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Coral Restoration Foundation employees work at a reef site called Pickles off the coast of Key Largo in November.
© Carolyn Van Houten/The Washington Post
In recent years, coral bleaching has been so bad that experts in the USA have significantly expanded their warning scale.
For more than a decade, marine experts have relied on an alarm scale from the National Oceanographic Atmospheric Administration (NOAA) that shows how much the ocean's heat is affecting corals and the risk of bleaching.
The highest level of the two-tier system, bleaching warning level 2, has meant a coral catastrophe for years.
That was enough - until last summer.
A blistering heat wave along Florida's 360-mile (579 km) long reef pushed water temperatures to unprecedented highs: from the mid to high 90s Fahrenheit (about 32 C), with some shallow waters reaching temperatures above 100 degrees Fahrenheit (approx. 38 °C).
In some places there was complete bleaching of the reefs, forcing restoration groups to remove some corals from the water.
Due to this unprecedented event, NOAA was forced to implement three new alert levels to reflect the higher mortality rate and extent of bleaching.
Previous levels were no longer adequate to represent the extreme effects of heat stress on coral reefs, said Derek Manzello, ecologist and head of NOAA's Coral Reef Watch program.
“What happened last year was really unexpected and incomprehensible,” Manzello said.
“We knew there would be more bleaching events because of warming oceans.
We knew they would be heavier.
What we did not expect was that such a serious event would occur so soon.
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How coral reefs respond to extreme heat
The new system is intended to give a better idea of how coral reefs respond to extreme heat.
When corals are stressed, they release algae that provide them with food and color and turn pale or white.
A bleached coral does not mean that the coral is dead, but rather that it is more vulnerable and may die.
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The new, most extreme category, Bleaching Alert Level 5, signals near-total coral die-off when at least 80 percent of corals in an area die due to prolonged extreme temperatures.
Bleach Alert Level 5 is five times higher than the first level, Bleach Alert Level 1, which indicates significant bleaching.
Manzello likened this highest level to a Category 5 cyclone. “It means drastic, severe and long-lasting impacts on coral reefs are expected,” he said.
The new bleaching warning level 3 represents the risk of cross-species mortality, and the bleaching warning level 4 represents severe cross-species mortality - 50 percent of corals or more.
Under the old system, Florida and most of the Caribbean were considered Level 2 alert for more than three months last summer. Under the new system, the Florida Keys would have reached Levels 4 and 5, and the Caribbean would have been declared Level 5, Manzello said .
Advanced coral warning system aims to improve forecasting
For those responsible for coral recovery, the enhanced warning system is a more sophisticated tool to better predict the various impacts of warm water on corals.
A blue tang fish swims among corals on November 29 near Key Largo, Florida.
© Carolyn Van Houten/The Washington Post
“Unfortunately, we were not prepared for how severe [the temperatures would be],” said Stephanie Schopmeyer, research associate with Florida Fish and Wildlife.
“These new alert levels will help us prepare for more serious events.”
The new warning levels are also an indication of what water conditions could look like in the future.
“It’s almost as if last year was a wake-up call,” Schopmeyer said.
“We cannot ignore climate change.”
Old scale ended where coral die-off began
The new warning system increases the limit for warming weeks, which are the weeks in which water temperatures are at least one degree Celsius above the average summer maximum.
Coral bleaching begins at about four weeks of warming - or four weeks in which temperatures are 1 degree Celsius above average highs.
Coral death begins at around eight degree weeks - at which point the old warning system ended.
The new scale fills in the gaps left by the last scale.
“We know that the frequency, severity and extent of bleaching events will increase because that is exactly what has happened over the last 40 years,” Manzello said.
“If such events become commonplace, coral reefs will find it very difficult to survive over the next 20 years.”
The bleaching occurred in 2023 as global air and ocean temperatures rose to record highs.
Scientists attribute the warming to rising concentrations of greenhouse gases in the atmosphere caused by the burning of fossil fuels, as well as the El Niño climate pattern, characterized by unusually warm waters in the tropical Pacific.
La Niña is expected to develop through August
National Weather Service forecasters recently issued a La Niña warning - the opposite of El Niño - that is expected to develop through August.
La Niña is characterized by colder than normal temperatures in the Pacific, fostering destructive hurricane seasons in the Atlantic.
La Niña could bring much-needed cooling to the world's coral reefs, although there have recently been massive bleaching events during La Niña on the Great Barrier Reef in 2022 and the Fiji Islands in 2023, Manzello said.
But the impending reversal of a historically strong El Nino could wipe out some of the cooler conditions that La Nina brought, said Cynthia Lewis, the director of the Florida Institute of Oceanography's Keys Marine Laboratory.
“It could be a very warm La Niña and not necessarily cooler like we like,” Lewis said.
About the author
Amudalat Ajasa
covers extreme weather for The Washington Post and writes about how extreme weather and climate change are affecting communities in the United States and abroad.
We are currently testing machine translations. This article was automatically translated from English into German.
This article was first published in English on February 14, 2024 at the “Washingtonpost.com” - as part of a cooperation, it is now also available in translation to readers of the IPPEN.MEDIA portals.