1000km stretch of the Great Barrier Reef devastated by coral bleaching

Ninety-five percent of the northern section of the Great Barrier Reef has been damaged by coral bleaching, an aerial survey has revealed.

"This will change the Great Barrier Reef forever," Professor Terry Hughes Terry Hughes, who led the survey team said told the ABC's 7.30.

The James Cook University academic said the bleaching affects around a 1000km stretch of the 2300km-long ecosystem.

Of 520 reefs he surveyed from Cairns to the Torres Strait only four were free of the bleaching, that turned the once colourful coral "snow white".

Professor Hughes could not say with any certainty how much of the bleached coral would die but expected to see at least half die in the coming months.

The bleaching can be caused by higher than average sea temperatures which stress the corals and cause them to expel or lose algal pigmentation that is key to their health.

Environment Minister Greg Hunt who saw the coral himself from an aircraft eight days ago said the flight showed him there was some "good news and bad news".

"The bottom three quarters of the reef is in strong condition," he said.

"[But] as we head north of Lizard Island it becomes increasingly prone to bleaching."

Professor Hughes said it could take ten years for surviving coral to return to normal but he also noted that the corals in the north of the reef were the most robust and had the greatest chance of recovering.

Reef scientist and University of Queensland Professor Justin Marshall said it was clear that the bleaching was a result of climate change.

"The world has agreed, this is climate change, we're seeing climate change play out across our reefs," he said.