What Is Coral Bleaching? Causes and Recovery Explained

What happens when coral bleaches?

Most reef-building corals live in partnership with single-celled algae called zooxanthellae. The algae live inside the coral’s cells, use sunlight to make sugars and pass most of them to their host, providing up to about 90 percent of its energy. They also give the coral much of its brown, green or golden color. Corals are animals, not plants, as explained in is coral a plant or an animal?

When the water gets too hot, the algae’s photosynthesis goes wrong and starts producing harmful, highly reactive forms of oxygen. To protect itself, the coral expels the algae or digests them, and the algae that remain lose their pigment. Without them, the coral’s thin tissue is almost transparent, and the white limestone skeleton shows through. That is the bleaching people see.

Some corals glow in bright blue, pink or yellow just before turning white. These are fluorescent pigments made by the coral itself, which act like a sunscreen and become visible or increase as the algae disappear.

What causes coral bleaching?

Heat is by far the biggest cause. Reef corals live close to the upper limit of the temperatures they can tolerate, so a rise of only about 1 °C (1.8 °F) above the usual summer maximum, lasting for several weeks, can trigger bleaching.

NOAA’s Coral Reef Watch program tracks this with a measure called degree heating weeks, which adds up how much and for how long water has been hotter than normal over the past 12 weeks. At 4 degree heating weeks, significant bleaching is likely. At 8, severe bleaching and widespread death are likely. In 2023, NOAA added higher alert levels, up to 20 degree heating weeks, because heatwaves had begun to go far beyond the old scale.

Other stresses can also cause bleaching, usually on a smaller scale:

• Unusually cold water, as during a cold snap in Florida in January 2010
• Strong sunlight, especially when corals are exposed at very low tides
• Floods of fresh water after heavy rain, which lower salinity
• Pollution, sediment and some diseases

Is bleached coral dead?

No, not at first. A bleached coral is alive, but it has lost its main food supply and is living on its reserves. Its polyps can still catch plankton, which helps a little. If water temperatures return to normal within a few weeks, the coral can take in new algae from the water or from the few that remained, regain its color within weeks to months, and survive.

If the heat lasts longer, the coral starves or is killed directly by heat, and its bare skeleton is soon covered by a fuzzy film of algae. Even corals that survive are weakened: they grow more slowly, may fail to reproduce that year and are more likely to catch diseases.

Global coral bleaching events

When bleaching hits reefs in all three major ocean basins, the Atlantic, Pacific and Indian Oceans, scientists call it a global bleaching event.

The fourth event was the largest on record. In Florida, a record marine heatwave in the summer of 2023 caused severe bleaching along the entire reef and killed nearly all the remaining elkhorn and staghorn coral, as described in what is happening to Florida’s coral reefs?

How long does it take reefs to recover?

Reefs can recover from bleaching, but it takes time. New corals must grow from surviving fragments or from larvae that drift in and settle, and full recovery of the fastest-growing corals typically takes at least 10 to 15 years. Slow-growing boulder and brain corals take decades.

The problem is that severe bleaching now returns more often. A 2018 study of 100 reefs worldwide found that the typical gap between severe bleaching events had shrunk to about six years, too short for full recovery. Some reefs do bounce back: after the 1998 event in the Seychelles, where more than 90 percent of corals died, about half of the reefs studied recovered. Many of the others were taken over by seaweed instead.

Which corals are most at risk?

Fast-growing branching corals such as Acropora and Pocillopora tend to bleach first and die fastest. Massive, slow-growing corals such as Porites often survive heat better. Corals in places that are naturally hot, such as parts of the Red Sea and the Persian Gulf, can tolerate higher temperatures than corals elsewhere, which gives scientists hope that some heat tolerance can spread or be encouraged.

What reduces the risk of coral bleaching?

The only long-term solution to mass bleaching is to slow ocean warming by cutting greenhouse gas emissions. Local action helps reefs survive and recover in the meantime: reducing sewage, fertilizer runoff and sediment; protecting parrotfish, sea urchins and other grazers that keep seaweed in check; creating marine protected areas; and restoring reefs with nursery-grown corals, increasingly chosen for heat tolerance. Visitors can help by not touching corals, using mooring buoys instead of anchors and choosing operators that follow reef-friendly guidelines.