A bleaching event is one of the fastest ways to kill a coral reef—and it’s the main way that corals react to the hotter oceans of climate change. Corals are tiny animals that live in huge, branching colonies of limestone. (Each “tree” of coral might contain hundreds of thousands of tiny, individual coral organisms.) Each coral polyp also contains a small amount of photosynthetic algae, which provides food to the coral and helps keep it healthy.

When the water gets just a few degrees warmer than usual, coral polyps expel their algae out of stress, and the entire branching colony turns white—that is, it bleaches. If the water does not cool down fast enough, the coral colony can then starve to death or get infected, and die. Even if the corals survive the episode, it takes about 10 years for them to fully recover.

“I studied coral reefs for 20 years before I first saw mass bleaching in 1998—the first global coral-bleaching event. It was also the first time that severe bleaching occurred along the Great Barrier Reef,” said Terry Hughes, an author of the new study and the director of the ARC Center of Excellence for Coral Reef Studies, the government agency that leads coral research in Australia.

“Since then then GBR has bleached three more times, in 2002, 2016 and 2017. We’ve gone from zero bleaching to back-to-back bleaching in just two decades,” he said.

The study finds that, in the early 1980s, bleaching events were rare, occurring at a rate of once every 25 to 30 years. By 2016, they had increased fivefold. Mass coral-bleaching events now strike about every six years, on average, far too fast for ecosystems to recover.

The region hardest hit, so far, are the reefs of the Caribbean and Western Atlantic, which include those in the U.S. Virgin Islands. The Western Atlantic began warming before other parts of the global ocean did, so more than half of the region’s reefs have bleached seven times since 1980. And the average reef in the Western Atlantic has bleached 10 times since 1980.

This devastation has targeted American reefs with particular ferocity. Half of all Caribbean coral reefs in the United States were lost in 2005, when sweltering waters swaddled Puerto Rico and the U.S. Virgin Islands.

Other parts of the world have so far avoided this blight. More than half of the reefs near Australia and in the Indian Ocean have bleached three times since 1980. But reefs in Australia seem to be getting worst fastest.

“The sense that everyone had, that, wow, this is a very different world than it was 30 years ago, was a correct impression,” said Lasker. “The nice thing about an analysis like this is it makes clear to people who make policy that this isn’t an impression, it’s actually occurring. It is important to do this.”

“Now, the impact it has on those officials ... that’s another issue,” he added.