Great reef catastrophe
Australian Institute of Marine Science senior scientist Hugh Sweatman says the reef's health has decreased dramatically.
Half the Great Barrier Reef's coral has disappeared in the past 27 years and less than a quarter could be left within a decade unless action is taken, a landmark study has found.
A long-term investigation of the reef by scientists at Townsville's Australian Institute of Marine Science found coral had been wiped out by intense tropical cyclones, a native species of starfish and coral bleaching.
The big concern going forward is that if nothing else changes than within 20 years the reef could be in a perilous state.
Researchers warned that while the World Heritage listed reef was a dynamic system — with coral cover rising and falling over time — if the mass die-off continued less than 25 per cent would exist in 2022.
"The big concern going forward is that if nothing else changes than within 20 years the reef could be in a perilous state," said institute senior scientist Peter Doherty .
At 214 reef sites surveyed, the coral cover halved from 28 to 13.8 per cent between 1985 and 2012.
Two-thirds of the loss occurred since 1998. Only three of the 214 reef sites exhibited no impact.
Coral damage on the Great Barrier Reef
Bleaching at Halfway Island. Photo: Ray Berkelmans, AIMS.
"Coral cover is the simplest index of reef health, and the health of the Great Barrier Reef has gone down dramatically," said institute senior scientist Hugh Sweatman.
"The coral provides shelter and food for thousands of organisms so you don't just lose the corals themselves you lose the species that depend on them."
The coral damage was most pronounced in the central and southern regions of the 2000-kilometre reef, with the remote northern section remaining largely unaffected.
Tropical cyclones accounted for 48 per cent of the coral die-off across the entire reef, followed by outbreaks of the crown of thorns starfish, which was responsible for 42 per cent of the loss. Bleaching contributed to 10 per cent of loss.
"You can dive on a coral reef one week when all you can see is living coral, each colony overlapping with its neighbours, but after the passage of a cyclone it looks like a cement road," said Dr Doherty.
Global warming models project increases in water temperatures will lead to more intense cyclones.
While crown of thorns starfish were a natural predator of coral — the adult animals feed on tiny polyps inside the coral skeleton — their impact over the past 25 years had been substantial.
A large outbreak that started around Lizard Island in 1994, spread the length of the reef over 15 years.
Flood waters carrying fertilisers and other agricultural nutrients into the ocean were thought to increase the survival of crown of thorns larvae because the runoff encouraged the growth of algae eaten by the offspring.
A crown of thorns starfish.
"The frequency of crown of thorns outbreaks on the Great Barrier Reef has likely increased from one in every 50-80 years before European agricultural runoff, to the currently observed frequency of one in about every 15 years," said the authors.
Warmer waters were also responsible for coral-bleaching events, where the tiny organisms living inside the coral skeleton "bleached" and died with the rising temperatures.
"The recent frequency and intensity of mass coral bleaching are of major concern, and are directly attributable to rising atmospheric greenhouse gases," wrote the authors, whose study is published in the journal Proceedings of the National Academy of Sciences.
"Bleaching mortality will almost certainly increase in the GBR, given the upward trend in temperatures," they said.
While the state of the world's longest reef system appeared bleak, it did have the ability to recover.
"What it needs is a decade or two to do it in," said Dr Sweatman.
"Right now the cyclones, the crown of thorns and the bleaching events are coming so thick and fast that they're not getting enough time to recover."
While storms and bleaching events could not be controlled in the short term, direct action on the crown of thorns starfish would offer an immediate remedy for the reef.
"If we could stop crown of thorn outbreaks tomorrow then we could get the coral cover on the reef back to 1985 condition in less than 20 years," said Dr Doherty.