  Since air-water and water-air interfaces are equally refractive, cloud
droplets and microbubbles dispersed in bodies of water reflect sunlight in much
the same way. The lifetime of sunlight-reflecting microbubbles, and hence the
scale on which they may be applied, depends on Stokes Law and the influence of
ambient or added surfactants. Small bubbles backscatter light more efficiently
than large ones, opening the possibility of using highly dilute micron-radius
hydrosols to substantially brighten surface waters. Such microbubbles can
noticeably increase water surface reflectivity, even at volume fractions of
parts per million and such loadings can be created at an energy cost as low as
J m-2 to initiate and milliwatts m-2 to sustain. Increasing water albedo in
this way can reduce solar energy absorption by as much as 100 W m-2,
potentially reducing equilibrium temperatures of standing water bodies by
several Kelvins. While aerosols injected into the stratosphere tend to alter
climate globally, hydrosols can be used to modulate surface albedo, locally and
reversibly, without risk of degrading the ozone layer or altering the color of
the sky. The low energy cost of microbubbles suggests a new approach to solar
radiation management in water conservation and geoengineering: Don't dim the
Sun; Brighten the water.
