Less rain, more intense storms: Climate change strengthens extreme urban floods

A new study reveals a surprising consequence of climate change: Cities may experience less rainfall overall yet face more severe extreme floods, due to shorter, more intense rainstorms. The research finds that 26% of the world’s urban areas, currently home to approximately 1.5 billion people, are projected to experience declining precipitation alongside intensifying extreme rainfall by the end of the century. Using advanced climate and hydrological simulations focused on central Israel, the researchers show that weak runoff events become weaker while extreme ones become stronger, and that urbanization amplifies this effect, highlighting the need to adapt urban planning and drainage infrastructure to these changing rainfall patterns.

A new study led by PhD student Raz Nussbaum, co-authored by Dr. Moshe Armon and supervised by Prof. Efrat Morin from the Hebrew University of Jerusalem, reveals a troubling consequence of climate change: Even as rainfall decreases, cities could face increasingly severe flooding.

Published in the Journal of Hydrology, the research first analyzed projections from 32 global climate models under a high-emissions scenario, finding that 26% of the world's urban areas, currently home to approximately 1.5 billion people, are projected to experience declining overall precipitation alongside intensifying extreme rainfall by the end of the century. The researchers then asked what this combination means for urban floods, a question that had remained largely unexplored.

They found a two-sided pattern: Weak runoff events become weaker, while the strongest ones, which are most likely to cause flooding, become even stronger. Moreover, urban development can amplify this phenomenon, leaving densely populated cities particularly vulnerable.

"Climate change is not simply making cities wetter or drier. It is fundamentally changing how rain falls" said Nussbaum. "Some regions are expected to receive less rain overall, but in shorter, more intense bursts. Our findings suggest that in these regions, the most extreme floods could become even more extreme. Urban flood protection will therefore need to be rethought, and not only because of climate change. Population growth and urban densification are expanding impervious surfaces, which intensify runoff, while exposing larger populations to flood risk."

Israel as a case study for a global challenge

To investigate this phenomenon, the researchers combined global climate projections with detailed rainfall and urban drainage simulations, focusing on the Nahal Ra'anana catchment in central Israel.

Using more than 5,300 simulations of heavy rainfall events under different climate and urbanization scenarios, the researchers examined how future storms could affect runoff in urban environments.

Their findings revealed that future storms in the eastern Mediterranean are expected to become more concentrated in both space and time. In the Ra'anana study area, maximum rainfall intensity over a 10-minute period increased by approximately 20% in the future climate simulations, even as longer-duration rainfall and total precipitation declined.

This creates a dangerous combination: Rainfall may become less widespread, but when intense storms strike urban areas, they can generate powerful runoff.

The researchers also examined the effects of urbanization by simulating different proportions of roads, buildings and other paved surfaces. They found that higher urban density further amplifies extreme runoff, particularly at the neighborhood level.

The explanation lies partly in the way cities are built. Roads, rooftops and paved surfaces prevent water from infiltrating the ground, causing rainfall to flow rapidly into drainage networks. When intense rain falls over a short period, existing drainage infrastructure may struggle to accommodate the resulting surge.

Rethinking how cities prepare for climate change

The findings carry important implications for urban planning in Israel and in cities worldwide facing the same combination of declining rainfall and intensifying extremes. According to the researchers, conventional approaches to flood protection, which rely heavily on historical rainfall patterns, may underestimate the risks associated with future extreme storms.

The study highlights the importance of adapting drainage infrastructure to short, intense rainfall events while integrating green spaces, water-retention systems and climate-resilient urban planning.

Although the detailed runoff findings were demonstrated in the Ra'anana catchment and require further investigation in other cities, the global climate analysis suggests that similar rainfall trends may affect urban areas across the Mediterranean, western and southern Africa, southeastern Australia and parts of the Americas.

The research underscores a critical message for policymakers: Preparing for a drier future does not mean preparing for milder floods. As climate change and urbanization progress together, cities may need to confront the seemingly contradictory challenge of managing both declining water availability and increasingly destructive rainfall extremes.