This story has been updated.

Last year the Great Barrier Reef — the largest coral structure on Earth — saw unprecedented bleaching due to extremely warm ocean temperatures. In major parts of the remote northern sector of the reef, two-thirds of the corals ultimately died.

This was the reef’s third and worst severe bleaching event — prior events occurred in 1998 and 2002. But now, scientists say, yet another event is unfolding that is also quite severe, meaning that the reef is experiencing its first back-to-back bleaching in two successive years.

“This one won’t be as bad as 2016, but it could be more comparable to 1998 or 2002,” said Terry Hughes, the lead author of the new study and director of the ARC Centre of Excellence in Coral Reef Studies at James Cook University in Townsville, Australia. “It’s an open question whether it’s the third- or second-most-severe.”

Bleaching occurs when stresses from temperature cause coral cells to push out the colorful algae, called zooxanthellae, that they rely on for energy. It isn’t an automatic death sentence for coral — that depends on whether the bleaching is mild or severe and how long the corals go without the algae. The hotter the waters and the longer they’re around, the greater the stresses and the more likely the corals are to succumb to them.

Hughes spoke from Australia on Tuesday as he was poised to take off on a seven-day flying mission to survey the damage across the enormous reef, which in total area is nearly the size of Montana. That’s the same thing he did last year when he and fellow scientists initially documented the extent of the severe bleaching event on the reef. In the final analysis, their surveys and additional research produced this map showing the dramatic consequences of the 2016 event:

Hughes is also, at the same time, the first author of a major paper in the journal Nature published Wednesday that surveys the bleaching of the reef so far and the lessons we can learn from that for extremely vulnerable corals in a fast changing climate.

The results of that analysis — which has no fewer than 46 scientific authors — are sobering.

“Globally what’s been happening is the number of these bleaching events is going up and up, and the time interval between them is shrinking,” said Hughes. “The end game if we don’t deal with emissions would be annual bleaching every year.”

The reef couldn’t withstand that because corals take many years to recover after severe bleaching that kills many individual animals — 10 to 15 years in many cases, but depending on the species, as much as 50 years. The likely result is that ever-more-frequent bleaching events will sharply change the composition of species living on reefs, creating a situation of “winners and losers,” Hughes said.

The study more or less says, therefore, that the Great Barrier Reef cannot be brought back to its state prior to the 2016 bleaching event.

“The assemblage structure of corals is now likely to be permanently shifted at severely bleached locations in the northern Great Barrier Reef,” the authors write.

Their study also has another punchline: No matter what you try to do to care for coral reefs locally, the researchers suggest that you can’t protect them from global warming without controlling temperatures themselves.

The Great Barrier Reef, much like the major U.S. reef located off the Florida Keys, is carefully protected by governments and individuals who try to preserve the quality of the water in the area and to defend the corals themselves from tourists and fishing activities. But the new study finds that, while these measures are clearly important, ever-warmer ocean temperatures will override any local attempts to make reefs more resilient.

“On the remote northern Great Barrier Reef, hundreds of individual reefs were severely bleached in 2016 regardless of whether they were zoned as no-entry, no-fishing, or open to fishing, and irrespective of inshore–offshore differences in water quality,” the study notes.

What this essentially means is that as the climate goes, so go coral reefs. The current, periodic bleachings are happening with about 1 degree Celsius of warming — and the world has pledged to keep warming to considerably below 2 degrees under the Paris climate accord.

Upon such progress do coral reefs depend, Hughes says.

“I’m confident that we’ll still have coral reefs if we can keep below 2. I don’t think we’ll keep below 1½,” he said. “I think we’ve got a narrowing window of opportunity, to put it optimistically — or to put it pessimistically, we’re running out of time.”