Great Barrier Reef: behind "best coral cover in 36 years"

Cosmos Investigation: “The best coral cover in 36 years” What the…?

What’s really happening on the reef? A special three part series. 

At first the Great Barrier Reef was collapsing from devastating back-to-back bleachings. But in August this year, scientists announced the “highest coral cover in central, northern reef in 36 years”, prompting The Australian to proclaim “The reef is strong, so stop the scare campaign”, while climate-sceptics on Sky News announced “it proves without a shadow of a doubt that many of [these scientific institutions] are completely untrustworthy.” 

Confused? Well, I’ve been a science reporter for 25 years and I’m confused. In amongst the daily news cycles, it can be hard to make head or tail of such conflicting reports. And like so many of us, I worry desperately about our glorious, irreplaceable 2,300km-long reef. So I decided to call the experts and try and put the whole picture into context for myself and you. In part one of my special investigation, I ask what’s really happening to coral – especially the magnificent old growth corals? And, spoiler alert: this story takes on more twists, turns – and cake – than I expected.

Jonica newby standing in front of the ocean
Jonica Newby. Credit: supplied

What set the cat amongst the corals was the release of the Australian Institute of Marine Science’s annual report of coral reef condition. Since 1985, they’ve measured coral cover on 80–130 representative reefs, in a survey known as the Long Term Monitoring Program, or LTMP. In 2022, it reported the highest coral cover in the northern and central reef since monitoring began, while the southern reef had dipped. To understand why this news from the north was such a surprise to me – and a boon to climate sceptics – we need to backtrack a few years.

“They cooked; they melted. It shows heat stress can kill a coral directly.”

Dr Terry Hughes

In the past seven years, the Great Barrier Reef experienced four significant bleaching events, of which 2016 was the worst. Dr Terry Hughes, former director of the ARC Centre of Excellence for Coral Reef Studies, conducted extensive aerial and underwater assessments and was shocked. 

“There was a 50% death of hard corals north of Cairns – 30% if you average over the whole reef [the southern reef was relatively unaffected by the 2016 event]. And half the corals that died, died quickly – in a week,” he tells me, aghast. “They cooked; they melted. It shows heat stress can kill a coral directly.”

Since then, according to Hughes, 80% of the 3,000-odd reefs that that make up the Great Barrier Reef have experienced at least one severe bleaching event. “Severe” means more than half of corals bleached – for context, Hughes has found that in severe bleaching, nearly all those corals die. Many reefs were hit that hard 2 or 3 times; some all 4 times. A further 18% of the reef experienced sub-lethal bleaching. Only 2% of the reef has escaped bleaching entirely. In other words, most of the reef has been hit hard at least once, with only 7 years or less to recover.

“It got down to 5% of what it was previously around Lizard Island,” recalls another renowned reef researcher, Dr Morgan Pratchett. “2018 was the all-time low.”

So how an earth did we get from there to “the highest coral cover in 36 years” in the northern and central reef by 2022?

Read more: Great Barrier Reef should be added to UNESCO ‘in Danger’ list says expert mission.

In a word: Acropora.

The regrowth is dominated by this one fast growing coral type. Acropora is a genus of corals which includes table corals, staghorn corals, branching corals – many of the colourful corals familiar to snorkellers. Acropora have always been the most abundant reef coral. And boy can they grow.

“Some Acropora grow up to 20cm a year!” explains Dr Mike Emslie, who’s in charge of the LTMP survey. “For example, table Acropora grow like dinner plates. When a new coral first settles, it doesn’t do much for a couple of years, and then it hits a phase of exponential growth on its periphery, physically covering a lot of the reef. And that’s the pattern we’re seeing over the last 5–6 years.”

The LTMP data shows that in 2020-2021, coral cover over the whole reef grew an astonishing 9%.

“How come? We’ve had a 5–6 year relatively coral-mortality-free period,” says Emslie. No cyclones, milder bleachings. “Honestly, I think it’s just down to luck.”

So if what’s grown back since around a third of hard corals died are these fast growing – some might say disparagingly “weedy”– species (even though they’re still great habitat), what has been lost from the reef? It’s with this seemingly straightforward question that my investigation fell down a mini-coral wormhole.

“Many of the old growth corals are gone, and they’ll never have time to grow back.”

Dr Terry Hughes

I’d assumed we are losing species diversity – the kaleidoscope of multi-form corals that makes the reef feel so magical. I’ve worried particularly about the “old growth” corals; those massive, ancient, slow growing corals of which the magnificent Porites is the grand old sentinel. Porites can grow to the size of a small house and provides unique habitat for fish and invertebrates, cleaning stations, and substantial larval and genetic riches. With an infinitesimal growth rate of 1cm per year, though, each giant is hundreds of years old.

“Many of the old growth corals are gone, and they’ll never have time to grow back,” says Hughes, his voice vibrating with distress. “Instead, the dead skeletons of old corals are being colonised by weedy, fast-growing, climate-vulnerable Acropora.”

“Think of a rainforest that burns down,” he explains. “It’s replaced by grasses or bushes. Is that recovery? And it’s really vulnerable to the next grassfire.”

Read more: The Great Barrier Reef – what does a new Labor government mean for its future?

This ongoing diminution of our reef is the account I got from many scientists, including Pratchett. It was the straightforward story I expected to write. But then I spoke with the LTMP’s Emslie, and … his data are not yet clear cut.

“It’s a really complex picture,” says Emslie. “You can’t take the reef as a whole.”

