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THE Great Barrier Reef has been so severely damaged by record ocean heat that it will never be the same again in our lifetimes or those of our grandchildren. With ever hotter ocean heatwaves set to occur every few years, the reef will have no chance to recover fully.
鈥淚n 30 years鈥� time, we鈥檒l still have a reef, but it will look very different,鈥� says Terry Hughes at James Cook University in Australia, whose team has conducted surveys of the reef to assess the damage.
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We already knew that the iconic reef was badly damaged by recent heat events. Hughes鈥檚 surveys show that the corals started dying at far lower levels of heat stress than expected. They also show that the structure of a third of the 4000 individual reefs that make up the Great Barrier Reef has been degraded, altering ecosystems.
The current damage began with a fierce ocean heatwave in early 2016, which directly killed many corals. Overall, 30 per cent of coral cover was lost, making it the worst die-off on record. A second heatwave at the start of 2017 then killed another 20 per cent. While some areas have , corals are still dying in the worst-hit regions.
Alarmingly, the corals鈥� tolerance of short periods of very high sea temperatures or of longer periods of less severe heat was just half as much as forecast by NASA and other research teams (Nature, ).
The corals also died faster than predicted. After sea surface temperatures reached record levels in March 2016, for example, millions of corals perished in just two weeks. 鈥淭hey simply cooked,鈥� says Hughes. 鈥淲e鈥檇 never seen anything like it.鈥�
Others died more slowly over the following months after bleaching 鈥� expelling the algae that provide much of their food, and also give them their colour.
Some coral species were harder hit than others. For example, in the northern section of the reef, which was worst affected in 2016, more than three-quarters of staghorn and table corals were wiped out, whereas most dome-shaped corals emerged unscathed.
This is problematic because dome-shaped corals don鈥檛 provide the same protection to fish as intricate staghorn and table corals, says Hughes: 鈥淭hey don鈥檛 create the same nooks and crannies for hiding in.鈥�
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This shift has already affected fish diversity, according to . They found a sharp decline in the number of butterflyfish, for example, which are highly dependent on staghorns.
In the best-case scenario, some of the corals will bounce back, says Hughes. So far, 2018 has been mild, allowing surviving fragments to start re-sprouting.
These survivors may be better equipped to handle future extremes, says Hughes. 鈥淭he ones left over should be tougher because they鈥檝e survived two mass casualty events now,鈥� he says. 鈥淚t鈥檚 basically been a huge natural selection event.鈥�
Line Bay at the Australian Institute of Marine Science agrees this is possible. Her team is investigating whether the corals that survived have genetic variants that help them cope better with heat stress.
However, even if the surviving corals can tolerate the same levels of heat that the reef endured in 2016 and 2017, they could struggle as ocean temperatures rise above that under climate change. Mass die-offs are likely to continue for at least the next century.
This means the reef won鈥檛 get a chance to recover fully. Even the fastest-growing corals take a decade to reach their full size, says Hughes, and severe heatwaves are predicted to strike every five years or so due to global warming. The mix of species making up a reef will also continue to shift, he says.
鈥淭he corals left over should be tougher because they鈥檝e survived two mass casualty events now鈥�
The fate of the Great Barrier Reef depends on how quickly we slow climate change by reducing greenhouse gas emissions, says Hughes. 鈥淭he endpoint for the reef won鈥檛 be extinction if we get our act together now,鈥� he says.
In January, the Australian government pledged an extra A$60 million in funding to help the reef. This will go towards removing crown-of-thorns starfish that eat the coral, reducing pollution running into the sea from nearby farms and researching ways to transplant healthy corals onto damaged parts of the reef.
The latter is controversial, says David Suggett at the University of Technology Sydney. 鈥淭here鈥檚 a school of thought that says we shouldn鈥檛 be interfering with the reef, we should just be solving the underlying climate change problem,鈥� he says. 鈥淏ut if you think of the reef like a patient with a terminal disease, I think it鈥檚 worth looking for ways to treat the symptoms while we鈥檙e still looking for the cure.鈥�
Selina Ward at the University of Queensland welcomes the extra funding, but says the Australian government isn鈥檛 doing enough to tackle greenhouse gas emissions.
Australia is doing less than most other rich countries to reduce emissions. Its government also supports coal projects, including the planned Carmichael coal mine just 300 kilometres from the Great Barrier Reef.
鈥淲e can鈥檛 have our government saying, 鈥榣et鈥檚 wait 10 years, let鈥檚 approve this huge coal mine, let鈥檚 extend the life of coal-fired power stations鈥�,鈥� says Ward. 鈥淭he reef just doesn鈥檛 have time for that.鈥�
This article appeared in print under the headline 鈥淏arrier reef鈥檚 last gasp鈥�
