Mass bleaching is a stress response of corals subjected to warmer-than-normal seawater temperatures during the warm season. During 1997--98 there were unprecedented numbers of reports of bleaching on many of the world's coral reefs. Observational evidence suggests an increase in frequency of mass coral bleaching events since the late 1970s. Two indices of warm season sea surface temperatures (SSTs; SST maximum anomaly and degree-months) are presented for 47 reef sites where bleaching occurred during 1997--98. The level of thermal stress at the vast majority of these coral reef sites during 1997--98 was unmatched in the period 1903--99. Warm season SSTs at these coral reef sites have significantly warmed over this period and the frequency of warm season SST extremes has increased since the late 1970s. Continued warming of tropical SSTs, as is likely due to the enhanced greenhouse effect, will increase the level of thermal stress to coral reefs. Increased frequency of bleaching events will reduce corals' capacity to recover and may significantly alter the make-up of present day coral reef ecosystems. ¿ 2000 American Geophysical Union