- Scientists say that coral reefs are currently undergoing a global bleaching event, with more than 54% of the world’s coral reef areas in the territorial waters of over 50 countries experiencing heat stress. According to one scientist, the percentage of areas dealing with bleaching-level heat stress “has been increasing by roughly 1% per week.”
- To assess the current bleaching event, scientists drew on satellite-derived sea surface temperature data and in-water measurements.
- Experts say the current El Niño, a phase in the El Niño–Southern Oscillation (ENSO) climate pattern, in combination with rising global sea temperatures, is responsible for the extensive coral bleaching.
- Mongabay interviewed scientists most familiar with coral reef bleaching data, and experts attending the 9th Our Ocean Conference in Athens, taking place from April 15-17.
ATHENS — The world is currently undergoing its fourth global bleaching event, with coral bleaching occurring in the territorial waters of more than 50 countries, according to scientists at the National Oceanic and Atmospheric Administration (NOAA) in the U.S. and the International Coral Reef Initiative (ICRI).
Since February 2023, more than 54% of the world’s coral reef areas have experienced heat stress that causes bleaching, making this the second most spatially extensive bleaching event on record. The largest bleaching event to date occurred between 2014 and 2017 when more than 56% of the world’s reefs experienced heat stress that led to bleaching. However, according to Derek Manzello, the coordinator of NOAA’s Coral Reef Watch, a program that uses satellite data to help identify bleaching events, the current event will likely surpass this previous record since “the percentage of reef areas experiencing bleaching-level heat stress has been increasing by roughly 1% per week.”
Global coral bleaching events also occurred in 1998 and 2010, when bleaching occurred in 20% and 35% of coral reef areas respectively.
“This should be viewed as another very urgent global warning on the state of ocean health,” Manzello told Mongabay in an email. “Coral reefs and other marine ecosystems are experiencing severe and devastating impacts as the oceans continue to warm.”
Coral bleaching happens when sea temperatures rise to a level that stresses the coral reefs, driving them to expel their life-sustaining and colorful algae, which turns them ghostly white. While corals can recover from bleaching, prolonged exposure to high temperatures will often result in coral mortality.
To assess the current bleaching event, NOAA and ICRI drew on satellite-derived sea surface temperature data, which measures the severity of possible bleaching. NOAA’s Coral Reef Watch uses five different alert levels, with level 5 being the highest and denoting “risk of near complete mortality.” According to Manzello, some parts of the eastern tropical Pacific and the central equatorial Pacific experienced many alerts of level 5 conditions during the period of the current bleaching event, and some parts of the eastern tropical Pacific even exceeded level 5 conditions.
In addition to sea surface temperature data, scientists also used in-water measurements collected by partners to assess the current global bleaching event, Manzello said.
Since the start of the event, scientists have confirmed “severe coral bleaching” in 54 countries and territories in all ocean basins. Fiji and Vanuatu were among the first countries to experience severe coral bleaching during the current event. Mass bleaching has also occurred in large swaths of the Caribbean and Florida; Brazil; the eastern tropical Pacific, including Mexico, El Salvador, Costa Rica, Panama and Colombia; Australia’s Great Barrier Reef; the South Pacific, including Fiji, Vanuatu, Tuvalu, Kiribati, the Samoas, and French Polynesia; the Red Sea; the Persian Gulf; and many parts of the Indian Ocean, including Tanzania, Kenya, Mauritius, the Seychelles, the French-administered islands of Tromelin and Mayotte, and Indonesia.
Greg Asner, managing director of the Allen Coral Atlas, a tool that uses satellite imagery to monitor coral bleaching through changes in brightness, told Mongabay in an email that he and his team “have not seen a bleaching event of this magnitude” since the Allen Coral Atlas went online in 2019.
Currently, data presented in the Allen Coral Atlas indicate that coral bleaching could be unfolding in many parts of the global ocean, including the Coral Sea, South Pacific Ocean, Indian Ocean, Brazil, and some parts of the eastern tropical Pacific, including the western coasts of Costa Rica and Panama and the Galápagos Islands.
Emily Darling, a coral reef scientist and director of coral reef conservation at the Wildlife Conservation Society (WCS), said the current El Niño, a phase in the El Niño–Southern Oscillation (ENSO) climate pattern characterized by warmer sea temperatures, in combination with rising global sea temperatures, is largely responsible for the extensive coral bleaching.
El Niño is “acting across a baseline of warming temperatures, and you’re getting these pushes above a threshold,” Darling told Mongabay at the 9th Our Ocean Conference in Athens on April 15. “The scary thing is that last year the Great Barrier Reef [bleached] during a La Niña — that’s the year that should be cooler. So it’s not just that these things are happening during these climate events — they’re happening all the time.”
Scientists recently confirmed that the Great Barrier Reef is undergoing its fifth mass bleaching event in eight years, which could result in high levels of coral mortality.
Manzello, too, noted that while strong El Niño events have “historically been the harbinger of severe bleaching events,” large-scale and severe coral bleaching has also occurred during La Niña periods.
“This suggests that ocean temperatures may have increased to the point whereby large-scale bleaching events may now occur during any phase of ENSO,” Manzello said.
“Nevertheless, the emergence of La Niña will more than likely bring respite to many different areas and lead to declines in the percentage of reef areas being impacted,” he added.
Manzello said the current El Niño is forecasted to transition to an ENSO-neutral phase by June 2024 before moving into a La Niña, which he called a “nugget of hope.”
To counter the effects of coral bleaching, many experts are turning to coral restoration, which often involves the growing and planting of healthy coral back onto reefs. However, experts say this won’t be a viable solution if global greenhouse gas emissions continue to rise, and climate change impacts continue to accelerate, including a persistent rise in sea temperatures.
“As someone who runs a coral restoration company, I’ll be the first person to say the best thing to do for coral reefs is not to hire us,” Sam Teicher, chief reef officer at Coral Vita, a project aiming to restore damaged and dying coral reefs, told Mongabay in Athens. “We’ve got to stop killing coral reefs. We have [to find solutions] for climate change, for pollution, for destructive fishing practices … those things aren’t happening fast enough.”
Darling said that experts need to “rethink what we’re doing to get coral reefs through the next bottleneck of climate change.”
“We know that certain [coral] species are more resilient, and we know that certain places are more resilient,” Darling said. In reefs where corals are proving to be more resilient, Darling said it’s paramount to mitigate local pressures, such as destructive fishing practices, to give the reefs “the best chance” to survive and recover when cooler water temperatures return.
“Corals can recover,” Darling said. “It has a lot to do with oceanography around these reefs … if there’s cool water that’s cooling off the corals physiologically, but then also bringing small animals that the corals can feed on with their tentacles.
“So there’s a lot of variability in nature, and there are still a lot of reasons to be really persistent in how we strategically conserve coral reefs,” she added. “The plan is to get them through the bottleneck of this unrelenting climate change and climate impacts and warming temperatures that we know is coming.”
Banner image: Fluorescing and bleached reef in New Caledonia in 2016. Image by The Ocean Agency / Ocean Image Bank.
Elizabeth Claire Alberts is a senior staff writer for Mongabay’s Ocean Desk. Follow her on Mastodon, @ECAlberts, Blue Sky, @elizabethalberts.bsky.social, and Twitter @ECAlberts.
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