The scientist who was branded alarmist for exposing the fate of coral reefs

Ove Hoegh-Guldberg was just 10 years old when he first saw the Great Barrier Reef. That year, 1969, most young kids around the world were getting their inspiration from Nasa’s mission to put an astronaut on the moon. But for Hoegh-Guldberg, the fine grey dust of the lunar surface had nothing on the other world below the gentle Queensland waves.

He remembers the copperband butterfly fish and its fringing iridescent colours whose beauty “defied logic”, as well as the “incredible” epaulette shark that uses its fins to walk on the sea bed.

These days, however, diving on reefs comes with the burden of knowledge that Hoegh-Guldberg’s 10-year-old self didn’t have.

“Maybe my depression is because ... I’m feeling a sense of failure,” he tells Guardian Australia in his home city of Brisbane.

The pioneering coral scientist is looking at 2023’s record-breaking temperatures in the ocean around the globe and is taking it personally.

In what is becoming an all-too-regular event, coral reefs like those he has spent a lifetime researching are turning white in the northern hemisphere. He’s nervous about what the coming summer might bring for the Great Barrier Reef.

“It’s 40 years of trying to get the science in place to solve the problems,” he says. “And with sea temperatures literally going off the rails, it really starts to look like we haven’t done it.”

For any objective onlooker, Hoegh-Guldberg’s career has been anything but a failure.

A pioneer of the scientific understanding of coral bleaching, the University of Queensland professor has written more than 400 scientific papers. His work has helped shape the world’s understanding of the risks the ocean’s richest ecosystems – home to a quarter of all marine species – face from global heating.

Related: ‘Hope has to be a strategy’: the scientist who refused to let the climate warmongers win

Hoegh-Guldberg was starting his PhD in California in the early 1980s when reports were starting to emerge of coral reefs turning white across wide areas.

Was it a disease? Was it pollution? Was it caused by excess sunlight? Were corals reacting to a change in the saltiness of water? “Everyone was sort of speculating, but no one had done the experiment,” he says.

In a series of what he calls “cooking experiments”, Hoegh-Guldberg took coral fragments and subjected them to different conditions in the lab.

What he and his colleagues found was that corals had a temperature threshold. Once those temperatures are breached, the corals start to expel the tiny algae that live inside them and give corals their colour and much of their nutrients.

He first saw a major bleaching event for himself in 1994 in Tahiti. The reef was so bright, he could see the bleaching from the boat before he got in the water. Hoegh-Guldberg says locals told him they had no term in Polynesia to describe what was happening.

Branded an alarmist

As the 1990s drew to a close, more bleaching events were being recorded and their severity was on the rise. In 1998, corals all around the world bleached.

“So the question is: how long before this becomes a problem?” says Hoegh-Guldberg. At the time, he thought the answer could be a century away.

But he took the outputs of climate models and matched them with the temperature thresholds of corals.

Rather than having a century or more, the models suggested as early as the 2020s some reefs could be bleaching six or more times a decade – a frequency far too high to give them time to recover.

“I thought I must have made a mistake. I didn’t believe it. I talked to the climate people that were giving me the support in terms of the models. And sure enough, no matter what way you cut it, you got bleaching events every year by 2040 ... 2050.”

It’s almost like you get an ulcer because you’re always on guard. That can build up over time and sort of depress you

Ove Hoegh-Guldberg

Hoegh-Guldberg wrote up the results in a paper. “Events as severe as the 1998 event, the worst on record, are likely to become commonplace within 20 years,” he wrote.

His findings were met with a storm of criticism. Some of his scientific colleagues thought he had gone too far, and in the conservative media he was branded an alarmist. He got threatening emails calling him a communist and saying they hoped he would die.

He was emboldened and confident in his science, but privately it affected him.

“It’s almost like you get an ulcer because you’re always on guard,” he says. “That can build up over time and sort of depress you. I’m a really very optimistic person. But it does knock you around a bit, there’s no doubt about it.”

‘Was there something I could have done?”

In 2022, the Great Barrier Reef saw its sixth mass bleaching. It was the first to take place in a supposedly cooler La Niña year and the fourth in the space of six years.

“Was there something I could have done?” he asks. “You know, could I have glued myself to a gate somewhere?”

But the thought of turning to activism comes and goes quickly. He’s more useful to the world, he says, as “the bald professor who comes along and talks about the details”.

The bleached coral at Heron Island on the Great Barrier Reef.
Ove Hoegh-Guldberg says the world heritage committee should have placed the Great Barrier Reef on its list of sites in danger. ‘If it walks like a duck and sounds like a duck …’ Photograph: AFP/Getty Images

Hoegh-Guldberg has spoken to governments and royalty about the crisis facing reefs (both the literal kind, such as the Prince of Monaco and the now King Charles, and the figurative, Sir David Attenborough). He has given his expertise to climate court cases, to multiple United Nations climate reports and government committees.

What gives him reason to stay optimistic, he says, is that some reefs around the world appear to be less exposed to global heating than others, thanks to the quirks of ocean currents. Concentrating on protecting those reefs from other impacts could allow them to hang on long enough for governments to cut greenhouse gas emissions and for temperatures to stabilise.

“If you can do that, then you start to preserve the stock,” he says.

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He believes, though, that if reefs are to be saved, it will take many generations before conditions return to those he remembers as a child. “We’ve got to think about this as multigenerational responses that we need to commit to,” he says.

For the Great Barrier Reef, he believes the world heritage committee should have put it on a list of sites in danger, despite consecutive governments lobbying against it.

Clearly, the reef is in danger. “If it walks like a duck and it sounds like a duck … ,” he says. “I think if you start to play with words you’re not doing a good service to the debate.”

He says the Australian government is still showing a “sort of schizophrenia” in allowing new fossil fuel projects to go ahead while claiming to be taking action on climate change.

“This is a planetary emergency,” he says. “This is so crucial to humankind. We’re not going to live in bubbles in the future. You know, we have to find some way of re-engaging with nature sooner rather than later.”

• In Weight of the world: a climate scientist’s burden, we hear how three pioneering scientists made their discoveries, the personal toll it took on them, and how, during the hottest year on record, they stay hopeful. Experience the full series here