With the potentially catastrophic, record-smashing El Ni帽o 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 Ni帽o 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 Ni帽os, such as the devastating reef mortality that occurred in 2024.
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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, 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 Ni帽a periods, which normally bring cooler weather to the reef.
鈥淲e鈥檝e had six events in the past decade,鈥� says Schoeman. 鈥淲e鈥檝e got a gradual increase in ocean temperatures, and the effect of El Ni帽o is stacking on top of that so that, even in a La Ni帽a year, you might get mass bleaching.鈥�
Even though there is recovery between these events, he says, the more frequently warming events occur, the more the coral suffers 鈥� and the more difficult it is for them to come back.
鈥淲e had a big event last year already, and if we have another one this year, and if it鈥檚 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,鈥� says Schoeman, a fundamental shift in the entire ecosystem.
鈥淵ou can think of that as a tipping point where getting there isn鈥檛 difficult, but getting out of it and flipping back is very difficult,鈥� he says.
Once the seaweeds dominate, he says, returning to a coral-dominated system isn鈥檛 just a matter of returning to normal temperatures.
at the Australian Institute of Marine Science (AIMS) says record global ocean temperatures are combining with El Ni帽o.
鈥淲e鈥檙e heading into a new era of heat stress,鈥� 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, 鈥渢his is obviously a record-breaking El Ni帽o, that probably won鈥檛 reach its peak until November鈥�.
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鈥檚 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 鈥� 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.
鈥淭he spatial footprint of severe heat and severe bleaching is expanding, and we expect that pattern to continue in the future,鈥� says Cantin.
Given the scale of the current El Ni帽o, 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 鈥� marine cloud brightening 鈥� won鈥檛 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.
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 鈥� 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 鈥� even unexpected ones 鈥� have been shown to have a significant impact.
Harrison cites suggesting that large reductions in sulphur dioxide emissions from shipping have increased the risk of coral bleaching 鈥� 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掳C of sea-surface temperature heating.
鈥淚 absolutely do believe marine cloud brightening could work and that the science so far has been very encouraging,鈥� says Harrison.
鈥淚t鈥檚 much harder to replace when it鈥檚 gone than to intervene now and protect the reef,鈥� 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.
鈥淚t鈥檚 not a replacement in any way, shape, or form for reducing emissions, but it鈥檚 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,鈥� says Harrison.
