James Kerry, Ph.D.Senior Marine & Climate Scientist

Having conducted the aerial surveys of the Great Barrier Reef during mass coral bleaching events, I know how gruelling it is to document such incidents.


Mass Coral Bleaching on the Great Barrier Reef, Again

March 12, 2024

The Great Barrier Reef has now bleached five times in the past eight Australian summers: 2016, 2017, 2020, 2022 and now 2024.

The Great Barrier Reef Marine Park Authority confirmed the most recent mass coral bleaching event on 8th March 2024. Such a call is only made when hundreds of kilometres of the Great Barrier Reef are undergoing some level of coral bleaching, which will have been confirmed via aerial surveys.

What is coral bleaching and what causes it?

Corals will bleach when the microscopic algae that live within them are overstimulated by abnormally warm waters. This causes the algae, which usually provide corals with most of their energy, to produce toxins that are harmful to the coral. As a result, the corals expel the colourful algae, and all that remains is the white coral skeleton now visible through a translucent coral skin, hence the term, “coral bleaching”.

Importantly, a bleached coral is not necessarily a dead coral, if the temperatures return to normal within a short enough time period, the algae can return and the coral may survive.

The climate crisis is the primary cause for this latest bout of mass coral bleaching, although it is not helped by the current El Niño phase that pushes temperatures even higher. The footprint of the coral bleaching closely maps the heat stress on the water, which satellites can measure from space. The longer and higher the heat stress, the more severe the coral bleaching, and the more likely the corals will die.

For coral reef ecosystems, mass coral bleaching is devastating. Not only are tens of thousands of individual corals lost, but as a foundation species on the Reef, corals provide food and shelter for all manner of fishes and other marine creatures, leading to knock-on effects that can unfold years and even decades later. From a human-centred view, healthy coral reefs are also important fisheries and tourism-based economies, while also having enormous spiritual and cultural significance for many indigenous communities throughout the tropics.

Assessing the impact

Having conducted the aerial surveys of the Great Barrier Reef during the mass coral bleaching events in 2016, 2017 and 2020, I know well what it means to document such events. The work itself is gruelling. Because the Reef is larger than two-thirds of the countries on earth, it takes over a week of flying to cover a representative sample of the more than 3,000 reefs that stretch from the Torres Straits below Papua New Guinea, right down to its southern extent some 2,500 km away. The aerial surveys flights themselves go for hours, zigzagging across the Great Barrier Reef to pick up as many individual reefs as possible, at each reef we would carefully record the numbers of corals that were bleached, as a percentage of the total.

What is most striking during a mass coral bleaching event, is just how much coral is impacted. Flying for hours, even days and not seeing a single coral reef without bleaching starts to weigh heavily on you, as an observer. It is important to document what is happening, both from a scientific perspective, but also alert the wider world to the seriousness of situation. But as you fly over reef after reef, you also become mindful that this is just a data gathering exercise, and there is nothing in that moment, that you can do to change the outcome.

Since the 2016 event, the coral bleaching has become so severe on some reefs, that we had to add additional bleaching categories to the original scoring method that was developed to assess mass coral bleaching, when it was first recorded on the Great Barrier Reef in 1998 and 2002. The need to evolve our methods when assessing climate impacts is a reflection of just how serious the situation is becoming. The National Oceanic and Atmospheric Agency’s (NOAA) Coral Reef Watch program also recently added two new alert levels to describe heat stress. These categories no longer address the risk of coral bleaching but rather coral mortality, with the most severe, “Alert Level 5” representing “near complete mortality”, the worst-case scenario.

For the 3,000 reefs on the Great Barrier Reef, this year’s event is still unfolding, but the heat stress and strong bleaching response appears to be concentrated in the northern and southern portions of the Reef. Because each bleaching event has a different footprint, each reef will also have its own bleaching history. Some reefs, will have had less than two years to recover since the 2022, others longer. What is clear though, is that no reef is been spared during the five events that have hit the Reef in the last eight years. Indeed, these events are becoming more and more common. From 2002 to 2016, the Great Barrier Reef had 14-year reprieve, it is unlikely to be so fortunate ever again.

A global mass coral bleaching event is upon us

The Great Barrier Reef, despite its size, is unfortunately only a part of the picture over the past year, which has seen mass coral bleaching occurring around the world. Coral reefs across the Gulf of Mexico, northern Atlantic Ocean, Caribbean Sea and Indian Ocean have also bleached. When this occurs in the three ocean basins – the Atlantic, Pacific and Indian, the event is deemed to be global in scale.

While tragic, the designation of another global mass coral bleaching event is sadly unsurprising. The world just registered its first 12-month period with an average temperature over 1.5 degrees Celsius above preindustrial levels. The IPCC previously assessed that, over a longer period of time, the rise of 1.5C is likely to be the tipping point for mass mortality of coral reefs. Scientists estimate that 90% of the world’s corals could be lost even at the target countries committed to reach under the Paris climate agreement. Sadly, to date, even these commitments are largely not being met.

While another news cycle about mass coral bleaching comes and goes, we would do well to reflect on the fact that the only way to ensure coral reefs are a part of our long-term future is to rapidly, and permanently, eliminate global greenhouse gas emissions.