Aerosol Emissions Dominate Observed and Modeled Hydrological Trends in Arid and Semiarid Regions

Arid and semi-arid regions are highly sensitive to hydroclimate changes. In recent decades, precipitation and evapotranspiration have declined across vast global drylands, posing critical challenges to water security and fragile ecosystems. However, these drying trends remain poorly understood and inadequately represented in current climate models. Here, using observations and CMIP6 multi-model simulations, we interpret hydroclimatic changes in (semi-)arid regions and associated model biases by developing a theoretical framework. From an energetic perspective, precipitation and evapotranspiration changes are directly linked to climate forcings through variations in atmospheric diabatic cooling (δQ), which is primarily governed by the response of surface sensible heat flux (δSH_down) to changes in surface shortwave radiation budget (δDSSR). Reanalysis and single-forcing simulations reveal that aerosol forcing—rather than greenhouse gases—dominates hydrological changes in dry regions. Since the 1970s, historical aerosol emissions have increased δDSSR and reduced δSH_down, the consequent decreases in δQ driving the observed drying trends. In CMIP6 simulations, the substantial underestimation of aerosol-induced solar brightening contributes to pronounced discrepancies with observations. By highlighting the critical role of aerosol effects, this work provides an effective approach for understanding and projecting dryland hydroclimatic responses to shortwave radiative forcings under broader scenarios.