Clouds which contain more particulates are better able to reflect solar radiation away from the earth's surface: the particulates are sites where water droplets condense and brighten the cloud. Particulates can come from many sources, both natural (sea spray, dust) and man-made (soot and other pollutants). Marine cloud brightening is a technique that has been proposed to spray particles of sea salt into low lying clouds to increase their reflectivity in this way. Generally this has been for planet-wide geoengineering to counteract the effects of global warming , but more recently this has been explored for use on a smaller scale to brighten the clouds over coral reefs and reduce the amount of sunlight reaching them . This still has to be used on a large area to have much effect, around 10,000 square miles , but it has the advantage of being the only known technique that can be easily applied to help corals on such a scale.
There doesn't seem to be much information available about how effective this is expected to be when deployed to help reefs.
- John Latham, Philip Rasch, Chih-Chieh (Jack) Chen, Laura Kettles, Alan Gadian, Andrew Gettelman, Hugh Morrison, Keith Bower, Tom Choularton, Global temperature stabilization via controlled albedo enhancement of low-level maritime clouds, Phil. Trans. R. Soc. A (2008)