Clouds are not only fascinating to watch for their myriad of shapes, but are also scientifically challenging because understanding their formation requires both knowledge of the large-scale meteorological environment and of the details of cloud droplet and ice crystal formation on the very small scale of micrometres. Formation of the ice phase in clouds remains enigmatic because ice crystal number concentrations can exceed by orders of magnitude the number concentrations of those aerosol particles acting as centers for ice crystals commonly known as ice nucleating particles (INP). To date, measurement devices for INP are rare and custom-made. In this lecture, Prof. Lohmann presents the progress that has been made by the global community in identifying which aerosol particles may act as INP and why. This is important in improving weather forecasting and climate prediction.

By scattering and absorbing sunlight, aerosol particles can cause a cooling of the lower atmosphere that partly offsets the warming by greenhouse gases such as carbon dioxide. Aerosol particles also influence the microphysics of clouds by acting as cloud condensation nuclei and INP. The magnitude and geographical distribution of the cooling caused by aerosol particles is much more uncertain than the greenhouse gas warming. This is because aerosol particles in the lower atmosphere have localized sources and sinks and only stay in the atmosphere for days to weeks. An additional uncertainty in climate is related to clouds. It is not yet clear, how clouds change in a warmer climate. In this lecture, Prof. Lohmann will also report on the progress that has been made in climate projections related to clouds and aerosols.

The Bert Bolin Lecture on Climate Research is given annually in May to commemorate professor Bert Bolin and his pioneering work for climate research at Stockholm University and internationally. The speaker is selected among prominent scientists within climate research by the Faculty of Science.