Cloud reflectivity modification is a proposed process of altering clouds to make them more or less reflective, in order to change their effect on climate.
There are two hypothesised forms: cirrus stripping and marine cloud brightening. High, cold cirrus clouds tend to allow sunlight through, yet reflect infra-red from the Earth's surface. Therefore, thinning or removing these clouds reduces their reflectivity to infra-red and results in a net cooling effect on the Earth's climate.
Conversely, low cloud tends to be warm and highly reflective to sunlight, especially when the cloud is white due to the presence of smaller droplets (because of the Twomey effect). Therefore, modifying these clouds to make them more reflective cools the climate. This proposed technique is known as 'marine cloud brightening' or 'cloud whitening' on low cloud.
By modifying the reflectivity of clouds, the albedo of the Earth would be altered. The intention is that this technique, in combination with greenhouse gas emissions reduction (and possibly other geoengineering techniques) will be sufficient to control global warming. The effect is expected to be fully reversible, as the cloud condensation nuclei particles precipitate naturally. However, like any planetary-scale project dealing with the complex climate system, there is a non-trivial risk of unintended consequences.
Read more about Cloud Reflectivity Modification: Basic Principles, Further Research, Reducing Cirrus Trapping of Outgoing Infrared Radiation, See Also
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