This mass bleaching event is the worst on record. Now scientists are hoping for hurricanes

By Rachel Ramirez, CNN

(CNN) — Unprecedented ocean heat has triggered the world’s worst mass coral bleaching event on record – a coral massacre so severe, reef experts are looking to one of nature’s most dangerous and destructive forces to provide relief: hurricanes.

Since January 2023, 72% of the planet’s reef areas have experienced heat stress capable of bleaching, surpassing 65.7% during the last global bleaching event in 2014 to 2017, according to latest data from NOAA shared with CNN.

Coral reefs in the Atlantic Ocean have been hit hardest, said Derek Manzello, coordinator for the National Oceanic and Atmospheric Administration’s Coral Reef Watch. “A well-timed tropical storm or hurricane can bring much-needed thermal relief to heat-stressed corals,” he told CNN.

The bleaching is being driven by record-breaking ocean heat fueled by planet-warming pollution and boosted by a “super” El Niño, a natural climate pattern marked by warmer-than-average ocean temperatures in the tropical Pacific.

Coral sheds its algae, which provides both its color and its food, when the water gets too hot for too long. This is called bleaching. If the water doesn’t cool down quickly enough, coral can starve and die.

The same ocean heat suffocating these corals is also one factor behind the threat of an unusually active hurricane season, which could provide a form of salvation for the reefs in the form of cooler water, coral experts say.

Hurricanes act like enormous ocean heat vacuums, feasting on warm water and moist air to strengthen. As they do, the storms help cool the ocean along their path, not only by consuming ocean heat, but also by churning up pockets of cold water from deeper in the ocean up to the surface.

The area of ocean cooled by a hurricane can extend more than 400 miles from the storm’s center, according to Manzello. “This means that storms can be a good thing for heat-stressed corals that are not within the direct damage swath,” he said.

“A hurricane, a storm or even just a week of cloudiness and rain can give these ecosystems a much-needed break when they’re under a really long period of stress,” said Dana Wusinich-Mendez, Atlantic and Caribbean team lead for NOAA’s Coral Reef Conservation Program.  “There hasn’t been much of a break since things heated up last July, so we’ll take every break we can get.”

The oceans are so hot right now, some scientists worry hurricanes may not be as effective, said Marilyn Brandt, professor at the University of the Virgin Islands’ Center for Marine and Environmental Studies.

“We know that stressful temperatures extend very, very deep,” Brandt, who co-authored a 2005 study with Manzello about how hurricanes can benefit bleached corals, told CNN. “So even a really massive storm may not have the cooling potential that it would have in the past because the temperatures have just gotten so hot and so deep.”

Category 1 and 2 hurricanes used to be enough to cool the water down to alleviate coral bleaching. Now it might take a stronger storm to bring the same level of respite, Brandt said. Relief is also fleeting, only lasting a week or so, she added. A full recovery from bleaching can take years, a timeframe that experts say is becoming more unpredictable as oceans warm.

Hurricanes are also a double-edged sword. Their powerful waves can break fragile corals apart, overturn entire colonies and significantly harm – if not kill – them.  And they worsen the coral health by exposing them to “a soup of contaminated water” made up of fertilizer, sewage and other harmful substances that run off from flooded land, Brandt said. This runoff leaves coral open for infection and bleaching and slows down recovery.

“Bottom line is that there will probably be more negative impacts of cyclones overlapping with bleaching than positive impacts,” Camille Mellin, a researcher at the University of Adelaide said, noting the cooling effects of cyclones are also likely limited to local scales.

Tropical cyclones might not even be around to help in a limited capacity if planet-warming pollution isn’t drastically reduced, a study released Thursday and led by Mellin found. Corals could bleach in spring, before hurricane season starts, by 2080, the study found.

The planet’s coral reefs are “facing death by a thousand cuts,” said Brandt. But she believes there’s a chance of survival – even if that means grasping at straws like hurricanes.

“At this point, it’s becoming so difficult for the corals to recover from these back-to-back bleaching events that at the end of the day, anything that cools down the water is probably going to help,” Brandt said. “So even though the corals are getting beaten up by the storms, it actually is helpful to cool the water down.”

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