Earlier this year, scientists warned that the Great Barrier Reef could be on the brink of its most widespread bleaching event ever recorded. That fear has now been realized.

New surveys conducted by scientists at Australia’s James Cook University and the Great Barrier Reef Marine Park Authority show that a summer of extreme heat has caused the reef, which is a World Heritage Site, to suffer a mass bleaching of unprecedented scale. Corals from the far north to the southern tip of the 1,400 mile-long ecosystem are experiencing severe impacts.

It was also one of the reef’s worst mass bleaching episodes in terms of intensity, second only to 2016, which killed half of all shallow-water corals on the northern Great Barrier Reef.

And unlike the summer of 2016, when an intense marine heat wave coincided with one of the strongest El Niño events on record, this past summer brought a bleaching event without any assistance from the Pacific climate oscillation.

El Niño events can elevate ocean temperatures in that part of the world, making bleaching events more likely. To scientists, this is another clear sign that human-caused climate change is now the primary driver behind these devastating events.

Mark Eakin, coordinator for the National Oceanic and Atmospheric Administration’s Coral Reef Watch program, described the rate of recurrence of these events as “truly disturbing.” Bleaching from the 2016 event was followed by a recurrence in 2017, when there was also an absence of an El Niño.

“In 2016 and 2017, the Great Barrier Reef had their first back-to-back bleaching events. Now we have the third bleaching event in five years,” Eakin wrote in an email.

“That is unprecedented on the Great Barrier Reef.”

Heat stress can be deadly to corals. Bleaching is a response to heat stress that occurs when corals spend too much time in water that’s too hot for them to handle. Exposure to prolonged heat causes the reef-building animals to temporarily evict their zooxanthellae, symbiotic algae in which the corals shelter in exchange for food.