Coral reefs put up "cloud umbrellas" to keep cool

We've recently seen a number of proposals for protecting coral reefs from the dangers of climate change, from giant fans that would pump cold water over the coral, to floating films that reduce the intensity of sunlight reaching it. It now turns out that corals already have a self-defense mechanism, after scientists discovered that the marine invertebrates can release aerosols into the atmosphere to create a protective "cloud umbrella."

It's no secret that coral reefs are under threat from the ravages of climate change. The world's biggest structure created by living organisms, the Great Barrier Reef, suffering mass bleaching events in both 2016 and 2017, with fears this trend will continue in 2018. These events have been due to above average sea surface temperatures during the summer, and with oceans predicted to continue to warm due to climate change, the severity and frequency of bleaching events is set to increase.

Now a team made up of scientists from Griffith University, Southern Cross University and the University of Southern Queensland has discovered that corals on the Great Barrier Reef have been using a previously unknown technique to try and keep cool. Field and lab work, and satellite data of an area around Heron Island roughly 100 sq km (37 sq mi) in size, revealed that during low tides, when the corals were under stress due to increased sunlight reaching them, they released volatile compounds into the water that ultimately ended up as aerosols in the air above them.

"Until recently, scientists didn't really appreciate that corals could produce these compounds that end up in the atmosphere and are converted through chemical reactions to aerosols," says Associate Professor Albert Gabric from the Griffith School of Environment and Science. "What happens to these aerosols when they get into the atmosphere is that they reflect incoming solar radiation and can also modify cloud microphysics – they can make clouds brighter and they can make clouds hang around for longer, so it's sort of like creating a natural umbrella for the reef."

Aside from the fact such "cloud umbrellas" obviously don't provide enough protection to prevent all bleaching, the researchers are also worried about the wider implications of the discovery. They know that the compounds produced by the corals are similar to those produced from the burning of fossil fuels and reflect incoming solar radiation in the same way, as well as changing the lifetime and reflectivity of clouds. What isn't known is what effects changes to future aerosol emission – be they natural or manmade – will have.

"The thing we know least about climate change is how both natural and anthropogenic aerosol emissions will change in the future," says Gabric. "This is critically important because aerosols appear to have a cooling affect, canceling or masking the warming due to greenhouse gases. And that's the trillion-dollar question – if countries stop producing these polluting aerosols what will happen to the underlying warming?"

Because such aerosols only last in the atmosphere for a few weeks, the researchers fear worldwide efforts to curb manmade emissions could actually result in an abrupt increase in temperatures in the short term as the cooling effect of the aerosols is removed. If that were the case, Gabric says is "could be disastrous" for the reef, and that geoengineering interventions, such as spraying an artificial aerosol in the form of sea salt into the air above the reef, may need to be considered.

"But on a large scale this would be quite expensive, so we need further research on how best to maintain and encourage natural coralline aerosol emissions," Gabric says.

The team's research appears in the journal Ambio.

Source: Griffith University