Though that bleaching event was reported at the time, scientists are just starting to understand how catastrophically transformative it was. A new paper, published Wednesday in the journal Nature, serves as a kind of autopsy report for the debacle.

After inspecting every one of its reefs, and surveying them on an almost species-by-species basis, the paper reports that vast swaths of the Great Barrier Reef were permanently transformed in the summer of 2016. The reef’s northern third, previously its most pristine section, lost more than half of its corals. Two of its most recognizable creatures—the amber-colored staghorn corals, and the flat, fanlike tabular corals—suffered the worst casualties.

But damage was widespread out across the entire ecosystem.

“On average, across the Great Barrier Reef, one in three corals died in nine months,” said Terry Hughes, an author of the paper and the director of the ARC Center of Excellence for Coral Reef Studies, the Australian government’s federal research program devoted to corals.

“You could say [the ecosystem] has collapsed. You could say it has degraded. I wouldn’t say that’s wrong,” Hughes said. “A more neutral way of putting it is that it has transformed into a completely new system that looks differently, and behaves differently, and functions differently, than how it was three years ago.”

“It’s a confirmation of our worst fears,” said John Bruno, a marine biologist at the University of North Carolina who was not involved in the study.

Yet it was not the end of troubles for the Great Barrier Reef. In the summer months of 2017, warm waters again struck the reef and triggered another bleaching event. This time, the heat hit the reef’s middle third. Hughes and his team have not published a peer-reviewed paper on that event, but he shared early survey results with me.

Combined, he said, the back-to-back bleaching events killed one in every two corals in the Great Barrier Reef. It is a fact almost beyond comprehension: In the summer of 2015, more than 2 billion corals lived in the Great Barrier Reef. Half of them are now dead.

What caused the devastation? Hughes was clear: human-caused global warming. The accumulation of heat-trapping pollution in the atmosphere has raised the world’s average temperature, making the oceans hotter and less hospitable to fragile tropical corals.

“People often ask me, ‘Will we have a Great Barrier Reef in 50 years, or 100 years?’ And my answer is, yes, I certainly hope so—but it’s completely contingent on the near-future trajectory of greenhouse-gas emissions,” Hughes said.

The Paris Agreement on climate change aims to prevent the world’s average temperature from rising by 2 degrees Celsius (3.6 degrees Fahrenheit). “One degree of that warming has already occurred since industrialization,” Hughes said. “That 1 degree has obviously made things uncomfortable for reefs—most reefs have bleached three or four times since 1998.”