D.he first scientific evaluation of the deadly July floods in the Ahr and Erft regions as well as on the Meuse in Belgium has established a clear influence of climate change on such extreme floods. There is now “a high statistical certainty that man-made climate change has increased the probability and intensity of such an event and that this development will continue with further global warming”. This is the conclusion of an analysis by the “World Weather Attribution” initiative, in which 39 scientists from 22 international research institutes took part.
The initiative was founded a few years ago by the German climate researcher Friedrike Otto, who works in Oxford, in order to clarify the question of how climate change is involved in extreme weather conditions. With this attribution or assignment research, more than four hundred weather events worldwide – in particular heat and cold waves, forest and bush fires and heavy precipitation events – have been investigated.
Climate models over an area of 130 by 130 kilometers
So far, you have the greatest security in the event of heat waves. On the other hand, it turned out to be difficult to determine the influence of climate change in connection with extreme precipitation because the data series are often incomplete and too short. Even with the devastating floods between Rhineland-Palatinate, North Rhine-Westphalia and Belgium, which cost the lives of more than 220 people from July 12th to 15th, the researchers had to make do with relatively short and difficult to interpret series of measurements.
“The observation data for Germany go back 90 years, those for Central Europe 80 years”, says the meteorologist Frank Kreienkamp from the German Weather Service, who was involved in the current attribution study. From the data series and historical sources alone, no trend could have been derived, which is why the scientists used various regional and in some cases high-resolution climate models for the first time, which can also depict individual thunderstorm cells over an area of 130 by 130 kilometers and the historical climate development as well as hydrographic Simulate the effects as precisely as possible.
Since the results of different computer models differ, the impact of climate change on the area varies. Accordingly, the previous warming of 1.2 degrees compared to the pre-industrial period increased the probability of such one to two-day extreme precipitation events between 1.2 and 9 times. In other words: if such extreme events occur in this western European region without climate change every 2000 years, they must now be calculated every 400 years or so. “The trend is clear,” said Kreienkamp when presenting the study, and this coincides with the findings formulated in the IPCC’s latest global climate assessment report: “Man has clearly warmed the world, and that leads to what we have seen here have “, so the DWD meteorologist.
Take a closer look at vulnerability
If the atmosphere continues to heat up to an average of two degrees, then even more dangerous extreme precipitation would be expected every 300 years. In the course of climate change, however, not only has the probability of occurrence increased, but also the intensity of precipitation: by three to nineteen percent since global warming began. According to the hydrologist Enno Nilson from the Federal Office of Hydrology, who was involved in the study, based on level measurements, it can be concluded that historical record amounts of rainwater flowed into the Ahr and Erft in a short period of time.
According to historical sources, “water levels of a similar magnitude” were reached as early as 1804. However, the hydrological records do not go back far enough, nor are they accurate enough, to be able to compare these extreme values with one another or to be able to derive a trend in the extreme values. The meteorological processes in mid-July, which originated from the low “Bernd” flowing in from the Mediterranean, are “easier to understand than the hydrological phenomenon”, according to Nilson.
Ninety liters of rain per square meter
However, it is precisely this hydrological effect that caused exorbitant amounts of water of ninety liters per square meter per day to fall, and which is likely to have caused the greatest part of the destruction. The narrow river valleys with steep slopes and a soil that was already saturated with water by rainfalls were factors that contributed to the destructive power of the rain. When presenting the study, the researchers of the World Weather Attribution Initiative therefore pleaded for a closer look at the “vulnerability” in the regions – in other words, the risk assessments for floods – in the regions most affected.
Many authorities and experts are slowly becoming aware of the increasing risk of flooding due to climate change. None of the experts expected that the extreme weather situation could relax again in the coming years, at least not as long as climate change continues unchecked as before.