The 4th Global Coral Bleaching Event: Unprecedented, unbounded, unrelenting

Increasingly frequent marine heatwaves have escalated the prevalence and extent of mass coral bleaching events. When marine heatwaves impact reef areas across all tropical ocean basins within a common period, global-scale coral bleaching can unfold. Here, we elaborate previous analyses to define set of objective criteria for defining a Global Coral Bleaching Event. Using this definition, we describe and compare the extent, intensity, and duration of accumulated heat stress contributing to each of the declared Global Coral Bleaching Events. In both global- and basin-scale analyses, the ongoing fourth Global Coral Bleaching Event is unprecedented in the extent and intensity of accumulated heat stress, which, based on our definition, has continued for a record duration of over six years. While the comprehensive ecosystem impacts resulting from the fourth Global Coral Bleaching Event are yet to be determined, verified field observations have confirmed coral bleaching from 83 countries spanning the northern and southern hemispheres of all ocean basins. However, an analysis of satellite-based heat stress accumulation patterns indicates that bleaching has likely impacted at least 97 of the 101 countries containing coral reefs. Following our objective analyses, there were only 175 days between the end of the third and the estimated start of the fourth Global Coral Bleaching Event; therefore the world has been experiencing global-scale coral bleaching-level heat stress for nearly the entirety of the last decade.