Coral bleaching is now so extreme, scientists had to expand their scale for it

For more than a decade, marine experts have relied on an alert scale from the National Oceanic and Atmospheric Administration to signal how much stress ocean heat is putting on corals and what risk there is for bleaching. The highest on the two-level system, Bleaching Alert Level 2, has for years represented coral catastrophe. That has sufficed - until last summer.

A blistering marine heat wave along Florida’s 360-mile-long reef pushed water temperatures to previously unseen levels: from mid-to-high 90 degrees Fahrenheit, with some shallow waters reaching temperatures above 100 degrees. Some locations experienced complete reef bleaching and forced restoration groups to pull some corals out of the water.

The unprecedented event forced NOAA to add three new alert levels to account for higher mortality rates and bleaching levels. The previous levels were no longer doing an adequate job of showing how extreme the heat stress impacts were on the coral reefs said, Derek Manzello, an ecologist and head of NOAA’s Coral Reef Watch program.

“What happened last year was really unexpected and off the charts,” Manzello said. “We knew there were going to be more bleaching events because of ocean warming. We knew they were going to become more severe. What we didn’t anticipate happening was such a severe event happening so early in time.”

The new system aims to give a better sense of how coral reefs respond to extreme heat. When corals are stressed, they release algae - which provide them with food and color - and turn pale or white. A bleached coral doesn’t mean the coral is dead, but rather that they are more vulnerable and can die.

The new most extreme category, Bleaching Alert Level 5, signals near-complete coral mortality, when at least 80 percent of corals in an area are experiencing mortality due to prolonged exposure to extreme temperatures.

A Bleaching Alert Level 5 is five times the amount of heat stress of the first level, Bleaching Alert Level 1, which signals significant bleaching.

Manzello compared that highest level to a Category 5 hurricane. It denotes that they “expect drastic, severe and long-lasting impacts on coral reefs,” he said.

The new Bleaching Alert Level 3 is associated with risk of multispecies mortality; and Bleaching Alert Level 4 is associated with severe, multispecies mortality - 50 percent of corals or more.

Under the old system, Florida and most of the Caribbean were deemed Alert Level 2 for over three months last summer. Under the new system, the Florida Keys would have reached a level 4 and 5, and the Caribbean would have been declared a level 5, Manzello said.

For coral restoration managers, the extended alert system serves as a more nuanced tool to better predict different warm water impacts on corals.

“Unfortunately, we weren’t prepared for how severe [temperatures were going to be],” said Stephanie Schopmeyer, associate research scientist with Florida Fish and Wildlife. “These new alert levels will help prepare us for more severe events.”

But the new alerts also serve as a warning for what water conditions could look in the future.

“It’s almost like last year was a wake-up call,” Schopmeyer said. “We can’t ignore climate change.”

The new alert system increases the limit for degree-heating weeks, which measures weeks when water temperatures are at least 1 degree Celsius above the average summertime maximum. Coral bleaching starts at around four degree-heating weeks - 0r four weeks where temperatures are 1 degree Celsius above maximum averages. Coral mortality starts at around eight degree-heating weeks - where the old alert system ended. The new scale fills in the gaps from where the last scale ended.

“We know bleaching events are going to increase in frequency, severity and magnitude, because that’s exactly what’s happened over the last 40 years,” Manzello said. “If events like this become commonplace, coral reefs are going to have a very challenging time surviving in the next 20 years or so.”

The bleaching occurred in 2023 as the global air and sea temperatures surged to records. Scientists say the warmth is the result of rising concentrations of greenhouse gases in the atmosphere from the burning of fossil fuels as well as the El Niño climate pattern characterized by abnormally warm ocean waters in the tropical Pacific.

Forecasters at the National Weather Service recently issued a La Niña watch - the opposite of El Niño - to develop by August. La Niña is characterized by colder-than-normal temperatures in the Pacific and encouraging destructive Atlantic hurricane seasons.

La Niña could provide a much-needed cool relief for the world’s coral reefs, although mass bleaching events during La Niña have recently occurred on the Great Barrier Reef in 2022 and Fiji in 2023, Manzello said.

But the impending flip from a historically strong El Nino could negate some of the cooler conditions brought by La Nina, said Cindy Lewis, the director of the Florida Institute of Oceanography’s Keys Marine Laboratory.

“It may be a very warm La Niña and not necessarily cooler like we like it,” Lewis said.