EXPERT REACTION: Worst summer on record for the Great Barrier Reef

Once again we are seeing a mass bleaching event over large parts of the GBR, with record levels of thermal stress over northern and southern parts of the reef. This is further demonstration of the rapid increase in bleaching frequency that has taken place since the first such bleaching event in 1998. The heat over the GBR is part of a nearly world wide ocean temperature extreme, with globally averaged ocean temperatures, over the last 12 months, warmer than ever measured by a record amount. In addition to the GBR, an unprecedented fraction of the planet has experience intense marine heatwave events. Many factors are likely at play in this global extreme, including the recent El Nino, reduced aerosols and an unlucky combination of heat inducing weather systems. But aggravating all this is the unrelenting march of human induced global warming.

We have been researching reef sites for coral cover and associated fishes for over 30 years, and visiting the area for over 45 years, and are currently on Heron Island with UTS students, witnessing a horrifying scene of widespread bleaching and recent death of corals of many species.  One huge coral bommie, hundreds of years old, is now white and bleached, hopefully not a tombstone for the Reef!  Climate action now!

This is the fifth mass coral bleaching event on the Great Barrier Reef in the past nine summers. Repeated disturbances limit the capacity of reef ecosystems to effectively recover between events, which are projected to become more frequent with climate change. Moreover, this year has seen record heat stress on nearly half of reef locations, particularly in the southern section of the reef where bleaching and mortality has been at the highest levels. In some areas, impacts from cyclones and flooding have exacerbated the effects of heat stress.

While actions to support coral survival and resilience remain important, the most important action needed is an urgent reduction of greenhouse gas emissions – the primary cause of climate change. Policy coherence is essential – we can't protect our natural heritage while also opening up new fossil fuel extraction activities. While Australia needs to act as part of the global community on reducing climate emissions, the impacts on our natural environment warrant taking a leadership role.

It is important to consider the impacts on our communities. Some peoples’ livelihoods rely upon a healthy Reef; others need to be offered just transitions away from extractive mining that conflicts with the premise of a healthy Reef in the long term. We all benefit from the goods and services that reefs provide, including coastal protection and as fish habitat.

Aerial surveys and in-water transects indicate the current mass bleaching event is one of the most extensive events to occur in the nearly 40 years of monitoring of the Great Barrier Reef by the Australian Institute of Marine Science. Three-quarters of the reefs surveyed contained some level of coral bleaching, with a third of the reefs surveyed showing prevalence of very high (61-90% of shallow corals bleached) or extreme bleaching (>90% of shallow corals bleached) across all three regions of the Great Barrier Reef, for the first time ever.

In-water surveys confirm mortality has started across all three regions of the Great Barrier Reef.
Ongoing in-water surveys over the next few months will help contribute to our understanding of the spatial extent and impacts to the reef from the record-breaking heat stress during the 2024 summer

Climate change remains the biggest challenge facing our environment (and the reef) today and will continue to be in coming decades. The current bleaching is a very clear and serious reminder that we need to keep working quickly and collaboratively on ways to help the reef withstand these pressures.
 
As the national science agency, we are committed to helping Australia’s ecosystems, and the communities and businesses that depend on them, build resilience and adapt to the impacts of climate change – the GBR is a nationally and internationally significant example.
 
Combatting multiple threats at once, including the coral-eating crown of thorns starfish pests that weaken the reef’s overall resilience and testing the latest technology for coral larvae deployment, are important and give the reef the best chance of recovery.

Researchers from the University of the Sunshine Coast have been running a long-term ‘Leaf to Reef’ study on the impacts of rehabilitation and climate change on the reefs surrounding Lady Elliot Island in the Southern GBR since 2020. Funded by the Great Barrier Reef Foundation, the research involves conducting fish, turtle and coral surveys once every four months.
 
The latest survey in February 2024 was the worst coral bleaching event that we have recorded so far, with temperatures on the reef flat reaching up to 31C. Correspondingly up to 70 percent of bleaching was recorded on the reef flat.
 
Concerningly, we also recorded extensive bleaching at depth, with the team recording massive bleaching in branching and plate corals at depths of up to 16m. The team will be returning in June to determine how much of the coral has recovered.
 
As Lady Elliot Island Reef is the very southern reef in The Great Barrier Reef, it has always been considered to be an 'ark', an area of safety as the waters further north warm and mobile animals start to move south to escape increasing temperatures. However, this isolated reef is also feeling the impacts of climate change highlighting that the need to limit our impacts on the planet by reducing greenhouse-gas emissions has never been more urgent.

The widespread bleaching of the Great Barrier Reef (GBR) and the resulting threat to coral survival are disasters for the diversity of reef ecosystems. This bleaching also significantly impacts the livelihoods of tens of thousands of Australians and the communities they reside in.

The frequency of marine heatwaves and mass bleaching on the GBR weaken its ability to recover and makes the entire ecosystem less resilient to a changing climate. The effects of bleaching range from short-term physiological damage to widespread mortality.

As an independent and authoritative science adviser, the Academy has long observed that climate change is the primary threat to the GBR and its connected systems. Scientists expect more severe, irreversible, and costly impacts unless bold action is taken to reduce emissions.

Simply continuing with a business-as-usual approach is no longer an option. It is crucial that Australia implements strong and effective national and local environmental laws.

These laws must prioritise scientific evidence and prevent practices that damage the GBR’s ability to adapt to climate change, including damage to the habitats in the GBR catchments.

