William Cotton Discusses Doped Jet Fuel
Use of commuter aircraft with their jet fuels doped with aerosol generators is another possibility. Also the use of UAVs or blimps for aerosol dispersal could be considered. Potential adverse consequences, however, are likely including impacts on precipitation, local cold temperature extremes (which would also impact fossil fuel demands) and the hydrological cycle. …
3.4 Seeding cirrus clouds or making more contrails
On an annual average clouds cover between 55 to 60% of the earth (Matveev 1984) and much of that cloud cover consists of middle and high clouds. It is thought that globally cirrus clouds contribute to warming of the atmosphere owing to their contribution to downward transfer of LW radiation. In other words they are a greenhouse agent. Human activity is already modifying the cirrus clouds through the production of aircraft contrails. Kuhn (1970) found that contrails depleted solar radiation and increased downward LW radiation but during the daytime their shortwave influence dominates and they contribute to a net surface cooling. Kuhn (1970) calculated that if contrails persist over 24h their net effect would be cooling. Others have concluded that they lead to surface warming (Liou et al. 1991; Schumann 1994) but Sassen (1997) notes that the sign of the climatic impact of contrails is dependent upon particle size. Global estimates of the effects of contrails are they contribute to a net warming (Minnis et al. 2004).
It has even been proposed to seed in clear air in the upper troposphere to produce artificial cirrus which would warm the surface enough to reduce cold-season heating demands (Detwiler and Cho 1982). So the prospects for seeding cirrus to contribute to global surface cooling do not seem to be very good.
The only approach that might be feasible is to perform wide-area seeding with soot or carbonaceous aerosols which would absorb solar radiation and warm cirrus layers enough to perhaps dissipate cirrus clouds (a semi-direct effect). This strategy would be similar to that proposed by Watts (1997) and Crutzen (2006) for implementation in the stratosphere. As noted by Crutzen (2006) only 1.7% of the mass of sulfur is needed to produce a similar magnitude of surface cooling. Application at cirrus levels in the upper troposphere would have the double benefit of absorbing solar radiation thus contributing to surface cooling and dissipating cirrus clouds which would increase outgoing longwave radiation. Of course, the soot that becomes attached to ice crystals will reduce the albedo of cirrus thus countering the longwave warming effect to some degree. In addition, there is evidence that soot particles can act as ice nuclei, thus contributing to greater concentrations of ice crystals by heterogeneous nucleation but possibly reduced crystal production by homogeneous nucleation (DeMott et al. 1994; Kärcher et al. 2007). Thus it would be best to engineer carbonaceous aerosol to be ineffective as IN.
The possible adverse consequences of such a procedure can only be conjectured at this time but are mostly likely to impact the hydrological cycle. Complex chemical, cloud-resolving, and global models are required to evaluate the feasibility of this approach and to estimate possible adverse consequences. The feasibility of this approach in terms of implementation strategies is probably comparable to seeding sulfates in the lower stratosphere. The costs would be similar to Crutzen’s estimates for stratospheric seeding.