July heavy rains and floods in western part of Germany: Evolution of a tragedy!

22 July 2021

Monica Ionita, Patrick Scholz und Klaus Grosfeld (AWI)

Floods represent the most widespread natural hazard on Earth, with losses of numerous humans’ lives and material damage of large proportions. According to the United Nations Office for Disaster Risk Reduction (UNDRR), floods constitute up to 43% of the total natural hazards. Over the interval 12.07 – 14.07.2021, western part of Germany, Belgium and the Netherlands have experienced historic rainfall (Figure 1) and flooding that killed more than 160 people (status as of 18.07.2021) and left more than 1,000 people missing during the last days. The record-breaking rainfall amounts which fell in less than 72 hours (Table 1) led the scientific community and general public to asks themselves: was the heavy rain events and subsequent flooding amplified by climate changes? Is the frequency of such events going to increase in the future? Was the catastrophic dimension foreseeable? Overall, the climate community agrees on one thing: due to climate change the frequency of suchlike extreme events (including floods, droughts, heat waves, cold spells, etc.) will increase in the future, thus we need to change our management and adaptation measures accordingly, besides a generally increased effort on climate protection. Future climate projections have already warned of a possible increase in extreme rainfall events which will lead to more frequent and more extreme floods events (IPCC, 2018). Under high-end climate scenarios with no significant climate-protection measures (e.g., an increase of 3 - 5° C in the global mean temperature by the end of the 21st century according to RCP8.5), climate change could triple the direct damages from floods during the 21st century (Alifieri et al., 2017), if no adaptation measures are taken.

Just over the last ~30 years, Germany has experienced a relatively high number of extreme summer floods (e.g. July 1997 – Oder, May 1999 – Danube, 2002 – Elbe and Danube, 2005 – Danube, 2013 – Elbe and Danube, among others), with catastrophic consequences for the people living near the flooded area, but also with exceptionally monetary losses (e.g. the Elbe floods of 2002 alone, caused losses totaling €17 billion) (For further information on the 2002 flood event in the town Grimma and how communities can prepare themselves see here.  These events were surpassed in everything that could have been imagined so far by the 12 – 14 July 2021 event, which might set up new records both in terms of number of casualties, and monetary losses, as well as in terms of flood peaks and rainfall amount. One of the most affected area was Ahrweiler (Rhineland-Palatinate), where more than 110 casualties were recorded. On the 15^th of July, at 4:00 a.m., a record-breaking water level of 574 cm of river Ahr was registered at the gauging station Altenahr, which is far less than one meter under normal conditions. The previous record water level was recorded on the 2^nd of June 2016, with a value of 371 cm (source: www.hochwasser-rlp.de).  

The July 2021 event was triggered by a weak, wiggly and shifted jet stream, which forced a low-pressure system to be stationary and to recirculate around the central part of Europe, leading to exceptionally high precipitation rates (Figure 1 and Table 1). By recirculating, this low-pressure system sucked warm and humid air from the Adriatic Sea and the Baltic Sea, which were extremely warm (~25°C in the Adriatic Sea and ~26°C in the Baltic Sea) and mixed this with cold air masses from the North Atlantic, leading to the formation of heavy rain clouds over central Europe. Driven by orography and the uplift of the air masses in the low-pressure system, humidity condensates in the clouds and precipitates in heavy rainfall. In some parts of North Rhine-Westphalia (NW) and Rhineland-Palatinate (RP), the amount of precipitation recorded in the first two weeks of July was ~250 % more than the normal climatological mean for this month. The highest maximum daily precipitation was recorded at Köln-Stammheim station (NW) on the 14^th of July 2021: 153.5 mm (Figure 1c), followed by Kall-Sistig station (NW): 144.8 mm and Dahlem-Schmidtheim station (NW) with a maximum daily precipitation amount of 129.2 mm.  A wiggly / slow jet stream is usually associated with anomalous weather and the occurrence of extreme events (e.g. floods, droughts, heatwaves). The jet stream (Figure 2a, Animation 1 – jet) is primarily driven by the difference in temperature between the polar and mid-latitudinal regions and the weakening of the zonal mean jets is generally due to the reduction of the equator-to-pole thermal gradient associated with changing surface conditions. A potential cause for the weakening of the jet stream might be the fact that the temperature over the Artic circle is increasing at a rate of two to three times higher than at the equator. Expression of the Arctic warming is the continuous retreat of summer sea ice extent which culminates nowadays at about half the area as of the beginning of the continuous recording via satellite measurements in 1979 (for more information sea www.meereisportal.de). In general, anticyclones are large systems, which stay at the same place for days and sometimes for week. Trapped between these two 'giants', was a low-pressure system over the central part of Europe (Figure 2b – T letter). Since the low-pressure system has been blocked between the two anticyclones, the rainfall and the consequent flooding was concentrated in the regions under the influence of the cyclonic circulation (Figure 2). From a synoptic point of view, the low-pressure system which led to the heavy rainfall was a large-scale weather situation of the type "Low Central Europe (Tm)", which usually brings heavy rainfall episodes and flooding, especially if it is associated with a low in altitude, like it was in the current case (Figure 2b and Figure 3).

Are events like this going to happen more in the future? Is the magnitude of such event going to increase even more? The answer is rather straightforward: Yes! We have to take into consideration that every degree of warming we add to the climate system means ~7% more water vapor in the atmosphere, which in agreement with the law of physics and the hydrological cycle, has to precipitate at some point somewhere. So, the question is not IF such events are going to happen more frequent, is actually WHEN and WHERE? Overall, the last couple of weeks have been the perfect and at the same time cruel example just how bad things can go. When looking at what happened in the last three weeks on Earth, in terms of extremes (e.g. the record breaking temperatures in Canada and death Valley (U.S) with temperatures above 50 ° C and wildfires, catastrophic flash floods and landslides in Japan, forest fires in California, heavy rainfall and flooding in Germany, deadly tornado in Czech Republic, record breaking temperatures in Antarctica (+18°C) and the list can go on) it is clear that we are in a situation of climate emergency and we have to keep our climate goals and come up with sustainable solutions. The flywheel of the climate crisis cannot simply be turned back. Science has clearly stated that we have only a few decades left to steer onto the path of a moderate climate increase that limits the global temperature rise to below 2°C if possible. In Germany, we have already reached an average warming of 1.6°C compared to pre-industrial times. It is time to act decisively, now.

References:
Alfieri, L., Bisselink, B., Dottori, F., Naumann, G., de Roo, A., Salamon, P., Wyser, K., Feyen, L., 2017:  Global projections of river flood risk in a warmer world. Earth Future, 5, 171–182.

IPCC, 2018: Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. World Meteorological Organization, Geneva, Switzerland, 32 pp.

Contact:
Monica Ionita, Patrick Scholz und Klaus Grosfeld
(Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven)