Devastating coral bleaching in 2024

The fifth mass bleaching event on the Great Barrier Reef in the past eight years was declared in April 2024. For the first time, all three sectors of the GBR are affected: south, middle, and north. It is part of the fourth global bleaching event according to the US National Oceanographic and Atmospheric Administration – the others were in 1998, 2010 and 2014-17.

At Lizard Island, bleaching became noticeable in early February as water temperature climbed and remained high for weeks. By late February, in shallow water, most Acropora corals – the ones that are very common and that provide much of the reef’s habitat structure – were fluorescing or fully bleached. By mid-March, in water shallower than 5 metres, virtually all Acropora corals were either bleached or dead. By mid-April, the death rate of those corals was estimated at more than 80%. Even though the water temperature is now dropping, we won’t know the outcome for the corals that are still alive for some months. They have been badly damaged and it remains to be seen how many will recover.

The reefs around Lizard were decimated between 2014 and 2017 – quite literally, less than one tenth of corals remained. This was caused by category 4 cyclones in each of 2014 and 2015, then catastrophic bleaching due to a marine heatwave in 2016. The water temperature in 2017 was just as high as in 2016, but by then, there were no susceptible corals left to be killed.

In the seven years since then, the reef in some areas around Lizard (but by no means everywhere) had recovered amazingly well. The recovery and its caveats are discussed in the 2023 Talbot Oration, Coral Reefs in Hot Water. It’s now clear that February 2024 was ‘peak recovery’ for this cycle.

Lizard Island Research Station has been bearing witness to events on the reef for more than 50 years. The timeline below shows all bleaching events in the area since 1973. It’s obvious that the frequency of these devastating events is increasing rapidly. In the past five years, from 2020 to 2024, there has been bleaching at Lizard Island every year. The concept of back-to-back bleaching events is still mentioned in the media as something that might happen in years to come, but it’s already happening. The only reason that none of the events between 2020 and 2023 resulted in mass death of corals is that the weather changed in the nick of time to drop the water temperature before too much damage had been sustained. Dumb luck, in other words. In 2024, we were not so lucky.

The current event is not as bad in this area as it was in 2016/17, but it comes a close second. This time, corals in deeper water have fared better, the outer barrier reefs are not badly affected, and some types of corals have not bleached as badly but others have bleached more. There is much to learn, and researchers at LIRS are onto it.

I will close with some words that resonate from a highly respected coral reef researcher, Peter Sale: