Skeletal Sr/Ca and O-18/O-16 ratios in corals from the Great Barrier Reef, Australia, indicate that the tropical ocean surface similar to 5350 years ago was 1 degrees C warmer and enriched in O-18 by 0.5 per mil relative to modern seawater. The results suggest that the temperature increase enhanced the evaporative enrichment of O-18 in seawater. Transport of part of the additional atmospheric water vapor to extratropical latitudes may have sustained the O-18/O-16 anomaly. The reduced glacial-Holocene shift in seawater O-18/O-16 ratio produced by the mid-Holocene O-18 enrichment may help to reconcile the different temperature histories for the last deglaciation given by coral Sr/Ca thermometry and foraminiferal oxygen-isotope-records.