When coral scientist Dr Zoe Richards left the Great Barrier Reef’s Lizard Island in late January, she was feeling optimistic.

Richards is a taxonomist. Since 2011 she has recorded and monitored 245 coral species at 14 locations around the island’s research station, about 270km north of Cairns.

In 2017 she saw “mass destruction of the reef”. Back-to-back mass bleaching in 2016 and 2017, and cyclones in 2014 and 2015, had wreaked havoc.

But in January, she saw thousands of new colonies of fast-growing Acropora corals that had “claimed the space” left by dead and degraded corals. In a three-year window without spiralling heat or churning cyclones, some corals were in an adolescent bloom – not mature enough to spawn, but getting close.

“It was an incredible recovery,” says Richards, of Curtin University. “But I knew if it was hit again, it would be trouble – and that’s exactly what happened.”

In 2020, mass bleaching returned to Lizard Island – perhaps not as badly as in previous years – but enough, says Richards, to turn the clock back on the recovery she had seen.

This summer has delivered a third mass bleaching for the reef in just five years. The back-to-back bleaching of 2016 and 2017 was mostly confined to the northern and central sections.

When bleaching is mild, corals can and do recover, although it can make them more susceptible to disease. But severe bleaching can kill corals. Estimates are that the 2016 bleaching killed about 29% of the reef’s shallow water corals and the 2017 event took another 19%.

Some scientists are now concerned global heating may have reached a point where tropical reefs bleach almost every year.

What this means for the reef in the coming decades is an area of live research and debate among scientists.

Can we fix it?

Scientists Guardian Australia spoke to say the reef’s fortunes hang on the answers to two questions.

The first is whether governments around the world will make deeper cuts to greenhouse gas emissions than they have already agreed and, if so, how close they will get to keeping global heating to 1.5C.

A second is whether efforts to first identify and then deploy a swathe of potential measures that could reduce the impact of rising temperatures will be successful.

What seems clear is that without some human intervention, the magic of the world’s greatest coral reef system will be lost.

Prof Peter Mumby, professor of coral reef ecology at the University of Queensland, is the chief scientist at the Great Barrier Reef Foundation – the once-small not-for-profit that was awarded a controversial $443m government grant in 2018.

He said the 2020 bleaching “is giving us greater pause, given it seems we can see quite frequent coral bleaching events earlier than people had previously expected”.

Mumby says bleaching events have been “patchy”, and the fact that some areas have escaped “means there’s an opportunity for management”.

What keeps the reef functioning as a single ecosystem is the way each reef connects to another through the way corals reproduce. They all either spawn, or produce larvae, that can float in the water column and settle on nearby reefs.

Mumby and colleagues have identified about 100 reefs along the GBR that are well spread, well connected to other reefs by ocean currents, and tend to experience cooler temperatures.

He says making sure those reefs stay as healthy as possible – in particular by managing outbreaks of the coral-eating crown of thorns starfish – could be crucial in keeping the wider reef viable.

The reef’s unrivalled size and diversity – almost 4,000 reefs, cays and islands stretching for more than 2,000 kilometres – gives it extra resilience, he says.

Climate change is still the reef’s biggest threat and society will need to focus on tackling it, “but there needs to be a way to adapt to how we manage reefs so that they can roll with the punches – we have to do both those things”.

The Australian Institute of Marine Sciences (AIMS) has produced an as-yet unpublished study, sent to the federal government, that reviews more than 160 different interventions that have been suggested for the reef, identifying about 40 that could be worth further study.

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Dr Lina Bay, a principal research scientists at AIMS, says one promising area of study is what’s known as “assisted gene flow”, where the spawn of corals with better tolerance for heat could be captured and then dispersed.

“Not all corals are created equal,” she says. “Some have a higher stress tolerance than others. Over many years we’ve shown that the variation in bleaching tolerance is hereditable – it gets passed from parents to offspring.”

She says these differences can exist even among the same species, meaning those corals can be selectively grown in a lab setting to promote more heat tolerance.

AIMS scientist Dr Neal Cantin has just finished a three-year experiment with one fast-growing coral species called Pocillopora acuta, which behaves like a weed by filling in the gaps when less hardy corals die off.

Starting with 90 parent specimens taken from three different parts of the Great Barrier Reef, Cantin and colleagues grew 7,500 offspring and then subjected them to rising levels of CO2 and temperatures of up to 2C warming.

Even at high temperatures, some of these corals survived, and they were able to tolerate higher levels of heat as the experiment went on.

Having a street-fighting weedy coral like this is important, says Cantin. Dead areas of coral reefs tend to get covered in algae, but Cantin says a weedy coral that can compete with the algae can then make room for slower-growing corals to also grow.

“The whole goal of a lot of these interventions is to work with species that can be successful on their own. We won’t be able to work with 600 species of corals, but we could probably work with 20 that fill the functional roles of a healthy reef community.

“You can’t deny bleaching events are becoming more frequent and more severe and they’re impacting across a bigger area than before. We can just document that demise, or we can learn from it and have some corals for future generations.”

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“It’s these silent extinctions that go on,” she says.

“The entire reef is operating like one big meta population with sub-populations that are connected to each other. If you successively take out nodes in that population, sooner or later you will end up with parts that don’t connect. It will be fragmented into subsets that will continue to erode in terms of diversity. It’s degradation of the [coral] community at a very large scale.”

‘At 3C, you basically have nothing’

Prof Ove Hoegh-Guldberg, of the University of Queensland, has done pioneering work on the study of coral bleaching going back to the mid-1980s.

He remembers Lizard Island as a “picture perfect” place to do research on corals in the late 80s, when his research there found rising temperatures caused corals to lose their “symbionts” – the algae that lives in the coral and gives them much of their nutrients and colour.

The Great Barrier Reef’s first major mass bleaching event happened in 1998. There was another in 2002, and again in 2016, 2017 and 2020.

Hoegh-Guldberg says: “We knew there was a temperature effect, and we knew that temperatures were going up. At the end of the 90s, I could put those two things together.”

The year after the reef’s first mass bleaching, Hoegh-Guldberg took climate models to forecast that if greenhouse gas emissions kept growing then, by 2020, “the average bleaching event is likely to be similar or greater than the 1998 event”.

As 2020 approached, the models showed reefs across the northern, central and southern regions would see between eight and 10 bleaching events per decade.

“I wished I’d been wrong” he says. “I think I said at the time that I’d have egg on my face if I was wrong. But there’s no egg on my face.”

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Hoegh-Guldberg says manually replanting corals is uneconomic at scale but there’s merit in helping the dispersal of coral larvae, pointing to a technique being developed by a scientist at Southern Cross University that captures millions of larvae in floating pools.

But he says the main game is keeping global heating down.

“Let’s say we get to 1.5C and then we can stabilise – that’s really the last call for reefs. Corals will come back and there will be winners and losers, but you’ll have a functional reef that supports fisheries and tourism.”

The problem is that right now, government pledges under the Paris agreement are enough to raise temperatures by 3C – not 1.5C.

“At 2C all the reef-building corals have plummeted and instead you are looking at the dominance of other organisms like algae. At 3C you basically have nothing.

“I’m fearful that in the next 10 years we will see the loss of coral across the planet at phenomenal rates,” he says. “That’s what keeps me up at night.”