Once upon a time, there was a city so dazzling and kaleidoscopic, so braided and water-rimmed, that it was often compared to a single living body. It clustered around a glimmering emerald spine, which astronauts could glimpse from orbit. It hid warm nooks and crannies, each a nursery for new life. It opened into radiant, iris-colored avenues, which tourists crossed oceans to see. The city was, the experts declared, the planet’s largest living structure.

Then, all at once, a kind of invisible wildfire overran the city. It consumed its avenues and neighborhoods, swallowed its canyons and branches. It expelled an uncountable number of dwellers from their homes. It was merciless: Even those who escaped the initial ravishment perished in the famine that followed.

Many people had loved the city, but none of them could protect it. No firefighters, no chemicals, no intervention of any kind could stop the destruction. As the heat plundered the city of its wealth, the experts could only respond with careful, mournful observation.

All of this recently happened, more or less, off the east coast of Australia. The Great Barrier Reef—which, at 1,400 miles long, is the longest and largest coral reef in the world—was blanketed by dangerously hot water in the summer of 2016. This heat strangled and starved the corals, causing what has been called “an unprecedented bleaching event.”

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.

A researcher assesses one area of the Great Barrier Reef that escaped with only minor damage after the 2016 bleaching event. (Gergely Torda / ARC Center of Excellence for Coral Reef Studies)

“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.”

“But the global-average target includes both land and sea, the poles and the tropics,” he continued. “The sea is warming at a slower rate than the land, and the tropics are warming more slowly than higher latitudes near the poles. So far, we’ve seen less than 1 degree of warming in the ocean—about 0.7 degrees Celsius. If we go to 2 degrees, we’ll see another 0.55 degrees on average in the tropics. I think that’s possibly doable in terms of still having functional coral reefs, but as we’ve already seen, the mix of species will be very different than it was just two years ago.”

It is about as hopeful a note as you can get out of him. And there’s one glaring problem: The world is currently not on track to meet the Paris Agreement goals. And the United States announced it would leave the treaty last year.

The new paper also advances its own idea of what the future of the Great Barrier Reef will look like. In short, it is highly unlikely that the Great Barrier Reef—the northern reef, especially—will resemble its old self in the lifetime of any living person.

“It’s like clear-cutting a redwood forest,” said Bruno, the marine biologist. “In 10 years, you’re going to have a lot of stuff on the ground, but you’re not going to have the old-growth forest back. Some of these corals were 10, 30 years old, but a lot of them were centuries old. In 100 years—if there is no more warming—they could return. But it’s a long time.”

“The most likely scenario,” write the authors of the paper, “is that coral reefs throughout the tropics will continue to degrade over the current century until climate change stabilizes, allowing remnant populations to reorganize into novel, heat-tolerant reef assemblages.”

“Novel, heat-tolerant reef assemblages”: That doesn’t exactly evoke the dazzling rainbow vistas familiar to viewers of Blue Planet or even Finding Nemo.

“I regret that I agree with the disturbing prognosis,” said Rebecca Vega Thurber, a coral scientist at Oregon State University who was not connected to the paper, in an email. “Unless we see evidence that these mass thermal anomalies are becoming less frequent or severe (which is counter to the evidence at hand), it is likely that the Great Barrier Reef (and reefs worldwide) will continue to degrade.”

How did this catastrophe happen so quickly? In part, this is just what heat can do. In late 2015, the Pacific Ocean lurched into one of the largest El Niño events ever recorded. El Niño is the shared name for a set of global weather symptoms that, in general, raise global temperatures and switch precipitation patterns.

El Niño–like conditions had struck the Great Barrier Reef thousands of times before, but the 2016 event had an accomplice: the slow, inexorable rise in global temperatures caused by climate change. Combined, global warming and El Niño sent ocean temperatures around the world soaring. Coral reefs in every ocean and hemisphere suffered in the blast.

It can be easy to forget, when gazing at their hard and gnarled exterior, that corals are animals. They do not make food on their own; like chimpanzees, golden retrievers, and sea cucumbers, corals have to find something to eat. But they approach this problem in a unique way. Tiny, photosynthetic algae live in coral tissue. The algae turn sunlight into food for corals, and corals give them a place to live in return. This symbiotic relationship between animal and plant undergirds every interaction in the tropical coral reef.

