Story highlights Only 7% of corals in the Great Barrier Reef unbleached, study finds

At some reefs, the "final death toll is likely to exceed 90%," scientist says

(CNN) A study measuring the extent of coral bleaching in Australia's iconic Great Barrier Reef is branding some of the northern reef's problem as "extreme."

As much as 93% of the 2,300 km (1,429 miles) reef suffers from some level of bleaching, according to the report from the ARC Centre of Excellence for Coral Reef Studies . Bleaching occurs when algae that live inside corals and give them their color are expelled -- either due to increased sea temperatures or extreme weather events.

"The bleaching is extreme in the 1,000 km (600 mile) region north of Port Douglas all the way up to the northern Torres Strait between Australia and Papua New Guinea," says Andrew Baird of the ARC Centre.

"At some reefs, the final death toll is likely to exceed 90%. When bleaching is this severe, it affects almost all coral species, including old, slow-growing corals that once lost will take decades or longer to return."