Heavy precipitation in late-March 2025 Cyclades storm likely influenced by both human-driven climate change and natural variability 

Contact Authors

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

• Emmanouil Flaounas, HCMR, Greece & ETH Zurich, Switzerland 📨em.flaounas@hcmr.gr  🗣️Greek, 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

Citation

• Alberti, T., Dafis, S., Flaounas, E., & Faranda, D. (2025). Heavy precipitation in late-March 2025 Cyclades storm likely influenced by both human-driven climate change and natural variability. ClimaMeter, Institut Pierre Simon Laplace, CNRS. https://doi.org/10.5281/zenodo.15125613

Press Summary

• Storms similar to the late-March 2025 Cyclades storm are up to 5mm/day ( up to 10-15%) wetter  in the present than they would have been in the past.

• The late-March 2025 Cyclades storm was driven by rare meteorological conditions.

• We mostly ascribe the  heavier precipitation and the stronger winds of the late-March 2025 Cyclades storm to human driven climate change and natural climate variability likely played a modest role

Event Description

In late March and early April 2025, severe storms struck several Greek islands, leading to significant flooding and damage. On March 31, the islands of Paros and Mykonos experienced torrential rains and hailstorms, resulting in submerged streets and vehicles being swept away. Authorities responded by closing schools and restricting traffic to emergency vehicles. Residents and tourists were advised to remain indoors as emergency services conducted rescue operations, successfully assisting stranded individuals without reported injuries. The adverse weather persisted into April, with Crete being notably affected. The port city of Chania recorded the highest rainfall over a 24-hour period, leading to widespread flooding. Rescue teams were deployed to assist those trapped in vehicles, and infrastructure repairs were initiated to address the damage. Other islands, including Rhodes, also faced severe conditions, with gale-force winds causing additional destruction.

The Surface Pressure Anomalies reveal a large negative (cyclonic) anomaly (up to -10 hPa) over Greek island. Temperature Anomalies show slightly positive anomalies up to 1 °C over the same region, in contrast with negative ones detected over Greece. Precipitation Data show intense daily precipitation (more than 40 mm/day) over Greek island. Windspeed data shows large areas of Greek island with sustained winds between 40 km/h and 60 km/h.

Climate and Data Background for the Analysis

The IPCC AR6 WG1 with a focus on the Mediterranean region in Sections 10.6.4, 12.4.1, and 12.4.5, highlights the increasing occurrence of Mediterranean cyclones during the summer season. These cyclones are fuelled by rising sea-surface temperatures (SSTs) in the Atlantic Ocean and Mediterranean, which have climbed by 0.25°C to 1°C since 1982–1998, contributing significantly to the intensity of these convective events. Despite the report's acknowledgment of the correlation between warmer SSTs, marine heatwaves, and summer Mediterranean cyclones, it refrains from making historical trend statements about extreme precipitation in the Mediterranean region. Furthermore, Chapter 11 of the IPCC AR6 report underscores the challenges in summarizing trends in severe convective storms across regions due to varying definitions and limited long-term data on phenomena such as tornadoes, hail, and lightning associated with such storms. Consequently, the report opts not to provide specific statements regarding changes in convective storms in the Mediterranean region. 

Our analysis approach rests on looking for weather situations similar to those of the event of interest observed in the past.  For this event we have high confidence in the robustness of our approach given the available climate data, as the event is very similar to other past events in the data record

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to Storm Darragh have changed in the present (1987–2023) compared to what they would have looked like if they had occurred in the past (1950–1986) in the region [15°W 34°W 30°N 45°N]. Surface pressure changes indicate that storms resembling the Cyclades Storm exhibit no changes in pressure anomalies in the present than in the past. Temperature changes display up to +2 °C variations over continental Europe, while no changes are detected over the Greek islands. Precipitation changes indicate wetter conditions (+5 mm/day) over Greek islands, as well as over Southern Italy. Windspeed changes suggest that these storms do not display significant variations. From the analysis in the urban areas we found that Rodos and Athens are getting wetter in the present than in the past during storms similar to the Cyclades Storm, while Chania and Athens also experiencing windier conditions. We also note that Similar Past Events display similar seasonal distribution between past and present periods, with a slight increase towards March occurrences for the present period.

Finally, we find that sources of natural climate variability, notably the El Nino Southern Oscillation, may have only partly influenced the event. This means that the changes we see in the event compared to the past may be mostly due to human driven climate change.

Conclusion

Based on the above, we conclude that storms similar to the late-March 2025 Cyclades Storm are up to 5mm/day ( up to 10-15%) wetter in the present than they would have been in the past. We interpret the Cyclades Storm as an event driven by rare meteorological conditions whose characteristics can mostly be ascribed to human driven climate change.