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

Building a Strong Online Presence with Katoomba Web Design

Katoomba web design is more than just creating a website that looks good—it’s about building an online presence that reflects your brand, engages your audience, and drives results. For local businesses in the Blue Mountains, a well-designed website a...

September Sunset Polo

International Polo Tour To Bridge Historic Sport, Life-Changing Philanthropy, and Breath-Taking Beauty On Saturday, September 6th, history will be made as the International Polo Tour (IPT), a sports leader headquartered here in South Florida...

5 Ways Microsoft Fabric Simplifies Your Data Analytics Workflow

In today's data-driven world, businesses are constantly seeking ways to streamline their data analytics processes. The sheer volume and complexity of data can be overwhelming, often leading to bottlenecks and inefficiencies. Enter the innovative da...

7 Questions to Ask Before You Sign IT Support Companies in Sydney

Choosing an IT partner can feel like buying an insurance policy you hope you never need. The right choice keeps your team productive, your data safe, and your budget predictable. The wrong choice shows up as slow tickets, surprise bills, and risky sh...

Choosing the Right Legal Aid Lawyer in Sutherland Shire: Key Considerations

Legal aid services play an essential role in ensuring access to justice for all. For people in the Sutherland Shire who may not have the financial means to pay for private legal assistance, legal aid ensures that everyone has access to representa...

Watercolor vs. Oil vs. Digital: Which Medium Fits Your Pet's Personality?

When it comes to immortalizing your pet’s unique personality in art, choosing the right medium is essential. Each artistic medium, whether watercolor, oil, or digital, has distinct qualities that can bring out the spirit of your furry friend in dif...

The Times Features

How much money do you need to be happy? Here’s what the research says

Over the next decade, Elon Musk could become the world’s first trillionaire[1]. The Tesla board recently proposed a US$1 trillion (A$1.5 trillion) compensation plan, if Musk ca...

NSW has a new fashion sector strategy – but a sustainable industry needs a federally legislated response

The New South Wales government recently announced the launch of the NSW Fashion Sector Strategy, 2025–28[1]. The strategy, developed in partnership with the Australian Fashion ...

From Garden to Gift: Why Roses Make the Perfect Present

Think back to the last time you gave or received flowers. Chances are, roses were part of the bunch, or maybe they were the whole bunch.   Roses tend to leave an impression. Even ...

Do I have insomnia? 5 reasons why you might not

Even a single night of sleep trouble can feel distressing and lonely. You toss and turn, stare at the ceiling, and wonder how you’ll cope tomorrow. No wonder many people star...

Wedding Photography Trends You Need to Know (Before You Regret Your Album)

Your wedding album should be a timeless keepsake, not something you cringe at years later. Trends may come and go, but choosing the right wedding photography approach ensures your ...

Can you say no to your doctor using an AI scribe?

Doctors’ offices were once private. But increasingly, artificial intelligence (AI) scribes (also known as digital scribes) are listening in. These tools can record and trans...