The Times Australia
The Times World News

.

High coral cover amid intense heatwaves and bleaching? Here’s how both can be true on the Great Barrier Reef

  • Written by Daniela Ceccarelli, Reef Fish Ecologist, Australian Institute of Marine Science

It was another difficult summer on the Great Barrier Reef. A serious marine heatwave caused the fifth mass coral bleaching event since 2016. Intense rain from Cyclone Jasper washed huge volumes of freshwater and sediment onto corals closer to shore, and Cyclone Kirrily crossed the central region. Some parts of the southern reef endured heat stress at levels higher than previously measured.

Has this summer’s bleaching killed many corals on the Barrier Reef – or will they recover? The answer is – we don’t know yet. In the latest Australian Institute of Marine Science coral cover report[1], released today, we report coral cover has increased slightly in all three regions, reaching regional high points in two of them.

How can that be? The answer is simple: lag time. Between 2018 and 2022, large areas of the Great Barrier Reef had a reprieve. Marine heatwaves and bleaching still occurred, but the damage was not too extreme. Coral began to recover and regrow.

Over the 2023–24 summer, the heat returned with a vengeance, triggering widespread coral bleaching. But bleached coral isn’t dead yet – it’s very stressed. The summer’s bleaching is only just winding up now, in August. We won’t know how much coral actually died until we complete our next round of surveys. We’ll be back in the water from September to find out.

bleached coral reef from air
From the sky, bleached corals appear white. This photo shows Pancake Reef, a rare estuarine coral reef near the town of 1770 in February 2024. Neal Cantin/AIMS, Author provided (no reuse)

How can we reconcile high coral cover and intense shocks?

Bleached coral is very stressed, but it’s still alive.

Corals respond to intense heat by expelling their tiny symbiotic algae, or zooxanthellae. In the process, they lose their colours and become bone-white. If the heat eases, the zooxanthellae can sometimes return, and the corals can bounce back.

But if temperatures stay high, corals die. A dead coral is not bone-white – it’s covered in light green fuzz, a sign of colonisation by filamentous algae.

What this means is it takes time to say a coral is truly dead.

For almost four decades, AIMS scientists have monitored the Great Barrier Reef. It’s no easy task to monitor a reef system the size of Italy.

To do it, our team spends 120 days at sea between September and June, across six separate trips.

The two trips we did during the peak of the mass bleaching event in February and March recorded bleached coral as live coral cover – because they were alive when we did the surveys.

So while our new report provides an update on the state of the reef, we cannot use it to describe the full impacts of this summer’s bleaching. It’s a reference point.

graph showing coral trends barrier reef Trends in hard coral cover, northern section of the Great Barrier Reef, 1986-2024. Australian Institute of Marine Science, CC BY-NC-ND[2][3] graph coral cover barrier reef Trends in hard coral cover, central section of the Great Barrier Reef, 1986-2024. Australian Institute of Marine Science, CC BY-NC-ND[4][5] graph coral cover barrier reef Trends in hard coral cover, southern section of the Great Barrier Reef, 1986-2024. Australian Institute of Marine Science, CC BY-NC-ND[6][7]

Our surveys found average hard coral cover in the year to June 2024 was:

  • 39.5% in the northern region (north of Cooktown), up from 35.8% last year

  • 34% in the central region (Cooktown to Proserpine), up from 30.7%

  • 39.1% in the southern region (south of Proserpine), up from 34%.

This year’s coral cover averages are higher than the last few years, but not by much. Statistically speaking, they’re within the margin of error.

By contrast, the reef recovered much more strongly during the less stressful years from 2018 to 2022. In the northern region, coral cover increased by 22.9%.

If we were living in ordinary times, corals would grow back over a decade or two, giving rise to more diverse reefs.

But as the world heats up, the reprieve from heatwaves and extreme weather is getting shorter and shorter. In recent decades, both size and frequency of events causing severe damage to the reef have increased[8].

How bad was this year’s bleaching?

This year’s marine heatwaves peaked in February and March, when researchers from AIMS and the Great Barrier Reef Marine Park Authority conducted additional surveys from the air and underwater.

What this showed was the 2024 mass bleaching event was one of the most serious and widespread so far. It took place against the fourth recorded global bleaching event[9].

Heat stress is cumulative – it gets worse the longer corals have to endure warmer water.

Some of the southern reefs were exposed to up to 15 degree heating weeks, a measure of the accumulated heat stress. Such high levels have never been recorded on the reef before.

Our aerial surveys[10] detected extreme levels of bleaching – affecting over 90% of corals on a reef – across all three regions of the reef, though not equally. Extreme bleaching was widespread in the southern region of the reef, but less so in the northern and central regions.

Reports of coral death on bleached reefs are beginning to arrive[11], but it’s too early to draw broad conclusions about the full impact of this event.

coral island and fringing reef, with bleaching Bleaching was clearly visible on the reefs fringing North West Island off Gladstone in February 2024. Neal Cantin/AIMS, Author provided (no reuse)

What will happen next?

