Last year – and again this year – the world rang with shock reports that mass coral bleaching was devastating Australia’s Great Barrier Reef (GBR), the largest and most famous of all coral ecosystems. Some despairing commentators pronounced the reef as almost or already dead, thanks to global warming. One media outlet even published its ‘obituary’.
At the other extreme, sceptics of man-made climate change dismissed the bleaching as either exaggerated or part of a natural cycle – or both. Adding to the confusion, many refutations of the GBR’s ‘death’ gave the impression that it is not in serious trouble. So who do we believe?
“Both are very far from the truth,” says David Wachenfeld, director of reef recovery at the Great Barrier Reef Marine Park Authority. “It’s not dead, and it’s not fine. It’s under extreme threat, parts of it are highly damaged, and it needs us to do more than we’re doing now. Collectively. Globally.”
One easily overlooked difficulty with discussing the GBR is its sheer size. Observations of one part twist easily into misleading conclusions about the whole. “The Great Barrier Reef is absolutely enormous,” says Wachenfeld. “I don’t think many people can understand something at that scale. They think of it as a single tourism destination like they might think of the Eiffel Tower. But so far – and I’m crossing my fingers here – no cyclone, bleaching event or anything else has caused severe impact over the whole Barrier Reef.”
There’s a reason explorer Matthew Flinders termed it the Great Barrier Reefs – not Reef – back in 1814. The ‘Reef’ is actually 2900 individual reefs in an area roughly the same size as Malaysia, with enough coral acreage to cover metropolitan London 15 times. Most visitors experience only the tiniest fraction – 7% of the reef receives 83% of the tourism. Stretching 2300 km along eastern Australia, collectively it forms the world’s largest living structure. Those billions of tiny corals are our planet’s only fauna visible from Earth’s orbit.
Yet just a quarter of a century ago, it was even bigger. Total coral coverage has halved since the 1980s, according to the Australian Institute of Marine Science. Leading factors such as agricultural run-off degrading water quality, and plagues of coral-eating crown-of-thorns starfish – which thrive on run-off – are the subject of recovery plans, and progress is being made.
But now the GBR faces an existential crisis beyond the power of Australia to address alone. “Global warming is the number one threat to the Reef,” Wachenfeld announced to media this March.
Mass coral bleaching had just hit the GBR for a second year in a row, which has never happened before. Bleaching occurs when heat-stressed corals expel the algae zooxanthellae that live inside them in a symbiotic relationship and give them their colour. They do this because zooxanthellae, when over-exposed to heat and light, produce oxygen in toxic amounts. However, without zooxanthellae, corals turn white and – if water temperature stays too high too long – begin to die, needing zooxanthellae to supply nutrients and recycle waste products. After they bleach, these stressed corals either slowly regain their zooxanthellae and colour as temperatures cool off, or else they die. Some corals bleach after only four weeks of a 1°C rise, and start to die after eight weeks.
Coral reefs usually recover from occasional smaller-scale bleaching, but extreme coral bleaching events were unknown as little as 20 years ago. The GBR has now had four – 1998, 2002, 2016 and 2017. Last year’s was the worst yet, although it didn’t bleach 93% of the total coral, as was commonly assumed. This was a misreading of the finding that 93% of the 911 reefs inspected had at least some coral bleaching, from minor to extensive. Even so, the result was the biggest-ever recorded coral die-off, overwhelmingly in Far North Queensland.
By the year’s end, follow-up surveys showed an overall 22% mortality, with 67% of corals dead in 700 km of shallow-water reef north of Port Douglas, but only 6% in reefs in the central zone between Port Douglas and Mackay, and just 1% south of Mackay. Surviving corals were regaining colour, and the two areas most popular with tourists, Cairns and the Whitsunday Islands, weren’t seriously affected.
Still, the damage was heartbreaking, especially since the relatively remote north had always been the least affected by human impact as it sees less than 3% of GBR tourism. “We flew for 4000 km in the most pristine parts of the Reef and saw only four reefs that had no bleaching,” coral reef ecologist Professor Terry Hughes said of the northern survey, calling it “the saddest research trip of my life.”
