The tourist brochure version: scuba diving on the Great Barrier Reef At least until this latest one. This time more than 1000 kilometres of reef has been subjected to some extent of bleaching. The pristine northern stretches between Cooktown and the Torres Strait have been hit the hardest, with images emerging of ghostly white reefs from places such as Lizard Island. The event's spread and intensity has again raised uncomfortable questions about the damage climate change is doing to Australia's most important natural tourism site. Professor Ove Hoegh-Guldberg, a prominent marine scientist who has studied coral reefs for decades, says he has little doubt about what is behind the bleaching.

A diver checking out the bleaching at Heron Island in February 2016. Credit:XL Catlin Seaview Survey "This event, I would say with 99 per cent certainty, is being driven by anthropogenic climate change," he says. Heron Island sits towards the southern end of the Great Barrier Reef's 2300-kilometre reach. It takes a stomach-turning two-hour boat ride from Gladstone to get there. Bleached coral at Heron Island on the Great Barrier Reef. Credit:Eddie Jim When you arrive though the plentiful corals surrounding the island look like the Reef" we all imagine – bathed in turquoise water, colourful and littered with endless fish, rays, sharks and turtles.

Dr Selina Ward, a coral reef ecologist at the University of Queensland, says Heron's biodiversity is a stark reminder there is still much to lose if the Great Barrier Reef is not looked after. Looking dead flat at Heron Island. Credit:Eddie Jim Bleaching does not necessarily lead to death. If water temperatures drop in time corals can start rebuilding their algae and recover within months. "We've lost 50 per cent of coral cover on the reef in 27 years, mostly due to cyclones, crown of thorns and bleaching," she says. There is still enough coral left to turn the reef around, she says, but only if we do right by it.

Bleached Coral at Heron Island. Credit:Eddie Jim Across the planet ocean temperatures have risen as a result of global warming. In Australia, average sea surface temperatures are a degree higher than in 1910. These elevated temperatures have driven corals closer to thresholds where bleaching conditions occur. When a weather event like El Nino emerges that threshold is often exceeded. Coral off Heron Island. Credit:Eddie Jim "The mass bleaching is a result of climate change and a strong El Nino exacerbating high sea surface temperatures that usually occur at this time of year," says Dr Russell Reichelt, chairman of Great Barrier Reef Marine Park Authority.

"This temperature trifecta has created heat stress and pushed corals beyond their ability to cope." Bleached coral off Heron Island. Credit:Eddie Jim Coral animals, called polyps, have a crucial symbiotic relationship with algae that live in their cells and provide them with energy. When hard corals are stressed they expel this algae, which turns their tissue translucent, and exposes their white skeletons. This leaves them vulnerable to starvation, disease, and potentially death. Dr Selina Ward, senior lecturer, School of Biological Sciences, University of Queensland, checking out the acropora aspera coral at Heron Island. Credit:Eddie Jim

Dr Paul Marshall, the former head of the climate change program at the marine park authority and now an adjunct professor at The University of Queensland, says the link between coral stress, bleaching and warming oceans is not in dispute. "It is simple equation, you heat up the atmosphere, you heat up the oceans. You heat up the oceans, the corals get stressed. All of the causal links are absolutely robust," he says. Beyond warming seas, climate change poses other problems for the reef, including increased ocean acidity, turbocharged storms and sea-level rise. Ward says rising ocean acidification is particularly troubling for corals because it undercuts their ability to lay skeleton, which forms the basis of reefs, and makes them more brittle and vulnerable to erosion. These climate pressures interact with each other, multiplying the risks. And they present problems not just for coral, but also for fish, other marine life and the overall ability of the ecosystem to function.

They also come against a backdrop of other environmental problems hampering the reef's overall resilience. Water pollution from farming runoff is chief among them. Marshall says of the climate threats, bleaching can do the most immediate damage. When it is severe enough, corals hundreds of years old can die. "That's something that has been really depressing for me," Marshall says. "There are corals that were here when Captain Cook sailed by, and they're dying under our watch and they're not coming back in anyone's lifetime." Bleaching does not necessarily lead to death. If water temperatures drop in time corals can start rebuilding their algae and recover within months.

Some coral species – there are over 600 on the reef – are also naturally hardier than others, so rather than total die-off bleaching can lead to species switching instead. According to the Australian Institute of Marine Science about 55 per cent of the reef was bleached during the 2002 event. About 5 per cent died. Given the right conditions new corals can also rebuild on dead reefs in about a decade. One study of 21 reefs in the Seychelles, on which 90 per cent of corals died during a 1998 global bleaching event, found that 12 were eventually restored. Ultimately it will take weeks for the full extent of the current bleaching event to emerge. The levels of coral mortality may not be known for longer still. Reichelt says: "experience tells us that corals can recover from major disturbances, but they need to be given time and the right conditions to do so."

