Fourth global coral bleaching event confirmed

The world is currently experiencing a global coral bleaching event, according to scientists at the US National Oceanic and Atmospheric Administration (NOAA). This is the fourth global event on record and the second in the last 10 years.

Bleaching is corals’ response to stress; symbiotic microscopic algae living inside the coral’s tissues are expelled, causing the coral to become pale or white. Coral bleaching can cause disease or reduced fertility, and while bleached corals are not dead, they will only recover if stressors, such as high water temperatures and poor water quality, ease and conditions return to normal.

Bleaching-level heat stress, as monitored by NOAA’s Coral Reef Watch (CRW), has been — and continues to be — extensive across the Atlantic, Pacific and Indian Ocean basins, coming off the back of months of record-breaking ocean temperatures caused by climate change and the El Niño climate pattern. CRW’s heat-stress monitoring is based on sea surface temperature data, spanning 1985 to the present, from a blend of NOAA and partner satellites.

“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 Dr Derek Manzello, NOAA CRW Coordinator. This is affecting communities in at least 53 countries, territories and local economies, with mass bleaching of coral reefs confirmed in the US, the Caribbean, Brazil, the eastern Tropical Pacific, the South Pacific, the Red Sea, the Persian Gulf, the Gulf of Aden, Tanzania, Kenya, Mauritius, the Seychelles, Tromelin, Mayotte and off the western coast of Indonesia.

Australia’s own Great Barrier Reef has also not been immune, with the Reef Snapshot: Summer 2023−24 showing the Reef is currently experiencing one of the most widespread and severe mass coral bleaching events it has ever seen. Australian Government surveys have found that almost half (46%) of the Reef has seen record levels of heat stress, while NOAA’s temperature data indicates that 80% of the Reef was subjected to bleaching-level heat stress in 2024 and this is increasing by approximately 1% per week. This is the Reef’s fifth mass bleaching event in just nine years.

“As the world’s oceans continue to warm, coral bleaching is becoming more frequent and severe,” Manzello said, echoing the predictions of years-old climate models. “When these events are sufficiently severe or prolonged, they can cause coral mortality, which hurts the people who depend on the coral reefs for their livelihoods.”

The International Coral Reef Initiative (ICRI), which NOAA co-chairs, and its international members are currently sharing and applying resilience-based management actions and lessons learned from the 2023 marine heatwaves in Florida and the Caribbean, including moving coral nurseries to deeper, cooler waters and deploying sunshades to protect corals in other areas. They are also helping to advance coral interventions and restoration by funding scientific research on best management practices.

“We are focusing on understanding the scientific mechanisms to prioritise actions that can positively impact ocean health,” said Professor Selina Stead, CEO of the Australian Institute of Marine Science (AIMS). “But the increasing frequency and extremity of marine heatwaves, driven by climate change, is testing the tolerance levels of coral reefs.

“That is why it is critical the world works to reduce carbon emissions. It is also important to ensure coral reefs are well managed at local and regional levels, using evidenced-based decisions, so they retain resilience in the face of increasingly challenging times.”

Image © AIMS | Eoghan Aston | Keppels Islands | 05.03.2024 under CC BY-NC-ND 4.0