Scientists have long known that reefs are in peril, but a new study published today in the journal One Earth quantifies coral losses around the world. The in-depth analysis reveals half of coral reefs have been lost since the 1950s. Scientists say climate change, overfishing and pollution are decimating these fragile ecosystems and putting communities and livelihoods in jeopardy. Their study, which is among the most comprehensive assessment of reefs and their associated biodiversity to date, underscores the rapid pace of global coral collapse.
“Coral reefs have been in decline worldwide—I think that's pretty commonly accepted,” says Tyler Eddy, a research scientist at Memorial University of Newfoundland who co-authored the study. “We didn't necessarily know the magnitude of how much, when we looked on a global scale, that reefs had declined.”
Coral reefs are biodiversity hotspots that provide habitat for fishes, protection for coastal communities and generate billions of dollars for fisheries and tourism. Part of the reason corals are dying is that they’re ultra-sensitive to changes in water temperature and acidity, says biologist Mary Hagedorn, who wasn’t involved in the study.
“Corals have skeletons, which makes them seem like rocks,” says Hagedorn, but they are animals with symbiotic partners. Coral polyps rely on colorful algae, called zooxanthellae, which live in their tissue and produce food the corals need to survive. When the polyps are stressed by changes in light, water temperature or acidity, they break that symbiotic relationship and expel the algae in a process called bleaching. Corals have a short window to regain their symbiotic algae, but if corals are stressed for too long, their death is irreversible. “There is not a reef on earth that has not been touched by some aspect of this global and local threat,” says Hagedorn.
Most coral assessments focus on specific regions or reefs, but Eddy and his colleagues from the University of British Columbia wanted to more complete assessment of coral losses. They used a combination of databases containing thousands of surveys of coral reef cover, marine biodiversity records and fisheries catch data to assess how each factor changed over time. They were particularly curious what dying corals meant for a reef’s “ecosystem services”—including providing habitat for diverse marine species, protecting the coast from storms and serving as a source of food and livelihood.
In addition to finding that half of living corals have died since the 1950s, researchers discovered that coral-reef-associated biodiversity dropped by 63 percent. Healthy reefs support thousands of different corals, fish and marine mammals, but bleached reefs lose their ability to support as many species. The scientists also found that catches of coral reef fishes peaked in 2002 and have been declining since then despite increasing fishing effort. And the study showed that the loss of coral species wasn’t equal across reefs—certain corals are proving more sensitive than others, leading some biologists to worry that some vulnerable coral species will be lost before they can be documented or preserved.
One challenge the team faced was finding detailed, accurate information about reef coverage in the 1950s. To deal with this limitation, they relied on coral cover estimates from their 2018 study on historical coral coverage. In the earlier work, the study authors asked more than one hundred scientists what they believed coral reef cover would have been at a given year based on existing evidence.
Eddy and his colleagues also documented the impact of the loss of coral reefs on coastal indigenous communities who have close cultural relationships with the reefs. Those communities lost ecosystems services, including reef-associated seafood they rely on for much of their diet.
The connection between human communities and reefs is a particularly important piece of this study, says ecologist Christina Hicks who wasn’t involved in the work. “It asks the question, ‘Yes, we are losing ecosystems, which is tragic, but what does do those losses mean, for people?’” she says. “Coral reefs play this really important function in supplying indigenous communities and local communities vital micronutrients, and if they lost them, it could lead to severe implications.”
The recent study didn’t assess what factors led to coral declines in recent decades, though overfishing and pollution from nearby land-based agriculture are common local stressors. Eddy and other coral experts agree the biggest threat to reefs is climate change, and note that the regions that contribute less to climate change often feel the worst impacts. Each year, the ocean absorbs around one-quarter of the carbon dioxide emitted from the burning of fossil fuels and becomes warmer, more acidic and less hospitable to corals.
“There are lots of strategies for saving coral reefs and for bringing down carbon emissions, and people often debate about what's most effective,” says Hicks. “What this study says is that it's even more vital that we act now, and that we act in all directions.”