Harmful bleaching of the world’s coral has grown to include 84% of the ocean’s reefs in the most intense event of its kind in recorded history, the International Coral Reef Initiative announced Wednesday.

It’s the fourth global bleaching event since 1998, and has now surpassed bleaching from 2014-17 that hit some two-thirds of reefs, said the ICRI, a mix of more than 100 governments, non-governmental organizations and others. And it’s not clear when the current crisis, which began in 2023 and is blamed on warming oceans, will end.

That’s deadly to corals, which are key to seafood production, tourism and protecting coastlines from erosion and storms. Coral reefs are sometimes dubbed “rainforests of the sea” because they support high levels of biodiversity — approximately 25% of all marine species can be found in, on and around coral reefs.

Coral get their bright colors from the colorful algae that live inside them and are a food source for the corals. Prolonged warmth causes the algae to release toxic compounds, and the coral eject them. A stark white skeleton is left behind, and the weakened coral is at heightened risk of dying.

The bleaching event has been so severe that NOAA’s Coral Reef Watch program has had to add levels to its bleaching alert scale to account for the growing risk of coral death.

Efforts are underway to conserve and restore coral. One Dutch lab has worked with coral fragments, including some taken from off the coast of the Seychelles, to propagate them in a zoo so that they might be used someday to repopulate wild coral reefs if needed. Other projects, including one off Florida, have worked to rescue corals endangered by high heat and nurse them back to health before returning them to the ocean.

April 23 (Reuters) - More than four-fifths of the world's coral reef areas have been affected by devastating mass bleaching spurred by record-high ocean temperatures, turning many once-colourful reefs a ghostly pale hue, scientific authorities said on Wednesday.

Bleaching is triggered by anomalies in water temperature that cause corals to expel the colorful algae living in their tissues. Without the algae's help in delivering nutrients to the corals, the corals cannot survive.

The world's fourth mass bleaching event, which scientists declared one year ago, has shown few signs of slowing down, according to the International Coral Reef Initiative and data from the U.S. National Oceanic and Atmospheric Administration which track reef health. Instead, it has grown to be the most widespread on record, with 84% of reef areas - from the Indian Ocean to the Atlantic to the Pacific - subjected to intense heat stress for a duration expected to cause bleaching as of March 2025.

Last year was the hottest on record and the first to reach over 1.5 degrees Celsius warmer than pre-industrial times, contributing to unprecedented ocean temperatures and triple the previous record number of marine heatwaves around the world.

"The magnitude and extent of the heat stress is shocking," said Melanie McField, a marine scientist working in the Caribbean. "Some reefs that had thus far escaped major heat stress and we thought to be somewhat resilient, succumbed to partial mortalities in 2024."

"Bleaching is always eerie - as if a silent snowfall has descended on the reef," she added.

Previous events in 1998, 2010, and 2014-17 saw 21%, 37% and 68% of reefs subjected to bleaching-level heat stress respectively.

Marine biologists had warned early last year the world's reefs were on the verge of a mass bleaching following months of record-breaking ocean heat fuelled by human-induced climate change and the El Nino climate pattern, which yields unusually warm ocean temperatures along the equator and in the Pacific.

In December 2024, a weak La Nina pattern, which typically brings cooler ocean temperatures, gave scientists hope that corals might recover, but it only lasted three months.