Does the Hook Structure Constrain Future Flood Intensification Under Anthropogenic Climate Warming?

Atmospheric moisture holding capacity increases with temperature by about 7% per °C according to the Clausius‐Clapeyron relationship. Thermodynamically then, precipitation intensity should exponentially intensify and thus worsen flood conditions as the climate warms. However, regional and global analyses often report a nonmonotonic (hook) scaling of precipitation and runoff, in which extremes strengthen with rising temperature up to a maximum or peak point (Tpp) and decline thereafter. The underlying cause of this hook structure is not yet well‐understood, and whether it may shift and/or regulate storm runoff extremes under anthropogenic climate warming remains unknown. Here, we examine temperature scaling of precipitation and storm runoff extremes under different climate conditions using observations and large ensemble hydroclimatic simulations over mainland China. In situ observations suggest a spatially homogeneous, negative response of relative humidity to warming climates over 34.6% of the land area, and the remaining hook‐dominated regions usually show a colder Tpp than that of precipitation or storm runoff extremes. The precipitation and streamflow series over mainland China's catchments throughout the 21st century are projected by a model cascade chain under a high‐end emission scenario (RCP 8.5), which involves 31 CMIP5 climate models, 11 CMIP6 climate members, a daily bias correction method, and four lumped conceptual hydrological models. The CMIP5 ensemble projects that the hook structures shift toward warmer temperature bins, resulting in 10%–30% increases in storm runoff extremes over mainland China, while the CMIP6 ensemble projects more severe flood conditions in future warming climates. Plain Language Summary: China is threatened by river floods and has frequently suffered from significant ecosystem damage and economic loss under a changing environment. Storm runoff extremes, an important indicator of flooding regime, have received increasing attention, especially regarding their response to a warming climate. Previous studies have observed a nonmonotonic relationship (called a hook structure) between storm runoff extremes and atmospheric temperatures, in which the extremes strengthen with warming up to a peak point and decline thereafter in a hotter environment. Here, we investigate the ability of such behavior to constrain the intensification of future storm runoff extremes, as well as the underlying mechanisms. We find that the hook structures are not fixed under climate warming but peak at warmer temperatures and higher storm runoff rates. As a result, future flooding risks in mainland China will not be constrained by the hook structures but progressively increase by roughly 10%–30%. Key Points: The temperature scaling of precipitation, storm runoff, and relative humidity are characterized over mainland China's catchmentsPrecipitation and storm runoff usually exhibit a hook structure, with a warmer peak point temperature than that of relative humidityThe hook structures are not fixed under climate change but shift toward warmer temperatures and higher extreme rainfall and runoff rates [ABSTRACT FROM AUTHOR]