One-third of global basins facing flashier floods in a warming climate

Flash floods present significant challenges for monitoring and forecasting due to their rapid onset. While extreme rainfall events, a primary driver of flooding, are becoming both more intense and frequent, yet there remains no consensus on whether floods will exhibit similar trends in a warming climate. Here we assess flash flood risk development over four decades across 741 basins globally using the flashiness index. Our findings reveal a shift towards flashier floods in over one-third of basins, predominantly concentrated in the mid-northern latitudes. We identify basins characterized by rising flood peaks and those marked by shorter onset times. By examining these types, we attribute the changes in basins with increasing flood peaks to increased extreme rainfall, whereas the causes in basins with shortening onset times are more complex involving multiple drivers. Regions with rising flash flood magnitudes are likely to require flood defense infrastructure capable of withstanding more severe flood events, while regions with shortening onset times may face challenges in implementing short-term early warning systems. By identifying regions prone to flash floods and highlighting global hotspots, the study offers valuable insights for policymakers to design effective flood management strategies. These findings underscore the urgency of implementing region-specific strategies to adapt to flashier floods in a warmer future.