Coral Bleaching

A relative of jellyfish and sea anemones, corals are invertebrates that belong to a large group of animals called Cnidaria. They are known as colonial organisms, which means many individuals live and grow while connected to one another.

Each individual coral is made of a polyp – a clear, tube-shaped structure attached to a reef at one end and open at the other. The open end has a mouth that is surrounded by a ring of tentacles.

Hard corals extract calcium from seawater to make limestone outer skeletons, which form coral reefs. Soft corals tend to be feathery in appearance such as sea fans and sea feathers.

Corals get most of their bright colours from microscopic algae that grows inside the polyps’ tissues. This algae, called zooxanthellae (pronounced zo-UH-zan-thuh-lay), helps corals by removing waste and using it to produce food in a process known as photosynthesis.

When corals are stressed, they expel the zooxanthellae that live inside their tissues. Without the algae to provide colour, corals appear transparent and reveal their white skeletons. This is called coral bleaching. Bleached corals are not dead, but are more at risk of starvation and disease.

Bleaching occurs when corals are under stress. A primary cause of coral bleaching on the Great Barrier Reef during summer is heat stress from raised water temperatures and increased UV radiation. A temperature increase of just one degree Celsius for only four weeks can trigger bleaching.

Deprived of zooxanthellae and therefore their food source, corals begin to starve. If raised water temperatures persist for long periods (eight weeks or more) corals begin to die.

Yes, corals can recover from bleaching over time, but only if temperatures drop and conditions return to normal. When this happens, the algae returns and the corals gradually regain their health.

Coral bleaching is a stress response and individual coral colonies suffer from a degree of bleaching in any given summer. This is a natural process and not of particular concern.

Large-scale marine heatwaves create mass coral bleaching events however, in which large numbers of corals bleach severely over a wide area. These events are typically associated with high levels of coral mortality.

As climate change causes ocean temperatures to rise, marine heatwaves and associated coral bleaching events are becoming more frequent and severe, and the Reef’s natural recovery processes are unable to keep up.

Yes. Severe, widespread coral bleaching is an issue on coral reefs around the world as a result of changes to the Earth’s climate.

No. The Great Barrier Reef is made up of 3,000 reefs and is home to thousands of species of marine life. Reports focusing on “how much of the Reef has died” imply finality. Reefs can be severely affected by bleaching but begin to recover as coral communities re-grow and new coral larvae settle on the reef.

The Great Barrier Reef is one of the richest and most complex natural ecosystems in the world. Climate change, which causes temperatures to rise on our land and in our oceans, is the biggest threat to the future of the Great Barrier Reef and coral reefs around the world.

We need urgent action on climate change to drastically reduce global emissions, but this alone is not enough. Action on climate change requires a dual effort to protect our reefs. While the world works towards reducing greenhouse gas emissions, we must make coral reefs more resilient and help them adapt to the warmer temperatures already caused by climate change.

There’s never been a more dangerous time for our reef systems, but the good news is we’ve never been in a better position to respond to the threat.

The time for action is now. Together, we can make a difference and protect our planet's most precious ecosystems for future generations.