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The Guardian
7 days ago
- Science
- The Guardian
WA's ‘longest and most intense' marine heatwave killed coral across 1,500km stretch
The 'longest, largest and most intense' marine heatwave ever recorded in Western Australia has killed coral throughout an area that stretches 1,500km, according to state and federal scientists. More than 100 scientists and marine managers will gather in Perth on Tuesday for a special meeting to discuss the devastating event that bleached and killed corals on remote reefs earlier this year. The marine heatwave that hit reefs from the World Heritage-listed Ningaloo to the remote Ashmore Reef left many scientists shocked. Sign up: AU Breaking News email A group of scientists from state and federal government departments, agencies and universities, who monitor coral bleaching in Western Australia, will issue a detailed update on Tuesday. The heatwave, which started to build off the north-western WA coast in August 2024, caused heat stress in corals across the state's northern reefs that lasted until May this year in 'the most severe coral bleaching on record for Western Australian coral reefs,' the update said. Coral bleaching describes a process whereby the coral animal expels the algae that live in its tissues and give it its colour and much of its nutrients. Without its algae, a coral's white skeleton can be seen through its translucent flesh, giving off a bleached appearance. Mass coral bleaching over large areas, first noticed in the 1980s around the Caribbean, is caused by rising ocean temperatures. Some corals also display fluorescent colours under stress when they release a pigment that filters light. Sunlight also plays a role in triggering bleaching. Corals can survive bleaching if temperatures are not too extreme or prolonged. But extreme marine heatwaves can kill corals outright. Coral bleaching can also have sub-lethal effects, including increased susceptibility to disease and reduced rates of growth and reproduction. Scientists say the gaps between bleaching events are becoming too short to allow reefs to recover. Coral reefs are considered one of the planet's ecosystems most at risk from global heating. Reefs support fisheries that feed hundreds of millions of people, as well as supporting major tourism industries. The world's biggest coral reef system – Australia's Great Barrier Reef – has suffered seven mass bleaching events since 1998, of which five were in the past decade. Dr James Gilmour, a senior research scientist at the Australian Institute of Marine Science that coordinates the monitoring group, said on some coral reefs that were inspected, more than 90% of corals had either bleached or died. 'When you have more than about 75% of corals that have bleached or died, it can be very hard to find any healthy corals,' he said. The update from the WA Coral Bleaching Group said the amount of bleaching and coral death on reefs ranged from medium (11%- 30%) to extreme (greater than 90%) across systems up to 1,500km apart. The worst-hit area was the Rowley Shoals, Gilmour said – a group of three remote reefs that had previously escaped the effects of global heating and had been a 'hope spot' for many scientists. 'The first time reefs bleach badly, the mortality tends to be quite high,' he said. Throughout the heatwave, scientists were monitoring a metric over reefs known as Degree Heating Weeks – a measure of accumulated heat stress. Gilmour said as a general rule, 8DHWs is high enough to cause some corals to die. During the marine heatwave, most WA reefs from Ningaloo north reached at least 15 DHWs, with some reefs off the Pilbara coast hitting 30DHWs. 'At pretty much all of these reefs, it was the highest heat stress ever recorded,' he said. The marine heatwave in WA coincided with the most widespread global bleaching event on record that started in January 2023 and is ongoing. More than 80% of the planet's reefs have seen heat stress high enough to cause corals to bleach. Sign up to Breaking News Australia Get the most important news as it breaks after newsletter promotion Previous marine heatwaves in WA have been associated with La Niña patterns of ocean heat in the Pacific that see warmer waters pulled south by a current that sweeps down the coast. This extreme marine heatwave in WA happened in the absence of a La Niña. 'We still got this dramatic warming, so we are only left with climate change [as a cause] after that,' said Dr Alistair Hobday, a marine heatwave expert at the CSIRO. Dr Claire Spillman, a member of the bleaching group from the Bureau of Meteorology, said the summer of late 2024 and early 2025 recorded the warmest ocean temperatures for the Australian region on a record going back to 1900. 'The increased frequency of extreme and record-breaking ocean temperatures is associated with global warming,' she said. The WA coral bleaching played out at the same time as significant bleaching was also hitting the Great Barrier Reef on the east coast. Scientists there have recorded the biggest annual drops in coral cover 'Climate change is driving these events, which are becoming more frequent, more intense and more widespread, giving our amazing, valuable coral reefs little time to recover,' Gilmour said. 'And they need 10 to 15 years to recover fully. 'The key to helping coral reefs survive under climate change is to reduce greenhouse gas emissions.'
