The Great Barrier Reef has lost half its coral cover in the last 27 years, and it could halve again by 2022 say researchers from the Australian Institute of Marine Science.

A study published today in the Proceedings of the National Academy of Sciences, found storm damage drove 48% of the loss, the crown of thorns starfish 42%, and bleaching 10%.

“This finding is based on the most comprehensive reef monitoring program in the world,” said Peter Doherty, research fellow at AIMS.

Dr Doherty said the monitoring included surveillance of more than 100 reefs since 1985 and from 1993 had incorporated more detailed annual surveys of 47 reefs.

The data show that reefs can regain their coral cover after severe events such as tropical cyclones, coral bleaching or crown of thorns starfish population explosions, 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,” said study author Hugh Sweatman.

Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland, agreed that the frequency of events was the key issue, and said action to address CO2 emissions was the only sensible remedy.

“What these results tell us is that the small change in the frequency of extreme events has a big impact on complex systems like the Great Barrier Reef, “ Dr Hoegh-Guldberg said.

He added that there was a climate component to all of the events, with warmer seas driving more intense storms, flood inundation events triggering crown of thorns starfish outbreaks, and mass coral bleaching driven by extreme heat events.

“The problem we are currently facing on the Great Barrier Reef is that today none of these stressors occur in isolation,” said coral biodiversity researcher Zoe Richards.

“This research clearly shows that over the last decade the rate at which coral cover has declined has accelerated and this reflects the cumulative impact of multiple stressors and the inability for communities to recover.”

“We can’t stop the storms, and ocean warming (the primary cause of coral bleaching) is one of the critical impacts of the global climate change,” said John Gunn, AIMS CEO.

“However, we can act to reduce the impact of crown of thorns,” he said. “The study shows that in the absence of crown of thorns, coral cover would increase at 0.89% per year, so even with losses due to cyclones and bleaching there should be slow recovery.”

Dr Hoegh-Guldberg said the only sensible and economic remedy was to reduce global CO2 emissions to zero within the next 10-20 years.

He said the connection between flood inundation and coastal run-off and the crown of thorns problem was very compelling.

“This makes an extremely large smoking gun: Warming of the planet increasing episodic flood volume which then impacts the food supply of larval crown of thorns starfish, which then leads to an increased frequency of outbreaks.”