What is marine cloud brightening?
Marine stratocumulus clouds scatter incoming shortwave radiation from the sun and so influence the amount of heat energy reaching the earth surface. Clouds are formed by water vapour condensing on small airborne particles known as aerosols. While aerosol concentrations are typically very high over the land (100s – 1000s per cm3 of air) they can be very low over the ocean (10s per cm3 of air). When marine stratocumulus clouds form over the ocean their brightness is dependent on the number of aerosols which form cloud condensation nuclei (CCN). If more aerosols successfully nucleate the cloud will be brighter and reflect more heat energy back into space. Brightness of a cloud is measured by its albedo, an albedo of 0 is clear sky, while an albedo of 1 is all incoming shortwave radiation reflected away.
Marine cloud brightening is a technology designed to increase the brightness of clouds by mimicking natural processes, in this case the generation of aerosols by sea spray. It requires the generation of small salt-water droplets which evaporate in the planetary boundary layer to give a nano-scale grain of salt. These droplets are generated near the sea surface and are dispersed in the planetary layer by atmospheric turbulence. The planetary boundary layer lies closest to the surface of the Earth, and is typically 1 to 2 km thick.
If droplets of seawater are produced of around 2 µm in diameter, on evaporation of the water the remaining salt crystal is around 20 times less in diameter (hence around 100 nm). Studies are underway by our project partners in Silicon Valley. They are looking at ideas to generate droplets of the optimum size to brighten clouds.
Our researchers at the Sydney Institute of Marine Science and the University of Sydney are researching how effective marine cloud brightening could be at reducing bleaching on the Great Barrier Reef. We are also researching ways the technology could be implemented on the reef, how it could be deployed for minimum cost and maximum effectiveness, social, legal, and ethical aspects, as well as whether there is potential for any unintended effects.