Droughts in Europe during the summer are more severe now than at any time in the past two millennia, a new study reveals.
Researchers studied ‘chemical fingerprints’ – carbon and oxygen isotopes – in European oak trees to reconstruct summer climate over the last 2,110 years.
They found that drought conditions suddenly intensified in 2015 beyond anything in the past two thousand years, likely due to climate change.
Europe experienced severe summer heat waves and drought spells in 2003, 2015 and 2018, which affected agricultural sectors and the wine and forestry industries.
Europe’s recent summer droughts have had ‘devastating ecological and economic consequences’ – and are set to worsen as the global climate continues to warm.
Pictured, landscape of dry earth ground and viaduct, extreme drought in Entrepenas reservoir, in Guadalajara, Castilla, Spain. Recent summer droughts in Europe are far more severe than anything in the past 2,100 years, according to the new study
The research has been conducted by an international team, led by experts from the University of Cambridge, who think studying trees can provide more accurate results than ‘limited’ computer modelling simulations.
‘We’re all aware of the cluster of exceptionally hot and dry summers we’ve had over the past few years,’ said study author Professor Ulf Büntgen from the University of Cambridge’s Department of Geography.
‘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.’
Most studies attempting to reconstruct past climates are restricted to temperature – but this team analysed stable isotopes in tree rings.
Chemical characteristics of the rings inside a tree can reveal what the weather conditions were like during each year of that tree’s life.
For the study, Büntgen and his colleagues 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.
The trees, from the genus Quercus, covered a period of 2,110 years, ranging from 75 BC to 2018.
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.
Example of a polished cross-section of an oak from the Czech Republic. Researchers studied ‘chemical fingerprints’ – carbon and oxygen isotopes – in 147 living and dead European oak trees to reconstruct summer climate over the last 2,110 years
‘Generally, our understanding is worse the further back we go back in time, as datasets looking at past drought conditions are rare,’ said Büntgen.
‘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.
Carbon values depend on the photosynthetic activity, which is influenced by the amount of sunlight present, while oxygen values are affected by the source water.
Fossil fuel emissions since the industrial era have also affected the isotopic composition of carbon dioxide (CO2).
Together, carbon and oxygen isotopic values closely correlate with the conditions of the growing season, the researchers say.
‘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 the University of Mainz, Germany.
Typically, trees are aged by measuring their girth – specifically the rings that develop over time that increase that girth.
Graph shows when 147 living, historical, archaeological and subfossil oaks (green bars). The photographs at the bottom show examples of archaeological remains, subfossil materials, historical constructions and living oaks, and the grey shading on the right refers to the industrial period during which human-made fossil fuel emissions affect the isotopic composition of carbon dioxide (CO2)
But trees involved in this study had already been aged. Looking at the isotopes within them revealed more about the climate at the time they were alive.
Over the 2,110-year period, the tree-ring isotope data showed there were very wet summers, such as AD 200, 720 and 1100, and very dry summers, such as AD 40, 590, 950 and 1510.
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 between 2015 and 2018, however, revealed that drought conditions in recent summers have far exceeded anything in the 2,110 years.
‘We’ve seen a sharp drop following centuries of a slow, significant decline, which is particularly alarming for agriculture and forestry,’ said co-author Professor Mirek Trnka from the CzechGlobe Research Centre in Brno, Czech Republic.
The magnified inset shows the anatomical structure of large earlywood vessels and homogeneous latewood fibres
‘Unprecedented forest dieback across much of central Europe corroborates our results.’
The researchers say that the recent cluster of abnormally dry summers is most likely the result of human-caused climate warming, and the associated changes in the position of the jet stream.
The jet stream is a band of strong winds between five and seven miles above the Earth blowing from the west to the east.
Atmospheric circulation over Europe and the position of the jet stream represent the dominant drivers of historical drought occurrence in the region, the team say.
A further increase in the frequency and severity of heat waves under projected global warming will likely result in a multitude of harmful direct and indirect impacts on human health’, they warn.
Professor Büntgen also pointed out that climate change does not necessarily mean 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,’ he said.
The results have been published in the journal Nature Geoscience.