Increasing hourly heavy rainfall in Austria reflected in flood changes

Climate change is expected to increase heavy rainfall with concomitant increases in flooding¹. Causes of increased heavy rainfall include the higher water-holding capacity of a warmer atmosphere and changes in atmospheric circulation patterns², which may translate into future heavy rainfall increases in most of Europe³. However, gathering evidence on the time evolution of past changes has been hampered by data limitations and measurement uncertainties, in particular for short rainfall durations, such as 1 h. Here we show an 8% increase in daily and 15% increase in hourly heavy rainfall over the last four decades by analysing a new dataset comprising 883 stations in Austria from 1900 to 2023. These increases are fully consistent between two independent networks and occurred after a retarding phase between 1960 and 1980. Hourly heavy rainfall changes are aligned with temperature increases with the sensitivity of a 7% increase per 1 °C of warming, in line with Clausius-Clapeyron scaling. Daily heavy rainfall changes, however, are aligned with atmospheric circulation indices with little correlation to air temperature, which suggests a bigger role of atmospheric circulation modes than previously thought. The daily heavy rainfall changes are remarkably consistent with observed flood increases of about 8% in large catchments. The hourly heavy rainfall changes are similarly consistent with flood changes in small catchments, although the flood increase is stronger (25% over the last four decades). Climate adaptation measures in flood management may therefore be more pressing for rivers draining smaller catchment areas than for large rivers.