tokens: transfer, to atmospheric chemistry and dynamics, and they influence—if not control—aspects such as the surface temperature and, thus, the potential habitability of a terrestrial planet (Marley et al. 2013 ). Understanding the impact of clouds is thus instrumental for the study of planetary climates. Clouds composed of carbon dioxide (CO 2 ) ice particles are important for cold, CO 2 -rich atmospheres. This includes, for example, the present and early Mars (Forget Pierrehumbert 1997 ), the early Earth (Caldeira Kasting 1992 ), or terrestrial exoplanets at the outer boundary of the habitable zone (HZ; Kasting et al. 1993 ; Forget Pierrehumbert 1997 ). Provided that condensation nuclei are available, such clouds would form easily in a supersaturated atmosphere. Such as in the case of water clouds (e.g., Kitzmann et al. 2010 ), the presence of CO 2 clouds will first result in an increase of the planetary albedo by scattering incident stellar radiation back to space, which leads to a cooling effect (albedo effect). In contrast to liquid or solid water, CO 2 ice is mostly transparent with respect to absorption in the infrared (Hansen 1997 , 2005 ). Thus, as argued by Forget Pierrehumbert ( 1997 ) or Kasting et al. ( 1993 ), a classical greenhouse effect by absorption and re-emission of thermal radiation is unlikely to occur for a cloud composed of dry ice. On the other hand, CO 2 ice particles can efficiently scatter thermal radiation back to the planetary surface, thereby exhibiting a scattering greenhouse effect. In their seminal study, Forget Pierrehumbert ( 1997 ) showed that this scattering greenhouse effect of CO 2 ice clouds alone would have been efficient enough to allow for liquid water on the surface of the early Mars. This strong heating effect was also found in later studies by, e.g., Pierrehumbert Erlick ( 1998 ), Mischna et al. ( 2000 ), or Colaprete Toon ( 2003 ). However, by using three-dimensional general circulation models, it has been suggested, that a limiting factor for the impact of CO 2 clouds on early Mars might be the cloud coverage, which could be well below 100% (Forget et al. 2013 ). In particular, the outer boundary of the classical HZ might also be influenced by the formation of CO 2 ice clouds and their corresponding climatic impact. For the Sun, Forget Pierrehumbert ( 1997 ) reported an outer HZ boundary of 2.4 AU for a planet fully covered with CO 2 clouds, which is far larger than the cloud-free value of 1.67 AU Kasting et al. ( 1993 ). The competing radiative cooling and heating effects are individually large, but partly balance each other. That means that small errors in the description of one of these radiative interactions can lead to large errors in a cloud’s net effect. So far, all previous studies on the climatic impact of CO 2 clouds used a simplified treatment for the numerical solution of the radiative transfer equation to determine the atmospheric and surface temperatures. In particular, two-stream
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