A new climate engineering (CE) method will be described where cirrus clouds are removed to cool the planet. Cirrus clouds have a warming effect on climate since they effectively trap outgoing longwave radiation (OLR) due to their cold temperatures. They also reflect sunlight, but the OLR effect is stronger. If the cirrus cloud optical depth in the thermal infrared window region can be reduced and their cloud coverage reduced, then more OLR can escape to space, cooling the planet.
This is accomplished by seeding cold (T < -40°C) cirrus clouds with efficient ice nuclei such as silver iodide (AgI) or bismuth tri-iodide (BiI3), which is as effective as AgI for T < -20°C (Dr. Bill Finnegan; private communication). At these temperatures most ice crystals appear to result from homogeneous freezing nucleation, but seeding cold cirrus with AgI or BiI3 would replace homogeneous nucleation with heterogeneous nucleation processes, resulting in fewer but larger ice crystals. The larger ice crystals have higher fall speeds, which reduces cirrus optical depth and decreases cirrus cloud coverage. This kind of seeding operation would require seeding an air mass prior to cirrus formation, allowing an optimal background concentration of ice nuclei to become established. This practice would initially produce more cirrus clouds in regions of clear sky supersaturated with respect to ice. But over time, with larger ice crystals forming with higher fall speeds, the flux of water substance to lower levels would increase, drying out the upper troposphere (UT). This reduces the UT relative humidity (RH). Thus, OLR increases for two reasons: less cirrus and lower RH in the UT. Global climate model (GCM) simulations have confirmed this.
Results from a recent cirrus field program (SPARTICUS) have been used to describe the effective ice particle sizes (De) and ice fall speeds in cirrus clouds in two GCM experiments. From the field experiment results, the De-T relationship for heterogeneous nucleation processes for T < -40°C was estimated. The observed De-T relationship is for heterogeneous nucleation processes for T > -40°C and for a mixture of homo- and heterogeneous processes for T < -40°C. These experiments were designed to estimate the maximum amount of cooling possible by seeding cirrus clouds. The two GCM experiments were carried out on the Community Atmosphere Model version 5 (CAM5) and the German GCM ECHAM5. The CAM5 results show weak cooling over the mid-latitude and Polar Regions (~ 0.5 W m-2) while the ECHAM5 results show stronger cooling (~ 2 W m-2) in these regions. Note that the cooling needed to neutralize a doubling of CO2 is about 3.7 W m-2. Thus, it appears possible that cirrus cloud CE could be part of an effective CE strategy, but the treatment of cirrus clouds in GCMs needs improvement before more quantitative estimates can be made.