The 2,300-kilometre (1,400-mile) World Heritage-listed reef suffered its most severe bleaching on record last year due to warming sea temperatures during March and April.
Scientists said today the impact will accelerate unless global greenhouse gas emissions are cut.
WHY DOES CORAL BLEACHING HAPPEN?
Corals have a symbiotic relationship with a tiny marine algae called 'zooxanthellae' that live inside and nourish them.
When sea surface temperatures rise, corals expel the colourful algae. The loss of the algae causes them to bleach and turn white.
While mildly bleached corals can recover if the temperature drops and the algae return, severely bleached corals die.
Initial aerial and in-water surveys showed 22 per cent of shallow water corals were destroyed in 2016, but it has now been bumped up to 29 per cent and with the reef currently experiencing an unprecedented second straight year of bleaching, the outlook is grim.
'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,' Great Barrier Reef Marine Park Authority (GBRMPA) chairman Russell Reichelt said.
'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 finalised, it's expected we'll also see an overall further coral cover decline by the end of 2017.'
Bleaching, which occurs when abnormal conditions such as warmer sea temperatures cause corals to expel tiny photosynthetic algae, draining them of their colour, also extended to deeper corals beyond depths divers can typically survey.
But mortality of those reefs could not be systematically assessed.)
Great Barrier Reef can no longer be saved
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