Cleaner air, drier land: the unintended drought consequences of near-term climate forcers mitigation

Abstract. Drought poses severe threats to water resources, ecosystems, and socioeconomic well-being worldwide. While greenhouse gas (GHG) emissions are the primary driver of global warming and the associated intensification of the hydrological cycle, the role of non-methane near-term climate forcers (NTCFs) — encompassing aerosols and their precursors, ozone-forming reactive gases, and other short-lived species — in modulating regional drought risk remains poorly constrained, particularly in arid and semi-arid regions where aerosol-induced radiative effects and circulation feedbacks interact non-linearly with the water balance. Here we employ seven Earth System Models from the CMIP6 AerChemMIP framework to quantify the contribution of NTCF emissions to drought evolution under two future pathways that share identical GHG forcing but differ in the stringency of NTCF controls: SSP3-7.0 and SSP3-7.0-lowNTCF. Using the Standardised Precipitation Evapotranspiration Index at 3-, 6-, and 12-month timescales, we characterise projected changes in drought frequency, duration, intensity, and severity by mid-21st century and identify the large-scale dynamical mechanisms driving regional responses. NTCF mitigation exerts negligible influence on global-mean drought tendency but produces strongly heterogeneous regional responses, with drought conditions worsening substantially across the Sahel, West Asia, Central Asia, and the Mediterranean, while Southeast Asia, Australia, and Central America experience significant drought reductions. These findings are robust across the majority of models and highlight that air quality policies carry regionally differentiated and sometimes counterintuitive hydrological consequences that must be considered alongside their climate and health co-benefits.