Researchers working on the so-called Cloud Brightening project say they use a turbine to spray microscopic sea particles to thicken existing clouds and reduce sunlight on the world's largest coral reef ecosystem.

The droplets evaporate leaving only tiny salt crystals that float into the atmosphere allowing water vapour to condense around them, forming clouds, says Daniel Harrison, a senior lecturer at Southern Cross University, who runs the project.

"If we do it over an extended period of time for a few weeks to a couple of months when the corals are experiencing a marine heatwave we can actually start to lower the water temperature over the Reef," Harrison said.

The project had its second trial in March, when the Reef off Australia's northeast is at its hottest, gathering valuable data on the atmosphere when corals are at most risk of bleaching.

A combination of light and warm water causes coral bleaching. By cutting light over the reef by six per cent in summer, "bleaching stress" would be cut by 50 per cent to 60 per cent on the undersea ecosystem, Harrison said.

But the benefits of cloud brightening would lessen over time unless other measures slowed the march of climate change.

"If we do have really strong action on climate change then the modelling shows that the cloud brightening is enough to stop the reef declining and to actually see it through this period while we reduce our carbon emissions," he said.

Australian Associated Press