In recent years, Central Europe has repeatedly experienced severe droughts – even in winter it rained less than the long-term average. But how extraordinary was this dry period? Researchers have now investigated this with the help of isotope analyzes of the annual rings of wood that is up to 2100 years old. It showed that although there have been drier phases time and again in the past, the extent of the droughts since 2015 has been unprecedented since the turn of the century. At the same time, the scientists identified an overarching trend towards a drier climate in Central Europe, which has persisted since ancient times.
Whether the great heat wave of 2003 or the extremely dry and hot summers from 2015 to 2018: In recent years people and nature in Central Europe have gone through several periods of heat and drought. In 2018 in particular, they led to sometimes severe crop failures, the rivers led to extremely low water levels and the trees in the forests are still suffering from the consequences today. This capricious weather raises the question of whether it is a normal climatic fluctuation, as it has happened again and again in the past, or whether there is more to it. “We are all aware of the exceptionally hot and dry summers of recent years, but we need precise reconstructions of the historical conditions in order to know how similar this is to previous events,” says first author Ulf Büntgen from the University of Cambridge.
Isotopes from annual rings allow a glimpse into the past
In order to gain more clarity, Büntgen and his team have combined the investigation of natural contemporary witnesses with the latest technology: They collected annual ring samples from 147 oaks that had grown in the Czech Republic and southern Bavaria over the past 2110 years. 21 of these oak trees were still alive, the remaining samples come from the wood of historical buildings, as well as from archaeological and fossilized wood finds. Using the sequence and shape of the annual rings, the scientists succeeded in arranging these samples in chronological order. In the next step, they then analyzed the oxygen and carbon isotopes in the tissues of the annual rings. “Unlike conventional tree ring studies, these annual ring isotopes give us much more accurate data to reconstruct the hydroclimatic conditions in temperate latitudes,” explains co-author Jan Esper from the University of Mainz. In order to calibrate their results, they first compared them with the well-documented climate data of the last century, before using them to reconstruct the climate of the past 2100 years.
The reconstruction initially showed, as expected, that there have been several phases of particularly dry or wet weather in the past. “There were times of unusually wet summers in antiquity and in the Middle Ages around the year 200, 720 and 1100,” the researchers report. “On the other hand, there were extended periods of drought around the years 40, 590 and 950.” These coincide with the end of the Celtic dominance over Europe and the time of the late ancient migration of peoples. However, dry periods from 1490 to 1540 and from the 1970s were even more striking, as Büntgen and his team report. The drought in the Renaissance period is also reflected in the fact that more than 70,000 artificial bodies of water, canals and fish ponds were built in the Czech Republic alone.
The most recent succession of drought summers is unique
Regardless of these fluctuations, the reconstruction also shows that the climate in Central Europe has gradually become increasingly drier over the past 2000 years. But even in comparison to the historical droughts and long-term trends, the drought of the recent past has a special position: “Our results show that what we have experienced in the last five summers is extraordinary for Central Europe,” says Büntgen. This sequence of summer droughts exceeds anything that has occurred in the past 2100 years. “After centuries of slow, significant decline in rainfall, we are now seeing a sharp drop, which is particularly alarming for agriculture and forestry,” says co-author Mirek Trnka from the CzechGlobe Research Center in Brno.
It has not yet been clarified in detail what causes the extraordinary dry spell of recent times. However, the scientists consider it very likely that anthropogenic climate change will play an important role in this. Because the decreasing gradient between the temperatures of the Arctic and the middle latitudes contributes to changing the large-scale air currents and in particular the jet stream that determines the climate in Central Europe. “Both the location of the summer jet stream and the position and duration of the corresponding high pressure cells over northern and central Europe can influence the spatial and temporal extent of European summer droughts,” explain Büntgen and his team. Because when the jet stream becomes weaker and more wavy, high pressure areas remain above us for longer – and that brings consistently sunny and dry weather in summer.
Source: Ulf Büntgen (University of Cambridge) et al., Nature Geoscience, doi: 10.1038 / s41561-021-00698-0