The worst coral bleaching event on record is destroying the most pristine part of the Great Barrier Reef in a "slow-motion train wreck", a reef expert has warned.
Professor Terry Hughes, the convenor of the National Coral Bleaching Taskforce, has just returned from aerial surveys that show devastating levels of bleaching in the northern part of the reef, from Cairns to the top of Cape York and beyond.
"The north has fried," Prof Hughes told AAP.
"This is an ongoing, slow-motion train wreck."
Of 520 reefs surveyed between Cairns and Papua New Guinea in recent days, only four appeared to be unaffected.
And 95 per cent of those reefs have been ranked in the two most severe categories of bleaching, meaning at least 30 per cent of their coral is affected, and in a vast number of cases more than 60 per cent.
The damage in the northern part of the reef far surpasses the previous worst bleaching event in 2002 when 18 per cent of reefs were ranked in the two most severe categories.
Coral bleaching is predominantly caused by abnormally warm sea temperatures.
