Climate change likely a key contributor to Europe’s devastating autumn floods

Europe saw record-breaking floods in autumn 2024. And climate change is likely playing a role in the increasing risk of these natural catastrophes. For FM clients and broker partners, the flood events serve as a stark reminder of the importance of resilience.

It starts with understanding the risk. As the year comes to an end, FM’s researchers are gathering some of the key lessons from this year. According to FM Research Area Director of Climate Risk and Resilience Dr. Angelika Werner, preparation and robust emergency warnings play key roles in resilience, particularly in a constantly changing risk environment.

In this article, Dr. Davide Panosetti, Lead Research Scientist from FM’s Climate Risk and Resilience team in Luxembourg, discusses the meteorological conditions and climate change contributors that led to the record rainfalls in Europe this year.

From mid-September to early November record-breaking rainfall across Central and Eastern Europe, central Italy, and Spain, caused rivers to overflow, leading to significant flooding. The hardest-hit regions recorded near-record rainfall in just a matter of days. For instance:

• Germany: In Saxony, rainfall exceeded 150 mm over the period, with some localized areas receiving up to 200 mm.
• Poland: The city of Wrocław saw 130 mm of rain, while areas along the Vistula River experienced over 140 mm.
• Czech Republic: The region of Ústí nad Labem recorded approximately 160 mm of rainfall.
• Austria: Salzburg and its surrounding areas received up to 180 mm of rain; some areas in Lower Austria received between 300 and 350 mm over just a few days, two to four times the average for September.
• Italy: Some areas in the Emilia-Romagna region exceeded 300 mm of rain in just under 48 hours.
• Spain: The eastern region of Valencia saw record rainfall and flooding, with the town of Chiva recording almost 500 mm of rain in just 8 hours.

The significant rainfall led to the rapid swelling of major rivers, including the Vistula and Oder in Poland, the Elbe in Germany, the Ebro and Duero in Spain, and the Danube in Austria, resulting in widespread flooding. In addition, several localities in the Alps saw their earliest snowfall on record.

Meteorological Conditions:

The primary factor behind the first wave of severe weather from September 12-19 was a slow-moving area of low pressure and an associated weather front that stretched almost 2,000 km at its peak. From September 12-17, this weather front stalled, “trapped” between two stationary areas of high pressure, a configuration that resulted in prolonged and intense rainfall across Poland, the Czech Republic, Slovakia, Austria, Hungary, Slovenia, and Romania. As a result, the air was forced to rise above the Alps and the Carpathian Mountains, while the storm drew moisture from the anomalously warm waters of the Mediterranean and Black Sea, intensifying the rainfall.

Then from September 17-19, the remnants of this weather front travelled to central Italy. Unlike the severe weather that affected Central and Eastern Europe, severe thunderstorms drove a large portion of the observed total precipitation amounts in Italy. The sea surface temperature anomaly, particularly in the Adriatic Sea, was a key contributor to the development and intensification of the thunderstorms. The Emilia-Romagna region, which saw catastrophic flooding in May 2023, was particularly hard hit.

Similarly, the key meteorological driver of the second wave of severe weather in late October-early November in Spain was a low-pressure system, locally referred to as DANA (“Depresión Aislada en Niveles Altos”, or “Isolated Depression at High Altitudes”). Eastern and Southern Spain are particularly susceptible to this phenomenon due to their position between the Atlantic Ocean and the Mediterranean Sea. In autumn, cold air from higher altitudes descends over the still warm Mediterranean Sea, leading to rapid weather deterioration and heavy rainfall.

The complex topography of Spain exacerbated the rainfall intensity locally. The broader orographic lifting from the Meseta Central and Sistema Ibérico not only caused the moist Mediterranean air to rise more quickly, but the narrow valleys also "trapped" isolated thunderstorms for extended periods, resulting in greater local rainfall accumulations.

The Impact of Climate Change on Flooding:

According to the latest global and regional climate model projections, there is "high confidence" that current trends will lead to more intense and frequent heavy rainfall events in the future under different greenhouse gas emissions scenarios as global temperatures rise. That, in turn, will increase the risk of flooding.

Moreover, the Mediterranean Sea has been warming at a faster rate than the global average. This was a key factor, explaining the extraordinary amounts of rain dumped by Storm Boris in September and the DANA in late October-early November. While weather systems like these are not new, they now occur in a warmer atmosphere that can hold more moisture, which in turn makes the precipitation more intense than if the same event had happened in the past.

Finally, one of the consequences of the faster warming of the Arctic compared to the mid-latitudes is a weaker and more meandering jet stream. An increased waviness of the jet stream can cause weather patterns to become more persistent, leading to prolonged periods of the same type of weather, such as extended cold spells, heatwaves, or heavy precipitation events. This was particularly evident in the late October-early November Spain floods.

Final remarks:

The autumn floods in Europe and broader climactic trends underscore the need for comprehensive and forward-thinking approaches to disaster preparedness and climate adaptation. Recent investments in flood defenses were critical in mitigating some losses, especially in major cities such as Vienna.

Fortunately, FM clients and broker partners have tools at their disposal to understand the risks and to help prepare for them. For example, FM’s climate suite of products includes a climate risk report to understand current risks, as well as a forward-looking climate change impact report. By drawing lessons from these events, we can more effectively protect our properties and economic activities from future climate challenges.