The Great Barrier Reef hides a massive world under the surface of the ocean. But while reefs may seem alien to us, they’re deeply tied to the rest of the Earth, and every year we’re they’re becoming increasingly vulnerable to climate change. A study in the journal Nature, published Wednesday, shows just how much warming oceans are endangering the Great Barrier Reef.

In the paper, researchers in the U.S. and Australia show that a heat wave in 2016 warmed ocean waters so much that 29 percent of the 3,863 reefs making up the Great Barrier Reef lost two-thirds or more of their coral. Using satellite imaging, the team, led by Terry Hughes, Ph.D., the director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland, Australia, tracked water temperatures around the reef, taking note of the areas where the temperature exceeded the temperature at which coral begins to get bleached. Correlating that data with the areas of the reef that had lost coral using the two maps below, the researchers came to an unsurprising, grim conclusion: The hotter the water, the sicker the coral.

coral heating and bleaching
These graphics show the change in coral cover between March 2016 and November 2016 (a), and degree heating weeks (a measure of heat stress) in the same area (b). The correlation between coral loss and excessive heat is clear.

While coral can survive some degree of heat variations, the sustained temperature increases in 2016 pushed much of the reef past the point of no return. “When corals bleach from a heatwave, they can either survive and regain their color slowly as the temperature drops, or they can die,” Hughes told the Guardian on Thursday. “Averaged across the whole Great Barrier Reef, we lost 30 percent of the corals in the 9-month period between March and November 2016.”

This study builds on a paper published by Hughes and his colleagues in 2017 that showed the extent of coral bleaching on the Great Barrier Reef by specifically mapping the relationship between water temperature change and coral loss. While the previous study showed a significant extent of coral bleaching on the Great Barrier Reef, the new paper shows that coral bleaching isn’t something that happens slowly over many years. It can happen in a single year, and it’s not clear how well the reefs can recover.

The Great Barrier Reef’s future depends, to a large degree, on two major factors: how resilient the coral is and how often these mass bleaching events occur. Coral can recover from mild bleaching, so many of the affected reefs could slowly grow back over decades. But if bleaching events happen during those periods, the situation could just get worse.

Unfortunately, with global sea surface temperatures on the rise, the odds are pretty high that there will be more of these mass bleaching events in our lifetimes. Only time will tell whether coral can adapt to this new normal.