Scientists have carried out a trial of prototype cloud brightening equipment on the Great Barrier Reef they hope could be scaled up to shade and cool corals and protect them from bleaching caused by rising global temperatures.
The experiment used a modified turbine with 100 high-pressure nozzles to spray trillions of nano-sized ocean salt crystals into the air from the back of a barge.
In theory, the tiny salt crystals are able to mix with low-altitude clouds, making them brighter and reflecting more sunlight away from the ocean surface.
The trial was not designed to test the effectiveness of cloud-brightening itself, but Daniel Harrison of Southern Cross University, who led the project, told Guardian Australia it had successfully demonstrated that the delivery system worked.
Between 25 and 28 March, Harrison and a small team of researchers from the university and the Sydney Institute of Marine Science carried out the experiment beside Broadhurst reef off Townsville, in Queensland.
Researchers from the University of Sydney and Queensland University of Technology were also on hand to test the prototype, developed in partnership with EmiControls of Italy. Several other researchers, including an EmiControls representative, were unable to join the crew as planned because of Covid-19 travel restrictions.
A separate vessel 5km away carrying atmospheric modelling equipment was able to detect the mist created by the prototype. Future experiments will measure if the salt particles do brighten clouds.
The cloud-brightening approach is just one of 43 concepts being funded under a $150m government-backed research and development program announced on Thursday.
Scientists are racing to find measures that could be used to reduce the impact of rising ocean temperatures on corals caused by global heating.
In 2020, the Great Barrier Reef experienced its third outbreak of mass coral bleaching in five years. Tropical coral reefs are especially sensitive to global heating. The UN’s Intergovernmental Panel on Climate Change says 70-90% of coral reefs will die as global heating gets to 1.5C.
Harrison said the technology, deployed in March under a permit from the Great Barrier Reef Marine Park Authority, had promise because it was relatively cheap, could be deployed at scale and kick-started a process that occurs naturally.
“Nature does most of the work for you,” he said. “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.”
Harrison said the technique was effectively boosting a natural process, because clouds mostly form over the ocean when moisture gathers around salt crystals stirred up by winds from the ocean surface.
He said while a cloud of mist was visible off the back of the boat, about 99% of the particles created by the nozzles were too small to see. However, they could be detected by atmospheric measuring equipment on the accompanying vessel.
“We thought we might only be able to detect it a couple of kilometres downwind, but we detected it 5km downwind,” Harrison said.
Scientists on the barge said they could see corals “bleaching around us” as they carried out the experiment.
Harrison and the core team drove 3,600km north to Townsville from Coffs Harbour, and back, camping and preparing their own food along the way to remain isolated from others not involved in the project as Covid-19 travel restrictions began to be rolled out.
Future research will also examine any downstream risks from the technique, and any local impacts on rainfall.
Manduburra traditional owner Usop Drahm, who joined the expedition, said: “We welcome scientific research where Indigenous people and the rest of Australia work together to maintain the reef ecosystem for future generations.
“This technology might help prevent bleaching and we like that it uses no chemicals and relies on natural processes.”
Harrison said while the trial was not set up to detect if clouds had been brightened, “the theory says [the particles] would have mixed up to the heights of low-level clouds about 800 metres up”.
The salt crystals would have remained in the air for only one or two days in the initial experiment, he said. The approach does not make clouds, but brightens those already in the atmosphere.
There are plans to scale up the experiment using more and larger turbines so that their output is about 10 times larger.
This, Harrison said, could cover an area of hundreds of square kilometres – a scale large enough to slightly cool ocean temperatures.
Within four years, Harrison said, it was hoped the project would show a brightening response in the clouds.
But he said the success of such projects would depend on action to cut greenhouse gas emissions.
He said as temperatures went up, the cloud brightening technique became less and less effective at protecting corals from mass bleaching.