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When 80 Percent of Corals Turn Ghost White
Imagine diving into what should be a vibrant underwater paradise only to find a skeletal wasteland stretching as far as you can see. By April 2024, 80% of coral colonies tracked by researchers had bleached white, creating scenes that marine biologist Professor Maria Byrne described as utterly devastating. The 2023-2024 marine heatwave was extreme in triggering coral bleaching and high mortality, with researchers tracking colonies over 161 days from the heatwave peak through winter cooling. What they witnessed was nothing short of an underwater apocalypse.
Reef sensors recorded a peak temperature of 30.55°C (86.99°F) at One Tree Island, temperatures higher than satellite readings could detect. The scale of the die-offs and the low recovery rate show how hard it can be for reefs to recover amid harsh bleaching events.
The Death Spiral Begins in February
Results revealed that 66 percent of colonies were bleached by February 2024 and 80 per cent by April, but the real horror story was yet to unfold. Marine scientists watched helplessly as what started as stress-induced bleaching transformed into mass mortality. By May, 44% of the bleached colonies were dead and 53% in July, with 31% of colonies remaining bleached and only 16% recovering.
In February, almost two thirds of the corals were ghostly white, with even the heat-resistant Porites affected, showing seven out of 10 colonies with signs of bleaching. This was particularly shocking since these corals are typically the last survivors in extreme heat events.
The Acropora Massacre
Some coral species didn’t just struggle during the 2024 heatwave – they were virtually annihilated. Mortality rates were much higher, at 95 per cent, for the Acropora coral, which experienced entire colony collapse. These branching corals, often called the rainforest builders of the reef, simply couldn’t withstand the unprecedented heat stress.
Some colonies were ‘hard to recognise’ according to researchers, due to how degraded they had become. The rapid transformation from living coral to skeletal rubble happened so quickly that scientists had to use natural landmarks like giant clams to track changes to the reef’s landscape.
Disease Follows the Heat
As if the scorching temperatures weren’t enough, weakened corals began succumbing to diseases they would normally resist. Goniopora genus corals were afflicted by not only bleaching but black band disease, caused when corals are placed under environmental stressors including high temperature levels. This deadly combination of heat stress and disease created a perfect storm of destruction.
The research highlights the complex interplay between heat stress, disease onset and coral mortality, with the rapid onset of bleaching and disease in corals previously considered resilient posing significant challenges. The reef’s natural immune system had essentially collapsed under the thermal assault.
Australia’s Ocean Surface Hits Record Heat
The bureau confirmed 2024 was Australia’s second-warmest year since national records began in 1910, with the national annual average temperature 1.46°C warmer than the long-term average. But perhaps more concerning was what was happening beneath the waves. Average sea surface temperature in the Australian region has warmed by 1.08°C since 1900, with 9 of the 10 warmest years on record occurring since 2010.
The year 2024 was the hottest on record in the South-West Pacific region, with temperatures approximately 0.48 degrees Celsius above the 1991–2020 average, marked by extreme heat and the largest marine heatwave event ever observed.
The Western Australia Fish Kill
While the Great Barrier Reef was dying, another marine catastrophe was unfolding on Australia’s west coast. Tens of thousands of fish have died off northwestern Australia, as a large and long-lasting marine heatwave intensifies, with the fish kill at Gnoorea Beach near Karratha concerning scientists. The hot mass of water was heading south toward the world-famous Ningaloo Reef.
This marine heatwave began in September, with temperatures up to 3°C warmer than usual off Broome, and by late December temperatures hit 4–5°C above normal at the surface. Right now, 28% of the world’s oceans are in heatwave conditions, based on surface temperatures.
The Climate Change Connection
These aren’t random weather events – they’re the direct result of human activity. Ocean surface temperatures were made, on average, at least 20 times more likely to occur as a result of climate change during marine heatwaves. The world’s oceans have taken up more than 90% of the extra energy stored by the planet as a result of enhanced greenhouse gas concentrations.
Climate change, fuelled by burning fossil fuels like coal, oil and gas, is driving longer, hotter and more frequent marine heatwaves, and we must give our oceans a fighting chance against further heat stress.
The Northern Reef’s Devastating Losses
Scientists found substantial losses of coral cover on 12 reefs ranging from 11% to 72% of pre-summer levels, with more than a third of hard coral cover lost across the Cooktown-Lizard Island sector – the largest annual decline for this sector in 39-years of monitoring. This represents decades of growth wiped out in a single summer.
In-water surveys of 19 reefs between Lizard Island and Cardwell found up to 72 percent coral mortality on 12 reefs, with one northern section losing more than a third of hard coral cover. The numbers were staggering even to veteran marine scientists.
The Fifth Mass Bleaching in Eight Years
In March 2024, the fifth mass bleaching event on the Great Barrier Reef was confirmed as part of the 4th global bleaching event, which began in 2023 and occurred during an El Niño year following the hottest year on record. Five mass coral bleaching events have occurred on the Great Barrier Reef over the past 10 years, all associated with marine heatwaves driven by anthropogenic climate change.
The 2024 event was the seventh mass bleaching event on the GBR since 1998 and the fifth since 2016, bringing severe conditions and rapid coral health decline not previously seen in the southern GBR region.
The Recovery Myth
The brutal reality is that corals are running out of time to recover between heatwaves. Scientists warn that heat intensities are now so frequent—every 2 years instead of every 10—that corals may not have a chance to recover. Rapidly recurring bleaching events do not give the reef ecosystem time to fully recover.
Some types of corals can remain bleached for months, remaining on a knife edge between survival and death, which is why returning and repeating surveys of the reefs in this vast, complex and dynamic system is so important.
The Global Scale of Destruction
The entire length of the Reef – over 2000 km – and over 1000 reefs has been surveyed, with bleaching recorded on 74 per cent of reefs in the Marine Park, half of which recorded high or very high levels of coral bleaching. This represents the most extensive bleaching event ever documented on the reef system.
The 2024 mass bleaching event forms part of the fourth global bleaching event impacting both hemispheres of the Atlantic, Pacific and Indian Oceans during 2023 and 2024, with mass bleaching now confirmed in 83 countries.
What This Means for Marine Life
This spells catastrophic news for the 25 per cent of marine life who are supported by corals globally, whether that be through providing shelter, food or a place to reproduce. The death of coral reefs creates a domino effect throughout the entire ocean ecosystem. The study showed a shift away from normal phytoplankton species towards smaller cells that aren’t easily consumed by larger animals, impacting the type and quality of food for copepods and krill, which in turn are eaten by fish.
The increasing frequency of marine heatwaves around Australia has contributed to permanent impacts on marine ecosystem health, including depleting kelp forests and seagrasses, a poleward shift in marine species, and increased occurrence of disease.
The Unstoppable Future
Climate models project more frequent, extensive, intense and longer-lasting marine heatwaves in future, with worsening impacts on coral reefs expected with continued warming, and the intensification much greater under high greenhouse gas emission scenarios. The underwater bushfire that consumed Australia’s reefs in 2024 is just a preview of what’s coming.
Prolonged ocean warming is driving changes in weather patterns and more frequent and intense marine heatwaves, threatening ecosystems and human livelihoods. Unless dramatic action is taken to reduce greenhouse gas emissions, the silent heatwaves will continue their deadly march through the world’s remaining coral reefs until nothing but ghostly white skeletons remain.
The 2024 marine heatwave didn’t just warm Australia’s waters – it delivered a death sentence to vast sections of the country’s most precious marine ecosystems. The underwater bushfire has left behind a landscape of skeletal remains where vibrant coral cities once thrived, and the next wave of destruction is already building beneath the surface.
