Water temps soar in Florida Keys. Can coral reefs stand another bout of extreme heat?

Florida’s coral reefs experienced the deadliest bleaching event in history last summer, a toll largely caused by record hot coastal waters.

Now, water temperatures in the Florida Keys are already approaching the coral danger zone — earlier and hotter than last year.

It was enough for federal scientists to issue the earliest ever coral bleaching watch, an indicator that the already struggling South Florida reef tract could facie another round of severe heat stress in the months ahead.

“It’s kinda crazy we are seeing these temperatures now. Before last year wouldn’t even think about it until August,” said Derek Manzello, a coral reef ecologist with the National Oceanic and Atmospheric Administration. “The concern is that if temperatures are this warm already it won’t take much for seasonal warming to push it over the edge to where the coral start getting stressed.”

Because some corals can recover from bleaching, it’s still too early to fully assess the toll from last summer or accurately predict how many of coral will survive back-to-back seasons of extreme heat. But the losses will likely prove severe. Last year, an estimated half of the juvenile corals perished in some underwater nurseries, where they are grown to support restoration efforts.

Scientists say the water has to to reach a ‘bleaching threshold’ of temperatures for a full month for corals to begin to feel stress and expel the algae that give them their dazzling colors and provide them important nutrients. Bleaching victims turns the hard exoskeletons or corals a pale white.

Last year, bleaching started in July, four to six weeks before the historic onset. That was driven in large part by record air temperatures in South Florida through the summer, helping super heat shallow coastal water. Whether bleaching will kick in that soon, or perhaps even earlier, will be shaped by weather in the weeks ahead.

“The heat stress that Florida saw last year was so extreme and so unexpected,” Manzello said.

READ MORE: Can Florida’s corals survive climate change? Fate of one small reef may hold the answer

Florida is one of many places globally with devastating coral bleaching. Since February, at least 63 other countries have had mass coral bleaching events.

One of coral reef’s biggest threats is climate change, according to NOAA. Human pollution, like burning fossil fuel for heat and energy, gets absorbed into the ocean warming it and changing the water’s chemistry. Also, increased storms and sea level rise from climate change can deposit sediment on the reefs and destroy them.

With water temperatures already so high, there is a threat to see the biggest global bleaching event in the record book, Manzello said.

So far, at least some popular reefs seem resilient. The water temperature at Cheeca Rocks, a shallower reef near Isamorada that draws snorkelers, hit 86 degrees Fahrenheit, six degrees higher than the average for this time of May. But despite the high water temperature there was a positive sign:

“Everything looked good, the coral all looked healthy, there was minimal disease virtually no bleaching,” said Allyson DeMerlis, a coral researcher for NOAA and Miami Rosenstiel School of Marine, Atmospheric, and Earth Science.

Along with NOAA, other groups leading reef recovery efforts, like the Coral Restoration Foundation, said they started planning for another difficult and challenging year as far back as November.

Phanor Montoya-Maya, the Coral Restoration Foundation’s restoration program manager, said his group is continuing to try to breed more coral with genetic diversity— a stock that someday could be replanted on natural reefs. But the Foundation has been on a voluntary coral planting hold since last June, waiting to see how conditions stabilize.

“It’s too soon for instance to trigger moving coral to deeper water or take coral out of the water,” Montoya-Maya said.

Another option they’re considering is relocating the coral further up north where the bleaching wasn’t as bad last year.

“Now we pretty much sit and wait on how reality is going to turn out to be,” Montoya-Maya said.

Ashley Miznazi is a climate change reporter for the Miami Herald funded by the Lynn and Louis Wolfson II Family Foundation in partnership with Journalism Funding Partners.