Record El Niño threatens Great Barrier Reef with ecological shift

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. Advertisement 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. “We’ve had six events in the past decade,” says Schoeman. “We’ve got a gradual increase in ocean temperatures, and the effect of El Niño is stacking on top of that so that, even in a La Niña year, you might get mass bleaching.” Emergency briefing on the nature and climate crisis Hear from three of the world’s leading climate scientists at New Scientist Live 2026 Even though there is recovery between these events, he says, the more frequently warming events occur, the more the coral suffers – and the more difficult it is for them to come back. “We had a big event last year already, and if we have another one this year, and if it’s bigger, then one of the concerns is that some areas of the reef could flip to an algal-dominated system rather than the coral-dominated system,” says Schoeman, a fundamental shift in the entire ecosystem. “You can think of that as a tipping point where getting there isn’t difficult, but getting out of it and flipping back is very difficult,” he says. 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. “We’re heading into a new era of heat stress,” says Cantin. 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”. 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. “The spatial footprint of severe heat and severe bleaching is expanding, and we expect that pattern to continue in the future,” says Cantin. Given the scale of the current El Niño, organisations like AIMS are coordinating with other authorities and readying for extensive surveys to quantify the bleaching. Unfortunately, one of the most promising mitigation tools to buy time for the reef while decarbonisation efforts continue – marine cloud brightening – won’t be ready to deploy this summer. Marine cloud brightening sprays seawater into the sky with water cannons, leading to brighter clouds that reflect sunlight away from marine ecosystems. Daniel Harrison at Southern Cross University in Australia is leading research and community consultation on marine cloud brightening in the Great Barrier Reef region. But trials are unlikely until 2028. Harrison says he and his colleagues have already shown that clouds can be artificially brightened – in earlier experiments, they have been able to increase the number of droplets in clouds by up to several hundred percent. They have also shown that human aerosol interventions – even unexpected ones – have been shown to have a significant impact. Harrison cites research suggesting that large reductions in sulphur dioxide emissions from shipping have increased the risk of coral bleaching – even while also reducing air pollution. The researchers found that an 80 per cent reduction in sulphate aerosols from shipping in the region of the reef leads to an extra 0.15°C of sea-surface temperature heating. “I absolutely do believe marine cloud brightening could work and that the science so far has been very encouraging,” says Harrison. “It’s much harder to replace when it’s gone than to intervene now and protect the reef,” says Harrison. Modelling, he says, shows that marine cloud brightening has the potential to be especially effective when combined with introducing heat-tolerant corals and controlling the numbers of crown-of-thorns starfish, which prey on hard corals. “It’s not a replacement in any way, shape, or form for reducing emissions, but it’s a way to buy time and maybe sort of put the reef on life support for some decades while we hopefully catch up with renewables and emissions-reduction efforts,” says Harrison.