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This article was made possible with sponsorship from Greenpeace Australia Pacific.

Creating brighter clouds over the Great Barrier Reef could help to protect it from devastating marine heatwaves. That’s the idea of engineers who are already testing the technology in offshore trials.

Brighter clouds reflect more sunlight back into space and so help to shade and cool anything below, including coral. Oceanographer and engineer Daniel Harrison believes this idea is the most promising of the cooling and shading approaches that he oversees as part of the Reef Restoration and Adaptation Program, a partnership of Australian universities, research agencies and park managers that is exploring ways to save the reef. So how do you create clouds that are brighter?

Clouds form when water vapour condenses around tiny particles in the air that nucleate droplet formation, explains Zoran Ristovski, an atmospheric scientist at Queensland University of Technology in Brisbane.

Too few nuclei and the droplets are large, creating dark clouds. But increasing the number of nuclei allows water to form into smaller droplets, creating white, fluffy looking clouds. “The smaller the droplets, the whiter the clouds and the better they are at scattering the light,” says Ristovski.

Ocean Cannons

He and Harrison have tested the idea over the ocean using repurposed snow cannons to shoot tiny droplets of ocean water into the air—around 900 trillion of them per second. That’s just the equivalent of a shot glass full of liquid but this proof-of-concept work is promising. The team say they have measured a significant increase in cloud condensation nuclei over several kilometres of water using the technique.

If they are able to scale up the technology, the idea is to use these machines to create clouds that keep the water cool throughout the hottest weeks or months of the summer.

Atmospheric models suggest that reducing sunlight over the reef by just six per cent could reduce water temperatures by 0.7 degrees Centigrade. That’s enough to prevent nearly all the coral death caused by bleaching at current levels of warming, says Harrison.

Other cooling and shading approaches include spraying salt crystals into the air to create a haze over the reef or covering the ocean with a thin biodegradable polymer film carrying particles of calcium carbonate. Some trials suggest this could block up to 30 per cent of the light from filtering through, although only over relatively small areas.

However, the scale of the challenge is huge. Critics point out that the Great Barrier Reef Marine Park is the size of 70 million football fields – or 350,000 square kilometres.

Then there is the question of unintended consequences. One possibility is that brighter clouds over the ocean could reduce rainfall on land. “The last thing you want is to save the reef but change the level of rainfall in northwestern Queensland,” says Ristovski. The clouds should disperse quickly once the machines are switched off but preventing problems like this in the first place will require careful modelling.

That’s why these ideas are at best “life-support” for the reef, says Harrison. The only way to save the coral is to tackle climate change.  “In a business-as-usual future, where we keep on burning fossil fuels, there is really nothing we can do to save the reef,” he adds.

For more in this series, visit The Future of the Great Barrier Reef hub.