The drought of recent summers has been unprecedented for at least 2 millennia

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In recent years, continental Europe has experienced a series of summers marked by exceptional heat and drought. Also, since 2014-2015, the hot season seems to have switched to a mode more characteristic of subtropical than temperate latitudes. New work now reveals that this chain of dry episodes has been unprecedented in magnitude for at least 2,000 years. The results were published in the journal Geoscience of nature this March 15.

To arrive at these conclusions, the researchers analyzed the carbon and oxygen isotopes preserved in the wood of European oaks. A finer method than simply evaluating the thickness or density of dark circles. “While the carbon values ​​depend on photosynthetic activity, the oxygen values ​​are affected by the water resource” says Paolo Cherubin, co-author of the article. “Together, they are closely correlated with the conditions present during the growing season”.

In total, more than 27,000 samples of wood from living or processed trees – for example, from archaeological sites – were taken into account. The data obtained made it possible to produce a reconstruction of the hydro-climatic variability in central Europe over the last 2110 years. It is the most precise and extensive series ever in the field. It reveals with unprecedented resolution the peculiarities of the summer climate from Roman times to the present day.

An unprecedented series of summers in the context of the last 2000 years

And the results are startling. Indeed, they show that if the region has been experiencing a drying trend for 2 millennia, the last summers are of unprecedented severity. The data also helps identify years that have experienced particularly wet or dry hot seasons in the past. Let us quote among others the year 200, 720 or 1100 very wet and the year 590, 950 or 1510 on the contrary very dry.

Variations in the drought index developed by the researchers between the year -75 and 2018. The higher the values, the more humid the conditions – and vice versa. In addition, different historical periods are indicated. Credits: Jan Esper / Ulf Büntgen et al. 2021.

“We are all aware of the cluster of unusually hot and dry summers we have had in recent years, but we needed precise reconstructions of historical conditions to see how these recent extremes compare to years in the past,” says Ulf Büntgen, lead author of the study. “Our results show that what we have experienced over the past five summers is extraordinary for Central Europe in terms of consecutive drought years”.

These droughts, coupled with record heat, unsurprisingly had environmental impacts and a huge economic blow. However, if we are to believe the climate projections and the hypotheses formulated by this study, the presence of similar events will grow with global warming. This does not mean that years – or series of years – more humid will not be able to punctuate this general evolution. We recall that in a warmer climate the water cycle accelerates and threatens in both dimensions: both dry and wet. In short, a world characterized by more extremes.

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