This September 2015 photo shows partially bleached coral in Kaneohe, Hawaii.  (Dan Dennison/Hawaii Department of Land and Natural Resources via AP)

The longest-lasting known global coral bleaching event has caused vast damage so far — including large swaths of dead corals in the northern sector of the iconic Great Barrier Reef.

Now federal scientists expect the coral bleaching to continue across key reefs in U.S. states and territories this summer, in some cases for a third straight year — a likelihood the scientists call “unprecedented.”

“All Northern Hemisphere U.S.-coral reefs are on alert for coral bleaching this year,” said Mark Eakin, who coordinates the National Oceanic and Atmospheric Administration’s Coral Reef Watch. “If we see bleaching in Florida or Hawaii this year it will be three years in a row.”

Here’s a global outlook for the coral bleaching risk in coming months from NOAA. In the images below, Alert Level 1 means “bleaching expected” and Alert Level 2 means “widespread bleaching and some mortality expected”:


(NOAA Coral Reef Watch)

And here’s a closer look at the Caribbean and Gulf region:


(NOAA Coral Reef Watch)

Coral bleaching occurs when the stresses caused by unusually warm ocean waters disrupt the relationship between corals and symbiotic algae, called zooxanthellae, which live inside coral cells and engage in photosynthesis, providing the corals with energy and food. The zooxanthellae give corals their color, but during bleaching, they are evacuated from the corals’ bodies, leading the animals to turn white.

This doesn’t immediately kill corals — it depends on the length and severity of the thermal stress. However, in many places around the globe of late, ocean temperatures have either been hot enough or hot for long enough to cause coral death — most notably in the Great Barrier Reef but also in many other tropical locations around the world.

By far the greatest threat, according to NOAA, is to reefs in Palau and the Federated States of Micronesia, which aren’t U.S. territories but are considered “freely associated” with the U.S. The agency says there is a 90 percent probability that these areas will see “widespread coral bleaching” as expected La Niña conditions set in.

“These are really spectacular reefs, and these are islands where the people are very dependent on their coral reefs, and our climate models are predicting very strongly that they’re going to be seeing severe bleaching on their reefs for much of the remainder of the year,” Eakin says. “That’s the place where we’re most likely to see really severe effects, maybe as bad as what was seen on the Great Barrier Reef, or worse.”

However, when it comes to reefs off the coasts of some U.S. states, there are also many reasons for concern.

While not under as strong of a warning as Palau and Micronesia, Hawaii and Florida have a different coral problem. In both states, reefs saw extensive bleaching in both 2014 and 2015. If that happens again in 2016, it will set a record for repeated assaults that corals can find it harder and harder to recover from.

“Many of these reefs have been hit now two years in a row, which absolutely is unprecedented for Hawaii, and a third year in a row is really uncharted territory for them,” Eakin says.

In Hawaii, 2014 saw the first archipelago-wide bleaching event ever recorded. For the Northwestern Hawaiian Islands, between 85 and 100 percent of corals died at the worst hit sites, according to the state’s Department of Land and Natural Resources. In 2015, the archipelago again saw widespread coral bleaching.

Florida, meanwhile, is home to the Florida Reef Tract, a 150-mile-long barrier reef that extends southward from Miami along the Florida Keys. It is the “the third largest barrier reef ecosystem in the world,” according to NOAA (the largest is the Great Barrier Reef). It, too, was hit by back to back bleaching events in 2014 and 2015, and in the wake of the 2014 event, the corals, already in a weakened state, were subsequently hit by a devastating disease outbreak, compounding the threat.

As if that isn’t enough, recent research has suggested that ocean acidification, which is also a by-product of global warming, is partly dissolving the limestone structures of the Florida Reef’s corals.

As with any forecast, it remains to be seen precisely how much warm waters affect reefs this year — there’s no such thing as a perfect prediction in a field like this. But all signs suggest that the world’s longest known coral bleaching event, which scientists have widely tied to climate change, hasn’t given corals a break yet.

“We’re looking at a larger area being affected by this long term bleaching event than has ever been affected in a global bleaching event before,” Eakin says.

Correction: This story previously stated that the Florida coral disease outbreak occurred after the 2015 bleaching event. It was after the 2014 bleaching event.