Australia’s Great Barrier Reef has recorded its steepest annual loss of hard-coral cover in nearly four decades, according to new data from the Australian Institute of Marine Sciences. The survey found coral cover in the reef’s northern and southern regions fell by roughly one-quarter to one-third over the past year, the largest drop since systematic monitoring began 39 years ago. Researchers attribute the decline to a mass bleaching event during the summer of 2024, the worst on record and the fifth such event since 2016. Heat stress turned large swathes of coral white across all three reef regions, leaving them vulnerable to disease and mortality and increasing volatility in coral cover levels. The findings are likely to intensify scrutiny of Australia’s management of the 2,400-kilometre ecosystem, which generates about A$6.4 billion (US$4.2 billion) annually through tourism. While the Great Barrier Reef is not on UNESCO’s endangered World Heritage list, the UN agency has recommended its inclusion, a move Canberra has long opposed over fears of reputational and economic fallout.