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How does coral bleaching happen?

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Quick answer

Coral bleaching occurs when unusually warm water stresses corals, causing them to expel the microscopic algae that live inside their tissues. These algae provide most of the coral's food and colour. Without them, the coral turns white. It can recover if the water cools soon, but prolonged stress leads to starvation and death.

Coral bleaching happens when heat stress makes corals expel the algae living in their tissues, which supply most of their colour and food. Without these algae, the coral turns white and, if the stress continues, can starve and die.

Step by step

  1. Corals build reefs and live in a partnership with algae called zooxanthellae inside their cells.
  2. The algae use sunlight to make food, giving the coral most of its energy and its colour.
  3. When the water gets too warm, the algae make harmful molecules that damage the coral.
  4. In response, the coral pushes the algae out of its tissues, leaving its white skeleton visible.
  5. If the heat continues, the coral has no main food source and slowly starves.
  6. If the water cools in time, the coral can take in new algae and recover its colour.

Key things to understand

  1. Corals are animals that live in partnership with tiny algae called zooxanthellae, which live inside their cells.
  2. The algae give the coral up to 90 percent of its energy through photosynthesis and also provide its colour.
  3. When water gets too warm, the algae produce harmful molecules that damage the coral, so the coral expels them.
  4. Without the algae, the coral's white skeleton shows through its clear tissue, making it look bleached.
  5. A bleached coral is not dead yet, but if the heat persists it starves and often dies.

Labelled diagram

  1. Healthy coral

    Coral animal with algae living inside its cells, giving it colour and food.

  2. Heat stress

    Water warms 1–2°C above normal summer high, stressing the coral.

  3. Algae expelled

    Coral pushes out the algae to protect itself from harmful molecules.

  4. Bleached coral

    Without algae, the white skeleton shows through clear tissue.

  5. Recovery or death

    If water cools, coral takes in new algae; if not, it starves and dies.

How coral bleaching happens

Real-world examples

  • The Great Barrier Reef in Australia has suffered several mass bleaching events, including back-to-back events in 2016 and 2017.
  • In 1998, a strong El Niño warmed oceans worldwide and caused the first recorded global bleaching event, hitting reefs in the Indian Ocean, Pacific, and Caribbean.
  • The Florida Keys reef tract experienced severe bleaching in 2023 when water temperatures soared above 32°C (90°F).
  • Reefs in the Maldives bleached heavily during the 2016 El Niño, turning once-colourful dive sites white.

Common mistake

✗ Myth: Bleached coral is dead coral.

✓ Fact: Bleached coral is still alive but under severe stress. It can recover if the water cools and it regains algae, though repeated bleaching weakens and eventually kills it.

People also ask

What causes coral bleaching?
The main cause is unusually warm ocean water, often during marine heatwaves linked to climate change. When water stays too warm for too long, corals expel the algae living in their tissues. Other triggers include pollution, too much fresh water, and very cold water.
Why does coral turn white when it bleaches?
Coral tissue is mostly clear, and the algae inside give it colour. When the coral expels the algae, the white calcium carbonate skeleton underneath becomes visible through the clear tissue. That is why bleached coral looks bright white.
How long does it take for coral to bleach?
Bleaching can start within a few days to a few weeks of sustained heat stress. The exact time depends on how hot the water gets and for how long. A short heat spike may cause mild bleaching, while weeks of warmth can bleach an entire reef.
Can bleached coral recover?
Yes, if the heat stress ends quickly and the water cools, the coral can take in new algae from the surrounding water and regain its colour. Recovery takes weeks to months. If the stress lasts too long, the coral dies.
Does coral bleaching affect humans?
Yes. Reefs protect coastlines from waves, support fish that millions of people eat, and draw tourists. When reefs bleach and die, fish populations drop, shorelines erode faster, and local economies that depend on reef tourism suffer.
What temperature causes coral bleaching?
Bleaching usually starts when water is about 1 to 2 degrees Celsius above the hottest normal summer temperature for that reef. Even a few weeks of this warmth can trigger it. Cold snaps, pollution, and too much fresh water can also cause bleaching.
Is coral bleaching caused by climate change?
Climate change is the main cause because it raises ocean temperatures, and mass bleaching events line up with marine heatwaves. Local factors like pollution and overfishing make reefs weaker, but the global trigger is warming water.

Part of the guide: Nature & Earth: Living Things and Rocks →