Changes in Precipitation patterns driving Romania Floods  mostly driven by human-driven climate change

Press Summary (First Published 2024/08/20)

• Depressions similar to those that caused the August 2024 Romanian floods generally show reduced precipitation (7 mm/day, or up to 15% less) over Eastern Romania in the present compared to the past. However, we observe a significant local increase in precipitation (up to 5 mm/day, or up to 10% more) over Constanta, one of the cities affected by the floods.

• This was a somewhat uncommon event.

• We mostly ascribe the changes in precipitation producing the Romanian Floods to human driven climate change and natural climate variability likely played a modest role.

Event Description

On August 30–31, 2024 flood events occurred along Romania's eastern coastline driven by a quasi stationary low pressure system (cyclone) over the Black Sea. The cyclone caused torrential rains that resulted in 100 mm of rain in just 24 hours across several coastal towns. Some localized areas reportedly received up to 220 mm of rain in less than 24 hours. Thus, according to the National Administration “Romanian Waters” 24-hour precipitation records were broken at Magalia (225,9 mm), Agigea (145 mm), and Tuzla (118 mm).  The towns of Agigea, Tuzla, Costinești, Eforie, Mangalia, and Vama Veche were particularly affected. Streets were submerged, homes were flooded, and in some areas, water levels reached as high as 1 meter. Vama Veche, a popular tourist destination, saw widespread power outages, complicating the rescue and cleanup operations.  Over 800 emergency calls were made, prompting a large-scale response from ISU Dobrogea firefighters and other emergency services. Despite the extensive damage, no fatalities were reported. Rescue operations focused on evacuating residents from flooded homes and vehicles, clearing debris, and restoring essential services. The flooding caused significant damage to infrastructure, including roads and bridges, making transportation difficult. 

The Surface Pressure Anomalies show a negative (cyclonic) anomaly, centered over the Black Sea, east of  Bulgaria and the northwest coasts of Türkiye. This depression was the main driver of the Romanian floods. Temperature anomalies show positive values over the same areas (except for eastern Bulgaria), with up to 4°C in eastern Romania and Moldova. Precipitation data show several areas of accumulation across the domain, with daily values up to about 30 mm in eastern Bulgaria at the border with Türkiye, and daily values up to 100 mm over south-eastern Romania. Wind speed data show that most of the domain was affected by relatively strong winds, with values around 20-30 km/h almost everywhere over south-eastern Romania.

Climate and Data Background for the Analysis

The IPCC AR6 WGI report states that the water cycle variability and extremes are projected to increase faster than the average change and in most of the tropical and extratropical 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 (including approximately the northern half of Romania). This increase will continue with high confidence for global warming levels higher than 1.5 °C.  This is well highlighted in panels (a) and (c) of Figure 12.9, that show positive changes of the 1-in-100-year river discharge affecting most of west and central Europe. This suggests an increment in intensity of extreme rainfall events that overload the small river catchments, ultimately increasing the risk of flooding. At the same time there is a medium confidence decrease in southern Europe (including approximately the southern half of Romania) for mid- and end-century under RCP8.5 and low confidence under RCP2.6. 

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 (at least 27 analogues). 

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the low pressure system leading to the Romanian Floods 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 [24°E 32°E 40°N 48°N]. The Surface Pressure Changes show that similar depressions are less deep than in the past. The Temperature Changes show up to +2°C warmer conditions in the present than in the past. The Precipitation Changes show no significantly increasing precipitation in Eastern Romania in the present than in the past with decreasing precipitation further south over Bulgaria. Windspeed Changes indicate no significant changes. We also note that Similar Past Events occur with a different seasonality in the past and present periods. In the past, similar events were mainly observed in July (followed by August and September), in the present these events are observed mainly in August (followed by September and July). Changes in Urban Areas reveal a significant increase in precipitation (up to 5 mm/day, or up to 10% more) în Constanța.  

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation may have influenced the changes in this event. This suggests that the changes we see in the event compared to the past may be due to human driven climate change, with a  minor contribution from natural variability.

Conclusion

Based on the above, we conclude that Depressions similar to those that caused the August 2024 Romanian floods generally show reduced precipitation (7 mm/day, or up to 15% less) over Eastern Romania in the present compared to the past. However, we observe a significant local increase in precipitation (up to 5 mm/day, or up to 10% more) over Constanta, one of the cities affected by the floods. We interpret the August 2024 Romanian Floods as an event whose local characteristics can mostly be ascribed to human driven climate change.

Contact Authors

-Bogdan Antonescu, Bucharest University,  📨bogdan.antonescu@g.unibuc.ro 🗣️Romanian, English

-Stavros Dafis, Data4Risk  &  National Observatory of Athens/meteo.gr 📨sdafis@noa.gr   🗣️Greek, English

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