Coral bleaching occurs when heat-stressed corals lose the algae that feed them, exposing their white skeletons. Bleached corals are still alive and can recover if the water cools soon enough. On Australia’s Great Barrier Reef, human-caused ocean warming is making marine heatwaves more severe, and repeated bleaching can leave reefs too little time to recover.
Jump to: Impacts and recovery · Bleaching history and seasons · Climate change · Causes · Coral bleaching for kids · Cairns & Port Douglas · Bleaching research · FAQs
Impacts and recovery
How bleaching affects corals and reef wildlife
Coral survival and reproduction. Colonies can regain algae and colour after cooling, but the energy shortfall can slow growth and reduce reproduction. When adult colonies die, fewer larvae are available to seed nearby reefs. Researchers recorded 89% fewer new coral recruits in 2018 than historical levels after the 2016 and 2017 events. Hughes and colleagues, Nature
Fish and habitat. Live branching corals provide food or shelter for some reef fish. As dead branches break down, the reef loses crevices that shelter small fish and invertebrates. The mix of species can change as habitat changes, with coral specialists generally most directly affected. Fast-growing corals can raise cover while slower-growing species and habitat structure take longer to recover. AIMS monitoring report
Social impacts of coral bleaching
Tourism, fishing and coastal protection. Damage at frequently visited sites can change where operators take guests and create uncertainty for businesses that depend on reef trips. Habitat change can affect reef-dependent fisheries over time. Where reef structure erodes, it can also weaken the physical barrier that reduces wave energy reaching shore. Reef Authority tourism strategy · Southern Reef bleaching study
Traditional Owners and Sea Country. Changes to species and habitats can affect customary fishing, access to culturally significant places and the passing on of knowledge. Traditional Owners lead work to care for Sea Country, including monitoring and conservation guided by their own priorities. Traditional Owners of the Great Barrier Reef
What can reduce the impact of coral bleaching?
Cut greenhouse gas emissions. Reducing global emissions limits future ocean warming and addresses the main driver of mass bleaching. Longer intervals between severe heatwaves give surviving colonies and new recruits more time to grow and reproduce. Coral Sea warming study
Reduce local pressures. Under the Australian and Queensland governments’ Reef 2050 Plan, catchment projects reduce sediment and nutrient runoff. On priority reefs, targeted crown-of-thorns starfish control protects live coral, including colonies that can supply larvae after a disturbance. Reef 2050 Plan · Reef Authority starfish control programme
Support recovery at selected sites. The Great Barrier Reef Foundation supports Coral IVF, which collects spawn, raises larvae in floating pools and releases them onto target reefs. The Coral Nurture Program works with operators around Cairns and Port Douglas to grow and attach coral fragments. Researchers are also testing ways to improve heat tolerance and deploy restoration at larger scales. These interventions can help selected sites recover while broader action addresses ocean warming. Foundation restoration projects · Cairns–Port Douglas projects · Reef Restoration and Adaptation Program
Protect coral during your visit. Keep fins clear of the reef, float over shallow coral and avoid touching or standing on it. Choose a High Standard Tourism Operator and ask whether the crew contributes observations to Eye on the Reef. If you report bleaching, include the date, location and a clear photo where possible. Reef Authority tourism programmes · Eye on the Reef Sightings Network
Bleaching history and seasons
| Summer or year | What was recorded |
|---|---|
| 1998 | First documented mass bleaching on the Great Barrier Reef, during the first global mass bleaching event. |
| 2002 | A second mass event affected reefs across the system. Localised bleaching also occurred in the south in 2006. |
| 2016 | Severe bleaching was concentrated in the north. |
| 2017 | A second consecutive event affected northern and central reefs, leaving some areas little time to recover. |
| 2020 | Widespread, variable bleaching reached southern reefs that had largely escaped the 2016 and 2017 events. |
| 2022 | The first mass event recorded during La Niña. The central Reef experienced the strongest heat stress. |
| 2024 | Extreme bleaching was recorded in all three regions for the first time. The most severe impacts were concentrated in the central and southern regions. |
| 2025 | A March aerial survey covered 281 reefs across the northern and central Reef and Torres Strait. Of the 162 inshore and mid-shelf reefs surveyed in the northern and central Reef, 41% had medium to high bleaching, meaning 11–60% of their coral cover was bleached. |
| 2026 | The Reef Authority recorded a regional event in the north and localised low levels of bleaching in the far north, central and southern regions. |
Reef Authority bleaching history, AIMS 2024–25 report AIMS bleaching event data and 2025–26 summer snapshot.
What the 2024 aerial surveys found
Flights began over the southern Reef on 23–24 February and continued across the Reef in March. The teams surveyed 1,080 reefs across the Marine Park and Torres Strait. Within the Marine Park, 73% of surveyed reefs had bleaching across more than 10% of their visible shallow coral cover. On 39%, bleaching affected more than 60% of that cover. Extreme bleaching meant more than 90% at an individual reef. About 46% of reefs across the Great Barrier Reef experienced record heat stress, and some surveyed inshore southern reefs reached 15 DHW. The flights mapped visible bleaching. Later underwater surveys measured survival and losses. 2024 aerial survey report and maps
What changed in 2025 and 2026?