During the cooler months, bleached corals can recover, but it’s not guaranteed. Bleaching makes it harder for corals to grow and reproduce, and leaves them more susceptible to disease. If their symbiotic algae return, some corals will recover, but many corals will not make it. We won’t know the death toll until after we do our next round of surveys.

While coral cover has increased and decreased over time, the variability has become much more erratic. Over the last 15 years, coral cover has had its highest highs and lowest lows[12] on record.

What we should take from this is the reef – the world’s largest living structure – is currently still able to recover from repeated shocks. But these shocks are getting worse and arriving more often[13], and future recovery is not guaranteed.

This is the rollercoaster ride the reef faces at just 1.1°C of warming. The pattern of disturbance and recovery is shifting – and not in the Reef’s favour.

Read more: Is the Great Barrier Reef reviving – or dying? Here's what's happening beyond the headlines[14]

References

  1. ^ coral cover report (www.aims.gov.au)
  2. ^ Australian Institute of Marine Science (www.aims.gov.au)
  3. ^ CC BY-NC-ND (creativecommons.org)
  4. ^ Australian Institute of Marine Science (www.aims.gov.au)
  5. ^ CC BY-NC-ND (creativecommons.org)
  6. ^ Australian Institute of Marine Science (www.aims.gov.au)
  7. ^ CC BY-NC-ND (creativecommons.org)
  8. ^ have increased (esajournals.onlinelibrary.wiley.com)
  9. ^ global bleaching event (www.noaa.gov)
  10. ^ aerial surveys (www.aims.gov.au)
  11. ^ beginning to arrive (theconversation.com)
  12. ^ highest highs and lowest lows (link.springer.com)
  13. ^ getting worse and arriving more often (esajournals.onlinelibrary.wiley.com)
  14. ^ Is the Great Barrier Reef reviving – or dying? Here's what's happening beyond the headlines (theconversation.com)

Read more https://theconversation.com/high-coral-cover-amid-intense-heatwaves-and-bleaching-heres-how-both-can-be-true-on-the-great-barrier-reef-235510

Times Magazine

Headless CMS in Digital Twins and 3D Product Experiences

Image by freepik As the metaverse becomes more advanced and accessible, it's clear that multiple sectors will use digital twins and 3D product experiences to visualize, connect, and streamline efforts better. A digital twin is a virtual replica of ...

The Decline of Hyper-Casual: How Mid-Core Mobile Games Took Over in 2025

In recent years, the mobile gaming landscape has undergone a significant transformation, with mid-core mobile games emerging as the dominant force in app stores by 2025. This shift is underpinned by changing user habits and evolving monetization tr...

Understanding ITIL 4 and PRINCE2 Project Management Synergy

Key Highlights ITIL 4 focuses on IT service management, emphasising continual improvement and value creation through modern digital transformation approaches. PRINCE2 project management supports systematic planning and execution of projects wit...

What AI Adoption Means for the Future of Workplace Risk Management

Image by freepik As industrial operations become more complex and fast-paced, the risks faced by workers and employers alike continue to grow. Traditional safety models—reliant on manual oversight, reactive investigations, and standardised checklist...

From Beach Bops to Alpine Anthems: Your Sonos Survival Guide for a Long Weekend Escape

Alright, fellow adventurers and relaxation enthusiasts! So, you've packed your bags, charged your devices, and mentally prepared for that glorious King's Birthday long weekend. But hold on, are you really ready? Because a true long weekend warrior kn...

Effective Commercial Pest Control Solutions for a Safer Workplace

Keeping a workplace clean, safe, and free from pests is essential for maintaining productivity, protecting employee health, and upholding a company's reputation. Pests pose health risks, can cause structural damage, and can lead to serious legal an...

The Times Features

Tricia Paoluccio designer to the stars

The Case for Nuturing Creativity in the Classroom, and in our Lives I am an actress and an artist who has had the privilege of sharing my work across many countries, touring my ...

Duke of Dural to Get Rooftop Bar as New Owners Invest in Venue Upgrade

The Duke of Dural, in Sydney’s north-west, is set for a major uplift under new ownership, following its acquisition by hospitality group Good Beer Company this week. Led by resp...

Prefab’s Second Life: Why Australia’s Backyard Boom Needs a Circular Makeover

The humble granny flat is being reimagined not just as a fix for housing shortages, but as a cornerstone of circular, factory-built architecture. But are our systems ready to s...

Melbourne’s Burglary Boom: Break-Ins Surge Nearly 25%

Victorian homeowners are being warned to act now, as rising break-ins and falling arrest rates paint a worrying picture for suburban safety. Melbourne residents are facing an ...

Exploring the Curriculum at a Modern Junior School in Melbourne

Key Highlights The curriculum at junior schools emphasises whole-person development, catering to children’s physical, emotional, and intellectual needs. It ensures early year...

Distressed by all the bad news? Here’s how to stay informed but still look after yourself

If you’re feeling like the news is particularly bad at the moment, you’re not alone. But many of us can’t look away – and don’t want to. Engaging with news can help us make ...