Past Acropora mortality on the Great Barrier Reef linked to climate variability and anthropogenic impacts

Understanding long-term coral reef dynamics provides essential context for assessing climate change impacts. Using uranium-thorium dating of dead coral in the Great Barrier Reef’s Mackay region, we reconstructed a 150-year history of Acropora spp. coral mortality. Results reveal decadal-scale variability superimposed on a century-long rise in mortality since ~1860, coinciding with expanding anthropogenic pressures. Major mortality episodes occurred between 1905-1970 and 1975-2005, with peaks aligning with El Niño events, cyclones and flooding. Although causality is difficult to isolate, mortality is significantly correlated with Niño 3.4 anomalies and positive phases of the Interdecadal Pacific Oscillation. Synchronous declines in the Palm Islands (~380 km north-west) began around European colonisation, alongside increased rainfall and lagoon freshening. The coherent cross-regional variability suggests regional rather than local drivers. We propose that intensifying marine heatwaves and declining water quality from catchment modification since colonisation together drive this long-term Acropora decline.