Global warming has pushed the world’s coral reefs into a fourth global mass bleaching event, which is on track to become the most extensive on record, US government scientists have confirmed.
About 54% of ocean waters containing coral reefs have experienced heat stress high enough to cause bleaching, according to the National Oceanic and Atmospheric Administration’s Coral Reef Watch.
A global bleaching event is defined as when at least 12% of corals in each of the major ocean basins – the Pacific, Atlantic and Indian Oceans – experience bleaching-level heat stress within a 12-month period. The statement also requires confirmed reports of bleaching.
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Coral Reef Watch also confirmed that the world’s largest coral reef system – Australia’s Great Barrier Reef – will experience its most widespread heat stress event on record in 2024.
The first global bleaching occurred in 1998, with 20% of the ocean’s reef corals exposed to a level of heat stress high enough to cause bleaching. The second event, in 2010, saw 35% reach that threshold, and the third from 2014 to 2017 peaked at 56%.
Dr. Derek Manzello, the director of Coral Reef Watch, told the Guardian that the current bleaching event is likely to soon surpass the previous, most widespread event “because the percentage of reef areas experiencing bleaching-level heat stress has increased by about 1% per week.”
The NOAA threshold for the onset of bleaching refers to the amount of accumulated heat corals are experiencing at any given time, known as degree warming weeks.
For example, 1 warm water is collected when corals are exposed to temperatures 1 degree above the usual maximum for seven days. Coral Reef Watch considers 4 SWWs as a bleaching threshold.
Coral reefs are rich in biodiversity and provide habitat for a quarter of all marine species, while covering less than 1% of the ocean’s surface. Reefs provide food and tourism income for millions of people and protect coastlines, but are considered among the most vulnerable ecosystems to global warming.
The current global event began in early 2023 and in the Northern Hemisphere, summer reefs across the Americas were bleached by record levels of heat stress.
Mass bleaching has been confirmed in the tropics, NOAA said, including Florida, the Caribbean, Brazil, many countries in the South Pacific, the Middle East and in parts of the Indian Ocean, from the west coast of Indonesia to the reefs off the coast of East Africa.
Coral bleaching describes a process in which the coral animal expels the algae that live in their tissues, giving them their color and many of their nutrients.
Without their algae, a coral’s white skeleton can be seen through their translucent flesh, giving it a bleached appearance.
The mass coral bleaching over large areas, first noted in the 1980s around the Caribbean, is caused by rising ocean temperatures.
Some corals also exhibit fluorescent colors under stress when they release a pigment that filters light. Sunlight also plays a role in causing bleaching.
Corals can survive bleaching if temperatures are not too extreme or prolonged. But extreme heat waves at sea can kill corals outright.
Coral bleaching can also have sub-lethal consequences, including increased susceptibility to disease and reduced growth and reproduction rates.
Scientists say the gaps between bleaching events are becoming too short for reefs to recover.
Coral reefs are considered one of the Earth’s ecosystems most at risk from global warming. Reefs support the fisheries that feed hundreds of millions of people, and also support important tourism industries.
The world’s largest coral reef system – Australia’s Great Barrier Reef – has suffered seven mass bleaching events since 1998, including five in the past decade.
Prolonged and severe bleaching can kill corals, but if temperatures drop quickly enough, the animals can recover. Research has shown that previously bleached corals have difficulty reproducing and may be more susceptible to disease after bleaching.
Manzello said global warming combined with a global El Niño has caused sea surface temperatures to rise. He said predictions made by scientists decades ago about the fate of corals in a warming world are now coming true.
“The bottom line is that as coral reefs experience more frequent and severe bleaching, the time they have to recover becomes shorter and shorter. Current climate models suggest that every reef on planet Earth will experience severe annual bleaching sometime between 2040 and 2050.”
Prof. Ove Hoegh-Guldberg, a pioneer in coral research and one of the first to link bleaching to global warming, said: “It’s a shock. We clearly need to prevent governments from investing in fossil fuels, otherwise we don’t have a chance [to save reefs].”
Earlier this year, Coral Reef Watch was forced to add three new alert levels to its global coral bleaching warning system to represent ever-increasing extremes.
Professor Tracy Ainsworth, the vice-president of the International Coral Reef Society, said the bleaching had spread to some of the most remote places on Earth.
“Globally, we are failing to protect coral reefs and the communities that depend on them. This is neglect on a global scale,” Ainsworth said.
The Great Barrier Reef is now suffering its fifth mass bleaching event in eight years. Data from Coral Reef Watch shows that 80% of the reef was exposed to bleaching-level heat stress by 2024, the highest ever recorded and above the previous record of 60% in 2017.
Dr. Roger Beeden, chief scientist at the Australian government’s Great Barrier Reef Marine Park Authority, said it was more important than ever to see global action on climate change.
“But the prognosis is not good for coral reefs as we know them, and the GBR is not immune. We are certainly not giving up on the reefs, but they are under great pressure.”