The upcoming review of the Reef 2050 plan needs to show ambition and align with the scale of the challenge—acknowledging that there can be no quick fix and putting all options on the table, from finding new ways to manage the GBR catchments to institutional arrangements, activating required resources, protecting and restoring habitats where possible, and drawing on Traditional Knowledges.

Over 1 billion people depend upon coral reefs for their livelihood. However, the survival of coral reefs as we know them is threatened by a variety of anthropogenic impacts, most notably sea water temperature increase driven by global warming. These elevated sweater temperatures lead to the phenomenon known as coral bleaching, where the coral expel their symbiotic algae that provide the majority of the energy to the coral. Coral will then starve and die if they lose too many algae or the high temperatures last for too long.

Currently, corals and the Great Barrier Reef are undergoing the 5th mass bleaching event in the last 8 years. These repeated mass bleaching events are now occurring more frequently, beyond coral's ability to repopulate impacted reefs. These events are not only occurring on the GBR but also on other reefs around the world. For example, the reefs of Lord Howe Island (the southernmost reef system in the world) were some of the first to show signs of coral bleaching this summer.

It is only with rapid decreases in fossil fuel emissions that we can hope to preserve these unique ecosystems for our children and grandchildren. While these stories are bleak, there is still some hope in that corals are recovering quicker than originally predicted. Hopefully, if we can take action we can allow coral reefs to recover.

The ocean acts as a vast heat sink, absorbing trapped atmospheric warmth and thus regulating the Earth’s climate system. Unfortunately, the accumulation of heat in the world's oceans over the past year has proven too much for corals, especially those inhabiting the Great Barrier Reef and subtropical eastern Australian reefs. Before the current bleaching event, coral communities on the Great Barrier Reef were recovering from a severe mortality event in 2016/17.

Certain fast-growing species had begun to replenish bare areas, but these very species, crucial for early colonization, are, unfortunately, the most vulnerable to bleaching. While the full extent of coral mortality is still being assessed, this fourth global bleaching event represents a devastating setback for the diversity, productivity, and health of the Great Barrier Reef. It is anticipated that millions of individual corals will perish, leading to local extinction events and the loss of genetic diversity. Additionally, surviving colonies may be so stressed that they lack the energy to reproduce during the annual coral spawning event later in the year.

Coral bleaching is occurring on a large geographic scale, an event termed global bleaching, including on the GBR. The event is not simultaneous or instantaneous, rather such global bleaching events are declared when bleaching effects build up to a threshold of impact such that 12% of corals in each of the three ocean basins exhibit bleaching (loss of brown-coloured symbiotic algae) leaving vulnerable white or sometimes colourful corals behind.

These events are often followed by mass mortality. Corals in the GBR have been experiencing heat stress for a number of months, and the rate of spread of bleaching impacts is still high. Bleaching can be a stress response to a number of environmental factors but sustained elevated temperatures are the main cause. In Australia, what were predicted marine heatwaves have raised ocean temperatures during the summer to the point of bleaching. Reefs at protected and long-studied sites such as those at Heron Island and Lizard Island are experiencing a bleaching event. The impact and recovery from this event will be well documented by the researchers working here.

Our research team are on One Tree Island in the southern Great Barrier and the impacts of this summer’s extreme conditions are immediately obvious, such as bleached white corals, fluorescing corals, recently dead corals, and the smell and taste of stressed corals as they give off a cocktail of chemicals.
 
However, it is not just the corals that are suffering. The team have observed bleached anemones (homes to clownfish), very few mobile invertebrates and early signs of changes in fish communities.
 
Initial observations of the marine sediments are that they are slimy and stringy, which may be indicative of coral mucus sloughing off into the sediments. There is a distinct sulphurous and sour smell to the sediments, noticeable even during sample processing back at the Research Station.

The Reef Snapshot report issued by the Great Barrier Reef Marine Park Authority is a factual government report, written in a way that avoids alarming language. But its findings should alarm us. 
 
It shows that the whole of the reef is now susceptible to coral bleaching. 
 
I was on the reef in January and February and was confronted by what I saw. The coral was pale, pale and paler and kept getting worse.
 
We know now that ocean temperatures in the southern reef have broken all records. This is not the result of a natural cycle.
 
What we can clearly say now is that the southern Great Barrier Reef has bleached seriously.
 
We have research teams right now working at our One Tree Island Research Station on the southern reef and will report our results soon.

The Southern Great Barrier Reef is experiencing extreme levels of thermal stress this summer and these effects are causing mass coral bleaching. At One Tree Reef, home of the University of Sydney’s 50-Year-Old One Tree Island Research Station, these effects are fully evident. 

I've been visiting the station since 2004 and have never witnessed anything like it.  Visiting the reef in March we could see the extensive bleaching from the boat. The snorkel that followed confirmed our fears where the difficult task was finding a coral colony that wasn’t stressed, bleached or already dead.

“We estimated more than 85 per cent of live coral outside the entrance to the lagoon was in this state.  At most sites, at least 70 per cent of the corals were stressed or bleached.  

My team is also looking at the interplay between corals and coral-eating fishes, a group that will also be heavily impacted. We noted, anecdotally, potentially higher levels of aggression among these fishes, perhaps due to a scarcity of food as the corals bleach and die.  

We will be following this up in two weeks’ time; looking more specifically at how the bleaching is affecting the behaviour and condition of this group of important reef fish.