Two of the world’s most ubiquitous species of reef-building corals seem surprisingly able to survive and even cope well with climate change, according to a new study—at least so long as global warming is kept below 2 degrees Celsius (3.6 degrees Fahrenheit), the target set by the Paris Agreement.
“We found hope,” says Rowan McLachlan, a coral expert at Oregon State University and lead author of the study published today in Nature Scientific Reports.
Hope has been a scarce thing lately on coral reefs. As a result of human-made greenhouse gas emissions, they face chronically warmer water, more intense marine heat waves, and an increasingly acidic ocean. That’s in addition to local stresses from pollution and overfishing.
The world has so far warmed by 1.1°C (1.98°F), and coral reefs have already suffered mass fatalities. The Great Barrier Reef, the world’s largest reef system, is currently in “crisis,” a recently published UN report said. That report, by the Intergovernmental Panel on Climate Change (IPCC), warned that some coral reef ecosystems could face irreversible damage if the world warms more than 1.5°C (2.7°F). A 2018 report by the IPCC concluded that at 2ºC (3.6°F) or more, 99 percent of all reef-building corals could be lost—meaning that living coral reefs would essentially disappear from the planet.
But that’s not what McLachlan and her colleagues observed when they subjected Hawaiian corals to a simulated 2ºC world for nearly two years. They found that two common coral species were especially resilient: Two-thirds of those corals survived the simulated future.
“We were expecting to see more mortality than we did, to find that the corals were just barely hanging on,” McLachlan says. “We were really shocked. They had really high survivorship.”
Within limits, it seems, some corals can acclimate to a warmer world.
Simulating tomorrow’s oceans
Oceans absorb some of the heat building up in the atmosphere. Heat waves amplified by climate change prompt corals to expel the symbiotic algae that nourish them—an effect called coral bleaching, which can ultimately kill them. Meanwhile, oceans also absorb some of the atmosphere’s excess carbon dioxide, making seawater more acidic, which weakens coral skeletons.
Marine heat waves killed over a third of the corals on Hawaii’s coral reefs in 2014 and 2015. In late 2015, to learn more about how both warming and acidification might jeopardize reefs in the future, McLachlan and her colleagues visited four reefs in diverse settings around Oahu. Using a hammer and chisel, they collected samples of three common coral species: rice coral, finger coral, and lobe coral.
The researchers placed the corals in 70-liter tanks—but not in a lab, as other experiments on coral resilience had done, but outside on Coconut Island, where they would be exposed to the same weather as a reef just offshore.They filled a total of 40 tanks with sand, rubble, reef fish, plankton, and other reef features. The idea was to simulate ocean conditions as realistically as possible.
“That’s why our experiment is different,” McLachlan says. “It’s more informative of how Hawaiian reefs might actually respond [to climate change].”
It’s also the longest such coral experiment ever to be conducted, she says.
For 22 months, the researchers subjected some corals to 2ºC of warming, some to acidified water, and some to both changes. A fourth set of tanks was left completely alone to serve as a control.
The tanks that tested both warming and ocean acidification together were the most realistic simulations of the future, says Andréa Grottoli, a coral biogeochemist at Ohio State and the senior author on the study. In all the tanks, she and her colleagues monitored a suite of physiological indicators to see how the corals were responding over time to their environment—and the results were heartening.
“We saw this long-term arc where you see stress responses, but after long enough there was acclimatization,” says Grottoli. The implication is that, with enough time to adapt to their surroundings, some corals may survive the stressful conditions wrought by climate change.
Overall, of the corals exposed to both conditions, 46 percent of rice coral, 56 percent of lobe coral, and 71 percent of finger coral survived. Many of the corals were even thriving.
“They weren’t just struggling. Two of the three species were doing really well,” says Grottoli. And her team may have underestimated the resilience of the third species, rice corals, she says. Rice corals are nourished not just by their symbiotic algae but by eating zooplankton, and in the experiments they were fed less zooplankton than they might normally get in the wild.
“The paper supports what has been observed in Hawaii,” says Ku'ulei Rodgers, a coral expert at the University of Hawaii at Manōa who monitors the state’s reefs and was not involved with the study.
“However, there is a limit to the rate at which acclimatization can protect corals from bleaching as temperatures continue to increase,” she says in an email, noting that the 2014-2015 marine heat wave in Hawaii also killed many lobe and finger corals.
“Although it is hopeful that some species will survive this century, unless drastic reductions in emissions occur, corals will eventually lose their fight for survival,” Rodgers says. Current policies to reduce emissions have the world on track to warm by about 2.7°C (4.86°F) by the end of the century, according to the Climate Action Tracker—substantially above the 2ºC simulated by McLachlan and her colleagues.
What do these findings mean for reefs?
Rice coral is common around Hawaii and in the waters of the north and central Pacific. But finger coral and lobe coral are found throughout the Pacific and Indian Oceans, and their ability to survive could be a sign that coral reefs in the future might be able to rebound from the brink of collapse, the study says. Lobe coral in particular is an essential reef builder in the Pacific Ocean.
Alan Friedlander, a coral reef ecologist at the University of Hawaii who was not involved with the study, argues that more marine protected areas are needed to ensure these climate resilient corals aren’t jeopardized by local pollution and degradation. Friedlander is chief scientist for National Geographic’s Pristine Seas initiative, which promotes marine protected areas.
“This work shows that coral reefs can survive and even thrive into the future if we can curb carbon dioxide emissions and manage local stressors like overfishing, sedimentation, and pollution,” he says in an email.
“Absent local stressors, there is hope moving forward,” says McLachlan. “If we can’t mitigate local stressors, the outcome for coral is much worse.”
But her team’s findings also may provide support for more proactive forms of reef management.
Given the dire state of corals today, some conservationists argue that it is no longer enough just to protect them from pollution and fishing and leave them alone—that active restoration work is needed. Knowing that a coral species like lobe coral can survive climate change means it’s a candidate for restoration projects that select hardy corals and plant them in degraded reefs, Grottoli suggests.
“Introducing a coral from somewhere else is now the lesser of two evils,” she says. “That kind of conversation is now on the table. Some conservationists wouldn’t have considered it a decade ago.”
As humanity struggles to eliminate the greenhouse gas emissions that might otherwise doom coral reefs worldwide, climate-resilient corals may offer a lifeline to the future.
“We have a chance to maintain reef systems long enough so when warming slows, reefs can catch up,” Grottoli says. “We have a window to work with.”