Australia's Great Barrier Reef has lost more than half its coral cover in the past 27 years due to storms, predatory starfish and bleaching linked to climate change, a study found Tuesday.
The research by scientists from the Australian Institute of Marine Sciences (AIMS) and the University of Wollongong warned that coral cover on the heritage-listed reef -- the world's largest -- could halve again by 2022 if trends continued.
Intense tropical cyclones -- 34 in total since 1985 -- were responsible for much of the damage, accounting for 48 percent, with outbreaks of the coral-feeding crown-of-thorns starfish linked to 42 percent.
Two severe coral bleaching events in 1998 and 2002 due to ocean warming also had "major detrimental impacts" on the central and northern parts of the reef, the study found, putting the impact at 10 percent.
Two-thirds of the loss had occurred since 1998, with the rate of decline increasing substantially and only three of the 214 individual reefs surveyed across the sprawling 345,000 square kilometre site escaping any impact.
"This loss of over half of initial cover is of great concern, signifying habitat loss for the tens of thousands of species associated with tropical coral reefs," the study said.
Author Hugh Sweatman said the findings, which were drawn from the world's largest ever reef monitoring project involving 2,258 surveys over 27 years, showed that coral could recover from such trauma.
"But recovery takes 10-20 years. At present, the intervals between the disturbances are generally too short for full recovery and that's causing the long-term losses," Sweatman said.
The study said cyclone intensities were increasing as the world's oceans warmed and bleaching deaths would "almost certainly increase" as a result of climate changes.
"The recent frequency and intensity of mass coral bleaching are of major concern, and are directly attributable to rising atmospheric greenhouse gases," it said.
"Mitigation of global warming and ocean acidification is essential for the future of the great barrier reef."
AIMS chief John Gunn said it was difficult to stop the storms and bleaching but researchers could focus their short-term efforts on the large, poisonous and spiny starfish, which feasts on coral polyps and can devastate reef cover.
The study said improving water quality was key to controlling starfish outbreaks, with increased agricultural run-off such as fertiliser along the reef coast causing algal blooms that starfish larvae feed on.
"We can't stop the storms but perhaps we can stop the starfish," said Gunn.
"If we can, then the reef will have more opportunity to adapt to the challenges of rising sea temperatures and ocean acidification."
He said researchers would try to "better predict and reduce the periodic population explosions" of the starfish and explore whether direct intervention methods could be useful.
According to the study, published in Proceedings of the National Academy of Sciences, coral cover would be able to rejuvenate by 0.89 percent every year without the starfish.
"So even with losses due to cyclones and bleaching there should be slow recovery," said Gunn.
UNESCO warned it was considering listing the reef as a heritage site in danger earlier this year due to the unprecedented gas and coal mining boom in northern Australia and increasing coastal development.
