Increasing hourly heavy rainfall in Austria reflected in flood changes.

Climate change is expected to increase heavy rainfall with concomitant increases in flooding. Causes of increased heavy rainfall include the higher water-holding capacity of a warmer atmosphere and changes in atmospheric circulation patterns, which may translate into future heavy rainfall increases in most of Europe. 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.