High precipitation in Autumn 2023 France and Italy floods likely enhanced by human-driven climate change and natural variability

Contact Authors

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

• Pascal Yiou,  IPSL-CEA, France  📨pascal.yiou@cea.fr  🗣️French, English

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

Citation

• Faranda, D., Yiou, P., & Alberti, T. (2024). High precipitation in Autumn 2023 France and Italy floods likely enhanced by human-driven climate change and natural variability. ClimaMeter, Institut Pierre Simon Laplace, CNRS. https://doi.org/10.5281/zenodo.14164143

Press Summary (First Published 202/11/27)

• Pressure patterns similar to those that caused the Autumn 2023 floods in France and Italy have deepened by 1 hPa to 2 hPa. This has resulted in wetter conditions, with an increase of 1 mm/day to 3 mm/day (equivalent to 15% to 30% more precipitation) along the French Atlantic coasts and the Italian Tyrrhenian coasts.

• Autumn 2023 France and Italy Floods were very uncommon events.

• Natural climate variability likely played an important role in driving the pressure pattern and the associated increase in precipitation linked to Autumn 2023 France and Italy Floods.

Event Description

In Autumn 2023, a sequence of storms, including Babet (18-21 Oct. 2023), Aline (21 Oct. 2023), Ciarán (28 Oct-4 Nov. 2023), Domingos (5 Nov. 2023), Elisa (9 Nov. 2023), and Frederico (15 Nov. 2023), led to exceptional floods in Central Europe, particulary in Northern France and Northern Italy. These storms brought about unprecedented rainfall levels of nearly 250 millimeters over three weeks, surpassing climatological averages. The impact was further intensified by the ground saturation from the storms and the flat terrain with limited drainage capabilities in some regions, as in Nord-Pas-de-Calais (France). The vulnerability to floods was heightened by the presence of silts and clay soils. The impact assessment by the Prefect of Pas-de-Calais reveals that 262 municipalities have been affected by floods, with 5,849 dwellings reported as impacted. Emergency services, including the Regional Health Agency and Department of Agriculture, have actively responded to the situation, providing assistance to vulnerable individuals and farmers. Firefighters conducted 20 interventions today, addressing issues such as pumping, securing areas, and debris removal. Economic repercussions are evident, with 98 businesses and 95 companies directly affected. While five minor injuries have been reported, 5,200 people still face water usage restrictions, and 45 households remain without electricity, primarily in Guînes and Clairmarais, with the exception of the school in La Calotterie, which remains closed. On November 22 The Caisse Central de Reassurance  estimated the damages of the floods in France to 550 MEUR.

During Storm Ciaran, a parallel catastrophe unfolded in Italy's Tuscany region. Unprecedented rainfall triggered floods, trapping residents in their homes, inundating hospitals, and causing car overturns. The storm led to at least two individuals reported missing and seven confirmed casualties, prompting the Italian government to declare a national state of emergency. Tuscany experienced wind speeds of up to 150 kilometers per hour and record-breaking rainfall not seen in at least half of a century. Cities like Prato, Pistoia, Pisa, and Campi Bisenzio faced inundation of streets, residences, hospitals, factories, shopping centers, and riverbanks. The aftermath left over 20,000 people without power for extended periods, with 9,000 users experiencing outages in the morning and 4,600 in the evening.

The Surface Pressure Anomalies reveal a large negative (cyclonic) anomaly over Western and Central Europe. In the context of Atlantic extratropical storms, this setup has the tendency to transport moist and warm air from Western Africa to central Europe, which in turn amplifies precipitation and wind strength.  Temperature Anomalies show warmer anomalies (about 1°C-2°C) over most of the domain analysed. Precipitation data show large amounts of rainfall over the English Channel and over Liguria and Tuscany (up to 20 mm/day which amounts to over 200 mm over the period considered). Windspeed data shows large areas of the English Channel and the Gulf of Genoa with winds between 40 km/h and 60 km/h on average over the considered time period.

Climate and Data Background for the Analysis

The IPCC AR6 WG1 report 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 localized decreases in precipitation. Extreme precipitation and pluvial flooding are projected to increase at global warming levels exceeding 1.5°C in all regions except the Mediterranean. (high confidence).

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-low confidence in the robustness of our approach given the available climate data, as the event is unusual in the data record.

ClimaMeter Analysis

Here we analyze (see Methodology for more details) how events similar to Autumn 2023 France and Italy Floods have changed in the present (2001–2022) compared to what they would have looked like if they had occurred in the past (1979–2000) in the region [-5°E 20°E 42°N 52°N]. Surface Pressure Changes indicate that current events are deeper by 1 hPa to 2 hPa compared to the past. Additionally, Temperature Changes reveal increased warmth across the analyzed region, with temperatures rising by up to +3°C over North France, the Benelux, and the Alps. Precipitation Changes suggest that these events are now 1 mm/day to 3 mm/day wetter over Northern France and Northern Italy, amounting to 30 mm to 60 mm over the entire period – approximately 15% to 30% more rainfall in the present compared to the past. Windspeed Changes evidence increased winds (2-4 km/h) over the Adriatic, leading to expectional high water level observed in Venice lagoon and Friuli Venezia Giulia, and over the Atlantic, leading fast moving storm surge events. Notably, Similar Past Events have become more frequent in March, November, and December, as opposed to their previous occurrences mostly in February and April. Analysis of Changes in Urban Areas indicates that Calais (France) and Florence (Italy) are experiencing increased precipitation and higher temperatures in the present compared to the past, while Bordeaux (France) shows no significant changes in precipitation.

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation, El Niño Southern Oscillation and the Pacific Decadal Oscillation, may have influenced the event. This means that the changes we see in the event compared to the past may be primarily due to natural climate variability. 

Conclusion

Based on the above, we conclude that pressure patterns leading to Autumn 2023 floods in France and Italy have deepened by 1 hPa to 2 hPa in the present with respect to the past. This results in wetter conditions, with an increase of 1 mm/day to 3 mm/day (equivalent to 15% to 30% more precipitation) along the French Atlantic coasts and the Italian Tyrrhenian coasts. We interpret the Autumn 2023 floods in France and Italy as an unusual event for which natural climate variability likely played an important role.