The Great Barrier Reef

Warming seas cause mass coral bleaching

Corals live close to their upper heat limit. A rise of one degree Celsius above the usual summer temperature, held for a few weeks, makes them expel the algae in their tissues, and without those algae they lose their colour and much of their food. Marine heatwaves that were once rare now return every few years, so bleaching arrives again before slow growing corals have rebuilt.

Crown of thorns starfish and cyclones strip the coral

Crown of thorns starfish eat living coral and multiply into outbreaks that spread along the reef. Tropical cyclones snap branching coral and can flatten a reef in a single night. Both are natural events, but they now fall on reefs already weakened by heat, so the gap between disturbances is shorter than the time coral needs to recover.

Farm runoff and fishing change the water and the food web

Rivers draining sugar cane and cattle country carry sediment, nitrogen and pesticides onto inshore reefs. Sediment clouds the water and cuts the light the algae depend on, while extra nutrients feed the plankton that crown of thorns larvae eat. Fishing removes predators and the fish that graze seaweed, so seaweed can hold space that coral would otherwise take.

Primary

• Corals bleach white when heat stress makes them expel the algae that feed them, and they die if the heat lasts.
• Hard coral cover in the northern reef fell from 39.8 per cent in 2024 to 30.0 per cent in 2025.
• The reefs that bleached most severely in the 2024 summer lost the most coral.

Secondary

• Dead coral skeletons break down, so the reef structure flattens and loses the holes that fish shelter in.
• Corals that survive bleaching grow and reproduce less well afterwards, so each recovery is slower than the last.
• Seaweed can spread across bare rock, which makes it harder for young coral to settle.

Social

• Traditional Owners whose sea country includes the reef watch places they have managed for generations change.
• Reef work is concentrated in coastal towns such as Cairns and Port Douglas, so a poor season is felt locally.
• Fishing communities face rules on where they may fish and what they may take under the Marine Park zoning.

Economic

• The reef added 6.4 billion Australian dollars to the national economy and supported more than 64,000 full time jobs.
• Tourism is the largest part of that value, so what visitors are able to see and reef condition are linked.
• Fishing, shipping and ports also use the Marine Park and have to be managed alongside tourism.

Environmental

• Hard coral cover in 2026 was 35.1 per cent in the north, 31.6 per cent in the centre and 26.4 per cent in the south.
• Mass bleaching has now struck in 1998, 2002, 2016, 2017, 2020, 2022, 2024 and 2025.
• Inshore reefs near river mouths carry the extra load of sediment, nutrients and pesticides from farmland.

Immediate

• Aerial and in water surveys map bleaching while it is happening, so managers know which reefs were hit.
• The reef authority publishes heat stress and bleaching updates through the hot months each summer.
• Divers cull crown of thorns starfish on the reefs that matter most for tourism and for ecology.

Long term

• The Marine Park zoning plan closes 33 per cent of the park to fishing and collecting.
• Australian and Queensland governments and private contributors have committed more than 5 billion dollars to reef protection.
• Water quality programmes pay and advise farmers to cut sediment, nutrient and pesticide runoff from the catchments.
• Long term monitoring surveys the same reefs every year, which is how any change in coral cover is known.
• In 2025 the World Heritage Committee decided not to place the reef on its list of sites in danger.