Global warming is a major threat to coral reef ecosystems, causing significant changes in coral assemblages. A study reveals that coral reefs respond non-linearly to heat stress, with increasing heat exposure leading to catastrophic coral loss and functional shifts. The 2016 marine heatwave on the Great Barrier Reef caused widespread coral mortality, with 40-90% of coral cover lost on reefs exposed to 4-10°C-weeks of heat stress. Exposure of 6°C-weeks or more led to a regional shift in coral composition, with some species dying rapidly while others survived. This shift transformed 29% of the Great Barrier Reef's coral assemblages, reducing biodiversity and ecological functions. The northern third of the reef, which experienced the highest heat stress, is unlikely to recover due to frequent marine heatwaves. Post-bleaching mortality has altered reef ecosystems, with some species dying and others surviving, leading to a radical shift in species composition and functional traits. The study highlights the need for understanding the relationship between heat exposure and coral mortality to predict future reef conditions. The 2016 bleaching event caused a 30% decline in coral cover across the Great Barrier Reef, with severe impacts on the northern third. The study also shows that coral mortality varies with heat exposure, with higher mortality at higher levels of stress. The findings emphasize the urgent need for climate action to prevent further degradation of coral reefs.Global warming is a major threat to coral reef ecosystems, causing significant changes in coral assemblages. A study reveals that coral reefs respond non-linearly to heat stress, with increasing heat exposure leading to catastrophic coral loss and functional shifts. The 2016 marine heatwave on the Great Barrier Reef caused widespread coral mortality, with 40-90% of coral cover lost on reefs exposed to 4-10°C-weeks of heat stress. Exposure of 6°C-weeks or more led to a regional shift in coral composition, with some species dying rapidly while others survived. This shift transformed 29% of the Great Barrier Reef's coral assemblages, reducing biodiversity and ecological functions. The northern third of the reef, which experienced the highest heat stress, is unlikely to recover due to frequent marine heatwaves. Post-bleaching mortality has altered reef ecosystems, with some species dying and others surviving, leading to a radical shift in species composition and functional traits. The study highlights the need for understanding the relationship between heat exposure and coral mortality to predict future reef conditions. The 2016 bleaching event caused a 30% decline in coral cover across the Great Barrier Reef, with severe impacts on the northern third. The study also shows that coral mortality varies with heat exposure, with higher mortality at higher levels of stress. The findings emphasize the urgent need for climate action to prevent further degradation of coral reefs.