Indeed, all the scientists commented how patchy the reef has become – one area algae-covered rubble, another flourishing with Acropora.  “We lost most of the corals across big chunks of the reef and no doubt lost old-growth coral. But I was swimming over massive Porites last week, so many are still there,” Emslie says.

When he breaks down his data, the cover of slower growing corals looks relatively stable over time, while the Acropora has surged down and up following cyclones or bleaching. “We’re definitely seeing a shift in species composition, but there’s no consistent pattern across the reef.”

“We’re definitely seeing a shift in species composition, but there’s no consistent pattern across the reef.”

Dr Mike Emslie

But when I go back to Dr Terry Hughes, he points me to a reef-wide study he published in 2020, which found that in every case, every species showed fewer large corals, fewer medium corals, and – at that point – fewer small corals than at the same sites in the 1990s. 

Huh? How can there be this discrepancy between the two versions? I’m confused again.  After harassing a few hapless scientists into a Saturday afternoon, I decide to regroup, bake out my frustrations by cake, and come back at this conundrum Monday. It’s clear I have to go deeper – and delve into data collection.

The LTMP collects information in two ways. First, they tow a snorkeller around the perimeter of a reef to ascertain total coral cover over a large area, but no species information. Second, they inspect 15 permanently marked areas per reef, each of 50m length, and count percentages of different coral types. Here’s the thing: when the survey was established the marked sites were set on the outer slope at 6-9 m depth – an area of high diversity. But it’s also cooler down there.

In contrast, Hughes’s research records more detail and includes the so called “crest” of the reef – ie the shallower 1–3m. It’s an area with substantial biodiversity, and one that’s the most vulnerable to sunlight, and heating.

“I’m really grateful that we have the long-term data, but your question about old growth coral falls in a gap in our methodology.”

Dr Dave Wachenfeld

So if the LTMP only looks at the deeper reef slope – and Hughes’s reef surveys are not done as regularly – are we missing some key aspects in our monitoring? Particularly when it comes to understanding what’s happening to the ancient Porites. The LTMP, vital as it is, doesn’t even record the size of the corals – so the Porites reported might be lots of young ones – or one big one. Can anyone tell me what’s really happening to the irreplaceable giants?

“You’ve got me thinking,” says Dr Dave Wachenfeld, Chief Scientist for the Great Barrier Reef Marine Park Authority when I call for help. “I’m really grateful that we have the long-term data, but your question about old growth coral falls in a gap in our methodology. I’m really wishing now that we’d tagged say 500 old-growth corals and followed them. Nobody expected we would need that.”

Read more: Ocean solutions.

Emslie says the LMTP is still trying to process their existing data on shifts in coral composition, and also trying to work out if it can add crucial information on size into their data collection.

Sure, there are many things that are not being adequately monitored on our massive reef, but this one area feels like it matters. I remember in the Californian fires, firefighters were physically wrapping giant old Sequoias in fire retardant clothes, desperate to ensure they survived. If we are not monitoring the underwater equivalents, how can we protect them?

What we do know is that any large, slow growing corals that die – and I’m convinced by Hughes’ data that many have already – will not be back in our lifetime, if ever. And the “weedy” Acropora are not only the first to come back, they’re also the first to die.

“They are the most vulnerable to cyclones, bleaching and [the invasive starfish] crown of thorns,” says Emslie.

What we know is that any large, slow growing corals that die will not be back in our lifetime, if ever.

What scares the scientists is how rapidly the events that kill coral are escalating. Cyclones are getting more powerful. Bleaching more frequent. There was a major bleaching event in 1998, in 2002, then 2016, 2017, 2020 and 2022. “And the last one was a La Nina year which is not supposed to bleach,” adds Emslie. “At what point is it every year?”

“I think what this latest LTMP survey tells us is good news,” reflects Wachenfeld. “It means the resilience of the reef is still marvellous. If I thought there was going to be no further global warming and we would stop at 1.2 degrees, we’d be sweet! But that’s not our reality. On current commitments we are going past 2.5 degrees by the end of the century. We’re going to hit 1.5 degrees in the early 2030s.”

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So what are these scientists’ best predictions for the reef going forward?

“Given the predominance of fast growing but vulnerable species, I think over the next decade or so we are going to see dramatic boom bust, boom, bust cycles,” says Pratchett.

“How low?” I ask.

“I think down to almost zero and back up again,” he says. “The worry is at some point – like an elastic band – the system snaps and doesn’t bounce back.” That’s what happened in the Caribbean where many coral reefs have functionally collapsed, leaving virtually no live reef building corals at all.

“The worry is at some point – like an elastic band – the system snaps and doesn’t bounce back.”

Dr Morgan Pratchett

Will we get down to just 1% of all corals worldwide surviving if we surpass 2°, as was shockingly predicted in a landmark IPCC report in 2019? I speak with the Dr David Mead, who set up the influential Reef Restoration and Adaptation Program and had oversight of their ongoing modelling. On their models, that level of annihilation is not unlikely if we hit 2°. Terry Hughes doesn’t think it will be quite as dire because of some natural selection for heat resistant genes, but it’s still, as he says “a ticking time bomb”.

A bomb that’s ticking away again as summer approaches.  Lizard Island water temperatures are already 2.5 degrees above what they should be…

All the scientists I spoke with feel they are trying to help cradle the reef through frightening times, so if we do manage to stabilise temperatures between 1.5° and 2.0°, there’s still enough viable coral to recover in some new steady form.

Morgan Pratchett sums it up. “The reef system is still functioning – so long as we do everything we can to stop the corals dying fast.”

So what are they doing that has any real chance of “saving” the reef?

Read part two as my Great Barrier Reef investigation continues.

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