The Guardian
06-06-2025
- Science
- The Guardian
WA reefs experiencing 'worst coral bleaching event' ever to hit state
From the World Heritage-listed Ningaloo all the way to Ashmore Reef about 1,500 km north-east, Western Australia's reefs are turning white. Now, teams of government scientists are reporting widespread coral death as they warn of widespread bleaching caused by increasing water temperatures.
The Guardian
06-06-2025
- Science
- The Guardian
‘It was our hope spot': scientists heartbroken as pristine coral gardens hit by Western Australia's worst bleaching event
The Rowley Shoals are on many a diver's bucket list. The three coral atolls, hundreds of kilometres off the Western Australian coastline, are teeming with pristine coral gardens that for a long time, unlike many of the world's reefs, had escaped the ravages of global heating. 'I've seen a fair bit of death and destruction, but Rowley Shoals was always the place that was still standing,' says Dr James Gilmour, a research scientist at the Australian Institute of Marine Science. 'Just the sheer abundance of life is incredible. It was our hope spot. It's the reef I love more than any other. So this was super emotional.' Starting in August 2024, an unprecedented heatwave has swept across Western Australia's reefs, turning corals white from the World Heritage-listed Ningaloo all the way to Ashmore Reef, about 1,500km north-east. Now, teams of government scientists are reporting widespread coral death, which they say is the worst bleaching to hit the state. There are still areas of live coral, and some bleached coral will recover, but as scientists gather data, the scale of mortality has left many shocked. At Rowley Shoals, Gilmour, who has been researching corals for 30 years, says a visit in mid-April presented a devastating and confronting scene. 'It was several weeks after the peak heat stress. Some corals were still bleached white, but most had died. We saw that over vast areas,' he says. 'The structure is still there but they're now all covered in algae. Everywhere was dead coral skeletons.' Coral bleaching describes a process whereby the coral animal expels the algae that live in its tissues and give it its colour and much of its nutrients. Without its algae, a coral's white skeleton can be seen through its translucent flesh, giving off a bleached appearance. Mass coral bleaching over large areas, first noticed in the 1980s around the Caribbean, is caused by rising ocean temperatures. Some corals also display fluorescent colours under stress when they release a pigment that filters light. Sunlight also plays a role in triggering bleaching. Corals can survive bleaching if temperatures are not too extreme or prolonged. But extreme marine heatwaves can kill corals outright. Coral bleaching can also have sub-lethal effects, including increased susceptibility to disease and reduced rates of growth and reproduction. Scientists say the gaps between bleaching events are becoming too short to allow reefs to recover. Coral reefs are considered one of the planet's ecosystems most at risk from global heating. Reefs support fisheries that feed hundreds of millions of people, as well as supporting major tourism industries. The world's biggest coral reef system – Australia's Great Barrier Reef – has suffered seven mass bleaching events since 1998, of which five were in the past decade. The sandy-bottomed reef lagoons – usually alive with colourful branching corals and fish filling every space – are now 'huge fields of staghorns, all dead,' Gilmour says. 'The outer slope drops from a few metres to 50 metres and it's like looking down the side of a cliff. You can usually see the life down there – the sharks swimming. But this time we looked down the side of the mountain and you didn't see life.' Gilmour says the temperatures at every reef north of Ningaloo reached as high as or higher than ever recorded. 'We've never had every major WA reef affected in a single event. This is the worst coral bleaching event recorded for WA reefs.' Dr Chris Fulton, a principal research scientist at AIMS, has been going to the World Heritage-listed tourism hotspot of Ningaloo since 2008. After a research visit in late January when corals were turning white, he and colleagues returned last week. 'It was a real shock and a lot of us were deeply affected,' Fulton says. 