Not So Great Barrier Reef. Half of 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 reef investigation by scientists at Townsville's Australian Institute of Marine Science found that of the 50% destroyed coral 24% had been wiped out by intense tropical cyclones, 21% by Crown of Thorns Starfish outbreaks and 5% by coral bleaching.

Even if nothing changes for the worse in the next 20 years the reef will be in a perilous state. Global warming models project increases in water temperatures will lead to more intense cyclones. The frequency of Crown of Thorns outbreaks has increased from one in every 50-80 years before European agricultural runoff, to the currently observed frequency of one in every 15 years. Runoff waters carrying fertilisers and other agricultural nutrients into the ocean were thought to increase the survival of Crown of Thorns larvae by encouraging the growth of algae eaten by the offspring. 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 growing frequency and intensity of mass coral bleaching is directly attributable to rising atmospheric greenhouse gases.

"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."

At 214 reef sites surveyed, the coral cover halved from 28% to 13.8% between 1985 and 2012. Two-thirds of the loss occurred since 1998. Only three of the 214 reef sites exhibited no impact. 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.