Coral δ ¹³ C Reveals Little Ice Age Dimming of Tropical Surface Shortwave Radiation Not Captured by Climate Models

Abstract Tropical low‐cloud feedback is the largest source of uncertainty in climate sensitivity, yet multi‐century records of surface shortwave radiation are scarce. We calibrate Porites coral δ 13 C against satellite photosynthetically available radiation (PAR) and reconstruct monthly PAR for the northern South China Sea during the Medieval Climate Anomaly (1129–1264 CE) and the Little Ice Age (1631–1771 CE). After correcting for the Suess effect and propagating errors via Monte Carlo resampling techniques, annual PAR during the Little‐Ice‐Age is ∼22% lower and seasonality slightly weaker. The dimming aligns with regional proxies for cooler, wetter conditions and is best explained by brighter low clouds, likely boosted by volcanic aerosol–cloud interactions. CMIP6/PMIP4 past1000 simulations, however, yield <0.2% change over the same interval, indicating that current models understate volcanic microphysics and tropical low‐cloud sensitivity. The coral PAR record thus provides a quantitative pre‐industrial target for evaluating tropical cloud processes and reducing uncertainty in equilibrium climate sensitivity.