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From space the Great Barrier Reef looks like a necklace beaded with thousands of turquoise gemstones. To the left of the satellite imagery is the ochre red of the mainland, brushed with the green of forest and canefields; to the right, the deep blue of the ocean. In the middle lies the spectacularly long thread, some 2300 kilometres, stretching from the Torres Strait in the north to Fraser Island in the south. It is the world’s largest natural construction, comprising thousands of reefs and islands that sustain a constellation of marine life. Dolphins, sharks, seahorses and a brilliant multitude of fish. The world’s second-largest reef – the Belize in the Caribbean – is 300 kilometres long.

The Great Barrier Reef was born after the fifth and most recent global extinction of reefs. These previous extinctions were likely the result of natural, epochal calamity – volcanic eruptions, asteroid collision – which unleashed toxic levels of carbon dioxide and sunlight-obscuring sediment and ash clouds. Today, experts believe we may be on the cusp of a sixth, entirely unnatural extinction. “We are precipitating the conditions for a mass extinction,” Professor John “Charlie” Veron, the world’s foremost expert on coral reefs, said earlier this year.

“Unless we wake up and deal with climate change sincerely and deeply then we really will have a Great Barrier Reef not worth visiting.”

Veron has given his professional life to coral reefs. The former chief scientist of the Australian Institute of Marine Science, Veron has famously discovered almost a quarter of the globe’s coral species and compiled an exhaustive classification of all reefs. Over many decades he has developed a reputation for fierce scholarly independence and unbridled passion. Sir David Attenborough calls him a friend. But for some years now, Veron has grown dispirited – a man prophesying mass extinction to very few ears.

In July 2009, Veron gave a presentation to the Royal Society in London, the famous academy of science established in 1660. It was entitled “Is the Great Barrier Reef on Death Row?”, a question to which Veron gave the qualified answer of “yes”. He apologised to the audience for the gloomy forecast, but seven years later it’s being fulfilled. “The numbers are what I said they would be,” Veron tells me. “I used to have the best job in the world. Now it’s turned sour. The forecasts are abysmal. We haven’t deviated from the track [detailed in the 2009 presentation]. It’s ultra gloomy, but accurate. I’m 71 years old now, and I think I may outlive the reef.”

1 . Mass bleaching

The first recorded mass bleaching of coral reefs was in 1981, the same year UNESCO placed the Great Barrier Reef on its World Heritage List. Healthy coral is symbiotically entwined with algae – zooxanthellae – that provide its colour and, photosynthetically, its energy. But when water temperatures rise, the algae begin producing toxic levels of oxygen. At this point, the coral sheds the algae like a cloak, revealing its stark white skeleton. Bleached long enough, the coral will eventually starve to death, leaving behind a kind of rubble. Bleaching is not necessarily fatal – if safe temperatures are quickly restored, it may regenerate. But repeated bleaching damages coral’s reproductive capacity – its drifting, regenerating larvae – and weaken it. Coral is also a sort of canary in the coalmine – its parlous state a prelude to further ecological decline.

Increased water temperature is not the sole cause of coral bleaching. Water pollution, unsustainable fishing, crown-of-thorns starfish and the sediment provoked by cyclones can all contribute. However, increased ocean temperatures are recognised as a primary factor in bleaching events. The United States’ National Ocean Service reports: “In 2005, the US lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event. The warm waters centred around the northern Antilles near the Virgin Islands and Puerto Rico expanded southward. Comparison of satellite data from the previous 20 years confirmed that thermal stress from the 2005 event was greater than the previous 20 years combined.”

Since 1981, there have been mass global bleachings in 1997-98 and again in 2001-02. In addition, the Great Barrier Reef was stricken in 2006. Studies showed that each bleaching episode coincided with increased ocean temperatures. “It is widely accepted that the No. 1 threat to the Great Barrier Reef is climate change,” says Professor Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland.

There have been scores of surveys conducted by marine scientists, and there’s a distressing uniformity to them – the Great Barrier Reef is under enormous threat, and great swaths of it are already grossly damaged. Professor Terry Hughes of James Cook University is the convener of the National Coral Bleaching Taskforce, and he reported last month that: “We’ve never seen anything like this scale of bleaching before. In the northern Great Barrier Reef, it’s like 10 cyclones have come ashore all at once … We have now flown over 911 individual reefs in a helicopter and light plane, to map out the extent and severity of bleaching along the full 2300-kilometre length of the Great Barrier Reef. Of all the reefs we surveyed, only 7 per cent have escaped bleaching entirely. At the other end of the spectrum, between 60 and 100 per cent of corals are severely bleached on 316 reefs, nearly all in the northern half of the reef.”

Underwater surveys confirmed the aerial analysis. “The reality is that the whole northern section is trashed,” Charlie Veron tells me. “Up near Lizard Island there’s hardly any coral at all. It looks like a war zone. It’s heartbreaking. And we like to think that the southern part has escaped. It hasn’t – it’s just not as bad. It was also saved by an unusual weather pattern – a normal monsoon set in against the El Niño. But there’ll be another in four to seven years, and each cycle is more severe than the last.”

None of this has escaped international attention. Just last week, The Washington Post and The New York Times carried prominent pieces on the grim future of the reef.

