This is the second such event in the last 10 years and comes at a time when global oceans have also recorded unprecedented heat
A bleached coral. Photo: iStock National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Watch (CRW) of the United States and the International Coral Reef Initiative (ICRI) have confirmed the fourth global mass coral bleaching event in 2023-2024. CRW and ICRI have recorded bleaching of corals in 53 countries, territories and local economies across five different ocean/sea basins between February 2023 and April 2024.
This is the second such event in the last 10 years and comes at a time when global oceans have also recorded unprecedented heat in 2023 and 2024. The earlier event had lasted from 2014 to 2017. The CRW declares a global mass coral bleaching event only when it records or gets inputs from all ocean basins of coral bleaching.
Hard corals (different from soft corals which do not have a shell) are marine animals with a tough shell covering them. Single-celled algae grow on the shell in a symbiotic relationship with the corals, giving them their characteristic colour. They usually band together to form colonies and structures known as coral reefs which become home to millions of marine animals and plants.
Around 25 per cent of all marine species are dependent on coral reefs during some part of their life cycle. When sea surface temperatures and ocean heat in general rise, the algae on the hard corals die-off. This makes the corals white.
This process is known as ‘bleaching’. Once bleached, the corals can become vulnerable to diseases and eventually die. If other stressors such as marine pollution and ocean acidification are kept under check and certain adaptation measures taken, corals can recover back to their original health.
Read Bleached to death
When corals die, all other marine species dependent on them for shelter, food and other eco system services also die off or move elsewhere. In such places, other life forms such as algae usually take over ecosystems.
This is why coral reefs are under threat from increasing sea surface temperatures and extensive marine heatwaves, along with other factors such as acidification of oceans and pollution. During a marine heatwave, temperatures in vast areas of sea and ocean surfaces increase and remain high for elongated periods of time.
The year 2023 was the hottest on record for the atmosphere and the oceans, according to the World Meteorological Organization (WMO) and the European Union’s Copernicus Climate Change Service (C3S).
Read Fourth global mass coral bleaching? Great Barrier Reef severely affected, corals seen dying
“On an average day in 2023, nearly one-third of the global ocean was gripped by a marine heatwave, harming vital ecosystems and food systems. Towards the end of 2023, over 90 per cent of the ocean had experienced heatwave conditions at some point during the year,” said the WMO in a press release on the State of the Global Climate 2023 Report.
The sea surface temperature records for July, August and September 2023 were broken by a wide margin. The El Nino conditions in the equatorial Pacific Ocean that began in July, added onto the general trend of warming over land and oceans that has been happening in the past decade due to accumulated greenhouse gases in the atmosphere. The warming trend of the oceans has also continued into 2024.
El Nino is the warmer-than-normal phase of the El Nino Southern Oscillation (ENSO) phenomenon in the equatorial Pacific Ocean. It is generally responsible for higher-than-normal temperatures on the surface of seas in most of the global ocean basins. During La Nina, which is the colder-than-normal phase of the ENSO, the exact opposite happens.
Among long-term patterns, ocean heating and mass coral bleaching are closely tied to the occurrence of El Nino events. Since 1950, an El Nino event that has occurred in the last six months of the first year and the first three months of the following year, happened seven times. Six of these seven times, the second year was warmer of the two and since 1997, each of these pairs of El Nino years has also witnessed mass bleaching of corals.
Localised coral bleaching events have also been witnessed in recent La Nina years, showing the growing impact of general oceanic warming. This happened in the Great Barrier Reef, the largest coral reef off the coast of Australia, in 2020 and 2022, both La Nina years.
“Ocean warming poses a serious threat to coral reef ecosystems around the planet, as bleaching events will increase in severity, frequency, and magnitude,” Derek Manzello, coordinator of CRW told Down To Earth. “The most recent climate models suggest that bleaching events may become an annual occurrence for most reefs somewhere around 2040-2050,” Manzello added.
“Long-term data on coral reef condition can put events like this into context. They allow us to quantify coral mortality, track recovery, and identify areas that don’t recover naturally and may need further protection or a helping hand through innovative interventions,” said Britta Schaffelke, manager (International Partnerships) at the Australian Institute of Marine Science and global coordinator of the Global Coral Reef Monitoring Network, in a press release by ICRI.
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