The news has gone from bad to worse to "very concerning" for Australia's Great Barrier Reef, with the world's largest reef system suffering through a second consecutive coral bleaching event that is proving even more destructive than first predicted.

Coral bleaching takes place as a result of abnormal sea conditions, including both warmer and colder temperatures, which place stress on the algae living inside the coral. This can cause the coral to expel the algae, which is critical to its health, leaving them white, withering and in danger of dying.

Prior to 2017, the Great Barrier Reef had suffered through three major bleaching events in modern history – 1998, 2002 and 2016 – and underwater and aerial surveys earlier this year indicated that 2017 would offer little reprieve, with scientists confirming back-to-back bleaching events were taking place. They had maintained hope that things would cool off quickly, but further surveys have now revealed that seems unlikely, along with the true extent of the current damage.

Scientists from the Great Barrier Reef Marine Park Authority have confirmed that 29 percent of shallow water corals died from the bleaching in 2016, an increase on the 22 percent they had predicted midway through that year. Deeper coral was also affected, but divers are unable to systematically assess mortality rates at those depths.

The scientists note that most of the mortality was confined to a northern section north of Port Douglas, and that there is evidence of strong recovery in the southern part of the reef due to the absence of bleaching and other impacts. These include the destructive crown-of-thorns starfish, which feast on the coral and make it more susceptible to storms, another impact, such as Cyclone Debbie that struck the reef in late March.

Warming oceans are expected to increase both the severity and frequency of mass bleaching events. According to the US Environmental Protection Agency, global sea surface temperatures rose at an average rate of 0.13° F (0.07° C) per decade between 1901 and 2014.

"We're very concerned about what this means for the Great Barrier Reef itself and what it means for the communities and industries that depend on it," said Great Barrier Reef Marine Park Authority Chairman Russell Reichelt. "The amount of coral that died from bleaching in 2016 is up from our original estimates and, at this stage, although reports are still being finalized, it's expected we'll also see an overall further coral cover decline by the end of 2017."

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