Aerosol Effects on Climate via Mixed-Phase and Ice Clouds

Clouds in Earth's atmosphere can be composed of liquid droplets, ice crystals, or a combination of the two. Clouds' thermodynamic phase is largely controlled by temperature, but other factors can also have a significant effect. Aerosols-i.e., particles suspended in Earth's atmosphere-affect cloud properties differently depending on cloud phase and can potentially have a strong influence on climate via any cloud type. Aerosol-cloud-climate interactions have been a topic of active research for more than two decades, but these interactions nevertheless currently represent one of the most uncertain forcings of climate change over the past century. Most research to date has focused on how aerosols can impact climate via liquid clouds, which are better understood and observed than their ice-containing counterparts. Thus, the problem of how liquid clouds mediate aerosols' effects on climate is a more tractable one. However, there is no a priori reason to think that mixed-phase and ice clouds are any less affected by changes in atmospheric aerosol composition than liquid clouds, and estimates of how aerosols can influence these ice-containing clouds have started to emerge. Laboratory and field work, as well as satellite observations, is now shifting attention to this new frontier in the field of aerosol-cloud-climate interactions, allowing for improved representation of ice processes in numerical models. Here, we review this recent progress in our understanding of aerosol effects on mixed-phase and ice clouds, focusing on the four underpinning research pillars of laboratory experiments, field observations, satellite retrievals, and numerical modeling of global climate. Evident from this review is the possibility of a powerful yet poorly constrained climate forcing, which is uncertain in terms of both its magnitude and its sign.