Contrail Cirrus Cloud Seeding and Thinning
“Jon Egill Kristjansson told us about the fine line a potential cloud seeder would have to walk by seeding small, homogenous ice nuclei in cirrus clouds, which then cool the planet by letting more long-wave radiation out, but making sure not to “over-seed“, as injecting too many nuclei would mean reduced solar reflection and subsequently more warming. We heard that although initial modeling on cirrus cloud “thinning” in this manner indicates that the method is scientifically feasible, as one member of the audience pointed out, very little is known about its technical feasibility. The creation of small, homogenous ice nuclei in cirrus clouds could cool the planet, but a multitude of questions remain regarding how, where and under what conditions such particles can be successfully created.”
TRM: Cirrus cloud seeding
- Idea: inject highly efficient ice nuclei into cirrus forming regions.
- Create competition effect between homogeneous and heterogeneous ice formation.
- Larger and heavier ice crystals can form
- Ice cloud depletion
- Removal of longwave trapping cirrus and upper tropospheric water vapour.
- Suggested seeding material:
- Bismuth tri-iodide, BiI3
- Cheap’ish and non-toxic.
- Seeding via commercial airliners?
- Advantage: seeding aerosol residence time is relatively short in the troposphere.
- Drawback: does not address ocean acidification issue.
Because of the competition of these various factors, the radiative forcing of cirrus cloud seeding varies between −1.8 and +2.1 W m−2. Thus, if cirrus seeding is not done carefully, the effect could be additional warming rather than the intended cooling.
Contrails during day cause cooling because of reflecting of sunlight back into space. During night, they trap infrared heat causing heating. So it is a balance between the two time intervals. We would like to have more CICs (contrail-induced cirrus clouds) during day and none during night.; FAA Scientist: We Want Clouds By Day, None By Night
“The maximum cirrus seeding potential would be achieved by removing all cirrus clouds,” they write. “If cirrus thinning works, it should be preferred over methods that target changes in solar radiation, such as stratospheric aerosol injections, because cirrus thinning would counteract greenhouse gas warming more directly.”
If the time and place of seeding is selected with care, the climate effect of cirrus thinning can be enhanced. For that, only the long-wave warming effect of cirrus clouds should be targeted, and their solar effect should be avoided. This can be achieved if seeding is limited to high-latitude winters or to nighttime seeding. 
Climate Change and Geoengineering: Artificially Cooling Planet Earth by Thinning Cirrus Clouds