Scientist warns record El Niño could tip parts of the Great Barrier Reef to seaweed

Figures from Neal Cantin of the Australian Institute of Marine Science (AIMS) ^[13] show how far each mass bleaching has reached. About 8 per cent of reefs bleached to the point of mortality in 1998, and about 35 per cent in 2016 ^[16][17]. That is roughly 4.4 times as many ^[22]. The 2016 deaths were largely confined to the far north ^[17]. In 2024 severe bleaching and mortality ran the length of the reef ^[18]. Sixty per cent of corals bleached, across what may have been 40 per cent of the reef, and many areas lost 70 to 90 per cent of their corals ^[18]. Those 2024 numbers are shares of corals within part of the reef, a different base from the earlier shares of reefs.

The geography has moved too. "The spatial footprint of severe heat and severe bleaching is expanding, and we expect that pattern to continue in the future," Cantin said ^[20]. He links it to record global ocean temperatures combining with El Niño. "We're heading into a new era of heat stress," he said ^[13]. He also said "this is obviously a record-breaking El Niño, that probably won't reach its peak until November" ^[15].

David Schoeman of the University of the Sunshine Coast makes his case on frequency. His count of six events in the past decade ^[7] works out to one about every 20 months ^[23]. "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," he said ^[8]. A bleached coral has expelled the algae that feed it, and it risks starving unless the water cools quickly ^[5]. Reefs do recover between events, Schoeman says, but the more often heat returns, the harder recovery becomes ^[9].

The seaweed scenario is a conditional warning. "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," Schoeman said ^[10]. "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 said ^[11]. Once seaweed dominates, he says, a return to normal temperatures alone will not restore the coral ^[12].

As stated, the warning covers "some areas" after a bigger event, with a return to coral described as very difficult ^[10][11]. His claim is narrower than a permanent loss across large parts of the reef, and it comes from one researcher. New Scientist's report does not include measurements of seaweed cover on any reef, or a threshold at which one tips.