In their current form, coral reefs have been around for around 60 million years. A quarter of all marine life, including over 4,000 species of fish, are dependent on coral reefs at some point in their life cycle. But these extraordinary ecosystems cannot survive the regular marine heatwaves caused by global heating.

In the 1980s, the first observed incidents occurred of coral reefs turning white over extended areas. Scientist Ove Hoegh-Guldberg discovered the cause. Corals are animals related to jellyfish and anemones. Their polyps take food from the water but they also depend on symbiotic algae which provide them with an additional food source through photosynthesis. Under heat stress, corals expel these algae, leaving their tissue a ghostly white. Without their algae, corals slowly starve, and may die. By the time of the first global bleaching event in 1998, Hoegh-Guldberg was able to predict that as early as the 2020s some reefs could be bleaching six or more times a decade - far too frequent to give them time to recover. The scientist was branded an ‘alarmist’. But in 2018 a landmark review by the Intergovernmental Panel on Climate Change (IPCC) found that if the earth warmed by 1.5C above pre-industrial levels this “will result in the further loss of 90% of reef-building corals compared to today”.  

In March 2023, global sea surface temperatures started breaking records, and have stayed that way since. The warming impact of El Niño has further exacerbated the heating caused by fossil fuel burning. These extreme ocean temperatures have led to what is believed to be the worst planet-wide coral bleaching event ever. This is the fourth global bleaching event on record and the second in the last 10 years. A global bleaching event is declared when at least 12% of corals in each of the main ocean basins - Pacific, Atlantic and Indian - experience bleaching-level heat stress within a 12-month period.

Coral reefs typically take at least 10 years to recover from episodes of mass bleaching. Researchers found that during 1986-2019, 84% of reefs were in regions where there was at least a decade between marine heatwaves. However, they predicted that at 1.5C of global warming, just 0.2% of reefs would have the benefit of a sufficient gap to recover between repeated marine heatwaves, and 90.6% of reefs would be likely to experience heatwaves every five years, or even more frequently.

Last summer, waters around Florida, in the Gulf of Mexico and the Caribbean warmed to unprecedented levels. Surface ocean temperatures around the Florida Keys soared to 38.43C. The extent and severity of this marine heatwave was unprecedented. As well as Florida, reefs in Panama, Colombia, El Salvador, Costa Rica, Mexico and six countries in the Caribbean suffered significant bleaching. The Coral Restoration Foundation reported that surveying Sombrero Reef, a site they had been working at for over a decade to restore coral health, they found 100% coral mortality. 

The Great Barrier Reef in Australia is the biggest coral system in the world, 2,300km long, it covers an area bigger than the size of Italy and is made up of about 3,000 individual reefs. It has just experienced the fifth mass coral bleaching event in just eight years. This year’s intense underwater heat has triggered the most severe heat stress ever seen on record. In past bleaching events on the Great Barrier Reef, the southern region has sometimes been spared worst of the bleaching. This time only 3% of southern reefs have not bleached at all. 

Scientists surveying the aftermath are described as ‘shellshocked’,and the reef as looking 'carpet bombed'. Professor Terry Hughes was quoted in the Guardian “bearing witness to the unfolding calamity he has dedicated his life to preventing."

“It’s fucking awful,” the softly spoken scientist says, emerging from the ocean. “They said the bleaching was extensive and uniform. They didn’t say it was extensive, uniform and fucking awful.

“It’s a graveyard out there.”

Photo credit, G.Torda, ARC Centre of Excellence for Coral Reef Studies, via Flickr