Safe havens for coral reefs will disappear as oceans warm

In 2015 and 2016, record ocean temperatures triggered coral bleaching events around the world — from Hawai‘i to the Caribbean to Australia — turning once-healthy polyps into ghostly skeletons. But some reefs managed to remain intact due to the cooling effects of upwelling and ocean currents. According to a new study, these special coral reefs, called “refugia,” could disappear in the very near future as human-induced climate change continues to heat up the world.

Presently, about 84% of the world’s shallow coral reefs are places of thermal refugia, defined in the paper as places that have 10 years to recover from heat stress. But when the world heats up to 1.5° Celsius (2.7° Fahrenheit) above pre-industrial levels — which could happen in as little as six years if emissions are not curtailed, according to the Intergovernmental Panel on Climate Change (IPCC) — coral reef refugia will dwindle down to 0.2%, the study says. At 2°C (3.6°F) of warming, which could be reached in as little as 20 years, the study projects that there will be no thermal refugia left in the world.

Lead author Adele Dixon, a marine biologist and climate scientist at the University of Leeds, U.K., said that while the study does not indicate that a loss in refugia will translate directly into mortality, it does show that corals will be exposed to a level of thermal stress they have not “been able to cope with in the past.”

“We’ve known for many years that 1.5° Celsius of warming will be catastrophic for coral reefs,” Dixon told Mongabay in a video interview. “And our findings emphasize that, and say, if anything, it’s worse than what we’ve predicted before.”

A related study found that for the sixth year in a row, the world’s oceans have become hotter than they’ve ever been in recent history due to human-induced climate change, as reported by Mongabay earlier this month. Another study found that extreme heat in the ocean has simply become the new normal. The increase in temperature has led to an increase in marine heat waves that intensify thermal stress on coral reefs.

Global distribution of exposure category in the 1986–2019 climate and at 1.5 and 2.0 ̊C of future global warming. Image by Dixon et al (2022).

‘Cascade of doom’

The research team used the latest climate model projections to measure thermal changes on reefs at a finer scale than ever before — at a 1-kilometer (0.6-mile) spatial resolution. This approach allowed them to understand differences in temperature within a small space, and take into account local-scale oceanographic features like upwelling and ocean currents.

The study paints an even grimmer future for coral reefs than what was reported in the 2018 IPCC report on global warming, which said that 70-90% of coral reefs will decline at 1.5°C of warming above pre-industrial levels, and that 99% of corals would be lost at 2°C of warming. The new study found that more than 99% of coral reefs will be exposed to frequent thermal stress (one thermal stress event every 10 years or more frequently) at just 1.5 degrees of warming, Dixon said.

“The fine-scale resolution takes away the hope that there will be something we didn’t capture with a bigger resolution that might be fine-scale refugia,” co-author Maria Beger, a coral reef ecologist and conservation scientist at the University of Leeds, told Mongabay in a video interview. “We used to think that 1.5° was not great for coral reefs, but that it was within a survivable space. And now I think we’re finding that it’s definitely not going to be a survivable space for most coral species and coral reefs as an ecosystem.”

The new study suggests that at 1.5°C of warming, only a few slivers of coral reef refugia will survive in the Coral Triangle region of the Western Pacific and around Polynesia in the South Pacific. But they are projected to disappear once temperatures reach 2°C.

If coral reefs die on a large scale, biodiversity will suffer, Beger said. For one, reefs are biogenic, meaning they are built by living organisms, so corals on a severely depleted reef will struggle to rebuild. Not only that, but reefs will have to deal with the impacts of sea level rise while trying to rebuild, she said.

Without the reefs, all of the organisms that depend on them for food and shelter — including fish, mollusks and crustaceans — will cease to exist, said Beger. Not only is this a threat to biodiversity, but people who depend on reefs for food will also be severely disadvantaged.

“It’s a cascade of doom. It’s coral Armageddon, I suppose,” Beger said. “What our predictions mean is that the future does not look good for reef biodiversity.”

Coral bleaching in American Samoa: Before (healthy Dec 2014), during bleaching (Feb 2015), and after (dead August 2015). Image by The Ocean Agency / Ocean Image Bank.

Can reefs still be saved?  

David Obura, the founding director of CORDIO East Africa, an organization focused on reef sustainability, who was not involved in this research, said the new paper helps emphasize the dire situation coral reefs are in. He said, however, that there is always a level of uncertainty in studies like this about when coral reefs might “fall off the cliff” — but added that we are fast approaching this point.

“It may be at 1.5 or 1.8 or 2.1 [degrees],” Obura told Mongabay in a video interview. “We don’t know. But we are heading towards more warming than that anyway.”

Dixon said a limitation of their study is that while it shows when reefs will be exposed to thermal stress, it doesn’t necessarily show how reefs will react to that stress.

“We’ve used an ecologically relevant threshold based on what’s caused bleaching in the past, but it’s a single threshold for the whole globe,” she said. “But we know that some regions might have a higher threshold than others. We know that some coral species might have a higher threshold than others. And so the actual response to this level of exposure is likely to be a lot happier.”

Even with this variation between coral reefs, the authors of the new paper argue that reef management needs to shift toward helping corals adapt to a warming ocean and even assist their migration to places that provide more favorable conditions. Beger said part of this process involves removing current stressors, such as destructive fishing practices and pollution, so corals are resilient and have enough genetic diversity to adapt into the future.

There are also many efforts to try and restore coral reefs. For instance, scientists in Australia are growing heat-tolerant corals in the lab and plan to plant them in parts of the Great Barrier Reef. There is also research that shows that giving coral reefs a kind of probiotic can help them recover from heat stress. Yet Beger said these kinds of strategies have limits and may not be enough to save the the world’s coral reefs.

Most importantly, Beger said we have to address the driver of climate change. “None of [these things] will be useful unless we bring emissions down,” she said.

According to the theory of planetary boundaries, there are nine Earth processes that help stabilize the Earth system, but each has a threshold that, once crossed, could push the planet into disorder. Climate change is a boundary that we already breached when we surpassed 350 parts per million (ppm) of carbon dioxide in the atmosphere in 1988. In December 2021, we had reached 417 ppm. Biodiversity is another planetary boundary that is said to have been transgressed, pushing the Earth into an unstable state. The loss of coral reefs and all of the species they support would exacerbate the issue even further.

With the odds stacked against coral reefs, Obura said he believes it’s important to manage existing coral reefs for the change that is set to come.

“Places that currently have coral reefs will have a new ecosystem in that space,” he said. “And so you have to invest in managing change in those places, not just for the biodiversity that will be … retained or will move in or adapt, but there’s a lot of people dependent on those places.

“We’ve spilt the milk, and there’s a lot of it and we can’t clean it up,” he added. “So let’s work out how to make the best of the current situation or with the emerging situations.”


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