2 . Assessment ignored

Earlier this year, Professor Ove Hoegh-Guldberg and Professor Will Steffen, head of the Climate Council, were approached by UNESCO to examine the scientific literature on the reef’s vulnerability – texts that would inform a UNESCO report examining how its World Heritage sites were being affected by climate change. Of the literature, Steffen told me: “It was in good shape and represented an accurate overview of the risks of climate change, focused on the effect of warming ocean waters on coral bleaching.”

But when Steffen and Hoegh-Guldberg saw the final report, they realised their assessment had been ignored – all references to the Great Barrier Reef had vanished. Last week, it was revealed that the Department of Environment had objected to all references to Australia, and demanded they be removed from the report. UNESCO obliged. Neither Steffen nor Hoegh-Guldberg were notified.

The department defended its actions to Guardian Australia last week, saying: “The department was concerned that the framing of the report confused two issues – the World Heritage status of the sites and risks arising from climate change and tourism.”

Steffen told me: “I haven’t found the department’s rationale very convincing. The report was very clear and accurate scientifically; it was not exaggerated, nor inflammatory. It even contained some text at the end that described some actions that the Australian government was taking to enhance the resilience of the Great Barrier Reef. This report was not about official designations – example ‘endangered’ – but simply outlined the risks posed by climate change on the basis of well-known and understood science.”

The move by the department has backfired – drawing greater attention both to the reef’s health and to the spectre of political interference with science. It’s extraordinary how frequently the Streisand effect – a canon of public relations wisdom that warns that attempts to quell information will be counterproductive – is ignored in political life. “I don’t think it was a good idea to remove this case study,” Hoegh-Guldberg tells me. “It’s a real lesson in making sure expert opinions aren’t clouded by this interference. And there are other ways to message this. The reef is in dire trouble, but it’s decades away before it’s no longer worth visiting. That’s the truth. But unless we wake up and deal with climate change sincerely and deeply then we really will have a Great Barrier Reef not worth visiting.”

It was precisely political interference with science that led Charlie Veron to resign his position as chief marine scientist under the Howard government. A decade on, he has lost none of his anger about the conditions that led to that resignation. “Science was being muzzled. So I left. There’s less money going solo, but I have my independence. And that’s crucial to a scientist. We don’t understand science in this country. It’s not a club. It’s not a religion. It’s the quest for accurate knowledge. Knowledge that is taken apart and put back together again.”

A great many scientists I’ve spoken to are gripped by an enchantment with the natural world that is unknown to most, and unsupportable by the contingencies of modern politics. For marine scientists, the reef is not a proxy in the interminable culture wars, but a beguiling place so intricately vital that it has sustained their curiosity and passion for decades. But this enlightened commitment is often ignored, jeered or vilified. “When I began in climate science,” Hoegh-Guldberg laughs, “I was accused of being a communist.”

It is painfully obvious that public discussions about climate change are polluted. Popularly and politically it has become a matter of ideology, not science. Charlie Veron tells me he believes that Australia and the United States are far behind the rest of the developed world in its acceptance of the science. It was a sentiment echoed by others I spoke to. But where some scientists see rhetorical restraint as a diplomatic necessity, Veron is blunt about his assessments of Canberra. “I spoke in Parliament House in Abbott’s time,” he tells me, “and there was just wall-to-wall denial about climate change. Except from Malcolm Turnbull. I spoke with him and he knew his stuff. He’d done his homework. Which makes his leadership all the doubly disappointing now.”

He moves on to the environment minister, who celebrated his department’s interference in the UN report this week by saying that “thanks to the Coalition” the reef was “off” the watch list.

“Now, I’ve spoken recently with Greg Hunt,” Veron tells me. “He rang me up. In that conversation I was gobsmacked by his lack of understanding. Like I was talking to a schoolkid. He’s emailing me now, and including staff, which is surprising because they reveal how ignorant he is. He politicises everything, which makes him very difficult to communicate with.”

Veron is aware of his stature in this country, and in conjunction with many other scientists has strove to have the reef’s health included in this federal campaign. There’s been success on that front, but he’s sceptical of Hunt’s proposals. “He’s announced funding for water quality and the [abatement] of crown-of-thorns. Well, water quality has nothing to do with it. It’s basically irrelevant. The bottom line is, each year the oceans are just a little bit warmer. El Niño gets more severe. The oceans equilibrate to warming, like a kettle equilibrating to the flame. And there’s a lag in this warming, of about 20 years. Even if we stopped emissions right now, the water temperatures would still rise.”

Labor has also pledged to improve water quality and fight swarms of crown-of-thorns, as part of a half-billion-dollar package. “The reef is in peril,” opposition leader Bill Shorten said in Cairns this week, and promised that the money would go to CSIRO research, “environmental investment and reef management”. But a private Queensland government report, leaked this week to the ABC, suggests that sufficiently improving water quality would cost $16 billion. The Turnbull government set aside $170 million in this year’s budget.

Compounding the headache is the fact that the worst damage to the reef is in remote parts where the question of “water quality” is moot – Charlie Veron argues that the pledges ignore the principal issue of carbon emissions and global warming.

At 71, Veron is still strident, curious and bewitched by coral. But he is also melancholic. As he did with his 2009 Royal Society audience, he apologises to me for his heavy heart.