- Coral reefs, vital for a billion people globally, face massive loss and potential collapse within decades due to rising ocean temperatures and acidification, even if emissions reduction targets under the Paris Agreement are met, scientists say.
- A new study challenges this prediction, suggesting reefs could adapt and avoid collapse, albeit with significant changes; it nevertheless highlights the need for ongoing reductions in local stressors and carbon emissions.
- The study tested the responses of experimental reefs, which included eight Hawaiian coral species and other reef-dwelling organisms, to various future ocean scenarios.
- However, some point out that such studies have limitations, such as not fully replicating reality or simulating the impact of further shocks on coral reef ecosystems, that reduce their predictive capacity.
Coral reefs support fisheries, protect coastal communities, and sustain tourism worldwide. However, rising ocean temperatures and acidification from the climate crisis threaten these ecosystems. Scientists warn that even if the Paris climate agreement targets on reducing emissions are met, massive coral reef loss is inevitable and reefs could collapse within decades, impacting a billion people who depend on them globally.
Yet the situation may not be quite so grim, according to a recently published study that challenges the 2023 prediction by the U.N.’s Intergovernmental Panel on Climate Change that coral reefs will decline by more than 99%. It suggests that reefs could instead adapt and avoid collapse, albeit with significant changes. The study authors say the findings nevertheless underscore the need to reduce local stressors and make rapid, deep cuts in carbon emissions to meet the Paris Agreement’s warming limit of 2° Celsius (3.6° Fahrenheit) above pre-industrial temperatures.
“Coral reefs are not inevitably doomed,” lead author Christopher Jury, a post-doctoral researcher at the University of Hawaiʻi at Mānoa’s Hawaiʻi Institute of Marine Biology, told Mongabay by email.
The new paper acknowledges that even if the world agrees to reduce carbon emissions, temperatures will keep rising, “such that reaching or even exceeding [Paris Agreement] targets now seems inevitable.”
“If we limit climate change to Paris Climate Agreement targets, or at least get relatively close, and also manage local stressors such as destructive fishing practices and coastal pollution, then coral reefs will continue to change, but global reef collapse may still be avoidable,” Jury said.
Experimental coral reef systems
To conduct the study, Jury’s team created 40 “mesocosms,” or controlled systems that replicate the diversity and environment of a wild coral reef. They filled the mesocosms with reef sand, rubble and unfiltered seawater, then populated them with eight of the most common Hawaiian coral species and a menagerie of reef-dwelling creatures, from microbes and algae to invertebrates and fish.
The coral species under study make up about 95% of the coral cover on Hawaiian reefs and are among the most common coral types in the Pacific and Indian oceans, regions known for their exceptional coral diversity, Jury said.
For two years, between 2016 and 2018, the scientists observed and statistically analyzed how these mesocosms responded to four different scenarios: Present-day temperature and acidity; higher temperature (+2°C); higher acidity (-0.2 pH); and a combination of both ocean stressors. These conditions align with the Paris Agreement’s 2°C warming target above pre-industrial levels; under a high carbon emissions scenario, this is projected to occur around the year 2075.
The goal was to see how the sampled coral species, the reef communities and the biodiversity of these systems would react to future climate scenarios.
Contrary to most projections, which predict collapse under future conditions, including those aligned with the Paris Agreement targets, the researchers were surprised to find that the experimental reefs exposed to all future ocean scenario treatments transformed into new calcifying systems with reduced yet significant coral cover and high biodiversity.
“We observed comparatively less severe responses than have been projected,” the paper states.
Why the results may differ from predictions
Jury’s team highlighted three key factors to explain why their study’s results differ from earlier projections about the future of reefs.
First, they wrote that previous studies have mainly focused on just three coral species — Acropora millepora, Pocillopora damicornis and Stylophora pistillata — missing the broader diversity of coral responses.
Second, predictions had suggested that calcification would stop due to increased warming and acidification later this century, but the experimental reefs didn’t behave as expected. The researchers found that many corals and other calcifying organisms showed greater resilience, partly because crustose coralline algae thrived under the future ocean conditions and became more prevalent — something that previous research had documented. The researchers say this shift in dominance helped the reefs maintain growth, although it marked a significant ecosystem change.
Third, the researchers noted that earlier predictions often focused on just one factor, like warming or acidification, without considering their combined effects. They also said that these forecasts overlooked how different reef organisms, many of which aren’t even formally described or named, might respond in various ways.
Meanwhile, Jury said the new study fully aligns with the recent IUCN global coral reef assessment, which finds 44% of reef-building coral species at risk of extinction. “It’s critical to consider that ‘elevated risk of extinction’ does not mean that extinction is inevitable,” he said. “Under a high end climate change scenario and unmitigated local stressors, it’s likely that we would see the extinction of many coral species. Critically, we do not have to take that path.”
The limitations of mesocosms
David Obura, director of the Kenya-based research organization CORDIO East Africa and co-chair of the IUCN’s Coral Specialist Group, told Mongabay by email he agreed that the new study results “align significantly” with their global coral assessment.
But he disputed the notion that the study’s findings offer a substantially different forecast from existing predictions. “As noted in the study, these ‘coral reefs’ (already very simplified and altered) have shifted to ‘novel calcifying ecosystems’ (i.e., not coral reefs), which, to me, is not far from a more global summary statement of approaching ‘coral reef collapse,’” he said.
Obura, who also chairs the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), pointed out that the eight dominant species from Hawai‘i sampled in the study represent a reef that has already declined in health and changed significantly in its community composition. These eight species may have a unique ability to persist in acidifying and warming waters that other species that would normally be more common in Hawaiian reefs may not have, limiting the study’s broader applicability.
Mesocosms also have downsides. “Small-scale artificial environments are notoriously poor at replicating reality,” Obura said, noting that Jury’s team is “highly capable and respected” and did “all possible to minimize this.”
“What the microcosm experiment doesn’t do is simulate the impact of further shocks, whether from a temperature spike, pollution, sedimentation, or some other impact, and how well the corals or system respond to that. And, of course, it misses all the corals that have already declined,” he said.
He suggested a 10-year experiment would be more informative, while acknowledging it would be logistically challenging.
Obura said we shouldn’t expect large-scale predictions, like coral reefs collapsing with a 2°C rise in temperature, to be fully reflected in small, controlled experiments. In reality, he said, different locations will show varied outcomes: Some reefs may collapse entirely, some may decline a little or a lot, and others may appear to be fine but lack certain components not considered in the study.
In the wake of the recent global climate conference, COP29 in Baku, Azerbaijan, Jury called for governments to “take more aggressive action” to secure the survival of coral reefs for current and future generations.
“We still have an opportunity to preserve coral reefs in a functional state,” he said, “but the window for that opportunity is rapidly closing, and depends heavily on climate change and local stressor mitigation.”
Banner image: Coral reef in Sombrero Island, Philippines. Image by Jett Britnell / Ocean Image Bank.
Citations:
Jury, C. P., Bahr, K. D., Cros, A., Dobson, K. L., Freel, E. B., Graham, A. T., … Toonen, R. J. (2024). Experimental coral reef communities transform yet persist under mitigated future ocean warming and acidification. Proceedings of the National Academy of Sciences, 121(45). doi:10.1073/pnas.2407112121
Cornwall, C. E., Carlot, J., Branson, O., Courtney, T. A., Harvey, B. P., Perry, C. T., … Comeau, S. (2023). Crustose coralline algae can contribute more than corals to coral reef carbonate production. Communications Earth & Environment, 4(1). doi:10.1038/s43247-023-00766-w
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