The massive bleaching hitting the Great Barrier Reef off the coast of Australia is likely that country's "biggest ever environmental disaster," says Dr. Justin Marshall, who has studied the reef for three decades.

Only 7 percent of the reef has escaped bleaching, according to researchers at the ARC Center of Excellence. Marshall, a professor at the University of Queensland, says the destructive phenomenon is happening in an area the size of Scotland.

"Before this mass bleaching started, we already were at the point of losing 50% of the coral cover on the Great Barrier Reef. This, I think, will probably take another 50% off what was left," Marshall says.

Over the course of the last six months, Marshall and his colleagues with the citizen science project Coral Watch have documented the degradation of reef structures near Lizard Island, one of the worst-hit areas.

They photographed the same formations of coral multiple times, showing clearly the pace of the destruction.