Southern Germany floods locally strengthened by human- driven Climate Change

Authors

-Erika Coppola, ICTP, Italy 📨coppolae@ictp.it  🗣️Italian, English

-Fred Kucharski, ICTP, Italy 📨kucharsk@ictp.it  🗣️German, English, Italian

-Davide Faranda, IPSL-CNRS, France 📨davide.faranda@lsce.ipsl.fr 🗣️French, Italian, English

We are grateful to Sebastian Buschow and Petra Petra Friederichs (Bonn University) for many suggestions that improved the exactness of the report

Press Summary (First Published 2024/06/07, Edited 2024/07/04)

• Floods similar to the June 2024 Southern Germany floods are locally up 1.5 mm/day (up to 10%) wetter in the present than they have been in the past.  This event also leads to 1.5 mm/day (up to 10%) dryer conditions along the Italian-Austrian Border.

• This was a somewhat uncommon event.

• We mostly ascribe the  heavy precipitation associated with the Southern Germany floods  to human driven climate change and natural climate variability likely played a modest role.  

Event Description

From June 1st to June 3rd, two storms, the first, stronger one named Orinoco, led to severe rainfall and flooding in Southern Germany  (Bavaria and Baden-Wuerttemberg) with more than 150 mm of rainfall in some places in 24 hours. This caused several deaths, thousands of evacuations, land-slides, dam destructions,  severe damages, and travel disruptions. 

On Thursday, May 30th, a trough from the Atlantic region moved south-eastward and entered Europe and the northern Mediterranean, leading to a deepening of the trough and lee-cyclone development over northern Italy on Friday, May 31st. This event is also referred to as Genoa cyclogenesis.  Because of the upper-level circulation, this kind of cyclone often travels north-eastwards into central Europe along a path that is referred to as ‘Vb-track’, resulting in a high potential for summer flooding in central Europe.  Indeed, the storm entered southern Germany in the night from Friday to Saturday June 1st, leading to the extreme rainfall and flooding event, enhanced by orographic lifting on the northern slopes of the Alps.  On Sunday June 2nd, yet another trough followed the first one and moved into the Mediterranean, leading to a very similar, but weaker lee-cyclone development that moved along a similar path as the previous one, causing a second heavy rainfall and flooding event on Monday, June 3rd. 

As a result the Rosenheim in Upper Bavaria district has declared a state of emergency and 5 casualties have been registered so far and several other people are still missing in Bavaria, where flood has hit several cities along the Danube like Regensburg and Passau. 

Climate and Data Background for the Analysis

The IPCC AR6 WGI report states that in both tropics and extratropics the water cycle variability and extremes are projected to increase faster than the average change and in most of the regions. In the extratropics during the warmer season, interannual variability of precipitation and runoff are increasing faster than the seasonal changes (Chapter 8). 

At the more local scale according to Chapter 12  there is high confidence of observed increasing trend of river floods in Western and Central Europe and this increase will continue with high confidence for global warming levels higher than 1.5 oC.   This is well highlighted in Figure 12.9 panel a and c where the change of the 100 year return time of the discharge is reported. The positive change is driven by the increase of extreme precipitation in the region where Bavaria is located. This overloads the small river catchments that are then  more prone to flooding.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event we have  medium-high confidence in the robustness of our approach given the available climate data, as the event is similar to other past events in the data record

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to Southern Germany floods in June changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001) in the region [5°E 15°E 46°N 51°N]. The Surface Pressure Changes show that similar events  do not display significant changes in the present climate than what they would have been in the past. The Temperature Changes show that similar events produce temperatures in the present climate at least 1 °C warmer than what they would have been in the past, mainly over the south western part of the region analyzed. The precipitation changes exhibit local variations, with increases of up to 1.5 mm/day (up to 10%) in the western region where flooding has been observed. Negative anomalies are observed along the Italian-Austrian Border. Windspeed Changes do not show any significant change. We also note that Similar Past Events previously mainly occurred in July, while in the present climate they are occurring more in June. Changes in Urban Areas indicate that Augsburg is experiencing increased precipitation (non significant) and higher temperatures in the present compared to the past, while Munich and Regensburg show no significant changes in precipitation. We underline that  another rapid attribution study, by DWD, found similar results.

Finally, we find that sources of natural climate variability did not influence the event. This means that the changes we see in the event compared to the past may be primarily due to human driven climate change. 

Conclusion

Based on the above, we conclude that depressions similar to those causing floods in Southern German are 1 °C warmer and locally up to 1.5 mm/day (up to 10%) wetter than they have been in the past. This event also leads to 1.5 mm/day (up to 10%) dryer conditions along the Italian-Austrian Border. We interpret these floods as an event whose characteristics can be ascribed to human driven climate change.