With NOAA confirming late last month that more than 83% of the planet's reefs have been hit by extreme ocean heat since January 2023, scientists are finding that even the most resilient of Australia's coral reefs are struggling – including the subtropical reefs of Lord Howe Island.
Paige Sawyers, a PhD candidate at UNSW Sydney, has been monitoring the reefs at the island alongside researchers from the University of Newcastle for more than a year.
She says while Lord Howe Island's coral reefs have avoided the coral bleaching events seen on the Great Barrier Reef and Western Australia's Ningaloo Reef this year, they are still struggling to recover from previous consecutive years of heat stress. This, she says, is what drives the decline of coral reefs.
"We know that recovery for these southern, subtropical reefs from bleaching and stress events is extremely slow," Sawyers says.
"So while this year we didn't see the bleaching temperatures of last year, we do have ongoing heat stress – which has occurred exactly when the reef needs to do its fastest recovery."
She says these conditions are concerning for reefs long-term, as sustained coral losses, disease and ecosystems change ecosystems permanently.
"We've never had warming this high for this long – we're not really sure what to expect after this. That's what's so scary."
The final frontier for reef research
Sitting off Australia's east coast, Lord Howe Island is home to the world's southernmost coral reef. It's often considered a more temperate and resilient reef than the Great Barrier Reef.
The island itself sits at the crossroads of five major ocean currents, including the East Australian Current, which brings tropical species down from the Great Barrier Reef. This convergence of currents means Lord Howe is a biodiversity hotspot, as well as a key site for understanding how coral reefs might adapt in a warming world.
Reefs are home to a third of all marine species. They support coastal communities with food and livelihoods – through fishing as well as tourism. And they serve as a natural buffer from storm surges and coastal erosion.
Yet, while iconic tropical reefs such as the Great Barrier Reef and Ningaloo have been studied for decades, subtropical systems like Lord Howe are understudied.
When Sawyers and her team first arrived at the island in January 2024, sea temperatures were already at 26°C, which is two degrees above average. Coral bleaching tends to occur at just a single degree above average.
By February 2024, as temperatures surged to 29–30°C, she watched as bleaching took hold across the site.
"All the susceptible species were bleached," Sawyers says. "And it happened so fast – just a two-degree spike, and suddenly every site we surveyed was impacted."
As the emotional strain on reef scientists grows, Sawyers says this needs to be part of the conversation, too.
"This was the first bleaching event I saw as it happened. It's devastating. You get attached to these sites."
Colliding impacts
Coral bleaching is a relatively new phenomenon – the Great Barrier Reef alone has experienced seven known mass coral bleaching events: in 1998, 2002, 2016, 2017, 2020, 2022 and 2024.
Corals are marine animals that, like most animals, get stressed by heat. When seas are warmer than usual, corals expel their microscopic zooxanthellae. These are symbionts that live within the coral's tissues, giving them their bright colours and supplying up to 90% of their energy through photosynthesis. Without zooxanthellae, corals begin to starve. They turn pale, they bleach.
When water temperatures drop, recovery can begin. But as the gap between abnormally high temperatures narrows, corals and coral reefs lose their window for recovery.
"It gets to the stage of something more like heatstroke," says Sawyers. "At first, they expel their zooxanthellae to try and conserve energy. But without recovery, the whole animal starts breaking down. There's tissue loss, with disease and opportunistic pathogens taking over."
Researchers saw signs of recovery at Lord Howe Island in April 2024. Some colour returned to the reef. Then, the following month, a sudden and severe low tide struck, exposing the top 20cms of the reef to air and sun.
"You could see a distinct line where the water level dropped," Sawyers says. "While corals within the water were fine, everything above it – every single coral that was exposed – was covered in brown algae. Dead."
This kind of compound event where sustained heat stress combines with something like low tide exposure is becoming more common. For corals already weakened by heat, even minor anomalies can be fatal.
Sawyers says it is "a perfect storm for reefs to suffer disease events, be outcompeted by algae and for the reefs themselves to change."
Marine heatwaves: longer, hotter and more widespread
When Sawyers returned to Lord Howe Island in February 2025, sea temperatures weren't as extreme as the year before – but they were still higher than average for months.
"The temperature has only just dropped down now," she says. "While a lot of the reef is okay, there is still algae across some sites due to the low tide mortality. Parts of the reef are still suffering."
What's happening at Lord Howe Island is only a snapshot of what scientists are observing across the country. On Australia's west coast, Ningaloo Reef has had Degree Heating Weeks (DHW) of 16. A DHW is a measurement of how long and how much water temperatures exceed the summer maximum. It is also used as a metric for bleaching severity.
Mass bleaching typically begins at 4 DHW. Mass coral mortality begins at 8 DHW.
In 2024, Lord Howe recorded 20 DHW, which is the highest ever for the region.
"These aren't just one-off spikes," says Sawyers. "Temperatures are staying above the historical maximum for four or five months at a time. That kind of stress is lethal."
The Northern Great Barrier Reef, once thought to be more resilient due to its past exposures to marine heatwaves, is currently experiencing widespread bleaching.
"Even the really resilient boulder corals of Ningaloo have turned bone-white," Sawyers says.
A glimmer of hope
While restoration in the form of coral nurseries, re-seeding programs and assisted evolution are being researched and implemented, Sawyers says the climate is changing too fast for even restored corals to adapt.
"Restoration is amazing," Sawyers says. "But we still have to address the root cause of coral decline, which is climate emissions."
Sawyers also studies the reefs surrounding Samoa.
"The same thing happened there last year with mass bleaching," she says. "And I thought for sure these corals were gone. But when we went back five months later, they'd recovered."
This unexpected resilience raises questions that fuel Sawyers' research. She says there's still much to learn and "that's where the hope is."
"We need to know what's out there before it disappears. We need to identify the survivors. Understand recovery. Go back to the basics," she says.
"What makes them survive?
"Because if we can answer that, maybe we can protect what's left."
Key Facts:
As the world's fourth — and most severe — mass coral bleaching event unfolds, a UNSW Sydney scientist says even Australia's lesser-known subtropical reef systems are not immune from the impacts of a warming ocean.
