The message has been repeated over and over this past year: The Great Barrier Reef is in trouble. Three major bleaching events since 1998 have left the reef in dire straits with another currently underway. As Damien Cave and Justin Gillis report for The New York Times, a new study of recent die offs details this damage, suggesting that the only way to prevent future severe bleaching is reducing global warming.

In 1998, a major bleaching event struck 43 percent of the 1,400-mile reef and in 2002, another 56 percent was hit reports Dennis Normile at Science. And just last year, 55 percent of the reef was severely impacted. In the latest study, published in the journal Nature, researchers combined observations during these three events, examining how the reefs responded after being bleached multiple times. Terry Hughes, who led the study, and his colleagues surveyed the reef from helicopters and coordinated with dive teams to investigate the current damage.

Bleaching happens during times of stress—like when water temperatures get too high—when coral polyps expel the specialized algae called zooxanthellae which give them their vibrant colors as well as important nutrients. Without the zooxanthellae, the coral appear bone white and can only survive so long—if the tiny creatures do not recolonize relatively soon after bleaching, the coral will die.

According to the latest study, only about nine percent of the Great Barrier Reef has avoided a bleaching event since 1998. While past researchers hypothesized that improved water quality could help coral bounce back from these events, according to the paper, water quality seems to offer no protection from bleaching during the last three events.

It was also believed that surviving bleaching could make coral species more resilient, helping them endure future events. While the researchers found that some types of coral did survive mild or moderate bleaching events better than others, they also found that severe bleaching, like the 2016 event, impacted coral species across the board whether or not they'd survived past bleaching events.

The study concludes that the increased frequency and severity of recent bleaching means the coral has little time to regenerate before the next bleaching event hits. That type of repeated bleaching means the reef does not have time to reach maturity, meaning the overall structure of the reef likely suffers permanent changes.

So what can be done? The paper concludes that there is only one solution: “urgent and rapid action to reduce global warming.”

The Great Barrier Reef is now undergoing another bleaching event this month, though it is not believed to be as severe as last year's. “None of us were expecting the water to be heating up again right now,” reef researcher Julia Baum of the University of Victoria in Canada tells Kristen Gelineau at the AP "I think it's beyond what any of us could have imagined. It's our worst nightmare.”

This year, the hardest hit regions are in the central reef—an area that escaped the worst of the bleaching last year.