In 2025, aircraft focused on reefs north of Townsville, where heat stress was highest. Offshore, 99% of the 96 surveyed reefs had low to medium bleaching. None of the 281 surveyed reefs reached the extreme category. Near Townsville, floodwater also caused coral mortality around the Palm Islands, where low salinity added to the season’s pressures. Follow-up monitoring found low mortality associated with the 2025 mass bleaching event itself. In 2026, late monsoon cloud and rain brought relief from rising heat, although cyclones and flood plumes damaged some reefs. AIMS bleaching event data · 2025–26 summer snapshot
Before and after at One Tree Island
Researchers tracked 462 colonies over 161 days at this southern reef. The share of tracked colonies that were bleached rose from 66% in February 2024 to 80% in April. By July, 53% of the bleached colonies had died. Mortality reached 95% in the tracked Acropora corals. This study shows how damage can continue to emerge as the water cools. Peer-reviewed One Tree Island study
How much live coral remained?
| AIMS monitoring region | 2023–24 | 2024–25 | 2025–26 |
|---|---|---|---|
| Northern | 39.8% | 30.0% | 35.1% |
| Central | 33.2% | 28.6% | 31.6% |
| Southern | 38.9% | 26.9% | 26.4% |
These are regional estimates of live hard coral cover, the share of surveyed reef surface occupied by living hard coral. The 2024–25 declines were driven chiefly by mortality from the 2024 heatwave, alongside cyclone damage and crown-of-thorns starfish predation. In the 2025–26 survey of 121 reefs, cover increased at 28%, held steady at 56% and fell at 16%. The central increase and small southern decline were within the estimates’ uncertainty. AIMS 2024–25 results · AIMS 2025–26 results and long-term graphs
Climate change and coral bleaching
The Coral Sea is starting from a warmer baseline. A 2024 study reconstructed January–March temperatures using coral skeleton records. The three hottest summers in its 400-year reconstruction were 2024, 2017 and 2020, in that order. Climate modelling attributed the rapid recent warming to human influence. Henley and colleagues, Nature
Some corals can acclimatise to a particular warming pattern. A 2016 study found that about three-quarters of historical heat-stress events followed a pattern in which mild heat primed corals ahead of stronger stress. In the study, that sequence more than halved coral cell death and symbiont loss. An extra 0.5°C of local warming could remove that protective sequence under the scenarios tested. The result describes the loss of a specific form of acclimatisation. Ainsworth and colleagues, Science
Weather still shapes individual events. El Niño can coincide with conditions that favour heat build-up. La Niña has historically brought cooler summers to the Reef, yet widespread bleaching still occurred in 2022. In 2026, late monsoon conditions helped cool parts of the Reef and eased heat stress. AIMS bleaching history · AIMS 2025–26 report
What causes coral bleaching?
Heat disrupts the coral–algae partnership. Reef-building corals host microscopic algae called zooxanthellae. These algae use sunlight to make sugars that feed the coral. Heat, particularly under intense sunlight, disrupts photosynthesis. The coral loses algae or their pigments, turning pale, white or sometimes bright pink, blue or purple as its own pigments become more visible. Reef Authority bleaching guide
Duration matters as much as temperature. NOAA’s Degree Heating Weeks (DHW) add up heat at least 1°C above the local climatological maximum monthly mean over the previous 12 weeks. A qualifying anomaly of 1°C for four weeks or 2°C for two weeks both produce 4 DHW, a level associated with significant bleaching risk. At 8 DHW, widespread bleaching and mortality of heat-sensitive corals become more likely. Species, depth, light and water movement influence the outcome. NOAA methodology
Other stresses can cause local bleaching or hinder recovery. Freshwater from floods can lower salinity enough to bleach shallow corals. Sediment can smother young corals, excess nutrients can encourage algal growth and ocean acidification makes it harder for reef builders to maintain their skeletons. Cyclones can also break colonies and crown-of-thorns starfish eat live coral tissue. Reef Authority summer snapshot
Coral bleaching explained for kids
A coral is a tiny animal. Lots of these animals live together and build a hard home. Tiny algae live inside them and make food from sunlight. When the sea gets too hot, the coral can lose those little food-makers and you can see its white home through its skin. If the water cools soon enough, the coral can get its algae back.
Coral bleaching around Cairns and Port Douglas
Conditions can vary between neighbouring reefs and even across depths at a single site. After the 2024 heatwave, AIMS recorded losses ranging from 17% to 60% of previous coral cover at monitored reefs near Cairns. Ask about the exact mooring or snorkel site and when the operator last surveyed it. For seasonal conditions, see seasonal weather patterns on the Great Barrier Reef. To compare nearshore islands and outer-reef trips, see snorkelling from Cairns and Port Douglas. AIMS survey findings
What a mathematical model adds
A 2025 study modelled how coral communities shift as corals lose or regain algae, grow and die. Calibrated against observations from Great Barrier Reef sites, the model highlighted coral growth and mortality during bleaching as important influences on recovery. The simulations also suggested that widespread visible bleaching can coexist with recovery when mortality stays low. A practical implication for monitoring is to follow colonies through cooling and measure which survive and which new corals establish. Ecological Modelling study
Frequently asked questions
The 1998 event is the starting point for the modern record of mass bleaching on the Reef. Individual colonies and local reefs can bleach whenever conditions are stressful enough. The timeline above tracks events recorded across large areas. AIMS bleaching history
Use the 2024 aerial survey maps for bleaching observed from aircraft, NOAA Coral Reef Watch for satellite-based heat-stress maps, and AIMS’s long-term graphs to see how live coral cover has changed since the 1980s. Check the map date and legend before comparing products. An alert on a heat map indicates risk. An aerial survey shows what observers could see at the time.