Coral Bleaching Explained: Why Reefs Turn White
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Coral reefs cover only a tiny fraction of the ocean floor, yet they are home to roughly a quarter of all known marine species at some point in their lives. They protect coastlines from waves, support fisheries and attract tourism that many communities depend on. In recent decades, reefs around the world have been hit by repeated episodes of coral bleaching. This explainer covers what bleaching is, what causes it, and what happens afterward.
Key points
- Corals are animals that live in partnership with tiny algae inside their tissues. The algae provide most of the coral's food and much of its color.
- When water stays unusually warm for weeks, the partnership breaks down and corals expel the algae, turning white. This is bleaching.
- Bleached corals are not dead, but they are starving and stressed. Many recover if temperatures return to normal quickly enough.
- Mass bleaching events affecting reefs across many ocean basins have become more frequent as oceans warm.
What is a coral?
A coral may look like a rock or a plant, but it is an animal. A coral colony is made of many tiny animals called polyps, each a soft body with a ring of tentacles. Reef-building corals secrete hard skeletons of calcium carbonate, and over thousands of years those skeletons pile up to form the reef structure.
The partnership that powers reefs
Inside the tissues of most reef-building corals live microscopic algae called zooxanthellae (pronounced zoh-zan-THEL-ee). This is a symbiosis, a relationship that benefits both partners:
- The algae get a safe home in sunlit water and nutrients from the coral's waste.
- The coral gets sugars and other nutrients that the algae make through photosynthesis. According to NOAA's National Ocean Service, the algae can supply up to 90 percent of the energy a coral needs.
The algae also give corals much of their brown, green and golden color. That explains why corals turn white when the algae leave: what remains is the nearly transparent coral tissue over a white skeleton.
What causes bleaching
Reef-building corals live close to the upper limit of the temperatures they can tolerate. When water stays about 1 °C above the usual summer maximum for several weeks, the algae's photosynthesis starts to malfunction and produce harmful compounds. The coral responds by expelling the algae.
Heat is the main driver of mass bleaching, but other stresses can cause it too, including:
- Unusually cold water
- Very strong sunlight, especially in calm, clear conditions
- Fresh water from heavy rain or river runoff
- Pollution and sediment
Short spikes in temperature are usually less harmful than long ones. What matters is how much heat builds up over time. NOAA's Coral Reef Watch program uses satellites to track this with a measure called Degree Heating Weeks, which combines how much warmer than normal the water is with how long the warmth lasts. Its alerts give reef managers advance warning of likely bleaching.
Bleaching is not the same as death
A bleached coral is still alive. But without its algae it has lost most of its food supply, so it is starving. What happens next depends on how severe and how long the heat stress is.
| Situation | Likely outcome |
|---|---|
| Mild, short heat stress | Corals may pale but often regain their algae within weeks to months |
| Severe or prolonged heat stress | Many corals die from starvation or disease; dead skeletons are soon covered in algae |
| Extreme heat | Some corals can die quickly, even before fully bleaching |
Even corals that survive bleaching are often weakened. They may grow more slowly, reproduce less and be more vulnerable to disease for some time afterward.
How reefs recover
Reefs can recover after bleaching, but recovery takes time. Surviving corals can regrow, and new coral larvae drifting in from healthier reefs can settle and start new colonies. Fast-growing corals can bounce back in several years under good conditions. Slow-growing, massive corals that may be centuries old take much longer to replace.
The challenge is that bleaching events are now coming closer together. If a reef is hit again before it has recovered, its coral cover can ratchet downward. Scientists also see shifts in which species dominate, with heat-tolerant or fast-growing types sometimes replacing more sensitive ones.
Recovery is helped by good local conditions: clean water, healthy populations of plant-eating fish that keep seaweed in check, and limited damage from fishing, anchors or coastal development.
Global bleaching events
Bleaching was rarely reported on a large scale before the 1980s. Since then, scientists have documented several global bleaching events, in which significant bleaching occurs across all three major ocean basins: the Atlantic, the Pacific and the Indian Ocean. Global events have included 1998, 2010 and 2014 to 2017, and NOAA confirmed another global event beginning in 2023.
These events have often coincided with strong El Niño conditions, which temporarily raise sea surface temperatures in many tropical regions. But El Niño acts on top of long-term ocean warming. Background warming means each El Niño pushes temperatures higher than similar events did decades ago, and bleaching can now also occur in non-El Niño years.
Australia's Great Barrier Reef, the world's largest reef system, has experienced multiple mass bleaching events since 1998, with several in the past decade. Reefs in the Caribbean, the Indian Ocean and Southeast Asia have also been heavily affected.
The outlook
Scientific assessments give a sobering but clear picture. The IPCC's Special Report on Global Warming of 1.5 °C projected that warm-water coral reefs would decline by a further 70 to 90 percent at 1.5 °C of warming, and that more than 99 percent would be lost at 2 °C. Those numbers are why corals are often cited as one of the ecosystems most at risk from climate change, and why the difference between 1.5 °C and 2 °C matters so much.
That does not mean reefs will vanish entirely or that efforts to help them are pointless. Some corals are more heat-tolerant than others, and some reef areas experience less heat stress because of local currents or upwelling. Researchers are studying which corals survive and why, and restoration projects are testing ways to grow and replant heat-tolerant corals. These efforts can help, but scientists generally agree that they cannot replace cutting greenhouse gas emissions.
Corals also face a slower threat from changing ocean chemistry, described in Ocean acidification explained.
The bottom line
Coral bleaching happens when heat stress breaks the partnership between corals and the algae that feed them. Bleached corals can recover if conditions improve quickly, but severe or repeated bleaching kills corals and reshapes reefs. The long-term future of reefs depends mainly on how much further the ocean warms, combined with local protection that gives reefs the best chance to bounce back.