October 2026 Storm Adam floods likely strengthened by human-driven climate change

Contact Authors

• Tommaso Alberti, INGV, Italy, tommaso.alberti@ingv.it 

• Davide Faranda, IPSL-CNRS, France, davide.faranda@lsce.ipsl.fr 

• Haosu Tang, University of Sheffield, UK, haosu.tang@sheffield.ac.uk

• Marco Zanchi, IPSL-CNRS, France, marco.zanchi@lsce.ipsl.fr 

Citation

• Alberti, T., Faranda, D., Tang, H., & Zanchi, M. (2026). October 2026 Storm Adam floods likely strengthened by human-driven climate change. ClimaMeter, Institut Pierre Simon Laplace, CNRS. https://doi.org/10.5281/zenodo.23564435

Press Summary

• Weather situations like Storm Adam now bring about 7% heavier rainfall, about 11% stronger winds and temperatures up to 2.5°C higher than they would have in the past (1950-1987).

• The rainfall increase is significant over south-eastern France, from the Cévennes and the lower Rhône valley to the Lyon region, while present-day situations are warmer over almost the whole western Mediterranean. Over eastern Spain, Corsica and Liguria the rainfall change is not statistically significant.

• The event was linked to a rare weather pattern, a deep upper-level low west of France drawing warm and moist Mediterranean air northwards.

• Approximately 8.5 million people and 372 billion USD (about €340 billion) in economic activity were exposed within the rainfall-hazard area. Of the resulting rain-linked damages, an estimated 2% is attributable to human-driven climate change.

• We attribute the intensification of the rainfall mostly to human-driven climate change, with a partial contribution from natural variability.

Figure 1. ClimaMeter analysis of Storm Adam (19 to 20 October 2026), over the western and central Mediterranean. (a, b) circulation (500 hPa geopotential height) and temperature anomalies; (c) precipitation and (d) wind speed during the event; (e, f, g, h) the present-minus-past changes in circulation, temperature, precipitation and wind speed, coloured only where statistically significant; (i) the months in which similar past events occurred; (j) changes at the selected cities (Valencia, Ajaccio and Genoa), with non-significant changes set to zero. The two dials summarise the attribution (left) and how rare the pattern is (right).

Event Description

Between 19 and 20 October 2026, Storm Adam, a cut off low pressure system, brought exceptionally heavy rainfall and widespread flooding to eastern Spain, particularly southern Valencia and northern Alicante, before affecting southern France, Corsica and north western Italy. The same meteorological event has been referred to by different names, including DANA, Mediterranean episode and libecciata, reflecting different regional descriptions of the phenomenon. 

In Spain, according to Agencia Estatal de Meteorología (AEMET), the rainfall in the Safor region (Valencia) was exceptional, reaching more than 600 mm in several locations with the most comparable historical episode dating back to 3 November 1987. The most intense rainfall occurred between 18 and 19 October, with  632 mm in 36 hours in Pego, (Alicante) and more than 500 mm at several locations of southern Valencia and northern Alicante. More than 800,000 residents were told to stay indoors, people were winched from rooftops by helicopter, as floods also hit Catalonia, Balearic Islands (Mallorca and Ibiza) and parts of Andalusia. At least six deaths were reported in connection with the storm in the Valencian Community, Catalonia and Andalusia.
In France, the Gard and the Hérault were placed under orange rain-flood vigilance, with reports of up to 500 mm of rain locally. Floods were also reported in the Gard, the Bouches-du-Rhône and the Var, where about 7,200 households lost power. In Corsica, the Solenzara station on the east coast registered about 380 mm of rain in a few hours, a value that would be a national record if confirmed, while in Liguria, more than 200 mm fell in the Savona area.

