“Recent history is repeating itself.”

It just seems like it can’t stop in the Great Barrier Reef. Once again, aerial photos reveal that large swaths of the world’s largest coral reef are fading. It is the fourth time in just six years that the reef has been hit by extreme bleaching.

Affected reefs

The Great Barrier Reef extends over an area of ​​344,000 square kilometers. That is larger than the United Kingdom, Switzerland and the Netherlands combined. During the aerial survey, scientists flew over 750 reefs and studied the corals that reside in shallow water—that is, to a depth of five meters. The analysis shows that many reefs have bleached severely. The bleaching observed from the air largely matched the spatial distribution of heat stress experienced in the reef this summer.

Fourth time

It’s disturbing news. Because, as mentioned, it is already the fourth time in just six years that the reef has had to deal with enormous bleaching. And that despite the cooling effect of La Niña, which was briefly heard last year. The last time the Great Barrier Reef was weighed down by a so-called mass bleaching event — a period of severe bleaching of many reefs over a large area at the same time — was in 2020, when scientists noticed severely bleached reefs in both the northern, central and southern parts of the Great Barrier Reef. “Recent history is repeating itself on Australia’s coral reefs,” says researcher Zoe Richards, of the Curtin University† “The fact that coral is bleaching again, even during a La Niña, is terrible news for marine life.”

Distribution

The degree of bleaching varies greatly between different reefs. The northern and central regions in particular are often hit hard. And this year, too, some mortality has been observed. Southern reefs are usually only slightly bleached. That also appears to be the case now. Some reefs are not even affected at all. This may have to do with the type of coral that grows there, the depth and currents, and the extent to which they have experienced heat stress. But despite that, this event should not be underestimated. “Whether severe or mild; it represents another setback in reef recovery,” emphasizes Richards. Either way, it will affect the health, productivity and diversity of the Great Barrier Reef.”

Bleached coral is alive
It is important to note that bleached coral does not necessarily mean dead coral. For light or moderate bleached reefs, there is a good chance that they will recover and survive. For example, this was the case in 2020, when very little coral mortality was recorded.

Weather patterns in the coming weeks will remain critical in determining the overall extent and severity of bleaching. In any case, sea surface temperatures have dropped over the past two weeks and are now less than 0.5 degrees Celsius above average in most of the park. Temperatures in some coastal areas around the northern city of Cairns and in southern areas around the city of Mackay are even below average for this time of year. Experts expect the heat stress to disappear in most of the park by the end of April.

Frequency

Follow-up research will have to show whether there is permanent damage and to what extent corals can recover from the event. And then it is hoped that it will take a while before the next period of extreme bleaching takes effect. Although the prospects are not very promising. “The announcement confirms that the frequency and intensity of mass bleaching episodes is increasing dramatically,” said Kathy Townsend, an expert in animal ecology and associated with the University of the Sunshine Coast† “Instead of once every ten years, we now see a mass bleaching event† This increase in frequency reduces coral’s ability to bounce back, making die-off much more likely.”

The fact that these periods now follow each other closer together has everything to do with the warming earth. “There is no doubt about it – these events are driven by climate change,” said Jodie Rummer, associate professor of marine biology at the University of Groningen. James Cook University† “Each grade is critical to the future of biodiversity. Climate change is accelerating, and our response to reduce greenhouse gas emissions in this critical decade must match this scale and urgency. Otherwise, the reef will experience more severe marine heat waves more frequently, causing massive periods of bleaching to occur annually from 2044 onwards. To give the Great Barrier Reef the best chance of survival, we need to replace coal, oil and gas with clean energy as soon as possible and aim for net zero emissions by 2035.”