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Tubbataha Reef, Philippines.Alexis Rosenfeld/Getty
This story was originally published in Yale E360 This is part of the Climate Desk collaboration.
The ecological genomicist Christian Voolstra began work on corals and reefs in Saudi Arabia in 2009. He was fortunate to be able to scuba dive on these beautiful reefs when he started his job. Things have changed. “I was just back in September and I was shocked,” says Voolstra, now at the University of Konstanz in Germany. “There’s a lot of rubble. The fish are missing. The colors are missing.”
It’s a sad but now familiar story. The Global Coral Reef Monitoring Network published the first-ever “First-ever” report earlier this month. report collating global statistics on coralsThis report details the status of reefs in 12,000 locations in 73 countries over the past 40 years. Overall, they report, the world has lost 14 percent of its corals from 2009 to 2018—that’s about 11,700 square kilometers of coral wiped out.
“If this had happened to the Amazon, if overnight it had turned white or black, it would be in the news everywhere,” says Voolstra. “Because it’s underwater, no one notices.”
Global warming is causing corals severe problems: Prolonged heat waves in the oceans, which are on the riseThis causes corals to expel symbiotic algae (called Zooxanthellae), making them vulnerable and weak. Although local pollution continues to be a problem, global warming is becoming the dominant threat to corals. 2018 was the International Panel on Climate Change reported that 1.5 degrees Celsius of global warming would cause global coral reefs to decline by 70-90 percent (warming currently standsAt 1.2 degrees Celsius A world 2 degrees Celsius warmer than ours would see corals disappear by more than 99 per cent.
There are some signs of hope. Global Coral Reef Monitoring Network report reveals that corals can recover worldwide if they are given about a decade to cool off from hot water. Some spots—particularly the Coral Triangle in East Asia, which hosts nearly a third of global corals—have bucked the trend and seen coral growth. There are signs that corals may be adapting to warmer environments. Research continues to grow on creative ways to improve coral recovery, from selectively breeding super-corals to spreading probiotics to stressed reefs.
“I’m hopeful,” says Voolstra. But it’s going to take a lot of quick action, he says, and even then we won’t be able to save all reefs. “That’s impossible. The point is you save some reefs so they can go through the dark ages of climate change.”
From 1978, when the Global Coral Reef Monitoring Network’s data collection began, hard coral on the world’s reefs held relatively steady for decades. 1998 saw the first global mass bleaching. This changed everything. Warm waters around the world caused in large part by a powerful El Niño wiped out about 8 percent of living coral globally, equivalent to a grand total of 6,500 square kilometers. “All the drama started in 1998,” says David Souter, coordinator of the Global Coral Reef Monitoring Network and a researcher at the Australian Institute of Marine Science in Townsville. “Corals are actually pretty good at sustaining short, sharp temperature increases, but when it starts to last months, we see real issues.”
Surprisingly though, global coral coverage had returned to pre-1998 levels in 2010. “That’s good news,” says Souter. “Even though reefs got knocked down, they got back up again.” When “old growth” corals are wiped out, the new ones that move in are often faster-growing, weedier species (just as with trees after a forest fire), says Souter. It’s great to have this growth, he says, but these opportunistic corals are often more vulnerable to disease, heat, and storms.
Since 2010, a global decline has been the main trend, bringing corals back to 1998 levels. That’s due in large part to two more global bleaching events, in 2010 and 2015-2017, from which corals haven’t been given enough reprieve. There has been a tiny, 2 percent uptick in live coral since 2019, though it’s too soon to say if that might continue. “If you were a really optimistic person you might say that this occurred even while temperatures are high, so maybe we’re seeing adaptation,” says Souter.
The average coral reef was around 30 percent coral and 15 percent macroalgae during the long, stable, and healthy period of corals in the 1990s to the early 2000s. That’s twice as much coral as algae. The ratio has dropped to about 1.5 since 2009 as reef macroalgae growth has increased by 20%. While seaweed also makes for a productive ecosystem, it’s not the same as the complex architecture made by reefs, and it supports different fish.
The so-called Coral Triangle of the East Asian Seas is a positive exception. This region holds almost a third of the world’s coral reefs—and it anomalously hosts More live hard coral and Less macroalgae today than in the early 1980s, despite rising water temperatures. That’s thought to be thanks to genetic diversity among the region’s 600 species of coral, which is allowing corals to adapt to warm waters. “Perhaps diversity has provided some protection,” says Souter, while a healthy population of herbivorous fish and urchins are keeping seaweeds down.
The other three main global regions for coral—the Pacific, holding more than a quarter of the global total; Australia, with 16 percent; and the Caribbean, with 10 percent—all host less coral today than when measurements started. “The Caribbean is a really tragic and desperate case,” says Voolstra, with only 50 or so species of coral and a new disease wiping them out.
Souter notes that it could get worse. “Reefs are probably, on average, better off than I thought,” he says. “The fact that the reefs retain the ability to bounce back, that’s amazing.”
