Cooling and shading

Marine cloud brightening

Marine cloud brightening aims to increase the amount of solar radiation (light and heat) that clouds reflect back into space, cooling and shading the reef below.

Cloud brightening replicates natural processes. It works by spraying microscopic sea water droplets into the atmosphere, allowing them to evaporate, resulting in clouds with a greater ability to deflect solar radiation. These ‘brightened’ clouds are less likely to produce rain, and would therefore remain above the reefs much longer than typical clouds, providing increased protection.

Similar technology – regional cloud modification - is already regularly used around the world, including by both Snowy Mountains Hydro and Tasmania Hydro to produce hydroelectricity. Cloud brightening stations can be thought of as similar to snow-making machines. A cloud brightening unit would require a fan for initial dispersal of the droplets into the atmosphere. The inputs are seawater and energy, which could be derived from renewable sources such as solar, wave or current energy. Modelling shows that to be effective, the units would need to operate over weeks to months when bleaching risk was predicted to be high.

Potential negative effects could include influencing local weather patterns – including potentially suppressing rain, reduced coral growth and impact on light-limited coral. Significant feasibility, engineering and experimental work is required.



Diagram by Dr Daniel Harrison, University of Sydney


Misting aims to mimic the effects of sea fog, which - like clouds - reduces the amount of sunlight and heat reaching the sea surface. It involves generating a mist by adding liquid particles to the atmosphere, similar to cloud brightening. Misting can be used at a local scale to provide shading, similar to surface films, or at regional scale, to provide shading and cooling of the sea surface temperature.

Preliminary modelling shows misting could be effective in protecting targeted areas, over short critical periods (days to weeks). It could be delivered through platforms mounted on small vessels. Potential negative effects include over-shading which may slow coral growth, and potential environmental impacts depending on the material used to generate the particles, of which there are many options. It would be ineffective during strong wind, however, bleaching is most likely to occur during calm conditions.

Misting is currently used in military operations. There are some commercially-available generators. Further work would be needed to scale these up and improve their reliability to run over extended periods.

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

Ultra-thin surface films

Ultra-thin surface films made from calcium carbonate (a key component of coral skeletons) and supported at the surface by buoyant organic materials, could reduce solar radiation. Similar technology is currently used to prevent evaporation in reservoirs and dams.

Tests so far indicate surface films remain stable for at least two days, reduce light by more than 20%, are not harmful to coral, and protect some species from bleaching. Such films require no permanent infrastructure and need only be applied periodically, when bleaching conditions are predicted.

The potential for negative effects such as over-shading, leading to reduced coral growth rate, are unlikely as the deployment periods are short. Further development and testing is required to improve light reduction, film longevity (stability and strength), deployment methods and to further assess environmental safety. Surface films could be applied to specific, high-value tourism or seed-reefs (which may in turn improve recovery of nearby reefs) by drone, airplane, sea vessel or automated buoy.



Photo by Phil Mercurio

Mixing and pumping of cool water

Mixing and pumping of cool water can reduce heat stress. 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 coral will be reduced and potentially, coral bleaching avoided.

This vertical mixing can occur naturally - ’upwelling’ - with evidence it has reduced coral bleaching. On a smaller scale, engineering structures such as ‘underwater fans’ could enhance vertical mixing to reduce surface water temperature. The relatively fast currents across reefs, and engineering costs would likely constrain this method to localised high-value sites. Alternatively, cooler water from the deep sea could be drawn, through pumping, to cool reefs. The larger temperature variation means less water would be required than the mixing method, however the engineering costs and challenges would be greater. The input of nutrient-rich deep water may boost productivity of reef systems, if these nutrients can be effectively understood and managed. However, these nutrients also pose one of the main ecological risks of introducing this method, as they could lead to algal blooms.

There is also a risk that some components of these nutrients could exacerbate bleaching. Reef systems would have to be extensively modelled, and the physiological mechanisms underpinning bleaching would need to be better understood to determine whether a safe balance of temperature reduction and nutrient limitation could be achieved. A solution could be to operate water cooling intermittently.


Photo by Christian Miller