Enlarge image
Altenahr in Rhineland-Palatinate shortly after the flood disaster in mid-July
Photo: Boris Roessler / dpa
First the drought, then the floods: Germany is increasingly affected by extreme weather. In 2018, 2019 and 2020, many parts of the country suffered from extreme drought and persistent heat, which drove farmers to despair and triggered forest dieback. This year almost 200 people were killed in devastating floods and thousands feared for their existence. In North Rhine-Westphalia and Rhineland-Palatinate, an average of 93 liters of rain fell per square meter per day, in some parts of Belgium even 106 liters. A rainy day is considered a heavy rain event from as little as 30 liters.
After such disasters, the same question always arises: How much climate change is there in the weather? Is it because of the way people live that they themselves are to blame for the fact that fields wither and rivers overflow? Answering that is not an easy matter - because extreme weather existed even before humans began to burn coal, oil and gas.
In a study published on Tuesday, 39 climate researchers examined exactly this question with a view to the floods last July. At first glance, your answer is clear: "In a warming world there is a clear tendency towards heavier precipitation," said Frank Kreienkamp, who worked on the study for the German Meteorological Service. The probability of such a flood disaster in the affected regions has increased by 1.2 to 9 times. Without man-made climate change, such an event in Central Europe would only take place around every 2000 years, according to Kreienkamp. But due to the global warming of around one degree that has already been achieved, the frequency is now even reduced to around 400 years.
In addition to western Germany, the study also examined France, parts of Belgium, the Netherlands, Luxembourg and Switzerland. Flood events of this kind are more common across the region. According to the study by the World Weather Attribution Group, which includes researchers from the University of Oxford, ETH Zurich, Columbia University and the research center, the more the warming progresses, the smaller the interval between the flood disasters would be Jülich and the German Weather Service were involved.
It does not necessarily rain more everywhere in the affected areas, rather the amount of precipitation is spread over ever shorter periods of time - this leads to heavy rain.
The water masses could not drain away and thus led to flooding.
Due to global warming in the region, the intensity of this extreme precipitation has already increased by between three and 19 percent, according to the study.
For such attribution studies, scientists simulate thousands of times on the computer how often exactly this weather situation would have existed in pre-industrial times and today.
At the same time, measurement data, for example from rain events, are also included in the calculations.
From this, the researchers then read a trend of the probability that floods or droughts would not have occurred without global warming - or would have been milder.
Problem factor in climate research: simulating precipitation
"The direction is clear, there is a plus everywhere," comments Enno Nielson from the Federal Institute for Hydrology on the rapid study.
The rains in July would have broken all historical records.
The study - even if the range is large - confirms the trend towards more extreme precipitation.
However, these numbers are rather imprecise when compared to similar studies on heat events or droughts.
There are reasons why the researchers were unable to quantify the impact of climate change on the floods in July.
According to Nielson, much precipitation data has only been available since the 1940s, and in some regions only since the 1960s.
Previously, only isolated level reports were documented.
Temperature measurements, on the other hand, have been around for a long time.
These series of measurements on precipitation are clearly too short to make a reliable statement as to whether and to what extent climate change played a role. Because even a comparison between the current level of around one degree of global warming and the pre-industrial period in the mid-19th century would require more data - but especially if you want to identify climate trends for centuries.
In the event of heat events as well as flood disasters, the World Weather Attribution Group uses an established approach that combines observed trends with climate models.
So far, the international research team has already carried out around 400 such studies, including the heat waves in Siberia and the bush fires in Australia and the heat wave in the USA this summer.
These statements were far clearer than the numbers now published.
"Heat and droughts are becoming more and more spacious and are reproduced much better by the climate models," DWD climate expert Kreienkamp told SPIEGEL.
The calculation is much simpler and therefore the statements are clearer.
Simulating rainfall, on the other hand, is more difficult.
Because these weather events are much more detailed.
While a heat wave can extend over several hundred kilometers and sometimes remains relatively stable for weeks, heavy rain only takes place in a very limited area and then moves on.
The authors of the study therefore thought a little bigger and, in addition to the areas that were badly affected in July, also included a much larger catchment area as far as the Alps and neighboring countries - and at the same time fed them into various climate models.
In the end, they had significantly more data and “more robust results” - but also larger bandwidths, i.e. high inaccuracies.
In addition, it usually takes researchers up to a year to produce really reliable results.
The figures presented today therefore only provide a first impression.
Seven percent more water vapor per degree of warming
Basically, however, this study confirms: the more the earth heats up, the more precipitation there is - although this does not increase uniformly everywhere.
According to climate researchers, the water content in the atmosphere could increase by six to seven percent per degree, which is why more rain is then possible.
It is an old physical law, the so-called Clausius-Clapeyron equation.
The idea behind it: the hotter the air above the earth's surface, the more water vapor it can absorb.
According to the data from the DWD, heavy rain has only increased slightly in Germany over the past 70 years, and the number of rainy days even tends to decrease in summer.
At the same time, the remaining precipitation is spread over fewer and fewer days - which in turn means heavy rain when more liters fall per unit of time and square meter.
The trend is therefore clear - even if there is great uncertainty as to where, how often and when such flood disasters will occur in the future.
But precisely because the prognoses are so difficult, but at the same time it is clear that with rising temperatures also extremes increase, action should be taken quickly.
"Basically, our study confirms the statements of the current World Climate Report," explains study author Kreienkamp.
In the sixth world climate report published two weeks ago, there was a separate chapter on extreme weather conditions for the first time.
The basis for this was again the numerous attribution studies that have been published in recent years.
The prospects are therefore worrying: In southern Europe, droughts could increase in the future, in the north more heavy rain - but an increase in heat waves is to be expected everywhere.
However, the authors of the study still speak of a probable tendency for precipitation - while there is »high evidence« for an increase in extremely hot weather events.
