Shorter storms, bigger impact: Rising flood risks in a warmer world

Climate change may lead to more precipitation and more intense floods, but a recent study highlights that the precise ways these shifts play out differ depending on the duration of rainfall events. 

Researchers from Austria have demonstrated that shorter rainfall occurrences, on a timescale of just a few hours, are especially affected by rising temperatures, whereas longer rainfall events lasting days reveal a more nuanced relationship with climate conditions.

The importance of timescale

The study shows that to understand more precisely this relationship, it is crucial to distinguish between different types of rainfall and flood events, such as short-term events occuring on a time scale of hours, and longer-term events which last several days. In each of these cases, climate change has a different impact.

Gathering data from over a century of records, the scientists were able to track precipitation patterns in Austria’s diverse climates. 

The findings suggest that these observations may be extended to other regions worldwide. They also indicate which areas can expect changes in flood probabilities – and how exactly they will be impacted. 

Exceptional datasets fuel discovery

Although climate change is affecting water cycles all over the globe, a look at Austria is particularly revealing. 

“We are in the very fortunate situation of having excellent data available,” said Günter Blöschl from the Vienna Institute of Technology, who led the research project.

Since 1900, two institutions – Austria’s meteorological service, now Geosphere Austria, and the Austrian Hydrography – have independently recorded rainfall. 

This activity, managed by the Ministry of Agriculture, yields especially reliable numbers, suitable as a reference for global forecasts.

With these datasets, the researchers delvied into detailed analyses of short- and long-term precipitation and flood patterns over time.

A rise in brief rainfall events

The scientists discovered that heavy rainfall lasting only a few hours has risen significantly – by about 15% – over the past three or four decades.

“This had already been predicted by climate models, albeit with uncertainties. We have now been able to confirm it,” Blöschl said.

Such a parallel increase was seen both north and south of the Alps. “This clearly shows that large-scale weather systems are not the decisive factor for these short-term precipitation events, because they would be different near the Mediterranean than north of the main Alpine ridge,” he explained. 

“Instead, the temperature increase caused by climate change leads to more intense precipitation locally. This is partly because warmer air can hold more moisture, but also because there is more energy in the system, and stronger warming at ground level leads to stronger upward movement of air masses. Then they also cool down faster, which leads to more rain.”

Because these intense downpours occur on relatively small scales, local temperature effects appear to override broader weather system influences. 

Consequently, short, heavy rains and flash floods can occur in many places worldwide. All that is required is a moisture-rich atmosphere and localized upward air movement.

Contrasting long-term rainfall events

In evaluating longer rainfall scenarios – spanning multiple days – the study found a different picture. Global climate processes like El Niño, driven largely by temperature shifts in the oceans, play a crucial role here. 

Such storms rely less on local temperature changes and more on large-scale atmospheric circulations. In Mediterranean countries like parts of Italy, Spain, and Greece, these multi-day precipitation bouts may actually become less frequent with climate change.

This contrast helps explain why floods don’t uniformly increase in every large watershed. Blöschl explained that flooding in smaller rivers with smaller catchment areas are strongly influenced by short-term intense rainfall. “The risk of short-term flooding is therefore much higher in areas close to such rivers.”

On the other hand, large rivers like the Danube, with huge basins, experience floods driven more by multi-day rainfall episodes. Floods in such systems may display modest changes or even remain stable in response to shifting climate conditions.

Lessons for flood risk assessment

By distinguishing these processes, the Austrian data reveals the nuanced ways climate change alters water flows. Short bursts of intense rain are broadly on the rise, while extended rainfall episodes rely on more complex factors, including global weather systems and ocean temperature anomalies.

In turn, communities and policymakers have to adapt flood prevention and infrastructure plans to local conditions. Regions prone to short, severe cloudbursts may require different planning and water management than areas where large-scale deluges dominate flood risk. 

With climate models increasingly indicating that short-term rainfall extremes will worsen worldwide, officials in many places may need to bolster flash flood readiness.

Over a century’s worth of data from Austria underscores that climate change is clearly influencing different types of rainfall and flood events, but understanding local climate drivers is crucial. In some locations, brief, intense storms will be the main threat; elsewhere, extended storm systems might prove more decisive. 

By looking carefully at timescales, meteorologists and decision-makers can prepare for the precise mix of hazards climate change brings – and hopefully reduce the harm these shifts may cause for people living in vulnerable river basins.

The study is published in the journal Nature.

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