Bleaching is one of the most misunderstood events in marine biology. A bleached coral is not a dead coral, and that distinction matters for what happens next.

By Clarisa Strohmeyer, Founder | Updated 18 September 2026 | 12 min read

Coral bleaching happens when a coral expels the algae living inside its tissue, usually because the water is too warm for too long. Those algae provide as much as 90% of the coral’s food, so a bleached coral is starving rather than dead. If the heat passes quickly enough, it can take the algae back and survive. If the heat persists, it dies. Everything else about bleaching follows from that distinction.

Key takeaways

Bleaching is starvation, not death. A bleached coral has expelled the algae that supply up to 90% of its food.
Thermal stress begins at about 1°C above the usual summertime maximum. Duration matters as much as temperature.
Corals can recover if the heat passes quickly enough. Whether they do depends on how long the stress lasted and what state they were in beforehand.
The 2023 to 2025 event affected 84% of the world’s reefs across 82 countries, the largest on record.

What is actually happening inside the coral

A reef-building coral is two organisms living as one. The animal is a polyp, a small soft-bodied creature that builds a limestone skeleton. Inside its tissue live single-celled algae called zooxanthellae.

The arrangement is mutual. NOAA describes it plainly: the coral offers the algae “a protected environment,” and in return, “as much as 90 percent of the organic material photosynthetically produced by the zooxanthellae is transferred to the host coral tissue.” The algae also give the coral its colour. A coral without them is not white because it has been drained of something. It is white because you are looking through transparent tissue at the limestone skeleton underneath.

Under stress, that partnership breaks down. In NOAA’s words: “Sometimes when corals become physically stressed, the polyps expel their zooxanthellae and the colony takes on a stark white appearance. This is coral bleaching.”

The coral is now alive, visibly intact, and cut off from most of its food supply. It can feed itself to a limited degree by catching plankton, but not enough to sustain it indefinitely. It is on a clock.

What triggers it

Heat, principally. Not heat in the abstract, but heat relative to what that particular reef is used to, sustained over time.

NOAA Coral Reef Watch measures this with two linked metrics, and they are worth understanding because they are what the alerts you see in the news are based on.

HotSpot is the difference between current sea surface temperature and the site’s Maximum Monthly Mean, which is the average temperature of its warmest month. Thermal stress begins at roughly 1°C above that usual summertime maximum. A reef in Cebu and a reef in the Red Sea have very different absolute temperatures; what matters is the departure from local normal.

Degree Heating Weeks accumulates that excess heat over a rolling 12-week window. One week at 2°C above the threshold and two weeks at 1°C above produce the same accumulated stress. This is why duration matters as much as peak temperature.

The thresholds are specific. At 4 °C-weeks, reef-wide bleaching is expected. At 8 °C-weeks, bleaching with mortality of heat-sensitive corals is expected.

Other things can bleach coral too: cold shock, freshwater from heavy runoff, disease, pollution, unusually intense light. But mass bleaching, the kind that hits entire regions at once, is a heat phenomenon.

Can bleached coral recover?

Yes, sometimes. This is the part most coverage gets wrong in one direction or the other.

If water temperatures return to normal before the coral exhausts itself, it can re-acquire zooxanthellae and regain both its colour and its food supply. Recovery is not instant and it is not free. A coral that has bleached and recovered is typically slower-growing, less likely to reproduce that season, and more vulnerable to disease for a period afterwards.

If the stress continues, the coral starves. Mortality is not uniform across a reef: fast-growing branching species tend to die first, while massive corals are more tolerant. This is why a reef that has been through a severe event often looks structurally different afterwards, even where coral cover partly returns.

The honest summary is that bleaching is a survivable injury whose outcome depends on how long it lasts, how healthy the coral was beforehand, and what else the reef is dealing with. A reef under fishing pressure and poor water quality recovers less well than one that is otherwise intact. That is the main reason local protection matters even though it does nothing about the heat.

How the alert levels work

NOAA’s Bleaching Alert Scale originally topped out at Level 2. During the most recent global event it had to be extended, which tells you something in itself. The upper levels now read:

Level 3 (12 to 16 °C-weeks): risk of multi-species mortality
Level 4 (16 to 20 °C-weeks): risk of severe, multi-species mortality, over 50%
Level 5 (20 °C-weeks and above): risk of near-complete mortality, over 80%

A measurement scale being rebuilt because reality exceeded its top value is not a small detail. It is the clearest single indication of how the last few years have gone.