Warm water severs this symbiosis. When corals are exposed to heat, they expel the colorful algae from their tissue. To human eyes, this causes them to lose their color—they “bleach”—but it also robs them of their food source. If temperatures do not soon return to safe levels, the corals starve and die.

These staghorn corals off the coast of Guam appeared bleached in October 2017. (David Burdick / AP)

At least, that is the conventional thinking. About 50 percent of all the coral that perished in the 2016 bleaching event died in the autumn and winter, long after temperatures had returned to normal. Those corals never regained their algae after evicting them, and they slowly starved to death.

“But about half of the corals that died did so in March, at the peak of summer temperatures,” Hughes told me. “We were surprised that about half of that mortality occurred very quickly.”

In other words, some corals did not even survive long enough to starve. “They died instantly, of heat stress,” Hughes said. “They cooked.”

Based off their work observing all of the Great Barrier Reef’s 3,863 constituent reefs by satellite, and then surveying many of them in person, Hughes’s team also advances new guides for predicting when the reef as a whole will bleach. They use a unit called degree-heating weeks, which multiplies the length of a heating episode (measured in weeks) by the size of its temperature anomaly. So if the temperature of a reef rises 2 degrees Celsius above average—and that warm spell lasts for two weeks—then the spell was four degree-heating weeks. If the temperature of a reef rises by 4 degrees Celsius, but only for one week, then the spell also was also four degree-heating weeks.

Corals are shockingly fragile to even small amounts of warmth, they found. At two degree-heating weeks, most corals in a local area will mildly bleach. At four degree-heating weeks, corals in an area will start to die. And if a local reef hits six degree-heating weeks, it will suffer “catastrophic mortality” comparable to what happened in 2016, Hughes said.

These distinctions are more than academic. The International Union for Conservation of Nature, the global organization that declares species “endangered” or “threatened,” is currently creating a similar scale for ecosystems. Its leaders hope that soon individual ecosystems might be discussed as “endangered,” in the same way that giant pandas and mountain gorillas are.

“I would classify the Great Barrier Reef, and all coral reefs around the world, as being endangered due to climate change,” Hughes said.

This finding, while a central finding of coral-reef science, may take a long time to sink in in Australia. “A lot of people believed that because [the Great Barrier Reef] was super diverse and had no pollution and was remote, it was resistant to bleaching,” said Bruno. But the scale of the 2016 event shows that “bleaching has nothing to do with virtue or management or having a commitment to resilience.”

“It’s just warming,” he said. “Once it warms, it’s toast.”

The Great Barrier Reef is almost late to this moment: Reefs around the world have already gone through a similar transformation in the past several decades.

“Coral cover in the Caribbean has been trashed for 20 years,” Bruno told me. “But it wasn’t like that in the ’80s. In the mid-’80s, you could go snorkeling in Florida and it was like flying over a Kansas wheat field—golden coral for acres and acres and acres.”

“Now that’s totally gone,” he said. “Coral cover in the Florida Keys is like at 3 percent.”

“This single event represents the mass destructive force that global climate change can have on coral reefs,” said Thurber. “Since the Great Barrier Reef is, for many, our gold-standard reef, its precipitous decline is all the more disturbing to those studying both pristine corals reefs and those that are on the fringe.”

The study fits into a streak of dreary findings for coral reefs. If the world were to warm by an average of 2 degrees Celsius, then ocean temperatures would consistently exceed their 2016 levels, even during non–El Niño years, a recent study in Nature Climate Change found. In another study, released in January, scientists surveyed observations of 100 coral reefs around the world going back 35 years. They found that mass bleaching events now strike five times more often than they did in the early 1980s.

Hughes said that the news will continue to get worse until governments and their citizens focus on the “root cause” of the problem. The Great Barrier Reef will continue to collapse and die until humanity stabilizes the amount of greenhouse-gas pollution in the air. But fixing that problem will require remaking the energy system, moving away from oil and gas and to solar, wind, and other renewable sources.

“But the federal government of Australia, like the federal government of the United States, still very much favors the continued development of the fossil-fuel industry,” Hughes sighed. “And that, to me, is a complete policy failure for the Great Barrier Reef.”