The news keeps getting worse for the the world's greatest coral reef system. Fresh on the heels of news that most of the Great Barrier Reef (GBR) has bleached comes the announcement that more than half of the coral in the reef has died this summer. Prospects look grim for most of the rest.

When corals are stressed by disease, pollution, or overheating, they expel their symbiotic microalgae. Microalgae give corals their beautiful colors. Without them, they become bright white in a process known as bleaching. Bleached corals are in danger, but not yet dead. If the source of their stress passes quickly, they can absorb new symbionts – sometimes finding microalgae more resistant to the stressor.

Professor Ove Hoegh-Guldberg of the University of Queensland, who has studied coral for over three decades, told IFLScience

Professor Ove Hoegh-Guldberg of the University of Queensland, who has studied coral bleaching over the last three decades, told IFLScience: “The symbionts are crucial to corals, passing on 90 percent of the energy they trap from sunlight to their host. Without its principal food source, coral is outcompeted by other organisms.”

If the bleaching event lasts too long, the corals become overgrown by opportunistic species that form the basis of far less productive ecosystems, which can be hard to displace once established. “The white corals become a scuzzy brown-green,” Hoegh-Guldberg said.

The contrast between a dead coral and one that is bleached but still alive is very clear. Ove Hoegh-Guldberg, Global Change Institute, University of Queensland

Bleached corals are so bright that aerial surveys show 93 percent bleaching. Picking up signs of coral death is harder, but Hoegh-Guldberg told IFLScience: “Dive teams have been looking at sample locations and are seeing well over 50 percent coral deaths.”

The extent of the damage varies with how far, and how long, temperatures exceeded normal maxima. “Inshore reefs where water has ponded have higher mortalities,” Hoegh-Guldberg said. “Where there are more currents, temperatures have been lower, but even a lot of the outer edge reefs have been very affected.”

The southern winter will bring relief, but it may come too late to save more than a small fraction of what was once a wonder of the world.

“From the tip of Cape York to the Whitsundays, the Great Barrier Reef in the east to the Kimberleys in the west and Sydney Harbor in the south, Australia’s corals are bleaching like never before,” Hoegh-Guldberg said in a statement. “This is the worst coral bleaching episode in Australia’s history, with reports of coral dying in places that we thought would be protected from rising temperatures.”

Bad as the news is, Hoegh-Guldberg does not think the reef is beyond salvation. “We will definitely see a degraded reef,” he told IFLScience. “However, if the world stops pumping out more CO 2 , temperatures will stabilize. Corals will be rare, but if we have not wiped them out entirely, they will eventually come back.”

Hoegh-Guldberg has led past studies protecting small reefs using shade cloth, something he said may be viable around tourist resorts, and replanting reefs with coral bred for heat tolerance. “The Great Barrier Reef is the size of Italy, so to contemplate replacing corals that have been lost is unrealistic,” he said. “However, if we grasp the problem of stopping our emissions, the problem is soluble.”