Italy Multiple Floods mostly strengthened by Human-driven Climate Change

Contact Authors

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

-Flavio Pons, IPSL, France  📨 flavio.pons@lsce.ipsl.fr  🗣️ Italian, English, French

-Tommaso Alberti, INGV, Italy  📨tommaso.alberti@ingv.it 🗣️Italian, English

-Alice Portal, CNR-ISAC, Italy 📨a.portal@isac.cnr.it 🗣️Italian, English, French

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

Press Summary (First Published 2024/10/22 

• Mediterranean depressions similar to that causing multiple floods in Italy in October 2024, show increased precipitation (up to 10 mm/day, or up to 25% more precipitation) and produce stronger winds (up to 6 km/h, representing a 10% increase in wind strength)  in the present compared to the past.

• This was a very exceptional event in terms of meteorological conditions.

• Natural variability alone cannot explain the changes in precipitation and wind associated with this very exceptional meteorological conditions

Event Description

On October 18 and 19, a wave of severe weather struck parts of Italy and central France, bringing storms and heavy rainfall. Emilia Romagna has been the hardest hit in Italy, with over 160 mm of rain falling in Bologna in 4 hours. Overnight, flooding also affected Ravenna, Modena, and Reggio Emilia, creating dangerous conditions across roads in the Apennines and causing more than 3000 evacuations in the whole region. In Pianoro, a car was swept away by a flood, resulting in the death of a young man. Irene Priolo, the acting president of the region, criticized poor emergency response services and urged residents to strictly follow instructions from local authorities, particularly regarding road closures and evacuations. More than 300 firefighting interventions took place across Emilia Romagna, particularly in Bologna, where the Ravone, Savena, Zena streams, and the Idice river overflowed. In Cadelbosco di Sopra, residents were evacuated after the flooding of the Crostolo stream, a tributary of the Po River, also flooded. 

From October 16 to 18 severe storms and heavy rainfall have caused floods in Liguria and Tuscany with 200 mm of rain in 18 hours and 100 mm in one hour respectively hitting the Genoa, Rapallo, Savona, and Chiavari cities in Liguria, and Livorno and Siena in Tuscany. Genoa's Sori Bridge was damaged and Siena railway station too.

Persistent heavy rainfall was also recorded from October 19 over Campania, Puglia, Basilicata, and Calabria, especially in the Ionian areas, and Sicily. In Sicily, emergency services rescued people trapped on rooftops and in vehicles as flash floods hit. The province of Agrigento saw the Salto river breach its banks, flooding parts of Licata. In Florence, the river Elsa also overflowed, and authorities have advised against non-essential travel. 

The Surface Pressure Anomalies show a significant negative (cyclonic) anomaly of 2 hPa over Central Mediterranean and Italian Peninsula, with Temperature anomalies displaying significantly warmer temperatures over Italy up to 3 °C and over Southern Mediterranean up to 1.5 °C. Precipitation data reveal high daily rainfall amounts, exceeding 40 mm per day over the Central Mediterranean and over Emilia Romagna. Windspeed data show moderate winds up to 20 km/h over almost all the Mediterranean.

Climate and Data Background for the Analysis

The  IPCC AR6 WG1 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).  The frequency and intensity of extreme precipitation have increased in the majority of land regions including Europe and in the Mediterranean with medium confidence and will increase with high confidence from a 2 °C. (AR6 WGI TS). As the IPCC AR6 states that with further global warming and proportional to the level of warming, starting from 1.5 °C, every region is projected to experience more and more concurrent multiple changes of climatic impact-drivers like the extreme precipitation.  Concurrent extreme at multiple location are more likely to occur at 2 °C compared to 1.5 °C (AR6 WGI SPM). The report also underscores the impact of climate change on storminess in Europe, aggravated by rising sea levels and intense precipitation. Anticipated alterations in atmospheric circulation patterns stem from the uneven warming of land and ocean, potentially resulting in diminished continental near-surface relative humidity and localised decreases in precipitation. In a warmer climate, individual midlatitude storms are expected to produce more precipitation, though changes in wind speed remain less certain. Studies, such as Ginesta et al. (2024), found that the intensity of recent storms, including both wind speed and precipitation, is likely to increase in the most impacted regions of Europe.

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  low confidence in the robustness of our approach given the available climate data, as the event is very exceptional in the data record 

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the Mediterranean depression of October 18-19 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 [0°E 22°E 34°N 48°N]. Surface Pressure Changes show no significant differences between the two periods. Temperature Changes show up to 1 °C over the Mediterranean, while no changes are observed over the Italian Peninsula. Precipitation Changes show that similar events are now producing wetter conditions over Italy, especially in the Southernmost part, with up to 10 mm/day in the present climate than in the past. Windspeed Changes indicate windier conditions up to 6 km/h over the Central and Southern Mediterranean. 

We also note that Similar Past Events occur with similar frequency as in the past, with a slight increase in September in the present climate. Changes in Urban Areas reveal that Bologna, Palermo, and Genoa see an increase in precipitation (up to 3 mm/day) in the present  compared to the past. Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation and the Pacific Decadal 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  contribution from natural variability.

Conclusion

Based on the above, we conclude that Mediterranean depressions similar to that causing multiple floods in Italy in October 2024, show increased precipitation (up to 10 mm/day, or up to 25% more precipitation) and produce stronger winds (up to 6 km/h, representing a 10% increase in wind strength)  in the present compared to the past. We interpret the multiple floods in Italy as an event driven by very exceptional meteorological conditions. Natural variability alone cannot explain the changes in precipitation and wind associated with this very exceptional meteorological conditions;