Climate change, other threats put 44% of warm-water coral reefs at risk of extinction

Two scientific efforts published late last year echo a grim reality for warm-water coral reefs: Over 40% of coral reefs are facing extinction due to climate change and other human-driven threats. Published in the journal PLOS One and the IUCN Red List of Threatened Species, the works demonstrate an increased threat to coral reefs since last assessed by the IUCN Red List of Threatened Species in 2008.

Beth Polidoro, an associate professor in the School of Mathematical and Natural Sciences at Arizona State University, presented the updated IUCN Red List of Threatened Species on reef-building corals at COP29 in November.  The global assessment, which showed that 44% of warm-water reefs are threatened, assessed 892 warm-water reef-building coral species.

“Our first assessment in 2008 was the very first time that we did a comprehensive assessment of all the coral species of the world,” said Polidoro, who contributed to the work through both her ASU affiliation and as the IUCN species survival commission coral red list authority coordinator.

Polidoro, who also serves as the program lead for marine conservation at the Julie Ann Wrigley Global Futures Laboratory’s Center for Biodiversity Outcomes, said a wholesale assessment of coral species offers a stronger understanding of overall ecosystem health.

“It's very well established that the more species you have — the better biodiversity that you have — the more resilient that ecosystem is to disturbance and stress,” she said. “The more naturally biodiverse the reef is, the healthier it is.”

Polidoro said the Red List assessment of coral reefs had originally been scheduled to take place every 10 years, but the COVID-19 pandemic slowed efforts. The Red List assessments of cold-water corals, which are found in different parts of the ocean, are currently ongoing. While this delay meant a disruption in their desired schedule, she said the timeline ultimately provided more accurate data made possible by new technological developments.

“When we did that first assessment, we were using the best available data at that time of coral cover loss to model habitat decline for these red list assessments. In the past several years, we've gotten better data,” she said. “In publishing this updated list, we’re utilizing newer technology, better climate change models and more accurate remote sensing than we had in 2008.”

The updated list showed that at least 340 species of warm-water reef-building corals are threatened with extinction. Of the total species measured, 56 species are categorized as “vulnerable,” 251 are “endangered” and 33 are “critically endangered.”

The Red List global assessment of reef-building corals, which are primarily found across the Indo-Pacific, also includes 85 Atlantic coral species. The study published in PLOS One is based on this Atlantic dataset. For this study, Polidoro collaborated with Luis Gutierrez, a biology and society PhD candidate in the School of Life Sciences. The two are listed as authors of the study, in addition to other contributors from ASU and other entities.

Gutierrez, lead author of the PLOS One study, said that while almost half of Atlantic corals are threatened by stressors related to climate change, disease and pollution, corals have proven to be a resilient species.

“There have been many surprising discoveries related to the resiliency of corals and their ability to survive and recover in the face of threats, they just need time,” he said. “Of course, there are other strategies we can implement to increase their odds of adaptation.”

Gutierrez and Polidoro point to several strategies in both the Red List and PLOS One paper, with solutions ranging from additional research on coral reef adaptation and overarching behavioral changes driven by human activities.

“The climate crises may not be able to be resolved locally, but reducing the threats of pollution, overfishing, disease, and other more local issues, potentially gives coral populations a fighting chance even in the face of continued rising temperatures,” Gutierrez said. “Reducing the number of threats corals are facing out on the reefs can make all the difference.”

Although he recognizes that coral reefs face unprecedented challenges, Gutierrez remains optimistic. Especially as a PhD candidate entering the field, he said he is excited to see how humanity’s understanding of coral resilience and vulnerability will evolve over time.

“While the results of our study may paint a bleak picture for the future of corals and coral reefs, there remains so much work to be done, not only in preventing their extinction but also in seeing new paths forward,” he said. “Whether it be from researchers, restoration practitioners, policy makers or individuals, there remains hope for protecting these unique species and vitally important ecosystems.”