Can cloud brightening rescue the Great Barrier Reef?

Last month, an ambitious project began in Australia to save the Great Barrier Reef from further bleaching due to global warming. By using harmless salt crystals to boost a naturally occurring process, researchers are hoping to protect the entire reef from further bleaching.

The co-called cloud brightening approach uses a modified turbine to spray minuscule salt particles into the air — the majority of them too tiny for the human eye to see — which, in theory, would mix with low altitude clouds and make them brighter. And these clouds would naturally reduce the temperature of the atmosphere and oceans by reflecting more of the sun’s energy away from the ocean.

Rising ocean temperatures are wreaking havoc on coral reefs. Indeed, ghostly white corals, once the backbone of a vibrant and colourful Great Barrier Reef, are one of the most shocking examples of the consequences of global warming. Record high temperatures in 2020 led to the third mass bleaching event in five years.

The project is part of an Australian government-backed initiative funding 43 different concepts to rescue the ailing Reef. However, the cloud-brightening strategy is viewed as one of the most promising approaches since it does involve the use of chemicals and instead relies on a completely natural process. According to Dr Daniel Harrison of Southern Cross University, who led the preliminary experiment, cloud brightening is one of the most innovative and promising methods for protecting large areas of the reef.

The concept is similar to the snow-making machines used by ski resorts to make snow, but instead of freezing to produce snowflakes, the much finer droplets evaporate leaving trillions of salt crystals that encourage more robust cloud formation.

“Nature does most of the work for you, Harrison explained. “Microscopic seawater droplets are sprayed into the air, evaporating leaving just nano-sized sea salt crystals which act as seeds for cloud droplets, brightening existing cloud and deflecting solar energy away from the reef waters when heat stress is at its maximum”.

“This makes a nano-sized salt crystal at hundreds of trillions per second. They get into a cloud and grow a cloud droplet that reflects a lot more sunlight”.

The team, including members from the Sydney Institute of Marine Science (SIMS), the University of Sydney, and Queensland University of Technology (QUT), deployed the first vessel off the coast of Townsville at the end of March to perform the first test at one-tenth of the scale they are aiming for.

The experiment was not designed to test whether the approach is effective, but to show that the approach does indeed work. Harrison says that in this respect, the team has succeeded. Next year, they plan to triple the size of the trial and to scale up even further the following year. Further research is still needed to examine the potential risks and to investigate the effects of cloud brightening on rainfall.

While the method is not foolproof, it is relatively cheap to implement and could buy some time to reduce emissions and slow down climate change. As Harrison explained: “Cloud brightening could potentially protect the entire Great Barrier Reef from coral bleaching in a relatively cost-effective way, buying precious time for longer-term climate change mitigation to lower the stress on this irreplaceable ecosystem”.