Coral bleaching 101

With the help of our Chief Scientist, we’ve put together some information on coral bleaching here to separate fact from fiction when it comes to what’s happening on the Reef right now.

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Q&A with Dr Dave

Coral bleaching 101

Myth-busting coral bleaching

True or false?

The Great Barrier Reef is dead

False. The Great Barrier Reef is made up of some 3000 reefs, spread over 14 degrees of latitude. The area is bigger than the United Kingdom, Switzerland and Holland combined. Reports which focus on “how much of the Reef has died” imply finality. For coral reefs this is not generally the case because a reef is an ecosystem, not an individual living thing. Reefs can be severely affected but begin to recover as coral communities re-grow and new coral larvae settle on the reef if environmental conditions are stable.

Mass coral bleaching occurred in 2019-20

True.  As Australia’s lead management agency for the Great Barrier Reef we can confirm mass bleaching did occur on the Reef during the 2019-20 summer, with widespread severe bleaching detected across many regions. We draw on the most current information from in-water and aerial observations and access the best available science and technology to understand current conditions.

Bleached corals are dead corals

False. Bleached corals are not dead corals — on mildly or moderately bleached reefs, the Reef has a good chance to recover and survive.

This is the worst coral bleaching event in 2019-20

False.  Out of 1036 reefs surveyed by scientists in a plane, 40% had no or negligible bleaching, 35% had moderate bleaching and 25% had severe bleaching. Based on these numbers, the 2020 event is not as bad as the 2016 event, but is worse than the events in 2017, 2002 and 1998. A key factor in describing the severity of a bleaching event is how much coral dies, not just how much bleaches. Because of on-water operational constraints due to COVID-19, it is likely that less data on coral mortality/recovery outcomes from the 2019-20 bleaching event was collected than would typically be achieved.

60 per cent of the Great Barrier Reef bleached in 2019-20

Partially true. Of the 1036 reefs surveyed by scientists in a plane, 60 per cent had either moderate or severe bleaching. However, this doesn’t mean all the corals on those reefs were bleached, nor does it equate to 60 per cent of the entire Reef. The aerial surveys accurately record bleaching to only a five metre depth, and bleaching severity generally declines with increasing depth.

Aerial surveys are a reliable way to determine bleaching levels

True. Aerial surveys are reliable. Bleached corals turn bright white or fluorescent colours, and this is very visible from the sky. Aerial survey flights are at low altitude, slow speed and targeted around low tide. This maximises the ability of the observers to quantify the bleaching. One important limitation of aerial surveys is that they can only reliably survey corals in shallow water (down to about five metres.) Given the size of the Great Barrier Reef – some 344,000km2 and bigger than Tasmania and Victoria combined – surveys from planes are the only way to cover a large area of the Reef. Aerial surveys are used in conjunction with surveys from Marine Park rangers and scientists, and reports through our Eye on the Reef program, to get a complete picture of what happens during a bleaching event.

Most reefs used for tourism aren’t severely affected

True. Most recognised tourism areas had no, negligible or moderate bleaching. High Standard Tourism operations contribute regularly into the Eye on the Reef system that provides the Great Barrier Reef Marine Park Authority with excellent real-time awareness of Reef health at high value tourism sites.

Coral reefs worldwide are being affected

True. Coral Bleaching is not just an Australian or Great Barrier Reef issue, it is a global problem affecting coral reefs world-wide as a result of changes to the Earth’s climate.

Bleaching is a natural process, the Reef recovers and it is all natural behaviour

True and false. Coral bleaching is a stress response and individual coral colonies will 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 in which very large numbers of corals bleach severely, on many different reefs over a wide area. These events are typically associated with high levels of coral mortality. As the Earth and its oceans warm with climate change, marine heatwaves and associated coral bleaching events are becoming more severe and frequent, and the Reef’s natural recovery processes are unable to keep up.

Nothing can be done to stop bleaching, and the Reef doesn’t need any help, it’s adapted successfully for thousands of years without interference

False. This mass bleaching reaffirms that climate change remains the single greatest challenge to the Reef and the strongest possible global efforts to reduce emissions are essential, along with local actions in the Marine Park and its catchment. Everyone everywhere can do their bit to protect the Great Barrier Reef for future generations to enjoy.