Warming leads to both earlier and later snowmelt floods over the past 70 years

Climate warming reduces snow cover in cold regions, altering snowmelt flood regimes with significant hydrological and ecological consequences. Existing evidence indicates that as climate warms, snowmelt tends to begin earlier in the season, leading to earlier snowmelt floods. Here we show that the timing of snowmelt floods can be either advanced or delayed under warming. Using streamflow observations from 1950-2020 and an event-based analysis that distinguishes flood-generating mechanisms across 2339 Northern Hemisphere, snow-affected catchments, we show that the earlier snowmelt effect can be substantially offset or even reversed by a decelerated snowmelt rate under warming. This results in delayed snowmelt floods in approximately 30% of the catchments, contributing to an overall minor shift on a hemispheric scale (-0.87 ± 2.4 days per decade). Our findings challenge the prevailing "warming leads to earlier snowmelt floods" paradigm, revealing a more complex pattern of changes in snowmelt flood in a warming world.