The article highlights the significant impact of global warming on coral reef ecosystems, particularly the Great Barrier Reef. The authors reveal that heat exposure is a critical factor in determining the resilience of coral assemblages, with low levels of stress initially having little effect but becoming increasingly detrimental at higher exposures. After the 2016 marine heatwave, corals began to die immediately when heat stress exceeded 3-4 °C-weeks, and after eight months, sites exposed to 4-10 °C-weeks lost between 40% and 90% of their coral cover. An exposure of 6 °C-weeks or more led to a regional-scale shift in coral composition, reflecting divergent responses to heat stress among different taxa. This abrupt shift transformed 29% of the 3,863 reefs in the Great Barrier Reef system, with the northern third experiencing the most severe temperature anomalies and unlikely to fully recover due to the increasing frequency of marine heatwaves. The study also shows that the severity of coral mortality was highly correlated with the amount of bleaching and heat exposure, and that the longer-term loss of coral cover increased progressively with higher levels of heat exposure. The functional traits of coral assemblages were also significantly altered, with a shift from fast-growing, three-dimensional species to simpler morphological characteristics and slower growth rates. The authors conclude that the 2016 bleaching event has triggered a geographic-scale collapse of coral assemblages, and that the prospects for full recovery are poor due to ongoing heat stress and the long recovery times required for weedy corals. The recurrence of mass bleaching during the recovery period is critical, given the increasing frequency of such events.The article highlights the significant impact of global warming on coral reef ecosystems, particularly the Great Barrier Reef. The authors reveal that heat exposure is a critical factor in determining the resilience of coral assemblages, with low levels of stress initially having little effect but becoming increasingly detrimental at higher exposures. After the 2016 marine heatwave, corals began to die immediately when heat stress exceeded 3-4 °C-weeks, and after eight months, sites exposed to 4-10 °C-weeks lost between 40% and 90% of their coral cover. An exposure of 6 °C-weeks or more led to a regional-scale shift in coral composition, reflecting divergent responses to heat stress among different taxa. This abrupt shift transformed 29% of the 3,863 reefs in the Great Barrier Reef system, with the northern third experiencing the most severe temperature anomalies and unlikely to fully recover due to the increasing frequency of marine heatwaves. The study also shows that the severity of coral mortality was highly correlated with the amount of bleaching and heat exposure, and that the longer-term loss of coral cover increased progressively with higher levels of heat exposure. The functional traits of coral assemblages were also significantly altered, with a shift from fast-growing, three-dimensional species to simpler morphological characteristics and slower growth rates. The authors conclude that the 2016 bleaching event has triggered a geographic-scale collapse of coral assemblages, and that the prospects for full recovery are poor due to ongoing heat stress and the long recovery times required for weedy corals. The recurrence of mass bleaching during the recovery period is critical, given the increasing frequency of such events.