Hochwasserlage Dezember 2023 bis Januar 2024

From 19 December 2023 to 5 January 2024, the northern part of Germany recorded high precipitation levels. As a result, a large-scale flood situation developed in large parts of northwest Germany.

The German Weather Service (DWD) has conducted and subsequently published a hydro-climatological classification of the precipitation event in question. However, the following assessment by ClimXtreme provides a detailed examination of specific elements of the continuous precipitation and resulting flooding from the perspective of the ClimXtreme research project. It serves to supplement the DWD report with results obtained using methods developed or further developed in the ClimXtreme project that are used for current research, also independently of the event analysed here.

Summary

• From Christmas 2023 until January 2024. there was a flood situation, especially in the area of the rivers Ems, Weser and Elbe, which led to a tense situation, primarily in large parts of northwest Germany,
• The flood was caused by large-scale continuous precipitation, particularly in northern Germany over Christmas (19-25 December 2023) and intensified by subsequent, albeit weaker, precipitation events at the beginning of January 2024, occurring on soils that had already reached saturation level. 
• The Christmas precipitation event was distinguished by its extensive spatial coverage and prolonged duration, spanning seven days. The event was linked to a weather situation (characterised by an extensive low-pressure system centred over southern Scandinavia), which, while not unusual in itself, persisted for an exceptionally long period of time.
• The specific spatial pattern of this week-long precipitation event was associated with both heavy continuous precipitation and the weather situation, described above, in the past.
• The subsequent precipitation events at the end of December and the beginning of January were considerably less intense. However, the temporal interaction of these events with the previous Christmas event resulted in the occurrence of extreme average precipitation intensities in some areas over prolonged periods, up to two and a half weeks.
• The flood, as the result of the precipitation, was also essentially characterised by its extensive spatial extent, with extreme river levels only being recorded in isolated cases. In the context of historical floods in Germany, the Christmas flood of 2023 ranks ninth in terms of spatial extent, with a discharge area of approximately 100,000 km². This ranking is based on a comprehensive analysis of all floods in Germany since 1955, where the discharges over two weeks had a return period of at least 10 years. It should be noted that the spatial extent referred to here is not the flooded area itself, but rather the area in which the flood discharges exceed a certain threshold value.
• The flooded area itself covered an estimated 1,000 km², encompassing over 40 districts, primarily in Lower Saxony and Bremen, but also in parts of Hessen and North Rhine-Westphalia. The event affected an estimated 18,000 to 30,000 people, while also impacting 4.6 km² of built-up area and 470 km of roads.
• The measured monthly precipitation of 164 mm, recorded in December 2023 for a highly impacted region in Lower Saxony (51.5°N - 53.5°N, 8.0°E - 11.0°E), occurs in Winter months on average only once every 120 years in the current climate. The results of an attribution study conducted by the German Weather Service (DWD) indicate that the probability of an event of this intensity has increased by a factor of 1.8 (with a result range of 0.1 to 140) due to the observed climate warming of 1.2°C since approximately 1900. Furthermore, the probability is projected to further increase given an anthropogenic climate change of 2 C, representing an additional 0.8°C warming. However, the study also demonstrates that these estimates are associated with significant uncertainties. Nevertheless, the estimates are consistent with the results of various studies, which demonstrate both an increase in mean winter precipitation and an intensification of extreme precipitation events in northern Central Europe.

To access the complete report, please click on the following link. Please note that this resource is only available in German.