With the potentially catastrophic, record-smashing El Niño still peaking in the Pacific Ocean, scientists are warning of a fundamental ecological shift that would see swathes of the Great Barrier Reef become dominated by seaweed instead of coral. During El Niño events, warmer ocean waters pool in the eastern and central Pacific. However, the phenomenon also dramatically disrupts the usual monsoon troughs, low-pressure zones that bring cooling clouds and rain to the Great Barrier Reef region.
This is why the worst Great Barrier Reef coral-bleaching events have coincided with past El Niños, such as the devastating reef mortality that occurred in 2024. Galapagos corals suggest climate change drives stronger El Niños Coral bleaching occurs when reefs turn a brilliant white after water temperatures become higher than the animals that comprise the colonies can tolerate. Heat stress leads the corals to eject the colourful algae that live within their tissues and provide them with food.
This puts them at risk of starvation and death, unless waters cool quickly. Now, David Schoeman at the University of the Sunshine Coast says that ocean temperatures have increased so much that coral bleaching is occurring even during neutral and La Niña periods, which normally bring cooler weather to the reef. Fishers detonate a bomb every hour on one Indonesian coral reef Once the seaweeds dominate, he says, returning to a coral-dominated system isn’t just a matter of returning to normal temperatures.
Neal Cantin at the Australian Institute of Marine Science (AIMS) says record global ocean temperatures are combining with El Niño. While it is impossible to predict what kind of temperatures the reef will be exposed to at the height of the upcoming Australian summer, or whether the worst of the conditions may be ameliorated by a strong monsoon season, he says, “this is obviously a record-breaking El Niño, that probably won’t reach its peak until November”. Coral reefs on a remote archipelago shrugged off a massive heatwave In 1998, says Cantin, about 8 per cent of the reefs across the Great Barrier Reef bleached to the point of mortality.
By 2016, that rate had risen to about 35 per cent, but mortality was largely restricted to the reef’s far north. What changed in 2024, he says, is that there was severe bleaching and mortality across the entire reef. Bleaching affected 60 per cent of corals across potentially 40 per cent of the reef.
In many areas, between 70 and 90 per cent of corals died. This included areas like the Swains Reefs, which are 150 kilometres offshore and surrounded by deep, normally cool water – a supposed buffer that gave bleaching researchers like Cantin a false sense of security. In 2024, the process that brought cool water to the reefs broke down, and there was severe bleaching.
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