Scientists confirm fourth global mass coral bleaching event

Bleached coral in the Maldives (Photo: The Ocean Agency/*Ocean Image Bank*)

It’s official – scientists have announced that the second global mass coral bleaching event of the decade is underway

Scientists from the National Oceanic and Atmospheric Administration (NOAA) and the International Coral Reef Initiative (ICRI), have announced that the fourth global mass coral bleaching event is underway, the fourth recorded since NOAA’s coral reef and sea temperature monitoring began in 1985, and the second in the last ten years.

According to NOAA, bleaching-level heat stress, as remotely monitored and predicted by NOAA’s Coral Reef Watch (CRW), has been — and continues to be — extensive across the Atlantic, Pacific and Indian Ocean basins.

The news follows the Australian Institute of Marine Science’s announcement in March that the Great Barrier Reef was experiencing its fifth mass bleaching event since 2016.

‘From February 2023 to April 2024, significant coral bleaching has been documented in both the Northern and Southern Hemispheres of each major ocean basin,’ said Derek Manzello, PhD, NOAA’s Coral Reef Watch coordinator. ‘As the world’s oceans continue to warm, coral bleaching is becoming more frequent and severe.

‘When these events are sufficiently severe or prolonged, they can cause coral mortality,’ added Manzello, ‘which hurts the people who depend on the coral reefs for their livelihoods.’

Since early 2023, mass bleaching of coral reefs has been confirmed throughout the tropics, including in Florida in the US; the Caribbean; Brazil; the eastern Tropical Pacific (including Mexico, El Salvador, Costa Rica, Panama and Colombia); Australia’s Great Barrier Reef; large areas of the South Pacific (including Fiji, Vanuatu, Tuvalu, Kiribati, the Samoas and French Polynesia); the Red Sea (including the Gulf of Aqaba); the Persian Gulf; and the Gulf of Aden.

NOAA reports that it has also received confirmation of widespread bleaching across other parts of the Indian Ocean basin, including Tanzania, Kenya, Mauritius, Seychelles, Tromelin, Mayotte and off the western coast of Indonesia.

Coral bleaching occurs when symbiotic dinoflagellates (a form of algae) known as zooxanthellae, which live within coral polyps providing them with nutrients through photosynthesis, are ejected when the ambient water temperature exceeds a threshold of toleration. As the zooxanthellae also give the naturally translucent coral its colour, its absence reveals the white reef-building substrate secreted by the coral below, giving the reef its ‘bleached’ appearance.

As NOAA points out – bleaching does not necessarily mean the corals will die. If the stress driving the bleaching diminishes, corals will take on new zooxanthellae and recover, so the reefs can continue to provide the ecosystem with the services that both its resident flora and fauna – and reef-dependent human communities – require.

Prolonged bleaching, however, can lead to coral mortality, which – especially on a widespread scale – can have a devastating impact on the economies, livelihoods and food security of human populations that depend on the reefs for their survival – from simple artisanal fishing to the revenue generated by dive tourism.

Bleached coral in Okinawa (Photo: The Ocean Agency/*Ocean Image Bank*)

While many reefs recover after bleaching events, there is a growing concern among scientists that they are becoming more widespread, and too frequent for successful coral recovery.

‘Climate model predictions for coral reefs have been suggesting for years that bleaching impacts would increase in frequency and magnitude as the ocean warms,’ said Jennifer Koss, director of NOAA’s Coral Reef Conservation Program (CRCP). ‘Because of this, the NOAA CRCP incorporated resilience-based management practices and increased the emphasis on coral restoration in its 2018 strategic plan, and funded a National Academies of Sciences’ study, which led to the publication of the 2019 Interventions to Increase the Resilience of Coral Reefs.‘

‘We are on the frontlines of coral reef research, management and restoration,’ said Koss, ‘and are actively and aggressively implementing the recommendations of the 2019 Interventions Report.’

Much work is being done to ensure the future resilience of coral reefs, including initiatives such as coral seeding, the practice of breeding coral spawn in labs before replanting them across damaged reefs, a process now widespread across the Caribbean region. During the 2023 mass bleaching event in Florida, NOAA also found that moving coral nurseries to deeper, cooler waters and deploying sunshades to protect corals in other areas were of great benefit to the reef’s recovery.

‘This global event requires global action,’ said NOAA in a statement announcing the latest mass bleaching. As part of that action, the International Coral Reef Initiative (ICRI), a partnership of 101 international members, co-chaired by NOAA and the US State Department are sharing and applying the data learned from previous marine heatwaves and bleaching events.

‘One of the best ways to assess the impact of a bleaching event is to track changes to a reef over time,” said Alexandra Dempsey, CEO of the Khaled bin Sultan Living Oceans Foundation, one of the ICRI’s leading members, which has been mapping the world’s coral reefs for more than a decade.

‘Reliable baseline data on the state of the reef—such as the data on coral cover, fish biomass, and species diversity we collected on the Foundation’s Global Reef Expedition—can help scientists assess the impact of the bleaching event and understand how the health of a reef changes over time.’

ICRI will be hosting a webinar on Tuesday 14 May 2024 to discuss the status of the 4th Global Bleaching Event. People wishing to atted can register their interest on the ICRI seminar website.