For the last three weeks, surveyors with CoralWatch at the University of Queensland have recorded some of the most pristine coral reefs in northern Australia undergoing an epic bleaching event. Divers near Lizard Island on the Great Barrier Reef north of Cooktown report that vast stretches of coral, roughly 90 to 100 percent, have turned bone white. And similar bleaching has also struck other nearby reef areas. This finding led the Great Barrier Reef Marine Park Authority, to launch its highest “Level 3” response to the event, which means increased monitoring of the coral.

Bleaching does not mean the coral has died. Instead, when water temperatures rise too much, the coral rejects tiny algae called zooxanthellae, reports Karl Mathiesen for The Guardian. Healthy coral cultivate the zooxanthellae, which provide most of the coral's food. The zooxanthellae also imparts color on the reef, so their loss leaves behind a bleached, bone white reef.

Coral can recover from mild bleaching events. But if temperatures remain high for too long, the coral will die. High temperatures also encourage other algae to move in, smothering the reef and preventing new coral from colonizing.

According to Tom Arup at the Sydney Morning Herald, ocean temperatures in northern Australia have been one degree Centigrade (1.8 degrees Fahrenheit) above normal since January, leading to the bleaching event. “I have never seen coral this heavily bleached,” Professor Justin Marshall, head of CoralWatch told Arup. “And we are seeing algae growing on parts, which means it has died.”

This most recent bleaching event is part of a devastating global trend, writes Brian Clark Howard for National Geographic, with both climate change and El Niño to blame. The recent bleaching isn't limited to the Great Barrier Reef, over the last year 12 percent of the world’s reefs have already bleached, and half of them may never recover, Clark Howard reports.

This most recent bleaching is similar to others that struck in 1997 and 1998 in which 16 percent of earth’s coral reefs were impacted, Professor Nick Graham, a marine ecologist at Lancaster University told Mathiesen. “This is the big one that we’ve been waiting for,” he says.

Models show that the ocean warming trend will likely last until early 2017, impacting the Indian Ocean and other parts of the Pacific, according to Graham. If ocean tempertures continue to warm or El Niño events become more frequent, the coral may not have time to regenerate.

Higher temperatures is just one threat corals face. Ocean acidification caused by climate change, damage from fishing trawlers, and chemical dumping are all taking a toll on the earth’s reefs.

Russell Reichelt, chairman of the Great Barrier Reef Park Authority says he’s hopeful that things will improve over the coming weeks as the arrival of the wet season cools off ocean temperatures. But in a press release, he says the event is a wake-up call.

“The health and future of the Great Barrier Reef is a priority for us—bleaching reinforces the need for us to continue working with our partners to improve the Reef’s resilience to give it the best possible chance of dealing with climate change impacts,” he says.