The concern is that if coral bleaching occurs more frequently, as it is projected to do, reefs won't have sufficient time to recover. Back in 1999 Hoegh-Guldberg forecast that rising greenhouse gas emissions would see most oceans become too hot for corals on a yearly basis by the 2040s and 2050s. In the most recent major assessment of the reef's health by the marine park authority reported that under moderate future emissions, bleaching conditions could be expected about once every five years for most parts of the reef by 2018. By 2052 to 2067 it could occur every year. It means some changes to the reef because of climate change appear inevitable (though there is some scientific inquiry into whether corals may be able to adapt somewhat to warmer and more acidic oceans).

Marshall says by 2050 the reef will ultimately be a different place and he paints four possible scenarios: Good water quality, lower emissions Under this future some areas escape the worst damage and big and old corals survive on some sites. Corals are able to grow and maintain themselves, but will be in a recovery phase more regularly. The reef can support diverse fish communities and habitat for other marine life. Overall there is less coral cover, more algae, more open space. But it is still beautiful. Good water quality, high emissions

Here not much of the old corals remain, with smaller, faster growing ones surviving instead. There is more open space and a lot of seaweed in it. The big "architecture" of the reef no longer exists, reducing its ability to support other marine life, especially the larger fish. This is akin to moving from an old growth forest to scrubby regrowth next to a farm field. Poor water quality, lower emissions A "dynamic tension" is established between seaweed and corals. Smaller corals grow among the seaweed, both competing for the same space. The seaweed increasingly gans the advantage as coral bleaching becomes more frequent. This environment still supports some marine life, such as smaller fish. But it is not beautiful.

Poor water quality, high emissions An effective wipeout. Only a fairly flat reef structure remains with very few corals growing and a lot of seaweed. Constant stress from coral bleaching and high levels of fertiliser and sediments entering from the land means corals are largely replaced by seaweeds, and the reef is unable to provide habitat for much fish life at all. None of these futures are ideal. Marshall says the sad reality is that future generations will inherit a coral reef that doesn't match the travel brochures of the 20th century. But he adds smart decisions today "can still secure a beautiful, productive reef for future generations." Here is where it gets tricky.

The federal and Queensland governments are already attempting, with mixed success, to address water pollution problems through multibillion-dollar commitments to tackle pesticide runoff from farms and damaging crown of thorns infestations. But what will largely decide the reef's fate is how fast the world cuts emissions and how high global temperatures are allowed to rise. Reichelt points to a consensus statement by the International Society for Reef Studies released last year which argues the average global temperature increase must be kept below two degrees in the short term, and below 1.5 degrees in the long term, to allow coral reefs to survive in perpetuity. It is in that light green groups are trying to link the fate of the Great Barrier Reef with new coal mine development. In their sights is the $21 billion Carmichael project in Queensland's Galilee Basin, which is backed by Indian company Adani and if built, would be Australia's largest coal mine. Last Sunday the Queensland government signed off on mining licences for Adani, which still needs to attract finance for the Carmichael project. Green groups argued this was a moment of enormous cognitive dissonance given the mass bleaching on the reef.

The Australia Conservation Foundation, which flew media to Heron Island this week to press its arguments, is also trying to make the link legally. It has launched court action to try overturn environmental approvals for Carmichael, arguing the federal government should have taken into account the damage that would be inflicted on the reef from the emissions from burning the mined coal once it was shipped to India The Queensland Resources Council rejects this connection. A spokeswoman says the emissions associated with the coal would be lower than other energy sources like "burning dung for cooking, which is one of the many high-emitting fuels that 300 million Indians without power are using." To say Carmichael coal would not have any impact on the reef requires some rejection of the well-established link between burning fossil fuels and global warming. But nor is it in isolation enough to push the planet beyond two degrees of warming. A 2014 expert study into the pollution associated with Carmichael coal found it would use up 0.53-0.56 per cent of the remaining global emissions that can occur and still see the world still avoid exceeding two degrees.

The study also notes the Carmichael coal emissions is among the highest from any single project in the world. Marshall says if the Great Barrier Reef is stave off climate change then Carmichael is not the only equation. The entire planet will have to cut emissions from all sources. "That's a great thing to anchor it in," Marshall says of the Carmichael-reef debate, "but really it is much bigger than that." "The future of the Great Barrier Reef, and reefs everywhere, depend on society's ability to totally shift away from fossil fuels." Fairfax Media travelled to Heron Island courtesy of the Australian Conservation Foundation.