Gathered together, the world’s coral reef systems would cover an area somewhere between the size of Oregon and Texas.  Scattered about the globe like species-rich ocean rainforests, they help nurture about half the world’s marine life, a bounty that sustains millions of reef fishers and their communities in the Global South, in a fishery worth about $6 billion.

But the amount of living coral cover on those reefs has declined by about half since the 1950s, along with associated biodiversity and fish catches, heightening concerns about food security, according to a new study published Friday in the journal One Earth. 

Climate change, overfishing, ocean acidification and pollution are driving the decline, the researchers found, after analyzing one of the most comprehensive global data sets on reefs and fisheries to date. The study analyzed data from thousands of reef surveys across all tropical reef regions, between 1957 and 2007. Since then, there have been several major global and regional coral reef mass bleaching events, which have caused more losses not measured by the study. Bleaching occurs when ocean water temperatures become too warm and cause corals to expel the algae living in their tissues, turning their color white.

The 60 percent decline in fish catches is particularly concerning because millions of people around the world, especially in Pacific Asia and Africa, rely on fish for protein and key nutrients, said co-author William Cheung, head of the Changing Ocean Research Unit at the University of British Columbia, who is also a co-author of fisheries- related sections of major global climate reports. 

To illustrate the global decline documented by the new study, Cheung said that a reef fisher who caught 10 fish per day in 1950 now only catches four fish per day. 

“With the same amount of effort they are catching less fish than before because there are less fish in the ocean,” he said. But, he added, there are regional exceptions, where fisheries have not declined as much, but even at protected and unpolluted reefs, the climate threat is intensifying.

“IPCC reports say greenhouse gases that have already been emitted are a substantial risk to coral reefs,” Cheung said. “We definitely need rapid climate action to reduce that threat. At the same time, we need to act locally to address threats like pollution.”

He said recent major science reports from the Intergovernmental Panel on Climate Change suggest that, somewhere between 1.5 and 2 degrees Celsius of global warming, there is a coral reef tipping point beyond which up to 90 percent of the planet’s reefs would be killed or severely damaged. Currently, Earth’s global temperature is about 1.1 degrees Celsius above the pre-industrial baseline, with many realistic projections taking that up to 1.5 degrees Celsius in the next few decades.

A 2019 IPCC report on oceans added more detail, indicating that some heat-tolerant corals will persist, and that the long-term fate of reefs also depends on how long global average temperatures stay at a dangerous level, Cheung said.

James Robinson, a co-author of a related article in the same journal, said there are important regional variations in the health of coral reefs. The destructive 1998 global mass bleaching destroyed coral reefs around the Seychelles. But they bounced back, leaving reef fisheries relatively unscathed. Low coral-cover fisheries in Kenya are also holding their own.

Together, the two journal articles help sharpen the global focus on “links between climate change and food security in the tropics,” Robinson said. His team’s related study “puts the lens of nutrients” over Cheung’s research to show how the decline of coral reef cover can affect public health by reducing access to critical nutrients that are not easily replaced.

“What this study really drives home is that Indigenous and coastal communities, in this case in the tropics, are being deeply and unfairly hurt by the world’s actions,” said Andres Cisneros Montemayor, who was a research associate at the University of British Columbia when he worked on the study, and is now an assistant professor at Simon Fraser University in British Columbia. 

“It’s heart-wrenching for us to see photos and video of wildfires or floods,” he said. “And that level of destruction is happening right now all over the world’s coral reefs and threatening people’s culture, their daily food, and their history. This isn’t just an environmental issue, it’s also about human rights.”

A Steady Decline

Lack of accurate historical data has been one of the longstanding challenges of accurately measuring the decline of living coral cover on the planet’s reefs, said Newfoundland-based co-author Tyler Eddy, with the Fisheries and Marine Institute of Memorial University of Newfoundland.

Eddy said that, in a separate but related study, the team also surveyed reef researchers globally to get a sampling of expert opinion on what the baseline global coral reef cover might have been without human impacts. Based in part on those experts’ own observations, that allowed the research team to compare the “modern record of coral reef cover with an estimate of what it may have been like historically.” 

“We know in 1950 there were already effects of the industrial revolution and climate change was happening, so this could be a baseline that was affected by the early days of climate change,” Eddy said. “We wanted to ground those early observations which were few and far between with a different technique.”

The detailed look enabled the researchers to see decadal changes, from a general decline in the 1960s and the beginning of the 1970s, to a reduced, but still declining rate, in the early 2000s. 

Observations for many regions were still sparse in the 1960s, and even in the early 1980s: The data set included only a few hundred observations per year. Thus, the early declines are still uncertain, but Eddy said, adding, “It’s been suggested that the most sensitive species declined early,” and were slowly replaced by more heat-tolerant species that slowed the loss in some regions. 

In a separate study, published in August in the journal Progress in Oceanography, Eddy and other scientists warned that most models are underestimating how much global warming will harm fisheries in the future. 

Fish and other marine animals will use more energy to cope with warmer water, leaving less scope for growth and reproduction. And in some regions, the productivity of plankton, at the base of the food chain, will decline, which means fewer and smaller fish. Those “worrying findings,” he said, reinforce concerns about food security in developing countries, especially those where people can’t easily replace lost food sources.

Eddy said one of the key messages of the study is that local interventions like protected areas and water quality management won’t be enough to protect reefs at a meaningful scale.

“I think they definitely play a role,” he said. “But when we look at the global scale, warming waters are the biggest threat corals face right now.” Recent research on the Great Barrier Reef by Australian coral expert Terry Hughes proves the point he added. 

Related: Latest Bleaching of Great Barrier Reef Underscores Global Coral Crisis

No Time to Recover

Hughes, who directed Australia’s coral reef research program until 2020 as one of the world’s leading coral researchers and an outspoken critic of Australia’s climate and energy policies, said global warming is causing so many marine heat waves that many coral reefs just don’t have time to recover. Since the world’s first pan-tropical bleaching hit 70 percent of the world’s corals in 1998, most reefs have bleached three or four more times.

“For example, the Great Barrier Reef bleached in 1998, 2002, 2016, 2017 and 2020,” Hughes said. “The most severe of these events, in 2016, killed 30 percent of the corals in just a few months. There has been some limited recovery since 2016 and 2017 on reefs that didn’t re-bleach in 2020.”

He added, “It takes about a decade for a decent recovery to unfold. A major concern is that the interval between one bleaching event and the next is shrinking—leaving less time for even a partial recovery.”

Hughes said the new study clearly documents a 50 percent loss of corals that “has important flow-on impacts on the broader ecosystem because of the dependence of many species on nooks and crannies provided by live corals.”

Healthy corals provide habitat and a nursery for many species, including fish that are important for the food security of tropical people.

“The loss of corals that the study found is consistent with the observed declines in reef fisheries,” he said.

Related: Climate Change Is Transforming the Great Barrier Reef, Likely Forever

Malin Pinksy, a marine ecologist and evolutionary biologist with Rutgers University who was not involved in the new study, said that, despite impressive efforts to pull together some of the best global datasets, researchers still do not know nearly enough about coral reef changes. 

“Coral reefs are the bejeweled crown encircling the ocean,” he said, “and this paper starkly illustrates how that crown is disintegrating.”