The Circulation Anomalies panel (Fig. 1a), the anomaly of the 500 hPa geopotential height (the pressure pattern about 5.5 km up), shows a deep low centred west of France over the Bay of Biscay, more than 100 m below normal, and higher pressure over Italy and the Balkans, a configuration that draws warm and moist air from the Mediterranean towards Spain, France and Italy. The Temperature Anomalies panel (Fig. 1b) shows air colder than usual over Iberia and warmer than usual, by 3°C to 5°C, over Italy and the central Mediterranean. The Precipitation Data panel (Fig. 1c) shows rainfall averaged over the two days of 60 to 80 mm/day over the Cévennes, the Gulf of Lion, Corsica and the Ligurian Sea, and the Windspeed Data panel (Fig. 1d) shows winds of 20 to 35 km/h over the western Mediterranean. At the resolution of the analysis (about 50 km), these daily values are much lower than the local rain-gauge totals recorded in Spain, France and Corsica.

A Summary of How ClimaMeter Works

ClimaMeter attributes an event by comparing it with its closest historical analogues, that is, past days whose large-scale weather pattern most closely resembles the event. The analogues are split into a past period (1950 to 1987) and a present period (1988 to 2025); the difference between the two sets shows how weather situations like this one have changed as the climate has warmed. All fields are detrended and deseasonalised before the search. A change is considered significant when it lies outside the range (2.5th to 97.5th percentile) of the changes obtained by randomly reshuffling the past and present analogues 1000 times. To prevent several consecutive days of the same episode from being counted as separate analogues, the retained analogues are required to be at least seven days apart. The two dials summarise, on the left, how much of the change is due to human-driven climate change versus natural variability, and, on the right, how rare the meteorological pattern is in the historical record.

Climate and Data Background for the Analysis

The IPCC Sixth Assessment Report (AR6, Working Group I, Chapter 11) concludes that heavy precipitation has become more frequent and more intense over most land regions with good observational coverage, and that human-driven warming is the main driver, since a warmer atmosphere holds about 7% more moisture per degree of warming. For the Mediterranean region, observed trends in heavy precipitation are mixed and assessed with low confidence, because of strong year-to-year variability and uneven long-term records.

Working Group I, Chapter 12 projects a drier Mediterranean in the mean, with less total rainfall and more frequent droughts, while the most intense rainfall events are expected to become heavier with further warming in many regions. The Working Group II Cross-Chapter Paper on the Mediterranean identifies flash floods in densely populated coastal areas, where the warm Mediterranean Sea supplies moisture to autumn storms and urbanisation amplifies runoff, as one of the key climate risks for the region.

ClimaMeter Analysis

We analyse how weather situations similar to Storm Adam have changed between the past (1950 to 1987) and the present (1988 to 2025) over [5°W to 25°E, 32°N to 48°N], identifying analogues from the 500 hPa geopotential height pattern. Circulation Changes (Fig. 1e) show a significantly deeper low over the Bay of Biscay and western France, by about 10 to 20 m, and higher pressure over Italy and the Balkans, indicating that present-day events favour a stronger southerly flow over the western Mediterranean. Temperature Changes (Fig. 1f) show present-day analogues up to about 2.5°C warmer in the area of heaviest rainfall, with significant warming over almost the whole domain.

Precipitation Changes (Fig. 1g) show about 7% heavier rainfall (by up to about 8 mm/day) over south-eastern France, from the Cévennes and the lower Rhône valley to the Lyon region, while the rainfall change is not significant over eastern Spain, Corsica and Liguria, and conditions are slightly drier over the Tyrrhenian Sea and southern Italy. Windspeed Changes (Fig. 1h) show winds about 11% stronger in the area of heaviest rainfall, with significant increases of up to about 4.5 km/h, notably over the Gulf of Lion and the Balearic Sea.

Similar Past Events (Fig. 1i) occur from September to November in both periods, but present-day analogues are more frequent in September and less frequent in October, suggesting that these events now occur earlier in the autumn. Changes in Urban Areas (Fig. 1j) show a significant warming of about 1.3°C in Valencia, 1.2°C in Ajaccio and 1.0°C in Genoa, with no significant change in rainfall or wind speed in any of the three cities. Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal 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. The right-hand dial points to a rare weather pattern, with close analogues in the 1950 to 2025 record; however, present-day analogues resemble the event significantly less closely than past ones, so the analogue-based attribution carries medium confidence.

