coral bleaching, a worrying symptom of record ocean temperatures

This phenomenon, which affects many reefs around the world, has consequences on underwater life and human populations, warns the American Oceanic and Atmospheric Observation Agency in its latest study.

The world’s coral reefs are dying. In a publication dated Monday April 15, the American Oceanic and Atmospheric Observation Agency (NOAA) warns of a massive episode of coral bleaching worldwide due to record ocean temperatures. This phenomenon of coral dieback threatens the very survival of reefs.

“As the world’s oceans continue to warm, coral bleaching is becoming more frequent and severe. When these events are severe or prolonged enough, they can cause coral mortality.”the researcher was alarmed Derek Manzello, who coordinates NOAA’s Coral Reef Observatory. This global bleaching of corals on a planetary scale is the fourth recorded by thegency since 1985, the third in fifteen years after 2010 and 2016.

Condemned to disappear with global warming

While the ocean surface temperature reached a record level in February (21.06°C), under the effect of global warming caused by human activities, the bleaching phenomenon is due to the rise in ocean temperatures. water in the seas and oceans. Due to thermal stress caused by heat, coral colonies, which are made up of tiny creatures called polyps, eject them symbiotic algae that they shelter and which provide them with nutrients. “Without these algae, which have various colors, these corals become transparent and we can see their limestone skeleton, which gives them this white tint”explains Jean-Pierre Gattuso, research director at the oceanography laboratory in Villefranche-sur-Mer (Alpes-Maritimes).

“Coral reefs are sentinels, they are the first to be affected in such a negative and global way in terms of climate.”

Jean-Pierre Gattuso, research director at the Villefranche-sur-Mer oceanography laboratory

at franceinfo

The phenomenon is not irreversible: Affected corals can survive if temperatures drop and other stressors (overfishing, pollution, human activities) are reduced. But the trend is not good.

In a report published in 2018, the Intergovernmental Panel on Climate Change (IPCC) predicted the disappearance of 70 to 90% of corals if the increase in average temperature reached 1.5°C compared to the pre-industrial era, and 99% at + 2°C. An even more pessimistic study, published in 2022 in the journal PLOS Climateexplained that more than 99% of corals would be unable to recover from marine heat waves with an increase in average temperature of 1.5°C. A bar reached for the first time over twelve consecutive months between February 2023 and January 2024, according to the European Copernicus Observatory.

Even if work is being done to move corals to deeper waters, where the temperature is cooler, or to replant them, Jean-Pierre Gattuso warns that the best way to preserve corals is through “a reduction in greenhouse gas emissions”.

Reservoirs of biodiversity and allies of human life

According to the National Oceanic and Atmospheric Observation Agency,he Great Barrier Reef off the coast of Australia is among the most affected reefs, as are those located in the Caribbean, Florida and the Red Sea. The South Pacific, where French Polynesia is located, is also experiencing coral bleaching, the consequences of which are multiple.

This phenomenon first disrupts underwater life and ecosystems, because many species take refuge in corals to feed and reproduce. “These coral reefs are formidable reservoirs of biodiversity, a bit like tropical forests on earth”supports researcher Jean-Pierre Gattuso. According to a publication from the Development Research Institute dating from 2022, they “host the greatest diversity of marine fish even though they cover only 0.1% of the ocean surface.

But these coral reefs are also allies for human life. According to the CNRS, “pMore than 500 million people depend on these ecosystems.” throughout the world, for their food with fishing, their employment with tourism or for their security, coral reefs acting as aprotection against the risk of submersion, notably by breaking the swell.