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.

Coral bleaching occurs when corals become stressed by changes in their environment most commonly elevated sea temperatures and increased UV radiation. In response, they expel the microscopic algae (zooxanthellae) that live within their tissues and provide them with most of their energy through photosynthesis. Without these algae, the coral’s tissue becomes transparent, revealing the bright white calcium carbonate skeleton underneath. While bleached corals are still alive, they are significantly weakened and more vulnerable to starvation, disease, and mortality if stressful conditions persist.

Coral bleaching is caused by environmental stress, most commonly from elevated sea temperatures. On the Great Barrier Reef, unusually warm ocean conditions during summer are the leading cause of bleaching events. When water temperatures rise by as little as one degree Celsius above the seasonal average for a period of four weeks, corals become stressed and expel the microscopic algae called zooxanthellae that live inside their tissues. Other factors such as poor water quality, pollution, and increased ultraviolet radiation can also contribute to the severity of bleaching and reduce the coral’s ability to recover.

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 can’t be stopped entirely, but we can reduce its impact by cutting emissions, limiting local stressors like pollution, and supporting reef restoration efforts. Protecting coral reefs also means reducing coastal runoff, establishing marine protected areas, and investing in research that helps corals adapt to rising temperatures.

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.