Cooling and shading


Marine cloud brightening

Marine cloud brightening is one of the most innovative and promising large-scale interventions being investigated by the RRAP R&D Program.

It could protect the entire Great Barrier Reef from coral bleaching in a relatively cost-effective way, buying precious time for longer-term climate change mitigation.

Marine cloud brightening harnesses natural processes, using sea spray to increase the amount of light and heat that clouds deflect from the sun, to cool and shade the coral reef below.

Microscopic sea water droplets are sprayed into the air, creating a mild fog which deflects solar energy. It also seeds the clouds, increasing their capacity to deflect sunlight.

‘Brightened’ clouds may remain above the Reef longer than typical clouds, providing increased protection.

Similar technology – regional cloud modification – is already used around the world, including by both Snowy Mountains Hydro and Tasmania Hydro, to produce hydroelectricity.

Cloud brightening equipment is similar to that of snow-making machines, except filtered seawater is sprayed into the air, to form microscopic droplets.  

The inputs are seawater and energy (which in the long term could come from renewable sources such as solar, wind or current energy).

Modelling shows that to be effective, cloud brightening units would need to operate over weeks to months, lowering the temperature a little each day, when bleaching risk was high.

Potential negative effects could include influencing local weather patterns, potentially mildly suppressing rainfall over the reef and adjacent land. However, cloud brightening can be easily stopped, with no long-term impact on weather patterns.

Cloud brightening would be deployed in collaboration with other intervention methods to ensure the Reef was continuing to adapt and increase its resilience to climate change.

Cloud brightening is intended to be a temporary protective measure while global action is taken to stabilise temperature and corals on the Great Barrier Reef have built sufficient resilience to thrive in a warmer environment.

Significant feasibility, engineering and experimental work is required, and regulatory and social licence issues must be addressed, before cloud brightening could be implemented. This is expected to take around 10 years.

Cloud brightening technology will be of global interest, as it will be likely to have many applications in a warming world.


Diagram by Dr Daniel Harrison, University of Sydney


Fogging can mimic the effects of sea fog, which – like clouds – reduces the amount of sunlight and heat reaching the sea surface and the coral reefs beneath.

A fog can be generated by adding liquid particles (seawater) into the air.

Fogging can be used at a local scale (individual reefs) to provide shading, or at regional scale to shade and cool coral reefs.

Preliminary modelling shows fogging could be effective in protecting targeted areas, over short critical periods (days to weeks).

It could be delivered using similar methods to cloud brightening.

Potential negative effects include over-shading which may slow coral growth. It would be ineffective during strong wind; however, bleaching is most likely to occur during calm conditions.

Fogging can be easily stopped, with no long-term impacts on weather patterns.

There are some commercially-available generators that would need to be modified to increase their scale and ability to run over extended periods.

Regulatory and social licence issues must be addressed, before fogging could be implemented. Fogging with seawater is expected to be very environmentally low risk.

Fogging and cloud brightening may work well together to reduce solar radiation on reefs across a wide range of atmospheric conditions.

Surface shields

Surface shields consisting of calcium carbonate – the same material as coral skeletons – can substantially reduce the amount of light penetrating the water surface and could protect some corals from bleaching.

The reflective calcium carbonate is held at the surface by a molecule-thick surface film.

The shields could be applied to individual, high-value reefs on clear, still days when bleaching conditions were at their worst.

Similar technology is currently used to prevent evaporation in reservoirs and dams, including those used for drinking water.

Sea trials show the surface shield can reduce light by more than 20 percent, while large-scale aquarium studies demonstrate stability over a day, no harmful effects on corals, and protection of some species from bleaching.

The surface shield need only be applied periodically (by airplane, sea vessel or automated buoy) when bleaching conditions are predicted.

Further development and testing is required.


Photo by Phil Mercurio

Mixing and pumping of cool water

Mixing and pumping cool water could reduce heat stress in corals. The surface water that surrounds coral reefs is warmer than adjacent deeper water. If, during the warmest times of the year, deeper, cooler water is mixed with warmer surface water, thermal stress of corals could be reduced and coral bleaching potentially avoided.

This vertical mixing can occur naturally—upwelling—with evidence it has reduced coral bleaching. The relatively fast currents across reefs, and engineering costs would likely constrain this method to localised high-value sites.

Investigations during the RRAP Concept Feasibility Study found that even for the most physically-favourably reefs, these methods were expensive and could not be scaled to a size that would have a meaningful positive impact on the Great Barrier Reef. This technology will therefore not be further investigated by the Reef Restoration and Adaptation Program.



Photo by Christian Miller