2015 suddenly intensified, beyond anything in the past two thousand years. This anomaly is likely the result of human-caused climate change and associated shifts in the jet stream.

Most studies attempting to reconstruct past climates are restricted to temperature, but stable isotopes in tree rings can provide annually-resolved and absolutely-dated information about hydroclimatic changes over long periods of time. Researchers from the Czech Republic, Germany and Switzerland studied more than 27,000 measurements of carbon and oxygen isotopic ratios from 147 living and dead European oak trees, covering a period of 2,110 years. The samples came from archaeological remains, subfossil materials, historical constructions, and living trees from what is now the Czech Republic and parts of south-eastern Bavaria.

For each ring in each tree, researchers at the CzechGlobe Centre in Brno 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.

“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,“ said co-author Professor Jan Esper from Johannes Gutenberg University Mainz (JGU).

“While carbon values depend on the photosynthetic activity, oxygen values are affected by the source water. Together, they closely correlate with the hydroclimatic conditions of the growing season,“ said co-author Professor Paolo Cherubini from the Federal Research Institute WSL in Birmensdorf, Switzerland.

Over the 2,110-year period, 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 show that for the past two millennia, Europe has been slowly getting drier.

The samples from 2015 to 2018, however, show that drought conditions in recent summers far exceed anything in the 2,110 years. “We have seen a sharp drop following centuries of a slow, significant decline, which is particularly alarming for agriculture and forestry. Unprecedented forest dieback across much of central Europe corroborates our results,“ said co-author Professor Mirek Trnka from the CzechGlobe Research Centre.

The researchers say that the recent cluster of abnormally dry summers is most likely the result of anthropogenic climate warming, and the associated changes in the jet stream position. “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,“ said first author Professor Ulf Büntgen from the Department of Geography at the University of Cambridge and also senior researcher at the CzechGlobe Centre.

Photo credit: U.S. Department of Agriculture, flickr/Creative Commons

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