The four global bleaching events

Mass bleaching on a global scale is a recent phenomenon. Four events have been recorded, and the trajectory is not subtle.

1998: 21% of the world’s reefs affected
2010: 37%
2014 to 2017: 68%
2023 to 2025: 84%, across 82 countries, territories and economies

The fourth event ran from 1 January 2023 to 30 March 2025 and was confirmed by the International Coral Reef Initiative in April 2025. It is the most extensive on record.

The interval between events matters as much as their size. Hughes and colleagues, publishing in Science in 2018, examined 100 reef locations worldwide between 1980 and 2016 and found that the median return time between severe bleaching events had fallen to six years. In the early 1980s it was measured in decades.

That shift is the real problem. A reef hit once every few decades has time to rebuild. A reef hit every six years does not. Recovery from a severe event takes longer than the gap now between events, which is how a reef can decline steadily without any single catastrophe being obviously responsible.

What this means for the Philippines and Cebu

Philippine reefs sit in the Coral Triangle, the most biodiverse marine region on earth, and they were not spared. The fourth global event reached them along with everywhere else.

We are careful here about what we claim. We have not conducted our own bleaching surveys, so we do not publish our own figures for Cebu. Published Philippine assessments are cited on our State of Philippine Reefs page, and where those numbers come from a specific study we name it.

What we can say about our own site: in July and August 2026 our founder spent six weeks in Cebu and photographed and filmed the reef at Kontiki, Mactan, where our first restoration mission is planned for Q4 2026. That is a visual record, not a bleaching survey, and we will not present it as one.

What actually helps

Being honest about bleaching means being honest that most of the responses people are offered do not address it.

Reducing emissions is the only thing that addresses the cause. Every other measure buys time or improves odds at the margin. This is not a reason to skip the other measures, but it is the order of magnitude.

Local pressure reduction improves recovery odds. A reef with healthy fish populations and clean water comes through a bleaching event better than one already under strain. This is the strongest argument for marine protected areas, and it is a real one.

Restoration does not prevent bleaching. Planting coral into warming water does not cool the water. Restoration can rebuild a specific site and is worth doing for that reason, but anyone presenting it as an answer to bleaching has the relationship backwards. We cover this in detail in Does Coral Reef Restoration Actually Work?

Monitoring matters more than it sounds. Most reefs have no baseline, so when a bleaching event passes through, nobody can say precisely what was lost.

A shrimp on sand beside coral rubble on a reef floor. — Reef life in Cebu.

Frequently asked questions

Does bleached coral always die?

No. A bleached coral is stressed and starving, not dead. If water temperatures return to normal quickly enough, it can regain its algae and recover, though usually with reduced growth and reproduction for a period afterwards.

How long can a coral survive while bleached?

There is no fixed number. It depends on the species, the severity of the heat and the coral’s condition beforehand. The accumulated-stress thresholds give a better guide than elapsed time: bleaching is expected around 4 °C-weeks and mortality of sensitive species around 8 °C-weeks.

What is the difference between bleaching and coral death?

A bleached coral is white because its transparent tissue is still there over the skeleton. A dead coral loses that tissue and is quickly colonised by algae, turning brown or green and fuzzy. Divers can tell the difference; photographs often cannot.

Can corals adapt to warmer water?

To a limited extent, and this is an active research area. Some corals host more heat-tolerant algal strains, and some populations show higher thresholds. Whether adaptation can keep pace with the current rate of warming is an open question, and claims in either direction outrun the evidence.

Is bleaching reversible at the reef scale?

Individual colonies recover. Whole reefs recover more slowly, and only if the interval between events is long enough. With a median return time now around six years, many reefs are being hit again before they have rebuilt.

Sources and further reading

What is Zooxanthellae? — NOAA National Ocean Service. Source for the 90% figure and the bleaching mechanism.
Coral Reef Watch methodology — NOAA. Source for HotSpot, Degree Heating Weeks, the 4 and 8 °C-week thresholds and Alert Levels 3 to 5.
84% of the world’s coral reefs impacted in the most intense global coral bleaching event ever — International Coral Reef Initiative, April 2025. Source for the four global events and their percentages.
Spatial and temporal patterns of mass bleaching of corals in the Anthropocene — Hughes et al., Science, 2018. Source for the six-year median return time across 100 reef locations.
World’s Fourth Mass Coral Bleaching Event Likely Ended in 2025 — NOAA NESDIS.
A Global Analysis of the Effectiveness of Marine Protected Areas in Preventing Coral Loss — Selig & Bruno, PLOS ONE, 2010.

Coral Bleaching Explained