Coral strontium/calcium ratios have been used to infer that the tropical sea surface temperature (SST) cooled by as much as 6 degrees C during the last glacial maximum. In contrast, little or no change has been inferred from other marine-based proxy records. Experimental studies of the effect of growth rate and the magnitude of intraspecific differences indicate that biological controls on coral skeletal strontium/calcium uptake have been underestimated. These results call into question the reliability of strontium/calcium-based SST reconstructions.