Increase in hourly precipitation extremes beyond expectations from temperature changes

Changes in precipitation extremes under greenhouse warming are commonly assumed to be constrained by changes in the amounts of precipitable water in the atmosphere(1-4). Global climate models generally predict only marginal changes in relative humidity(5), implying that the actual amount of atmospheric precipitable water scales with the water vapour content of saturation, which is governed by the Clausius-Clapeyron relation. Indeed, changes in daily precipitation extremes in global climate models seem to be consistent with the 7% increase per degree of warming given by the Clausius-Clapeyron relation(3,4), but it is uncertain how general this scaling behaviour is across timescales. Here, we analyse a 99-year record of hourly precipitation observations from De Bilt, the Netherlands, and find that one-hour precipitation extremes increase twice as fast with rising temperatures as expected from the Clausius-Clapeyron relation when daily mean temperatures exceed 12 degrees C. In addition, simulations with a high-resolution regional climate model show that one-hour precipitation extremes increase at a rate close to 14% per degree of warming in large parts of Europe. Our results demonstrate that changes in short-duration precipitation extremes may well exceed expectations from the Clausius-Clapeyron relation. These short-duration extreme events can have significant impacts, such as local flooding, erosion and water damage.