Marine cloud brightening (MCB) is one form of cloud engineering. The idea is to make marine clouds reflect more solar radiation back into space, cooling the surrounding region. This would be achieved by spraying droplets of seawater into the sky, with sea salt crystals providing additional seed nuclei for water droplets to condense.16 This condensation into more droplets makes the clouds whiter and more reflective. A range of spray technologies have been considered.17
Various researchers have also proposed thinning and dispersing high-altitude cirrus clouds, which contribute to warming by trapping a disproportionately large amount of terrestrial radiation that would otherwise escape into space.18 Injecting these clouds with particles of bismuth tri-iodide allows the formation of large ice crystals within the clouds. These large ice crystals fall out more rapidly, shortening the clouds' lifespan. Cirrus reduction can improve the transmission of long-wave terrestrial radiation into space.19
Whereas SAI has a global effect, cloud engineering techniques produce more localised cooling, allowing them to be used in a targeted way. For instance, MCB could be used to cool major coral reefs, which suffer bleaching when water temperatures become too high.20 A field test of this has been conducted over the Great Barrier Reef, though there is no publication of the results from these experiments.21 The sea spray used may also produce a cooling effect in regions with little or no cloud, by forming reflective aerosol particles.22
