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Highlights d Marine heatwaves lead to rapid coral mortality and microbial biofilm formation d Microbial metabolic activity results in rapid dissolution of the coral skeleton d Dissolution reduces skeletal hardness and density and increased porosity

The global coral bleaching event of 2014-2017 resulted from the latest in a series of heat stress events that have increased in intensity. We assessed global-and basinscale variations in sea surface temperature-based heat stress products for 1985-2017 to provide the context for how heat stress during 2014-2017 compared with the past 3 decades. Previously, undefined ''Heat Stress Year'' periods (used to describe interannual variation in heat stress) were identified for the Northern and Southern Hemispheres, in which heat stress peaks during or shortly after the boreal and austral summers, respectively. The proportion of reef pixels experiencing bleaching-level heat stress increased through the record, accelerating during the last decade. This increase in accumulated heat stress at a bleaching level is a result of longer stress events rather than an increase in the peak stress intensity. Thresholds of heat stress extent for the three tropical ocean basins were established to designate ''global'' events, and a Global Bleaching Index was defined that relates heat stress extent to that observed in 1998. Notably, during the 2014-2017 global bleaching event, more than three times as many reefs were exposed to bleaching-level heat stress as in the 1998 global bleaching.

Anomalously high sea surface temperatures (SST) have led to repeated mass coral bleaching events on a global scale. Existing satellite‐based systems used to monitor conditions conducive to bleaching are based on low‐resolution (0.5°, ∼50 km) SST data. While these systems have served the research and management community well, they have inherent weaknesses that limit their capacity to predict stress on coral reefs at local scales, over which bleaching severity is known to vary dramatically. Here we discuss the development and testing of ReefTemp, a new operational remote sensing application for the Great Barrier Reef that assesses bleaching risk daily using: high‐resolution (2 km) SST, regionally validated thermal stress indices, and color‐graded legends directly related to past observations of bleaching severity. Given projections of sea temperature rise, ReefTemp is timely as it can accurately predict bleaching severity at a local scale and therefore help to give focus to future research and monitoring efforts.