'You have the desert going right to the water's edge and you can just step off the shore in to a spectacular reef that you don't get anywhere else. These natural features are comforting. So imagine if they've all been painted white. Every shape and size of coral colony are being affected with bleaching and mortality.' Fulton spends hundreds of hours a year diving and, usually, the water temperature isn't something that registers. 'But we were all struck by the massive heat in the lagoon,' he says. Ningaloo has bleached badly before, in 2010/11, but Fulton says this year is worse. Temperature loggers in the water showed it was up to 3C above normal – levels that can be devastating for corals. Sign up for Guardian Australia's breaking news email After surveying one personal favourite spot, known as the Oyster Stacks, Fulton says he emerged 'bawling my eyes out'. 'I couldn't believe how bad it was – but then there's a resolve to document what's happening.' Fulton says the seaweed meadows used by fish as nurseries have fared OK, but the fish that feed on coral – such as the Chevron butterfly fish – are crowding around the few surviving corals. 'They're often the pretty fish, but they're usually the first to go. They literally starve to death, and we're starting to see that already. I'm not optimistic they're going to survive.' One bright spot, says Fulton, is that some individual corals across a wide variety of types had managed to survive when others of the same species had died. 'I'm astounded by that,' he said. 'So it's a genuine source of optimism.' Elsewhere along the WA coastline, the story of coral death is repeated. At the Kimberley Marine Research Station, intern and recent university ecology graduate Tara Thomsen, from Melbourne, says even though temperatures have started to fall, there is still bleaching 'I've found it pretty heartbreaking, coming to this beautiful part of the world with pristine areas but seeing in some places the reefs reduced to rubble. It's pretty sad,' she says. Phillip 'Bibido' McCarthy, coordinator of Bardi Jawi Rangers, says there are 50 or 60 small islands off the Dampier Peninsula, many with big reefs. 'We've had a big impact right through the coastline,' McCarthy says. 'We can see the whiteness even off the boat ramp. It's quite terrible. Our resources come from the ocean. These habitats are where the fish grow. I'm 56 but I've never seen anything like this.' Dr Thomas Holmes coordinates the marine science program at the WA government's Department of Biodiversity, Conservation and Attractions. Reefs have been monitored from the air and in the water. He says the heatwave started to reach levels to bleach corals in December. At Ningaloo, bleaching is still unfolding. 'I'm not afraid to use the word unprecedented,' he says. 'We have never seen this in recorded history, whether it's the period of time – it started getting hot in December and some places are still bleaching – or in how hot it's got. And it's unprecedented in scale.' Dr Claire Spillman, principal research scientist at the Bureau of Meteorology, confirms marine heatwave conditions started in WA as early as August 2024 and are ongoing in central and southern parts of the state's waters. Australia's ocean areas have warmed on average by 1C since 1900, and several WA ocean areas have seen their hottest months in this latest heatwave. 'Warming events like the one we are seeing now off the WA coast are becoming more frequent,' Spillman says. Helping fuel the heat, too, has been an accumulation of warmer water in the far western Pacific, which pushed down the WA coast to become part of the Leeuwin current running south. About 90% of the extra heat trapped by rising levels of greenhouse gases has been absorbed by the ocean. Gilmour says the sheer scale of ocean heating is something corals in the region have never had to deal with. 'When you're out there it looks like everything is dead, and it's overwhelming. For Rowley Shoals it will take 10 years [for some recovery] – if we don't get another severe bleaching event. But of course, we are going to get one. 'What really worries me and others is not so much the loss of the corals and reefs, it's that we have reached the point where all these ecosystems are in the same situation. 'And what does this mean? 'This is what 1.5C above the preindustrial [average temperature] means. Things will get a lot worse before it gets better, and that's what makes me sad.'
