Parts of Australia's Great Barrier Reef will never recover from the impact of unseasonably warm waters, scientists warned on Thursday.
Huge sections of the reef, stretching across hundreds of miles of its most pristine northern sector, were recently found to be dead, killed last year by overheated seawater.
Warm water caused the coral to expel living algae, triggering it to calcify and turn white, a process known as bleaching.
The bleaching in 2016 was the worst on record.
The research, published in the journal Nature, said bleaching events should no longer be studied individually, but as threats to the reef's survival.
"Given time, coral can recover from bleaching but the problem comes when you get repeated events. With less time between them, capacity for the coral reef community to recover diminishes rapidly," Janice Lough, senior principal research scientist at the Australian Institute of Marine Science, told Reuters.
Scientists described the recent disaster as the third worldwide mass bleaching of coral reefs since 1998, but by far the most widespread and damaging.
The conclusion is a major blow for Australia's tourism industry, with the reef attracting A$5.2 billion ($3.9 billion) in spending each year, a 2013 Deloitte Access Economics report estimated.
The outlook for the Great Barrier Reef has further darkened with evidence of an unprecedented second consecutive bleaching event this year, researchers at James Cook University said.
"We're hoping that the next two to three weeks will cool off quickly, and this year's bleaching won't be anything like last year," said Terry Hughes, director of ARC Centre of Excellence for Coral Reef Studies.