Coral bleaching is the process by which corals expel the symbiotic algae called zooxanthellae that live in their tissues, causing them to turn white. The most common trigger is elevated sea surface temperature, although changes in salinity, light, bacterial infection, and pollutants such as oxybenzone from sunscreen can also cause bleaching. Without the zooxanthellae, which supply up to 90 percent of the coral's energy through photosynthesis, bleached corals begin to starve.
How much coral did the 2016 Great Barrier Reef bleaching event kill?
The 2016 bleaching event killed between 29 and 50 percent of the Great Barrier Reef's coral. Near Port Douglas, on average more than half of the bleached corals died. A November 2016 survey of 62 reefs found that long-term heat stress had caused a 29 percent loss of shallow-water coral.
What is the 2023-2025 global coral bleaching event?
The 2023-2025 global coral bleaching event, which began in February 2023, is the fourth confirmed global bleaching event. It has affected reefs in at least 82 countries and territories across all major ocean basins. By April 2025-84 percent of the world's reefs had been exposed to bleach-level heat, with coral mortality reaching up to 93 percent in some areas along the Pacific coast near Mexico.
How does ocean acidification contribute to coral bleaching?
Ocean acidification occurs when carbon dioxide from burning fossil fuels dissolves into seawater, lowering the ocean's pH and reducing the availability of carbonate ions that corals need to build their calcium carbonate skeletons. This weakens reef structures and makes corals more susceptible to erosion. Acidification also works in combination with thermal stress to amplify bleaching effects, as demonstrated in an eight-week experiment at Heron Island, Australia, where 40-50 percent of Acropora bleached under high-CO2 conditions.
What are super corals and how are researchers using them to restore reefs?
Super corals are corals that live and thrive in naturally warmer and more acidic environments. Ruth Gates and Madelaine Van Oppen conducted experiments at the Gates Coral Lab at the Hawaii Institute of Marine Biology to breed corals capable of withstanding the stressors that currently cause mass die-offs. Research also found naturally heat-tolerant corals near Ofu Island in American Samoa and in the Andaman Sea. Growing corals in research tanks takes at least 10 years before the corals are mature enough to breed.
What is the economic value of coral reefs and how does bleaching affect it?
Coral reefs contribute approximately $2.7 trillion annually to the global economy, including $36 billion from tourism alone. Experts estimate reef services are worth up to $1.2 million per hectare, averaging $172 billion per year globally. A study by Chen and colleagues found that reef commercial value falls by almost 4 percent for every 1 percent decline in coral cover, and a model by Speers and colleagues calculated direct fishery losses from decreased coral cover at $49-69 billion under high greenhouse gas emission scenarios.