In good news for coral reefs, scientists identify where the toughest ones are

It's rarified to acquire goodness tidings virtually coral reefs. 

Consider the majority of the world's reefs just went through more than two years of very stressful heat in a mass bleaching event, which was only recently declared over. And the relief will likely be short-lived, as the Pacific-warming phenomenon El Niño is now here — and it's looking like a strong one. 

But new research being presented at this week's Our Ocean Conference in Mombasa, Kenya, says there's a surprising strength to these organisms, even as climate change and other factors drive up ocean temperatures. 

"Coral reefs are often framed as ecosystems beyond saving," said study co-author Emily Darling, director of coral conservation at the Wildlife Conservation Society. "This research shows otherwise." 

Currently under peer review, the analysis found close to 166,000 square kilometres of reefs around the world that were potentially climate-resilient — meaning they could withstand those hot temperatures for various reasons. 

Working with the tech non-profit SkyTruth, the researcher mapped these resilient reefs, the majority of which were in five countries' coastal waters: Bahamas, Cuba, Australia, Indonesia and the Philippines. The analysis built on 2018 research, which focused on 50 reefs that, if properly protected, had the best chance of survival.

In pinpointing exact areas to support, experts say the research can serve as a focus for conservation efforts for communities who depend on reefs as nutritional and economic lifelines.  

"It provides some hope," said Craig Dahlgren, executive director of the Perry Institute for Marine Science, and who was not involved in the analysis. 

"Things are bad, but we do have places [where] there's something about them and the corals there that we can learn from, build off of, and help ensure there's a future for coral reefs," he said. 

Good news about coral reefs and their climate resilience

For coral, bleaching looks like death, but it's just the beginning. Environmental stress, such as heat, causes coral to expel the symbiotic algae that lives within — turning them a ghostly white. This makes it more susceptible to disease and eventually, death. 

Looking at resilience didn't just mean finding out reef populations survived, but figuring out — through tens of thousands of field observations over decades and machine learning — what is driving the survival. 

For some reefs, it's as simple as location. 

"These are places where coral reefs just naturally avoid the impacts of heating and climate change," Darling said, giving an example of corals she studied off the coast of Mozambique, where upwelling of cold water prevents the water from getting too hot. 

But for some reefs, like in Kenya, there's no avoiding it.

"They're kind of living in a bathtub where once that hot water comes in, it sits there," Darling said of corals located off the coast of Kenya. But this is where you can also find resilience, as some corals withstand and resist the heat, allowing the reef as a whole to survive a bleaching event. 

Dahlgren, with the Perry Institute, has seen this up close in the Bahamas during the intense heat of 2023.  

"Two coral colonies, same species, pretty much identical in size, shape, configuration. One of them's bleached white, one of them is perfectly healthy," he said, attributing this resilience to individual factors such as the specific microbes and algae inside.  

The final mode of resilience are those that do get hit hard — but come back. Stacy Jupiter, executive director of the Global Marine Program at WCS, saw this first-hand in Fiji following Cyclone Winston in 2016.

"When we did surveys four years later, we saw baby corals that were all about the same size," Jupiter said during a media briefing. "So the reefs were rebounding in terms of their coral cover."

The 2019 Intergovernmental Panel on Climate Change special report on the ocean and cryosphere said the dangers of losing a reef include losing food, tourism and coastal protection from storms. But making all reefs off-limits would inhibit a billion people who use the services they provide.  

The challenge for many countries is knowing exactly where to invest what little they can — as many of the island nations that have reef ecosystems often do not have the resources or economic freedom to protect these often vast areas from the effects of climate change or fishing.

"Climate-resilient reefs are not spread evenly," said Joseph Maina, a co-author of the coral study and scientist at Macquarie University in Sydney, Australia. "And countries need to understand …those differences such that when they plan where future conservation investment should go, they consider this uneven distribution." 

Dahlgren sees the work as helping to give "the biggest bang for the buck" to conservation funding — such as establishing marine protected areas or active coral restoration. 

While climate change, driven by humanity's burning of fossil fuels, is accelerating ocean heat and thereby the intensity of these bleaching events, Dahlgren warns it's not the only threat corals are facing. 

And with El Niño threatening to kick-start more bleaching around the world, experts are concerned that even those resilient corals are being hit too often. 

Coral reefs experiencing mass bleaching event

"Shrinking recovery windows between bleaching events makes me quite nervous," Darling said, adding that nature needs time to adapt and change, even when there is that potential to resist or recover.

But it's also important to protect corals that show a diversity of resilience within a reef, says Darling, as it helps to balance risk in case one form of strength fails.

"It's like a financial portfolio," he said. "It's better to have more redundancy in the same area."

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