Corals across several countries are bleaching and dying en masse from unprecedented levels of heat stress, prompting fears that an unfolding tragedy in Central America, North America and the Caribbean could become a global event.
US government scientists have confirmed reefs in Panama, Colombia, El Salvador, Costa Rica, Mexico and six countries in the Caribbean, including the Bahamas and Cuba, are suffering significant bleaching, alongside corals in Florida that began turning white almost a month ago.
“I don’t think any of these places have seen heat stress like this before,” said Dr Derek Manzello, coordinator of Coral Reef Watch at the US National Oceanic and Atmospheric Administration.
“This will only get worse until there is a global reduction in greenhouse gas emissions. This is essentially a big field experiment. The big fear is there will be catastrophic mortality.”
Coral reefs are home to more than a quarter of the world’s marine species despite taking up about 0.1% of the ocean floor and are considered one of the most susceptible ecosystems to global heating.
Noaa’s Coral Reef Watch this week also had its highest level of coral bleaching warning over reefs off north Vietnam and southern China.
Manzello said in many places ocean temperatures were as high or higher than ever recorded by satellites.
“This mass bleaching is taking place over a huge area,” he said, referring to the bleaching in North and Central America.
“We are marching towards a Caribbean-wide coral bleaching event in the next month if things don’t change.”
The world’s ocean temperatures have been at record high levels for months.
Noaa data suggests many regions that are bleaching still have weeks before their usual summer temperature peak, suggesting the situation could worsen without sustained cloudy weather or major storms.
Dr Ian Enochs, head of the coral program at Noaa’s Atlantic Oceanographic and Meteorological Laboratory, has inspected the Cheeca Rocks reef that used to be one of the healthiest and most vibrant in the Florida Keys.
“I was not prepared for what I saw there,” he said. “Every single coral I saw was affected – either bleached or severely paled. It is pretty hard to wrap your brain around. It’s a lot to handle.”
Species of soft coral were “disintegrating” in the hot water. “The tissue was falling off them,” he said.
“If I take any hope or strength from this, it is that I hope it motivates and unites people.”
Projects to restore small reef areas by planting and growing corals in the region, including in Costa Rica, have also reported bleaching. In Florida, some sites have reported total loss of all corals.
‘Increasingly hard for reefs to recover‘
Depending on the species and the amount of heat, corals can recover from bleaching but scientists say they are more susceptible to disease and don’t reproduce as well in the years that follow.
To assess the level of heat stress accumulating on corals, scientists use a measure called degree heating weeks (DHW). For example, if a reef faces temperatures 1C above the average maximum for two weeks it is being subjected to 2DHW.
As a rule of thumb, at 8DHW reefs experience severe bleaching with a risk of mortality for some corals. Some reefs in the Florida Keys have hit 15DHW. Since the Noaa data started in 1985, the region had never seen levels above 10DHW.
“We started hitting values never achieved before, and it’s happening earlier than ever before. It’s very alarming for me,” said Manzello.
Dr Lorenzo Álvarez-Filip, a coral scientist at the National Autonomous University of Mexico, is monitoring bleaching of corals at Puerto Morelos near Cancún – the northern part of the Mesoamerican Barrier Reef System, which stretches from Mexico to Honduras, Guatemala and, in Belize, is world heritage-listed.
He noticed corals bleaching in early June, at least three months earlier than had been seen in previous bleaching events.
“We have reports of bleaching from here to Belize – that’s [more than] 400km,” he said.
Álvarez-Filip said the area has never seen a mass coral mortality event from heat stress in the past but it was now “very likely” to occur in the coming months. “I wish I could say something different,” he said. “It is very painful.”
He said an outbreak of coral disease killed an estimated 80 million corals in Mexican waters over a few months in 2018 and 2019. He had grave concerns the corals that had survived could now perish in the heat.
Massive coral bleaching event! Firsts effects of #ENSO on the reefs of Huatulco, Mexican South Pacific.
— Francisco Medellín (@FRAME5_7) July 14, 2023
1. Orthophotography of San Agustín reef
2. A panoramic view of bleached Pocillopora corals.
#ElNiño #ClimateChange #Coralbleaching #CoralReef
📸 ARBIOLAB and SIGALT pic.twitter.com/EO92TNoelV
Functioning reef systems were crucial for the region’s economy, Álvarez-Filip said, as they supplied food, protected the coastline and brought in tourism.
In Panama, Dr Sean Connolly, of the Smithsonian Tropical Research Institute, said there were reports of bleaching on the Caribbean and Pacific side of the country.
He said the frequency of heat stress events suggested climate models had underestimated the severity of extreme events.
“We are seeing a world where these kinds of temperature extremes are happening so frequently that it’s going to become increasingly hard for reefs to recover in the time periods in between.”
Fears of ‘massive global event’
From 2014 to 2017, a global mass bleaching event started in the northern hemisphere and continued on reefs in the south, including the Great Barrier Reef.
“We are looking at the unfolding of yet another global mass bleaching event,” Connolly said.
Prof Tracy Ainsworth, vice-president of the International Coral Reef Society, said the levels of heat stress facing some reefs was “unsurvivable” for corals.
“This is multiple reefs in multiple countries. There’s a concern we’re going into another massive global event,” she said.
Some corals that were at risk of death were decades old. She said the reefs become “permanently changed” from the heat. “There are decades of growth in some of these massive corals,” she said. “Once they die, they die. We can’t assume that a recovery happens because we’re now in the second decade of these events.
“I’ve been talking about [the risk from climate change] for my entire career and so you can’t get any more prepared than coral biologists, but it’s still so shocking.”
She said for many people who worked, studied and loved reefs, the events unfolding would be traumatic.
“This is sort of incomprehensible. We don’t even study this kind of heat stress. We say unprecedented a lot. But this is unprecedented.”
