“Almost none of this reef has made it through 2015 and 2016,” Cobb said, calling the event “the wholesale destruction of the reef.”

By any measure, 2016 was not a good year for coral reefs. El Niño raised ocean temperatures worldwide, devastating corals the world over. The Great Barrier Reef—the sprawling system off the coast of Australia, and among the world’s most biodiverse reef systems—suffered a particularly debilitating year. Miles and miles of the coral reef bleached so severely, and for so long, that they died.

On Monday, news broke that it happened again. For the second year in a row, warm ocean temperatures are bleaching the Great Barrier Reef. The white splotches of ocean floor indicative of the phenomenon run even farther south—some 500 kilometers—than they did last year. The bleaching occurred even though there is no worldwide El Niño this year: The reef is ailed not by a rare climatic phenomenon but by the baseline warming of the oceans.

Until this decade, back-to-back bleaching events like that simply didn’t happen.

“It’s new. It is so new. It’s a complete change in the phenomenon that all of us study,” said Ruth Gates, a professor at the Hawaii Institute of Marine Biology and the president of the International Society for Reef Studies. “We knew that this day would come—we’ve been seeing the thermal-tolerance threshold for corals get closer and closer, and we knew it was pushing over the limit for coral survival.”

“There will now be years where it doesn’t take an El Niño event to reach the bleaching threshold. This is going to be statistically more likely in a warming world,” said Cobb.

The intensity and duration of bleaching events is ultimately leading to a change in the study of coral reefs overall. Instead of focusing on reefs in situ, scientists are increasingly having to study how reefs recover from warming oceans and other forms of environmental disaster.

“We are in a different moment with coral reefs right now. We’ve had this global insult on reefs. The choice now is to study recovery because that’s what we are doing, because that’s what we have to do,” said Gates.

The reef that Gates knows best—the coral reef in Kāne’ohe Bay, right next to the institute where she works—was one of the first in the world to suffer a back-to-back bleaching. In 2014, a warming Pacific pushed the Kāne’ohe Bay corals to warm; in 2015, the sea bleached them again. “We were not really expecting it to be a bleaching year then and we didn't expect it to be a bleaching year the following year,” she told me.

Since then, she has been monitoring the health of the reef and watched it recover. Scientists still don’t know how repeated bleaching events—especially in back-to-back years—will affect the long-term health of a coral reef. Kāne’ohe Bay has recovered faster and more vigorously than Gates expected, but it is a considerably less biodiverse reef than the Great Barrier Reef. Much of Gates’ research focuses on expanding coral resilience between reefs. (There was a wonderful New Yorker profile on her work last year.)