The role of clouds in the climate system

Abstract
Clouds are important for global climate since they have a strong impact on solar and terrestrial radiation as well as on the formation of precipitation. The different types of clouds in the atmosphere are linked to the climate system by a multitude of dynamical and thermodynamical processes including numerous feedback mechanisms. In present-day climate, on average, clouds cool our planet, the net cloud radiative forcing at the top of the atmosphere is about -20 Wm^-2. One of the most interesting questions concerning clouds is: how will they respond to a change in climate? A slight change in cloud amount or a shift in the vertical distribution of clouds might have a considerable impact on the energy budget of the Earth. The Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states clearly that cloud processes and related feedbacks are among the physical processes leading to large uncertainties in the prediction of future climate. The main reason for this is that many microphysical and dynamical processes controlling the life cycle and radiative properties of clouds are not adequately implemented in global climate models. The interaction of aerosols and clouds and the resulting radiative forcing (indirect and semi-direct aerosol effect) is one of the major fields of active cloud research at present. This chapter introduces various aspects of the cloud-climate relation and summarizes the discussions of topics currently under scientific debate.