Which is rejected due to persistent drought

Impending mega-drought : As dry as it has been in 2100 years?

The sad sight of nature in late summer 2015 should still be remembered by many. The drought that hit parts of Central Europe at the time was unusual. Scientists working with Ulf Büntgen from Cambridge University have now come to a drastic assessment: A study based on isotope data from tree rings has shown that the summer droughts in Central Europe since 2015 have been far more severe than in the 2100 years before.

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A study published almost at the same time by researchers at the Alfred Wegener Institute for Polar and Marine Research (AWI) sees the most recent dry periods, however, still within the scope of natural variability.

Consequences of anthropogenic climate change

The Cambridge researchers see the accumulation of extreme heat waves and dry periods in the summers of recent years as a result of man-made climate change. A changed position of the polar jet stream, which repeatedly stalled in 2018, for example, could lead to persistent extreme weather conditions. This has already been shown by other studies.

But climate change does not mean that it will be drier everywhere: "In some places it may be wetter or colder, but extreme conditions are becoming more frequent, which could be devastating for agriculture, ecosystems and society as a whole," says lead author Ulf Büntgen.

For the international study published in the journal "Nature Geoscience", the researchers evaluated a huge data set of tree rings in order to be able to represent the hydroclimatic conditions in Central Europe since Roman times.

Büntgen and his colleagues analyzed more than 27,000 measurements on tree rings from 147 oaks, covering the period from 75 BC. Until 2018 covered.

The samples come from archaeological remains, historical building material and living trees from areas of the Czech Republic and southeastern Bavaria.

The researchers were not satisfied with merely measuring the width of the tree rings in order to come to conclusions about the historical climate, but instead extracted stable carbon and oxygen isotopes from the rings.

They assume that the isotopes reflect the physical conditions and the response of the trees to them. This gives you a very precise picture to reconstruct hydroclimate conditions.

The isotope data of the tree rings would show that there were very humid summers in Europe, such as in AD 200, 720 and 1100 - but also very dry summers such as in AD 40, 590, 950 and 1510. Overall, a tendency towards an increasingly drier climate can be seen in the past two millennia.

The samples from 2015 to 2018 would also have shown that the drought in these years was more drastic than in the 2100 years before. "After centuries of slow, significant decline, we have seen a drastic collapse, which is particularly alarming for agriculture and forestry," said co-author Mirek Trnka.

Second study: droughts still vary naturally

In the AWI study in the specialist magazine “Communications Earth & Environment”, however, Monica Ionita-Scholz and Gerrit Lohmann come to the conclusion that the supposed “record year” 2018 as well as the very dry years 2003 and 2015 were still within the limits of natural variability .

“Our colleagues have used data from the Czech Republic, which happens to be a hotspot for drought in the last decade, but they deduce drought on a European level, which is very dangerous,” Monica Ionita-Scholz told Tagesspiegel.

Drought is a very dynamic phenomenon, you can't just use one country to describe it on a European level.

The AWI researchers used a large number of data sets: tree rings, documentary evidence, paleora analyzes, historical records of temperature and precipitation as well as water levels in rivers, while the Cambridge study only refers to tree rings.

For the AWI study only data from Germany and Switzerland were used, the conclusions were only based on this region - Central Europe.

According to this, past mega-droughts in Central Europe were longer, more violent and less warm than today's drought periods. There were extreme periods of drought between the years 1400 and 1480 and 1770-1840.

"At that time, however, these affected completely different landscapes, with a significantly higher proportion of natural mixed forests, rivers and wetlands," said the researchers.

Here, too, an alarming result

Despite the different historical classification of the most recent drought phases, the AWI researchers also come to a rather alarming result. Because a look into the past millennium shows that droughts in Germany could become even more extreme.

When multiple factors, including natural ones, come together, an extreme drought could occur. "In addition to rising temperatures, this includes solar radiation and certain weather and flow conditions in the North Atlantic, as forecast for the future," the researchers write.

Prepare for extreme drought

“We have to be prepared for the fact that in the course of climate change in Germany there may be extreme droughts in the future, which will cause enormous damage to modern agriculture and forestry,” says Monica Ionita-Scholz.

In science it is widely assumed that the North Atlantic ocean circulation will weaken. "If there is then a phase of low solar activity due to the natural variability, this could cause pronounced mega-droughts that lasted for decades, as occurred in the past millennium - an enormous challenge for society and politics," says Monica Ionita-Scholz.

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