Substantial impacts from the 2024 summer reverse five years of coral cover gains on the Great Barrier Reef

Abstract Anthropogenic climate change is fuelling repeated marine heatwaves and has triggered the fourth global mass coral bleaching event in 2023–2025, causing widespread coral mortality worldwide. Here, we document the impacts of this event on the Great Barrier Reef (GBR), where heat accumulation peaked in February–March 2024 and caused mass coral bleaching for the seventh time since 1998. We use data from the Australian Institute of Marine Science (AIMS)’s Long-Term Monitoring Program (LTMP), which during the 2024–2025 season recorded coral cover on 124 reefs distributed across the full latitudinal and longitudinal extent of the GBR. Coral cover declined to 30.0% (26.6–33.8% Credible Intervals [CIs]; a 24.8% relative decline since 2024) in the Northern GBR, to 28.6% (25.7–31.6% CIs; a 14% relative decline) in the Central GBR and to 26.9% (23.1–30.9% CIs; a 30.6% relative decline) in the Southern GBR. For the Northern and Southern GBR, this was the largest single-year decline in hard coral cover since monitoring began in 1986. Acropora corals contributed between 40 and 80% of the increases and 48–70% of the decreases in hard coral cover over the 39 monitoring years. Despite the pivotal role Acropora play in accelerating post-disturbance recovery through their rapid growth, their vulnerability to disturbance also contributes to an increasingly volatile ecosystem, where hard coral cover oscillates between record lows and record highs. Climate change is already fuelling increasingly frequent and intense heatwaves, and conditions for reef recovery will continue to deteriorate without urgent reductions of greenhouse gas emissions.