Exposure

To estimate the population and economic assets exposed to climate change-influenced conditions, we overlay the ClimaMeter rainfall-hazard map onto gridded datasets of population density and economic activity using the methodology developed in Faranda et al 2026 (ERL). The hazard map retains only the grid cells where present-day conditions during events like the studied one are significantly more extreme than in the historical baseline (1950 to 1987). Onto this area, we overlay gridded population and gridded gross domestic product both at a spatial resolution of about 50 km.

We then aggregate the population and economic value within the hazard area and categorize them into three severity classes. The categories are based on how unusual the local rainfall was during the event relative to the 1950-2025 record:

• Moderate: falling between the local 98th and 99th percentiles (representing the most extreme one to two per cent of days);

• Severe: falling between the 99th and 99.5th percentiles (the most extreme half to one per cent); and

• Extreme: exceeding the 99.5th percentile (the rarest and most intense conditions).

Importantly, these figures measure exposure, the people and assets in harm’s way potentially impacted, not the realized losses.

About 8.5 million people and 372 billion USD of economic activity fall within the rainfall-hazard area, out of a domain total of about 279 million people and 9.1 trillion USD. The hazard area lies mostly over south-eastern France, from the Cévennes and the lower Rhône valley to the Lyon region, with a small area at sea south-west of Sardinia; eastern Spain, Corsica and Liguria lie outside it, because the rainfall change there is not significant. By severity class, roughly 1.3 million people are in the moderate class, 2.2 million in the severe class and 4.9 million in the extreme class. In economic terms, about 57 billion USD are in the moderate class, 130 billion USD in the severe class and 185 billion USD in the extreme class (Fig. S5).

Impacts and Vulnerability

The methodology for calculating the climate-linked damage share is adapted from Faranda et al. 2026 (QJRMS). In this case, each grid cell represents the geographical spread of the event. We first calculate how much more intense the hazard is today compared to historical analogues, defined as the present-minus-past difference of the analogue composites. Afterwards, we restrict this estimate to the hazard core, representing the cells where the event was most severe and where the change is statistically significant. Applying this relative intensification to the exposed population and assets splits the local impact into two components that sum to 100 per cent in every affected cell: (1) a baseline share, which the weather extreme would have caused anyway, and (2) a climate-change share, the part intensified by the human-driven warming. Because these results are fractions independent of absolute monetary losses, we report percentages only, while the values below are estimates.

We quantify the uncertainty on this attributable share from the analogue permutation test. For every grid cell, randomly reshuffling the past and present analogues 1000 times gives the spread of the present-minus-past change (the delta) expected from the finite number of analogues. We shift the delta by the distances of the 10th and 90th percentiles of that spread from its median and recompute the climate-attributable damage for each, bracketing the central estimate. Because the low and high percentiles are applied at every cell at once, the resulting range (reported in brackets) is a conservative envelope of the uncertainty in the attributed hazard change rather than a strict confidence interval.

Aggregated over the exposed assets, about 2% (10-90th percentile 1 to 3%) of the rain-linked damage is attributable to human-driven climate change (Fig. S6). Crucially, this percentage represents the climate-driven fraction of realized losses, rather than a proportion of the total exposed assets. The local rainfall extremes recorded in eastern Spain and Corsica, which are not resolved at the scale of the analysis, and the role of river flooding and urban drainage are not represented in this rainfall-based estimate.

Conclusion

Weather situations similar to Storm Adam now bring about 7% heavier rainfall south-eastern France , about 11% stronger winds and temperatures up to 2.5°C higher than in the past (1950 to 1987), with significant rainfall increases over south-eastern France but no significant rainfall change over eastern Spain, Corsica and Liguria. About 8.5 million people and 372 billion USD of economic activity lie within the rainfall-hazard area, and about 2% (10-90th percentile 1 to 3%) of the rain-linked damage is attributable to human-driven climate change.

Additional Information

The supplementary figures below are technical diagnostics. They can be understood by reading the methodology and reference literature provided by the ClimaMeter consortium at https://www.climameter.org/consortium.