Coral conservationists around the world are working to protect corals and actively restore them in the face of harsh conditions. One recent study, led by Lisa Boström-Einarsson of James Cook University in Australia, trawled through the literature and found more than 360 coral restoration projects across 56 countries. Most are focused on transplanting bits of coral from a flourishing spot to a struggling one, or “gardening” baby corals in nurseries and planting them out. They also include innovative efforts such as using electricity to prompt calcification on artificial reefs (an old but still-controversial idea), and using a diamond blade saw to slice tiny, fast-growing microfragments off slow-growing corals.
Other researchers are also involved in piloting projects. spray coral larvae onto reefs that need it most—this should be faster and easier than hand-planting corals, but it’s unclear yet how many of the larvae survive. “If it works, it will produce much greater gains more rapidly,” says Souter.
Boström-Einarsson and colleagues found an encouragingly high average survival rate of 66 percent for the restored corals in these 362 projects. These happy statistics mask other disturbing facts. Nearly half of the projects were located in a few countries. Most lasted less than 18 month and averaged 100 square meters. Worse, coral gains were often temporary. In one case in Indonesia, a three-year project dramatically increased coral cover and fish—which were then decimated by a heat wave six months after the project ended.
Voolstra says such efforts are still worthwhile as they raise awareness about corals. There are other options. some techniques that could make them far more effective and far bigger in scale.
One bold strategy is to selectively breed corals in order to create super-strains best adapted to a warmer world—but this work is still very preliminary. “Corals take longer to breed and raise up than cows, so we have been betting more on finding heat-resistant individuals that are already out there than on making new ones in the lab,” says Stephen Palumbi at Stanford University, a marine biologist who focuses on corals around the Pacific Island nation of Palau. Palumbi has developed a tank that runs small samples of coral through a heat test on site, and is now working to make it cheaper — in part, he says, by borrowing components from the home brewing industry. Voolstra has also developed a tool to conduct stress testing on-site; he was awarded $4 million by the Paul Allen Foundation to take his effort global.
Heat tolerance, though, isn’t the only thing that corals need. The corals that can withstand heat may also be less resistant to disease or grow slower. “We need to understand this better,” says Voolstra.
Another strategy is to modify the organisms living around corals and help them grow, including the symbiotic bacteria and zooxanthellae. Getting corals to adopt heat-tolerant zooxanthellae is a great idea that could theoretically have a huge impact, says Voolstra, but it’s hard to do. The union is like an intimate marriage, and it’s difficult to shift. Changing corals’ bacteria, which tend to live on a mucous layer on the outside of the corals, is easier, and seems to boost overall coral health. “They bleach the same way but recover better,” says Voolstra. Recent research by Raquel Peixoto (King Abdulla University microbiologist) showed that lathering corals in probiotics could improve coral survival after a heat wave by 40 percent. “It’s still experimental and proof of concept,” says Peixoto, who is experimenting with robotic submarines that could drop slow-release probiotic pills onto reefs to release bacteria slowly over weeks.
Another option is the idea of brightening clouds over a reef in an attempt to shield them from extreme heat. “It’s totally left field,” laughs Souter, but should work the same way as cloud seeding for agriculture: A sprayed mist of seawater encourages clouds to form and shields the ground from direct light. This year researchers trialed the idea; they haven’t yet published their results. Scaling up the idea would be a huge undertaking if it works. They anticipate that they will need to build a thousand stations each with hundreds of sprayers, in order to lower the solar radiation by about 80%. 6.5 percent over the Great Barrier Reef during a heat wave. It is not clear if the effort would be worthwhile, or what the net effects on ecosystems throughout this region would be.
Voolstra believes that more research is needed to verify the effectiveness of these strategies in real-world situations. “Then you put truckloads of money into whatever really makes a difference,” he says. Peixoto states that different reefs require different solutions. That makes all these strategies vital. “It’s all hands on deck.:
Voolstra supports investing in sanctuaries. These are areas, like the Northern Red Sea that corals have adapted to hot waters, but are being threatened by other factors such as sewage, pollution and fish farms. Local efforts to address non-climate-related coral hazards can be very successful. The Belize Barrier Reef Reserve System (Brazil Barrier Reef Reserve System) was created. taken off the list of World Heritage sites in danger in 2018, for example, after a push to protect that ecosystem and ban oil development.
If protecting a handful of refugia from humans doesn’t seem like a big enough effort, last year researchers also launched a project to build an emergency “Noah’s Ark” for corals across global aquaria, keeping their genetic diversity alive in tanks on land.
Voolstra says that the IPCC’s 2018 declaration that 99 percent of corals would die in a world that is 2 degrees Celsius warmer was shocking. He stated that the goal now is to reduce that 99 per cent to 90 per cent or less so that reefs have at most a chance to bounce back. “Whatever we do, it gets much worse before it gets better.”
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