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Coral scientists fear a global bleaching event, with Florida leading the way

Coral on the ocean bed in the Straits of Florida near Key Largo, Florida, in September 2021. Record warm ocean temperatures this summer are leading to what scientists fear may be a global bleaching event.
Chandan Khanna
Coral on the ocean bed in the Straits of Florida near Key Largo, Florida, in September 2021. Record warm ocean temperatures this summer are leading to what scientists fear may be a global bleaching event.

Experts say they are observing “thousands upon thousands” of miles of reefs undergoing bleaching due to heat stress caused by record high ocean temperatures.

No immediate end is in sight to the unprecedented marine heat wave stressing the state’s coral reefs, raising fears the heart-rending losses may portend a global bleaching event that could affect reefs from Florida to Colombia, scientists of the National Oceanic and Atmospheric Administration said Thursday.

The scientists said the record temperatures affecting the state’s corals, including those protected as part of the Florida Keys National Marine Sanctuary, which includes the only barrier reef in the continental United States, are remarkable not only for their intensity but duration.

Since April, NOAA scientists have tracked a steady rise in ocean temperatures. In the Florida Keys, the temperatures have been higher than previous records for 29 days between July 9 and Aug. 16. The scientists say the heat stress developed earlier than ever before by five to six weeks.

“There is a big concern among the coral reef scientific community that we are potentially walking into another global bleaching event, based on what we know and what history has taught us,” said Derek Manzello, coordinator of NOAA’s Coral Reef Watch Program. “This is a very serious event, and Florida is just the tip of the iceberg.”

It will be months before scientists fully understand the scope of the problem, but they say they are seeing “thousands upon thousands” of miles of corals undergoing bleaching as a result of heat stress in Belize, Colombia, Costa Rica, Cuba, El Salvador, Mexico, Panama, Puerto Rico and the U.S. Virgin Islands, with Florida most impacted.

The heat wave is forecast to last through at least October, although a cooling event like a hurricane could change that, they said.

Heat affects corals, which are sedentary animals, by breaking down their relationship with the microscopic algae that lives inside them, gives them their color and provides them with food.

When the water is too warm, the corals eject the algae, leaving the corals to turn white. It is possible for corals to survive bleaching if the water temperature normalizes in enough time, but the event can leave the corals weakened and susceptible to disease.

The problem is expected to grow worse and more widespread as the global climate warms. The ocean absorbs 90 percent of the excess heat associated with climate change, and scientists say marine heat waves are intensifying worldwide.

Reefs are crucial to marine biodiversity and are important economic drivers, drawing snorkelers, scuba divers and anglers. They also serve as natural buffers protecting coastal communities from the pounding waves of storms and hurricanes.

To a lesser extent, corals are also threatened by acidification, the other impact on the oceans caused by the greenhouse gases warming the planet. Oceans absorb about 25 percent of the carbon dioxide in the atmosphere.

The carbon dioxide dissolves in water and forms a mild acid that neutralizes calcium carbonate and bicarbonate, which corals and other invertebrates use to build their hard shells and skeletons. Acidic ocean water can even dissolve these shells.

In Florida, the mass bleaching event has sparked a rush to rescue the ailing corals from the hot waters and relocate them to on-land tanks, where they can be preserved. Thousands of corals have been relocated from the Florida Keys National Marine Sanctuary, said Andy Bruckner, the sanctuary’s research coordinator.

He said the effort also involves monitoring to identify corals that are showing resilience to the extraordinary temperatures.

“By next spring we will have a sense of how severe the impacts were from this event. But right now, other than the fact that we are seeing bleaching everywhere, we don’t have a lot more information,” he said.

“We do need to worry.”

This story was produced in partnership with the Florida Climate Reporting Network, a multi-newsroom initiative formed to cover the impacts of climate change in the state.