- Written by Georgina Ranard
- BBC climate reporter
Corals around the world are turning white and even dying due to the devastating effects of recent record ocean heat.
According to the National Oceanic and Atmospheric Administration (NOAA), this is the fourth major coral bleaching event worldwide.
Bleaching occurs when corals become stressed and turn white because the water they live in is too hot.
Corals support marine life and fisheries, generating trillions of dollars in revenue each year.
Ocean heat records have been declining in recent months, but this is the first global evidence of how the phenomenon is affecting marine life.
The National Oceanic and Atmospheric Administration (NOAA) has confirmed mass stress in all oceans (Atlantic, Pacific, and Indian Oceans) after receiving reports from scientists around the world for several weeks.
Bleached coral may look beautiful in photos, but scientists who have dived to study the reef say that up close, the coral is clearly diseased and decaying.
Scientists from the United States, Australia, Kenya and Brazil told BBC News they were disappointed and even angered to see their beloved corals threatened or killed by warmer oceans.
The first red flags came in the Caribbean last year, when beachgoers noticed the water off the coast of Florida was as warm as a hot tub.
That heat moved to the southern hemisphere. It currently affects more than half of the world's corals, including Australia's Great Barrier Reef, Tanzania, Mauritius, Brazil, Pacific island coastlines, the Red Sea, and the Persian Gulf.
Last August, the world's average ocean temperature broke an all-time record and has been above average almost every day since.
Climate change is causing sea surface temperatures to rise as greenhouse gases released when burning oil, coal and gas are absorbed by the ocean.
El Nino, a natural weather phenomenon, has also contributed to rising temperatures since June last year, but there are now signs that it is weakening.
Scientist Neil Cantin spent 10 days in February flying a plane over the Great Barrier Reef for the Australian Institute of Marine Science. The United Nations Heritage site is 2,000 km long, or roughly the length of the US east coast.
“For the first time, we have recorded very high levels of bleaching in all three areas of the Great Barrier Reef Marine Park,” Dr Cantin said. Many corals are likely to die at these levels, he added.
Corals are essential to the earth. Also known as the Architect of the Sea, this island builds huge structures that are home to her 25% of all marine life.
Stressed corals will likely die if they experience temperatures 1 degree above their thermal limit for two months. If the water temperature is 2°C higher, they can survive for about a month.
When coral dies, fish and other creatures that rely on coral sounds to get around can have a hard time finding their way home.
Scientist Anne Hoggett has been diving for 30 years on Australia's Lizard Island, the beautiful coral reef featured in the Netflix film Chasing Coral. She has again witnessed widespread bleaching since February.
Like many researchers, she was shocked to see coral turn white during the first mass bleaching in 1998. “Right now I'm just angry that something like this is allowed to happen again,” she says, speaking at the Australian Museum's Lizard Island Research Station.
Corals can recover from heat stress, but it takes time – ideally several years. When they become weak, they become more susceptible to diseases and die easily.
“If given a chance, corals are actually resilient and can recover. But as bleaching becomes more frequent and more intense, that chance is really narrowing.” says Dr. Emma Camp of the University of Technology Sydney, Australia.
The last major global bleaching event was from 2014 to 2016. Since then, ocean temperatures have become so warm that NOAA has had to introduce three new heat alert levels.
David Obura, an ecologist from Kenya, receives messages from hundreds of rangers, scientists and fishing communities who have witnessed bleaching in the Indian Ocean. It started in Madagascar in February and has since spread to Tanzania and the Comoros.
He says fishermen know corals well and can quickly notice when something is wrong. They worry about the future of the fishery, he says, because if the coral dies, it will affect the feeding patterns of fish and, in turn, their livelihoods.
Research published last week offers hope that corals living in the cool, deep waters of the Great Barrier Reef (30 to 50 meters deep) can survive longer than their shallower counterparts as the planet warms.
NOAA's Jennifer McWhorter, who co-authored the study with the University of Exeter, said the study shows that deep-sea corals can withstand global warming of up to 3 degrees Celsius above pre-industrial levels. Says.
But all the coral scientists BBC News spoke to said they must accept that coral reefs are forever changed and that small-scale restoration efforts will not save the world's corals.
Only rapid global reductions in greenhouse gas emissions, which limit ocean warming, will ensure that at least some coral remains, they say.
“It's like going from coral providing homes and buildings to marine life to just scaffolding. What would really want to live in a scaffold?” Dr. McWhorter says. Coral reefs are an early warning system of the effects of global warming on nature. “We need to learn from this so we don't do the same thing to other ecosystems,” Dr. Obra says.
Graphics by visual journalist Erwan Rivault