The droughts that have been registered in summer in Europe since 2015 are the most serious in the last 2,100 years, according to a reconstruction of the hydroclimatic conditions carried out by a team of scientists. To reach this conclusion, the researchers have used tree trunks and analyzed the evolution of their growth rings, which offer abundant information about the climate.
The work, which has been published in the journal Nature Geoscience and is directed by the University of Cambridge (United Kingdom), suggests that this phenomenon is due to anthropogenic climate change and variations in jet streams.
“We are all aware of the cluster of exceptionally hot and dry summers that we have had in recent years, but we needed precise reconstructions to see how these phenomena compare to previous years,” explains Ulf Büntgen, lead researcher on the paper, professor in the department of Geography from the University of Cambridge and an expert in dendrochronology.
The results, says Büntgen, show that the summers of recent years have been “unusual” because of how dry they have been consecutively.
“In general, our knowledge of previous droughts is worse as we go back in time,” says the researcher. In this case, the data prior to the Middle Ages are “vital” because they allow us to know the variations in droughts at this time and their impact on the functioning and productivity of ecosystems and societies.
The chemical ‘footprints’ of oaks
Oak chemical ‘fingerprints’ To analyze the historical records, the team studied 27,000 chemical ‘fingerprints’ of 147 European oak trees from the Czech Republic, Switzerland and Germany. The samples come from living and dead trees, archaeological remains, subfossil materials (more recent remains) and historical constructions made with this type of wood.
These footprints are found in the stable isotopes of carbon and oxygen , present in the inner rings of these trees and that provide data on hydroclimatic changes over long periods of time.
Research has been able to construct the largest and most detailed dataset on summer hydroclimatic conditions in central Europe, from Roman times to the present day.
In this graph you can see the evolution of humidity over the centuries:
Thus, the isotopes of the oak rings “provide more precise data to reconstruct the hydroclimatic conditions of these areas”, defends the co-author of the study, Jan Esper, from the University of Mainz (Germany). Research has been able to construct the largest and most detailed dataset on summer hydroclimatic conditions in central Europe, from Roman times to the present day .
The values of these isotopes reflect the physical conditions of the place and the responses of the trees to the climatic conditions, rather than the net growth of the stem, which is obtained with standard measurements of the tree rings. In this way, carbon values depend on photosynthetic activity, while oxygen values are affected by the water source.
The results of both isotopes “are closely correlated”, details Paolo Cherubini, co-author of the work and professor at the Federal Institute for Snow and Landscape Research (WSL) in Birmensdorf (Switzerland).
Two millennia of data
The dendrochronological data of the study show that there were very humid summers in the years 200, 720 and 1100 of the present era , and very dry times in 40, 590, 950 and 1510 AD Apart from these years “out of the ordinary”, the Research indicates that Europe is an increasingly drier continent.
Researchers suggest that this abnormal summer drought period is due to human-caused global warming and associated changes in jet streams.
But the samples from the 2015-2018 period, nevertheless, show that the drought conditions of these summers exceed everything that happened in the previous 2,110 years.
“We have seen a sharp increase [in dryness] after centuries of slow and significant growth, which is especially alarming for agriculture and forestry,” says Mirek Trnka, co-author and member of the CzechGlobe Research Center in Brno, Czech Republic ). “The unprecedented forest retreat in much of Central Europe corroborates our results,” he continues.
The researchers suggest that this abnormal period of summer droughts is due to anthropogenic global warming and associated changes in jet streams. “Climate change does not mean that everything is going to get drier and drier: some places may get colder or wetter, but extreme weather conditions will be more and more frequent,” concludes Büntgen.