Cirrus cloud thinning

In 2021 the IPCC described CCT as a proposal "to reduce the amount of cirrus clouds by injecting ice nucleating substances in the upper troposphere." However it reported low confidence in the cooling effect of CCT, due to limited understanding of cirrus microphysics, its interaction with aerosols, and the complexity of seeding strategy. CCT may also increase global precipitation.

Basic principles

Typical cirrus clouds may be susceptible to modification to reduce their lifetime and optical thickness, and hence their net positive radiative forcing (in contrast to the typical low, warm liquid clouds). Material to seed such modification could be delivered via drones or by aircraft. Scientists believe that cirrus clouds in the high latitude upper troposphere are formed by homogeneous freezing, resulting in large numbers of small ice crystals. If effective ice nuclei were introduced into this environment, the cirrus may instead form by heterogeneous freezing. If the concentration of ice nuclei is seeded such that the resulting cloud particle density is less than that for the natural case, the cloud particles should grow larger due to less water vapor competition and attain higher settling velocities. By seeding with aerosols, ice crystals could grow rapidly and deplete water vapor, suppress nucleation and any growth of ice crystals by homogeneous nucleation. The net effect would be a reduced optical thickness and a reduced cloud lifetime, allowing more infrared radiation to be emitted at the top of the atmosphere, as the ice particles sediment out. Less upper tropospheric water vapor and infrared radiation in the atmosphere would consequently cool the climate.

Bismuth tri-iodide (BiI3) has been proposed as the seeding material, as it is effective as ice nuclei for temperatures colder than -10 °C, non-toxic and relatively inexpensive compared to e.g. silver iodide. The seeding aerosols would need to be added regularly, as it would sediment out along with the large ice crystals.