In context: Located off the coast of Queensland, Australia, the Great Barrier Reef is the world's most expansive coral reef system, spanning more than 2,300 kilometers and comprising 2,900 individual reefs and 900 islands. It also serves as an early warning sign of the widespread effects of global warming on the planet's health.
A new report from the Australian Institute of Marine Science (AIMS) confirms the deteriorating state of the Great Barrier Reef (GBR). The study assesses coral cover – and by extension, reef health – one year after the reef experienced its fifth mass bleaching event since 2016.
The BBC notes that the institute surveyed 124 individual coral reefs between August 2024 and May 2025. Scientists began these surveys in 1986 and now report the most significant annual decline in coral cover across vast sections of the Great Barrier Reef.
The northern region of the Great Barrier Reef lost about a quarter of its record coral cover – the most severe single-year drop ever recorded there. Meanwhile, the southern region saw its steepest annual decline, mainly due to unprecedented heat stress in 2024. The central reef was less affected by bleaching, showing smaller declines in coral cover.
Coral bleaching occurs when corals lose their symbiotic algae and photosynthetic pigments, turning them white. Bleached corals are not dead but face high stress and become more vulnerable to starvation and disease. If water temperatures rise even one degree Celsius above their thermal limit, stressed corals may survive only a few months.
Rising ocean temperatures linked to climate change and human pollution primarily cause coral bleaching. The AIMS report confirms the Great Barrier Reef experienced unprecedented heat stress, triggering the most extensive and severe bleaching events on record. The reefs also face heightened threats from a surge in coral-eating crown-of-thorns starfish.
The Great Barrier Reef, like coral reefs worldwide, ranks among the most vibrant ecosystems below sea level. Though they cover just one percent of Earth's surface, coral reefs host a quarter of all known marine species. They are vital underwater ecosystems supporting the livelihoods of over a billion people – making their preservation crucial.
The continued loss of coral reefs – especially the Great Barrier Reef – not only threatens marine biodiversity and coastal livelihoods but also weakens one of the planet's natural defenses against climate change, pushing us closer to more extreme environmental and economic instability.