Widespread coral “bleaching” in the iconic Great Barrier Reef has forced Australian authorities overseeing the region to raise their response level to “severe” — the highest in their coral bleaching response plan. The move was triggered by recent underwater surveys in the Cape York region, which showed that some reefs in the region had suffered up to 50 percent mortality.

The true extent of the bleaching event on the barrier reef is still largely unclear as the severity of bleaching varies greatly across the approximately 133,000 square miles occupied by the Great Barrier Reef Marine Park.

“We still have many more reefs to survey to gauge the full impact of bleaching, however unfortunately, the further north we go from Cooktown, the more coral mortality we’re finding. The corals in the remote far north of the Reef experienced extremely hot and still conditions this summer, and were effectively bathed in warm water for months, creating heat stress that they could no longer cope with,” the Marine Park Authority’s Chairman Russell Reichelt said in a statement released Sunday. “A level three response level means we’re stepping up surveys in response to the coral mortality to help us better understand the effects of various pressures on the Reef and help guide management actions.”

Coral reefs, which are delicate marine biodiversity hotspots where corals grow in symbiosis with algae, become bleached when they are stressed — due to a rise in ocean temperatures, acidity, or both. This expels the symbiotic algae living in corals’ tissues, causing them to turn completely white.

The Great Barrier Reef, which is the largest living structure on the planet, consists of nearly 3,000 individual reefs and stretches over 1,400 miles along Australia’s northeast coast. The region serves as a habitat for nearly 100 species of jellyfish, 3000 varieties of molluscs, 1,600 types of fish, over 130 varieties of sharks and rays, and more than 30 species of whales and dolphins.

“We can turn this around. The Reef can recover but we must speed up the shift to clean, renewable energy and we must build reef resilience by reducing runoff pollution from farms and land clearing,” Richard Leck, a spokesman for the World Wide Fund for Nature (WWF) — which, on Monday, released a video showing the extent of bleaching in near the reef’s Lizard Island — said in a statement.

“Australia must speed up the transition to clean energy — like solar and wind — by setting a target of 100 percent renewable electricity by 2035. And a legal cap on chemical pollution running into the Reef's waters would build Reef resilience and help it survive while the world tackles climate change,” Leck said.