What used to be the most pristine section of the Great Barrier Reef is experiencing “the worst mass bleaching event in its history,” which could have disastrous consequences for the marine ecosystem, scientists say.

Aerial surveys of more than 500 coral reefs in a stretch of the Coral Seafrom Cairns in northeastern Australia to Papua New Guinea show that the overwhelming majority of them are severely bleached, the ARC Centre of Excellence for Coral Reef Studies said Tuesday.

“This has been the saddest research trip of my life,” Terry Hughes, convenor of the National Coral Bleaching Taskforce, said in a news release.

“Almost without exception, every reef we flew across showed consistently high levels of bleaching, from the reef slope right up onto the top of the reef. We flew for 4,000 km in the most pristine parts of the Great Barrier Reef and saw only four reefs that had no bleaching. The severity is much greater than in earlier bleaching events in 2002 or 1998.”

The ARC Centre has released video captured during the aerial surveys, which clearly shows large swaths of bleached corals in the ocean. 

The bleaching occurs when rising sea temperatures cause the corals to expel tiny algae called zooxanthellae. The corals turn white once those algae are lost. They can recover from the bleaching if the sea temperature drops in time. Otherwise, the corals can die.

Along the Great Barrier Reef, which is home to approximately 1,500 species of fish, 4,000 types of mollusc and various other sea creatures, coral destruction can have a massive environmental impact. 

Hughes said that scientists are “already reporting up to 50 per cent mortality of bleached corals.” He said it’s too early to tell what the overall outcome will be as research teams continue to monitor the situation.

Scientists say that climate change has been causing abnormally high sea surface temperatures and causing corals to bleach during the summer months. Previous mass bleaching events along the Great Barrier Reef occurred in the summers of 1998 and 2002.

The Great Barrier Reef is listed as a UNESCO World Heritage Site for its “outstanding universal value.”