Climate change is physically reshaping the Great Barrier Reef, a new study shows, and parts of the reef system are likely in the midst of an irreversible decline.

Scientists found that coral bleaching that hit the Great Barrier Reef during a marine heat wave in 2016 transformed the structure of large swaths of the reef system, likely forever.

While previous research had shown widespread coral die-off in the reef that year, the new paper, published in the journal Nature, is the first to systematically link the mortality of different coral species to water temperatures. It found that about 30 percent of the Great Barrier Reef lost at least two-thirds of its coral cover in response to the 2016 event.

"When you lose that much coral, it's the ecological collapse of that reef system, at least for now," said Mark Eakin, coordinator of Coral Reef Watch at the National Oceanic and Atmospheric Administration and a co-author of the paper. "It'll stay that way if the reef does not have long enough to recover."

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When water temperatures rise far enough above normal, coral species expel the symbiotic algae that live on them and give the corals their bright colors. Bleaching weakens the coral, making it more susceptible to disease and death.

As global temperatures surged to record highs over the past few years, warming ocean water brought the most extensive and longest-lasting bleaching on record. Some research has suggested that climate change has started overwhelming even healthy reefs.

The Coral Species Hit Hardest Are Vital for Other Marine Life

The new study looked at what happened to specific coral colonies in the Great Barrier Reef system off Australia in the aftermath of the bleaching of 2016, and found that die-offs occurred with even less heat stress than expected. The worst-hit sections—in the northern part of the 1,400 mile-long reef system—saw the coral cover decline by more than 80 percent.

The die-offs didn't hit all species equally. The authors found that faster-growing, branching species such as staghorn coral were particularly hard hit. These species also harbor much of the ecological diversity of the reef, so their loss could have profound implications for the fish and other creatures that inhabit those waters.

"It was a flattening or homogenization of the coral reef ecosystem," Eakin said. "That has an impact on the rest of the ecosystem."

Coral Bleaching Is Happening More Often

The multi-year bleaching event that damaged reefs in several parts of the world has abated, but its effects could linger for years. A recent study by many of the same authors found that bleaching events that once occurred every 25 or 30 years a few decades ago are now happening every six years on average.

The likelihood of a full recovery of the Great Barrier Reef's corals is poor, the study said, in part because many of the surviving coral colonies were weakened so much that they continue to slowly die. The reef experienced severe bleaching again in 2017.

"Even in the least disturbed and healthiest reef system, after a severe mortality event like this it takes 15 years for the fastest growing corals to come back," Eakin said. "Unless we get climate change under control, we're going to see marine heat waves killing corals more quickly than the systems can recover."