Cooling the reef
Marine cloud brightening can work in two ways to help reduce bleaching on the Great Barrier Reef. Firstly, when the correct type of clouds are present, if they are brightened by providing additional nano sized salt crystals as condensation nuclei over several months during the summer the ocean surface waters can be cooled. Secondly, the ‘shading’ of the brighter cloud will relieve stress on corals, as it has been shown that bleaching is also related to irradiance of corals which are under temperature stress.
The GBR lagoon water flows through the reef and is warmer in summer months. If the solar radiation is reduced over the reef, such as by marine cloud brightening, the surface waters will be cooler. Clouds are formed by water condensing on small particles in the air, known as cloud condensation nuclei (CCN). There are some unanswered questions about the existing number of CCN over the reef, how many CNN are needed to sufficiently reduce the surface temperature and how often the conditions are right for cloud brightening and, therefore how effective cloud brightening would be in reducing the temperature.
The CSIRO in collaboration with government and industry has developed a comprehensive computer model of the Great Barrier Reef waters, called eReefs. This high resolution and well calibrated model is being used to investigate the effects of marine cloud brightening over the Great Barrier Reef in an investigation funded by the Myer Foundation.
One variable of relevance is the residence time of water in the reef lagoon. Because the marine cloud brightening will be localised to the reef, exchange of reef water with water from outside the reef will dilute the effect of the MCB. Luick et al. (2007) found that the water has a residence time in the GBR of about a month to a year.
Initial calculations indicated that a reduction of only around 5 to 10% of solar energy is needed to cool the water by 1 degree C. This is much less than the cooling one can feel on a sunny day when a cloud passes overhead. Theoretical calculations by Salter et al. (2008) predict that this reduction in solar radiation could be achieved with droplet densities within the range of naturally occurring concentrations. In a process that mimics the natural generation of sea spray from the ocean, the droplet generators would aim to use renewable energy (solar, wave, or current) to pump seawater, filter it, and emit it into the atmosphere as tiny invisible droplets that form cloud condensation nuclei. The droplet generators would be remotely controlled using cellular or satellite networks. Existing coral bleaching risk warning systems based on satellite remote sensing could be used to decide when the clouds need to be brightened.