Hakai Magazine

Close-up of a soft coral branch (Dendronephthya sp), Great Barrier Reef, Australia
Some research from Australia’s Great Barrier Reef, where this soft coral grows, suggests that the physiology of corals can help cool the waters around them. Photo by Roberto Rinaldi/Minden Pictures

Climate Change Could Be Breaking a Natural Coral Reef Thermostat

Some ocean algae spew gases that encourage cooling clouds; but when they get too hot, they seem to stop.

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by Richard Kemeny

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The 2,300-kilometer Great Barrier Reef fringing Australia’s northeast coast might function like a giant thermostat, regulating the temperature of its environment. Yet as scientists work to uncover more about this built-in mechanism, climate change might be breaking it.

In the late 1980s, a group of scientists hypothesized that some phytoplankton and algae, including the zooxanthellae that live in corals, employ a biological feedback loop that might control the water temperature around them. When stressed by heat, zooxanthellae produce compounds that make a gas called dimethyl sulfide (DMS), which creates an aerosol layer just above the water. The fine particles in this aerosol layer scatter sunlight. They also act as the nuclei for the formation of tiny water droplets that tend to make low, bright clouds that shade and cool the water’s surface.

While all the steps in the feedback loop are known, it isn’t clear whether coral algae can actually regulate the temperature of their environment in this way. For instance, they may not trigger the production of enough DMS to make a difference, or perhaps something more complex is going on and temperature dampening doesn’t happen at all.

Rebecca Jackson, an oceanic and atmospheric scientist at Griffith University in Australia, studied nearly two decades of data from the Great Barrier Reef to find out more. Jackson’s team analyzed all the available data on coral stress, sea surface temperature, and the thickness of the aerosol layer to see what patterns emerged. Aerosol amounts increased when water temperatures went up with the seasons and other natural variations, they found. Meanwhile, other work has shown that the water temperature over the long term has increased a little less over the reef than elsewhere. Together, these findings support the idea that the corals were stimulating aerosol production and that some form of self-regulation is at play, the researchers argue.

Jackson’s team is currently looking at whether they can actually see an effect on low cloud cover from the amount of aerosols in the region. “This is exciting as it may provide some direct evidence,” Jackson says.

Importantly, there was also a worrying trend in the data. Once water temperatures shot up past the tolerance level for corals—about 27 °C—aerosol levels began to fall. This, the researchers argue, suggests that the mechanism has a tipping point after which it no longer works.

Jackson hypothesizes that as the algae living in corals get increasingly stressed, they use up almost all the compounds that make DMS to soak up other harmful compounds in their system, leaving very little to be released into the environment. After this point, temperatures continue to rise and corals become even more stressed—a positive feedback loop that might explain increased mass bleaching events. In their study, the researchers found that aerosol levels dropped significantly just before each of six mass bleaching events.

Peter Liss, an environmental scientist and professor emeritus at the University of East Anglia in the United Kingdom, thinks the results are interesting but speculative for now. If the DMS feedback loop only works sometimes, and is “broken” other times, he notes, that might explain why it has been so hard to actually observe it in the real world.

The findings could help predict the future resilience of corals to climate change and feed into research on proposed geoengineering techniques—some scientists have suggested spraying aerosols into the air above coral reefs to protect particularly vulnerable or ecologically important regions from bleaching, for example. “It’s important that we understand if and how natural aerosol-cloud feedbacks work so that we can accurately inform these types of strategies,” says Jackson.