Scientists from the Australian Institute of Marine Science in Townsville have found that the loss of coral is caused mainly by cyclones, and crown-of-thorns starfish.
Coral bleaching is also to blame.
The study released today in the Proceedings of the National Academy of Sciences, argues that stopping the progress of the crown-of-thorns starfish is crucial to the reef's recovery.
Presenter: Nance Haxton
Speakers: Jamie Oliver, researcher director of the Australian Institute of Marine Science; John Gunn, CEO of the Australian Institute of Marine Science
NANCE HAXTON: The Great Barrier Reef is the world's largest coral reef, stretching more than 3,000 kilometres up the Queensland coast from Bundaberg to the tip of Cape York.
But scientist John Gunn says the future of the ecosystem could be under threat if the loss of coral is not stopped.
JOHN GUNN: Accumulative impacts of storms and crown-of-thorns and two bleaching events have had a quite devastating effect over the last three decades.
NANCE HAXTON: Was it a surprise to the scientists how much of the reef has been lost?
JOHN GUNN: A surprise no, concerning, yes obviously. We're very concerned that this is a bit of a crossroads for the reef and these data are very authoritative.
NANCE HAXTON: What could this mean for the Great Barrier Reef, does it mean that it, would you describe it as under threat, essentially?
JOHN GUNN: I can't pretend that if we had this type of impact continuing and we had some of the possible impacts of climate change in the future that the Great Barrier Reef really is at threat.
NANCE HAXTON: John Gunn is the CEO of the Australian Institute of Marine Science in Townsville that has monitored the reef system over the past 27 years.
The resulting study released today found that just over half of the Great Barrier Reef's coral has died over that time. Mr Gunn says damage to the reef is patchy, with some areas affected more than others.
JOHN GUNN: So there are parts of the reef that are still pretty much as we'd like the whole of the reef to be and they give us some hope that that's what we could achieve with the whole of it and these are areas north of Cooktown and they're pretty healthy reefs, in fact they're beautiful.
It's the areas that really have these cumulative impacts, the three factors that we take account of in the study that have really come under sort of major pressure. But even there, there are reefs that are still very, very lovely to visit.
And I, it's for our tourism industry that brings $5 billion a year into the Australian economy, maintaining those beautiful reefs and making sure that others become healthier over time, is also a very, very important message.
NANCE HAXTON: Last week AM reported a Climate Commission study that found global warming was putting increasing pressure on the Great Barrier Reef, potentially causing more bleaching events.
AIMS research director Dr Jamie Oliver says stopping the crown-of-thorns starfish could be the key to the reef's long term survival.
JAMIE OLIVER: Now this is a native species which outbreaks in enormous proportions, killing off large proportions of the reef and this is something that we may be able to take some direction, action on.
If we can at least disrupt these outbreaks, that may give the reef a chance to recover from the other factors that we describe, such as cyclones and coral bleaching.
NANCE HAXTON: It sounds like that really is going to be the key factor in helping the Great Barrier Reef to recover.
JAMIE OLIVER: Indeed, I think a multi-pronged approach looking at the water quality, looking at direct action, perhaps increasing or improving, that is directly moving starfish or killing them around particular areas, together with looking at the impacts of disease and perhaps using that as control will make a big difference.
Basically, doing whatever we can to protect the reef and keep it strong.
NANCE HAXTON: Dr Oliver says the research clearly found that if left to its own devices, the ramifications for the Great Barrier Reef are dire.
JAMIE OLIVER: Left unchecked, if the trends continue, we would anticipate that they would reduce by another half by 2022. So this is something that we need to be able to take very seriously and to look at the causes before this decline and see what action can be taken.