Corals on the Great Barrier Reef experienced a catastrophic die-off following the extended marine heatwave of 2016, transforming the ecological functioning of almost one-third of the 3,863 reefs that comprise the world’s largest reef system. These findings, reported online in Nature this week, reinforce the need for risk assessment for reef ecosystem collapse, especially if global action on climate change fails to limit warming to 1.5-2 °C above pre-industrial levels.

Terry Hughes and colleagues map the geographical pattern of heat exposure and resultant coral death along the 2,300-km length of the Great Barrier Reef following the extreme marine heatwave of 2016. They find that although many corals died immediately from the heat stress, others died more slowly following the depletion of their zooxanthellae - the yellowish brown symbiotic algae that live within most reef-building corals. Coral death was highly correlated with the amount of bleaching and level of heat exposure, with the northern third of the Great Barrier Reef most affected. The coral die-off also led to radical changes in the composition and functional traits of coral assemblages on hundreds of individual reefs, with mature and diverse assemblages transformed into more degraded systems.

The authors note that a full recovery to the pre-bleaching assemblages is unlikely to occur, because many surviving coral colonies continue to die slowly, and the replacement of dead corals will take at least a decade even for fast-growing species. Moreover, the Great Barrier Reef experienced severe bleaching again in 2017, causing further extensive damage. As such, coral reefs throughout the tropics are likely to continue to degrade until climate change stabilizes, allowing remnant populations to reorganize into heat-tolerant reef assemblages, the authors conclude.