What is coral bleaching?

Coral bleaching happens when corals get stressed and expel the colourful marine algae living in their tissues (zooxanthellae). It’s a response to changes in conditions, for example in water temperature.

Why has the reef been bleaching again?

Heat stress is a major cause of coral bleaching and climate change is making marine heatwaves hotter, longer and more frequent. In the last year, daily global sea surface temperatures were at record highs for the whole year. Corals respond well to water at a consistent 23°-29° Celsius. Most can cope with higher temperatures for one to two days but if a there is a marine heatwave for a week or more, they are likely to bleach.

Can corals recover from bleaching?

Under moderate levels of heat stress, corals can recover if the temperature returns to normal quickly and stabilises. Some corals cope better than others. But it can take decades for reefs to fully regrow. Under sustained marine heatwave conditions, many corals will die.

Is bleaching part of a natural cycle?

Yes. Coral bleaching is a natural phenomenon. But what’s different now is the increasing frequency of bleaching events. Under stable conditions, corals may take 10 to 15 years to replace. With five bleaching events in eight years there is much less time for corals to recover between events and, as a result, it’s pushing the reef to its limits.

Is the reef dying?

There is no doubt that the world-heritage-listed Great Barrier Reef is in serious decline. We can all still have fantastic trips to the reef and there’s a lot to wow tourists with, but scientists and divers who’ve been visiting the same locations over many years are seeing long-term changes. The reef is so vast that there will always be pockets that survive in some form. What scientists are now starting to see is that the composition of the reef is changing. Fast-growing corals are recovering from disturbances but slow-growing corals, which may take hundreds of years to replace, are not getting the opportunity to regrow. The reef recently avoided an “in danger” listing by the United Nations World Heritage Committee but the committee stressed it was considered under “serious threat” from global heating and water pollution and asked Australia to demonstrate what it’s doing to address the health of the reef.

What about “record” coral cover?

Coral cover is a term used to describe the proportion of reef occupied by coral, and in 2022 scientists recorded the highest levels of coral cover since monitoring began 36 years ago. However, in recent years in the Wet Tropics, coral cover has generally been in poor condition, although a lower number of disturbances such as crown-of-thorns starfish and tropical cyclones has allowed coral cover to improve. But coral cover is just one measure used to assess the health of this vast ecosystem. For a complete picture of reef health, other variables are assessed like coral diversity, coral recruitment (how much new coral there is) and the incidence of disease or macroalgae.

What can we do?

The short answer is we need to stop burning fossil fuels and cut our emissions to get to net zero as soon as possible. Locally, significant investment is also being directed into reducing other stressors on the reef including improving the quality of water running off the land, reducing crown-of-thorns starfish outbreaks and reducing over-fishing, as well as work to regenerate coral in popular tourism locations. We can all help by reducing litter and minimising waste, reducing emissions and restoring nature.

Fast facts:

• Corals are colonies of tiny animals called polyps.
• They have a symbiotic relationship with microscopic algae called zooxanthellae that lives in their tissues.
• When coral is stressed, it expels zooxanthellae and loses its colour (bleaching).
• Ocean temperatures have warmed by 1° Celsius since 1900.
• The Great Barrier Reef was designated as a world heritage site by UNESCO in 1981.