Abstract Summary
Marine cloud brightening is an intervention which could potentially mitigate bleaching of heat stressed coral reefs. The proposal is that providing additional cloud condensation nuclei (CCN) in the form of nano sized salt crystals derived from evaporated sea water droplets to the marine boundary layer will increase the reflectivity of low lying marine stratocumulus clouds. By increasing the cloud albedo, incoming shortwave solar radiation reaching the sea surface is reduced, with the integrative effect that over some days to months ocean mixed layer temperatures are lowered compared to otherwise. A potential synergistic benefit is decreased irradiance (shading) during times of heat stress. The response of the Great Barrier Reef hydrodynamic heat flux budget and coral biological response to scenarios of various intensities and scales of imposed cloud brightening is considered using the CSIRO developed eReefs biogeochemical ocean model. Reduction to the shortwave radiation input term is partially offset by reductions in longwave radiation, sensible heat flux, and latent heat flux loss terms, resulting in net cooling. The reduction in sea surface temperatures achieved is found to depend on the amount of suitable cloud cover (the first and second indirect aerosol effect), concentration and reflectivity of aerosols (the direct aerosol effect) and is a function of the amount of time a given water parcel spends under ‘brightened cloud’. Thus, there are regional and inter-annual differences in both hydrodynamic and biological response to cloud brightening.
