Credit: XL Catlin Seaview Survey Soft coral decomposing and falling off the reef, captured by the XL Catlin Seaview Survey at Lizard Island on the Great Barrier Reef in May 2016.

Coral reefs, already reeling from a two-year global bleaching event that has left large swaths of ocean biomes dying or dead, will likely continue to suffer during a third year of warmer oceans, researchers warned Monday.

Key reefs, particularly those around the U.S., will likely face widespread destruction as the third global bleaching event in recorded history continues, the National Oceanic and Atmospheric Administration said.

Warm oceans have wrought havoc on the planet's coral since mid-2014, spurred by a particularly strong El Nino, which waned earlier this year.

“This is the most widespread, longest coral bleaching event ever to occur," Mark Eakin, coordinator of NOAA's Coral Reef Watch, said during a news conference.

Bleaching occurs when the sensitive algae that live within coral leave, usually when oceans become too warm. The colorful structures turn ghostly white, and if temperatures don't ease, coral often die.

Such an event along the Great Barrier Reef off Australia's coast has caused 93 percent of the structure's coral to show signs of bleaching. More than one-third of the coral in some parts of the Great Barrier has already died.

Credit: XL Catlin Seaview Survey A dying Giant Clam on the Great Barrier Reef.

NOAA scientists said reefs around Palau and Micronesia have a 90 percent chance of showing signs of bleaching this year. Hawaii, Guam, the U.S. Virgin Islands and areas throughout the Gulf of Mexico are also on high alert for damage through fall.

More than 70 percent of U.S. reefs have already been exposed to long-lasting high temperatures that can cause bleaching, NOAA said.

Scientists have linked the growing intensity of bleaching events to climate change. Man-made global warming has warmed the oceans, causing seas to rise, ice sheets to melt and corals to bleach. When events like El Nino show up, effects are compounded and can cause worldwide events, like the one ongoing. Two previous bleaching events were in 1998 and 2010.

“Rising temperatures due to climate change have pushed corals beyond their tolerance levels,” Ove Hoegh-Guldberg, director of Australia's Global Change Institute, told The New York Times.

Coral can recover from bleaching if water temperatures drop back to normal. But scientists have warned repetitive, years-long bleaching events act as a one-two punch to a species already beaten down from past traumas.

The state of coral reefs, scientists say, is an early indicator of planetary health and an urgent call to halt climate change.

Coral "is the bellwether for the changes that are occurring in our oceans," said Jennifer Koss, NOAA's coral reef conservation program director. "If you think of coral as the canary, they're chirping really loudly right now. ... The ones' that are still alive, that is."