More than 60 percent of the world's corals have been damaged in a mass bleaching event that began last year and continues to unfold into the northern summer.
Bleaching caused by heat stress is a major health threat to coral reefs, nicknamed the rainforests of the sea because of their intricate ecosystems that host a wide variety of marine life.
"Crazy haywire" ocean temperatures – in the words of Derek Manzello, of the US National Oceanic and Atmospheric Administration – have triggered the fourth global mass coral bleaching event in memory.
"This event is still growing in size and impacts," Manzello told journalists. "I'm very worried about the state of the world's coral reefs. We’re seeing [ocean temperatures] play out right now that are very extreme in nature."
Data from the EU's Copernicus Climate Change Service shows that sea surface temperatures have been at record highs for the past 13 months.
Fuelling the extreme temperatures is the naturally occurring El Nino weather pattern, as well as energy trapped in the atmosphere and the oceans by greenhouse gases.
Visual proof of trauma
The ongoing mass coral bleaching is the world's fourth on record, with three others occurring between 1998 to 2017.
While records have been toppled over the past year – with 2023 declared the hottest ever – the whitened, damaged coral offers visual evidence of how the heat is hurting marine life.
Coral bleaching occurs when symbiotic algae, known as zooxanthellae, exits the coral tissue because of overly warm water temperatures.
“The algae are what feeds the coral and give it its colour, so without the algae the corals are colourless, or bleached, and you can see through to the white skeleton,” Australian marine biologist Tess Moriarty told RFI.
Once the algae’s gone, the coral loses its main source of food produced through photosynthesis.
Damaged coral can bounce back after being bleached, but too many bleaching events will prevent the coral from regenerating.
Fragile coral then becomes susceptible to diseases and other infections.
Great Barrier Reef hit hard
Scientists studying Australia’s iconic Great Barrier Reef, which has suffered its fifth mass bleaching event since 2016, are looking for signs of coral recovery.
The Australian Institute of Marine Science said in March that its aerial surveys showed that almost two-thirds of the reef had been affected.
Heat stress was worst in the southern part of the UN heritage-listed reef, where sea-surface temperatures peaked at 2.5°C above average.
“As the Great Barrier Reef ecosystem is so large, the size of Italy, the heat stress across it isn’t uniform,” said researcher Neal Cantin, who led the aerial surveys.
It will be months before the consequences of the reef’s worst ever mass bleaching become clear.
While coral reefs cover only 0.2 percent of the oceans, they are home to 30 percent of marine biodiversity. In the Great Barrier Reef alone, there are more than 1,500 species of fish and hundreds of species of corals.
The UN has warned that up to 90 percent of coral reefs would be lost by the middle of the century once the world reaches 1.5°C of warming.
Temperature increases of 2°C would come at the cost of nearly all corals.
