Coarse climate model grids underestimate coastal humid-heat extremes

Humid-heat extremes reach intensities close to the limit of human survivability around the Arabian Peninsula. Here, we analyze fully coupled kilometer-scale Icosahedral Nonhydrostatic (ICON) and Integrated Forecasting System-FinitevolumE Sea ice-Ocean Model (IFS-FESOM) simulations from the nextGEMS project, with original resolutions of around 9 kilometers, and compare them with station observations, ERA5, and CMIP6 projections. IFS-FESOM reproduces well the observed upper-tail wet-bulb temperature (TW) and lies near the high end of the CMIP6 distribution over coastal regions, while ICON simulates weaker extremes. Spatial smoothing suppresses localized coastal maxima, reducing daily-maximum TW extremes by up to 8°C and lowering exceedance frequencies across multiple thresholds. Native resolution fields also show stronger mid-century increases in exposure under SSP3-7.0 than their coarse-gridded counterparts. These results suggest that conventional coarse resolution projections may underestimate localized coastal humid-heat risk, although the magnitude of this effect remains model dependent.