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Hope, but no free pass, as Pacific corals show tolerance to warming oceans

A coral reef in southern Pacific.

A coral reef in southern Pacific. Image by Tracey Jennings / Ocean Image Bank.

  • New research suggests that coral reefs in the Pacific islands of Palau are becoming increasingly tolerant to thermal stress brought on by climate change.
  • The study found that Palau’s coral reefs appeared to suffer less bleaching over three successive marine heat waves in 1998, 2010 and 2017.
  • While the findings provide some hope for coral reefs, one expert says the study has some limitations in providing a clear picture of how corals respond to different heat events.
  • Scientists also say that reducing carbon emissions is essential to safeguard coral reefs — and to secure the planet’s future.

As climate change raises global sea temperatures, coral bleaching events are becoming more frequent and intense around the world. Yet researchers say some coral reefs may have found a way to cope with this thermal stress, offering a “glimmer of hope” for the future.

In a new study published in Nature Communications, a team of international researchers used temperature data and field observations to find that a remote coral reef system in Palau, a Pacific nation of more than 300 islands, has become increasingly tolerant to thermal stress. They noted that Palau’s coral reefs increased their thermal tolerance by 0.1° Celsius (0.2° Fahrenheit) each decade since the late 1980s. However, according to the research, it’s unclear if this rising tolerance rate will match the pace of ocean warming.

Palau’s reefs experienced powerful marine heat waves in 1998, 2010 and 2017, yet each successive event led to less coral bleaching, the study found. Similar events have previously been recorded on Australia’s Great Barrier Reef, the coral reefs of Southeast Asia, and in Mo‘orea in French Polynesia.

The researchers say there could be several explanations for the Palauan corals growing tolerant to heat. One possibility is that the reefs are becoming dominated by hardier coral species while more sensitive corals are disappearing. A second explanation is genetic adaptation — that is, natural selection may favor the genes in corals that are more likely to survive heat stress. A third suggestion is that coral reefs acclimate to thermal stress, managing to withstand high-level thermal stress after exposure to low-level heat. This acclimatization could happen to the corals themselves and the symbiotic algae that live with the corals, the study suggests.

“This apparent increase in tolerance that we’ve mentioned in this recent study could be due to a lot of things,” study lead author Liam Lachs, a researcher at Newcastle University in the U.K., told Mongabay. “I think the next big challenge is trying to disentangle what the drivers were and if there are any associated risks, trade-offs, or costs. For instance, losing all of the thermally sensitive species is obviously a big cost because you also lose the ecological function of those species on the reef.

“I think this work opens up a lot more questions than it answers, which I guess is always the case with science,” he added.

A Palau island.
According to the research, it’s unclear if this rising tolerance rate will match the pace of ocean warming. Image by denAsuncioner via Flickr (CC BY-ND 2.0).
A coral reef in Palau.
Palau’s reefs experienced powerful marine heat waves in 1998, 2010 and 2017, yet each successive event led to less coral bleaching. Image by Kuruman via Flickr (CC BY 2.0).

Lachs said in a statement that the study offers a “glimmer of hope.” Still, he said it also highlights the urgent need for reducing carbon emissions “to mitigate climate change and secure a future for these vital ecosystems.”

Large swaths of the world’s oceans are currently experiencing marine heat waves in response to climate change and the current El Niño climate pattern that’s playing out. While Lachs said that Palau’s coral reefs seem to be doing fine at the moment, other parts of the world, including the southeastern United States and the Caribbean, are experiencing intense thermal stress. Some corals are already bleaching; more is expected to occur in the coming months.

Emily Darling, a coral reef scientist at the Wildlife Conservation Society (WCS), who was not involved in the study, praised the research for finding that corals in Palau may have a biological mechanism for dealing with heat stress.

“This study is important because, for one of the first times, it’s comparing the impact of repeated mass bleaching events on corals,” Darling told Mongabay, “and it has a really nice data set of both underwater monitoring — so what is actually happening for the corals in water — and then comparing that to what is actually happening with the bleaching environment, both the temperature as well as the light, which we know are two real key stressors for corals.”

However, she said the study needed more “nuanced indicators or metrics” to provide a clearer picture of how corals respond to different heat events. Additionally, she said the study didn’t look at total coral cover, a standard coral reef health metric. An examination of entire coral cover would have enabled the researchers to understand if the corals appeared to bleach less in each successive event due to there simply being less coral, or if more tolerant corals replaced the sensitive ones, she said.

Darling said the study also showed that corals’ increased tolerance to heat stress will only buy us time but won’t be a long-term solution to marine heat waves brought on by climate change.

“This study is hopeful … but it’s not a free pass for corals,” Darling told Mongabay. “It’s not an excuse for business as usual. It is a sign that nature is incredibly resilient, but we need to get our act together now if we want to have coral reefs, healthy, functioning oceans, as well as a functioning planet for all of us to live on.”

Banner image: A coral reef in southern Pacific. Image by Tracey Jennings / Ocean Image Bank.

Elizabeth Claire Alberts is a senior staff writer for Mongabay. Follow her on Twitter @ECAlberts.

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Lachs, L., Donner, S. D., Mumby, P. J., Bythell, J. C., Humanes, A., East, H. K., & Guest, J. R. (2023). Emergent increase in coral thermal tolerance reduces mass bleaching under climate change. Nature Communications, 14(1). doi:10.1038/s41467-023-40601-6

Guest, J. R., Baird, A. H., Maynard, J. A., Muttaqin, E., Edwards, A. J., Campbell, S. J., … Chou, L. M. (2012). Contrasting patterns of coral bleaching susceptibility in 2010 suggest an adaptive response to thermal stress. PLOS ONE, 7(3), e33353. doi:10.1371/journal.pone.0033353

Hughes, T. P., Kerry, J. T., Connolly, S. R., Álvarez-Romero, J. G., Eakin, C. M., Heron, S. F., … Moneghetti, J. (2021). Emergent properties in the responses of tropical corals to recurrent climate extremes. Current Biology, 31(23), 5393-5399.e3. doi:10.1016/j.cub.2021.10.046

Pratchett, M. S., McCowan, D., Maynard, J. A., & Heron, S. F. (2013). Changes in bleaching susceptibility among corals subject to ocean warming and recurrent bleaching in Moorea, French Polynesia. PLoS ONE, 8(7), e70443. doi:10.1371/journal.pone.0070443

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