Why the huge north–south divide? Do the cooler waters that southern corals inhabit offer better protection against heat-induced bleaching? No. “Corals are very tightly adapted to their local conditions,” says Wachenfeld. “A month at 1°C above average in February is a different temperature at Heron Island [south] than at Lizard Island [north] – it’s a lower temperature – but it’s still the same amount of stress to the corals.”
In other words, coral bleaches at cooler temperatures in the south than in the north. The south’s reprieve last year was Cyclone Winston, which brought a tropical rain depression that acted as a buffer, dropping sea temperatures below local norms and increasing cloud cover.
“We dodged a bullet,” says Sara Keltie, naturalist-guide at Heron Island, where reefs are still vibrant. Best known for nesting turtles and an eco-resort, Heron Island has the GBR’s oldest scientific research station, where marine biologists have been studying just how much climate change it can withstand.
Last year’s mass bleaching coincided with an El Niño, a natural climatic cycle featuring raised sea temperatures. However, Keltie says, because we’re warming the ocean with carbon dioxide (CO2), “corals are getting closer to their bleaching thresholds, so when an El Niño comes through, a greater proportion of the species are pushed over.”
Some have argued that the coral will eventually move south over time. According to Wachenfeld, this thinking ignores the impact that climate change has already had on the GBR. “Corals have been around for 400 million years,” he points out. “They’ve seen climate change before. But climate has never changed as quickly as we are changing it now. So the fact that animals have adapted and coped over geological time-scales in the past doesn’t mean they will in the future, now we’re changing things faster.”
“The second problem is that hundreds of millions of humans rely on coral reefs for food, for coastal protection from wave energy, for income from things like tourism. It’s no good to tell a human, look, your reef might die in 20 years because of climate change, but don’t worry – in 5000 years it might come back again.”
A third problem is the other threat to coral reefs posed by climate change, which is also happening too rapidly for corals to manage. “Climate change is making the ocean more acidic,” says Wachenfeld. “About 30% of the carbon dioxide we put into the atmosphere as we burn fossil fuel dissolves in the oceans.”
Carbon dioxide in seawater forms carbonic acid, which releases hydrogen ions that bond with the free-floating carbonate ions needed by hard corals to make their calcium carbonate skeletons. The more carbon dioxide in the sea, the less able hard corals are to build reefs. Hydrogen ions will even start dissolving hard coral and shells to get carbonate if the free-floating supply runs short.
On Heron Island, a long-term experiment has observed the impact of various acidities and temperatures on coral reef mesocosms (ecosystem replications). Two futures were tested – a 4°C rise in global temperature from the pre-industrial average, which is expected by the year 2100 if nothing is done to slow carbon dioxide emissions; and a 2°C rise, which is the cap on warming sought by the Paris Agreement, the international accord for addressing climate change that took effect last November. Ocean acidity from atmospheric carbon dioxide levels that would produce these temperatures was also examined.
“Every single coral in both future scenarios went bone-white,” says Keltie. “But they followed two different trajectories. In the do-nothing scenario, the corals starved to death and started to dissolve. The only organism that flourished was Cyanobacteria.”
While a 4°C future appears fatal, there is at least hope for coral reefs in a world that meets the Paris Agreement targets. “In the do-something scenario, some of the corals survived – slower-growing corals like boulder corals, which have lower energy demands,” says Keltie. “At the end of the experiment they were still growing and reproducing.”
For Wachenfeld, who sees the Paris Agreement as a positive step, world efforts can’t stop there. “None of the forecasts I’ve seen are below 2°C by 2100,” he points out. “The most recent consensus says in the short term global warming could get to +2°C, but to keep healthy reefs into the future we have to bring it back to +1.5°C. Even at 1.5, coral reefs will be under severe stress – the Reef has just had its worst-ever bleaching at about +1.
“The Reef is very much alive, but it’s also very much under pressure and desperately needs more help.”
Surveys this year will inspect 1150 reefs. Whether die-off is worse this time, or affects different areas, time will soon tell. And time is something the Great Barrier Reef could well be running out of.