Tropical sea surface temperatures (SSTs), as thermodynamically recorded in Barbados corals, were 5-degrees-C colder than present values 19,000 years ago. Variable tropical SSTs may explain the interhemispheric synchroneity of global climate change as recorded in ice cores, snowline reconstructions, and vegetation records. Radiative changes due to cloud type and cloud cover are plausible mechanisms for maintaining cooler tropical SSTs in the past.