Cold cloud thinning covers proposed climate interventions that looks to seed ice clouds with efficient ice nucleating particles, so they trap less outgoing longwave radiation. With cirrus cloud thinning, seeding aims to shift ice formation away from homogenous to heterogenous freezing nucleation, producing fewer but larger ice crystals that fall out faster, giving thinner, shorter-lived clouds. More recently, mixed-phase cloud thinning has sought to target supercooled clouds over the polar oceans in the winter, where cloud glaciation could reduce longwave radiation from reaching the surface and so help slow sea ice loss. Model estimates of the resulting cooling varies from net negative to net positive when clouds are overseeded, and much of this spread comes from processes that are difficult to constrain, such as the competition between nucleation modes, pre-existing ice, small-scale updrafts, and gravity waves.

This session focusses on work that helps improve our understanding of the ice cloud processes that decide whether cold cloud thinning could work, and what its side effects might be. We welcome laboratory studies of ice nucleation and possible seeding particles, field experiments including targeted glaciogenic seeding, in situ and satellite observations of cirrus and mixed-phase clouds (including early results from EarthCARE), and modelling from process- to global-scale climate models. Natural analogues are of interest, since mineral dust, volcanic eruptions and aviation all perturb ice clouds in ways that let us test seeding hypotheses in the atmosphere. Contributions on detectability, attribution, risks, and wider impacts are also welcome, and we encourage early career researchers to submit.