Coral reefs are some of the richest and most imperiled ecosystems on the planet. But maybe they aren’t as close to the brink as we thought.
Scientists have been warning that without dramatic reductions in climate pollution, most of the world’s reefs will be dead this century, cooked by underwater heatwaves.
An elaborate recent experiment in Hawaii, however, suggests that while coral reefs will suffer, they won’t all be obliterated, so long as humans manage to meet international targets to reign in global warming.
“This was a very surprising result, since almost all projections of reef futures suggest that the corals should have almost entirely died,” said Christopher Jury, a post-doctoral researcher at the Hawaiʻi Institute of Marine Biology (HIMB) at the University of Hawai‘i at Mānoa, a renowned center for coral research.
There’s little doubt that warming waters are already taking a huge toll on coral reefs. A heatwave in 2016 killed at least half of the shallow-water reef building corals in Australia’s Great Barrier Reef. The reef has since been hit with four more heatwaves that triggered mass bleaching, where high temperatures cause coral polyps to reject the symbiotic algae living in their tissue. Because the algae is a key source of food, bleaching puts coral at risk of starving. Last year water temperatures spiked above 100°F in the Caribbean, killing some corals outright.
Predictions that coral reefs are on the verge of disappearing hinge on a combination of laboratory studies and monitoring of survival for key individual species of coral. But that can overlook the ways an entire coral reef community functions. To get a more complete picture, Jury and his colleagues created what amounted to 70-liter aquariums at a research station at the edge of the island of O’ahu.
These 40 tanks were populated with chunks of 8 different coral species and a menagerie of other reef inhabitants. Water was piped in from a nearby bay. Some tanks were fed with regular seawater at a normal temperature. Others got water that had been heated or had the chemistry altered to mimic higher temperatures and acidity levels expected later this century.
“Rather than focusing on just one or two species in isolation, we included the entire complement of reef species from microbes, to algae, invertebrates, and fish, under realistic conditions they would experience in nature,” said Rob Toonen, a marine scientist at HIMB and one of the senior authors on the study.
Over the course of two years, the researchers tracked the evolution of these miniature reefs. They gathered information about how many corals survived and how much carbonate was getting created or lost as coral polyps built their exoskeletons.
By the end, even in the reefs subjected to a combination of higher temperatures and more acidity, coral survival fell by 35%, rather than the total wipeout that might have been expected. The amount of area covered by corals rose from 3% at the start to more than 20% in those tanks, compared to 40% coverage in the tanks that were kept at today’s conditions. And those same reefs hit with the double whammy of heat and acidity still created new exoskeleton material, though at 56% of the rate in the regular tanks, the scientists reported in late October in the Proceedings of the National Academy of Sciences.
“These experimental reef communities persisted as new reef communities rather than collapsing,” said Jury.
That’s not so say they were unscathed. In addition to growing more slowly and having lower survival rates, the experimental reefs with hotter water also became less diverse, as some corals fared better than others. One species, Pocillopora meandrina, commonly known as cauliflower coral, almost vanished. Another, Porites evermanni, seemed virtually unfazed by the heat. The remaining species fell somewhere in between.
The researchers offer a few reasons why these mini-reefs fared better than expected. Many past studies focused on more heat-sensitive corals, rather than a broader array of coral species or the other organisms that make up an ecosystem. That greater diversity could explain why these reefs continued to grow.
Amid the hopeful signs is one huge caveat: The experiment only tested temperature increases of 2°C, roughly in line with the goals in the climate treaty negotiated in Paris in 2015. The central target is to keep average global temperatures this century to “well below” 2°C above pre-industrial levels.
But in late October, the United Nations issued its latest update on how well countries were doing to meet that target. Currently, it warned, the world is on track for an increase of 2.6-3.1°C.
For Jury, the new findings add additional urgency to changing that trajectory. “The recognition that coral reefs are not doomed if we take appropriate action on climate change and local stressors reinforces the need to accomplish these goals,” he said.
Jury, et. al. “Experimental coral reef communities transform yet persist under mitigated future ocean warming and acidification.” Proceedings of the National Academy of Sciences. Oct. 29, 2024.
Image: Courtney Mattison artwork at the Florence Griswold Museum
