Abstract. According to everyone’s experience, predicting the weather reliably over more than 8 days seems an impossible taskfor our best weather agencies. At the same time, politicians and citizens are asking scientists for climate projections severaldecades into the future to guide economic and environmental policies, especially regarding the maximum admissible emissions of CO2. To what extent is this request scientifically admissible? In this lecture we will investigate this question, focusing on the topic of predictions of transitions between metastable statesof the atmospheric or oceanic circulations. Two relevant exemples are the switching between zonal and blocked atmosphericcirculation at midlatitudes and the alternance of El Niño and La Niña phases in the Pacific ocean. The main issue is whetherpresent climate models, that necessarily have a finite resolution and a smaller number of degrees of freedom than the actualterrestrial system, are able to reproduce such spontaneous or forced transitions. To do so, we will draw an analogy betweenclimate observations and results obtained in our group on a laboratory-scale, turbulent, von Kármán flow, in which spontaneoustransitions between different states of the circulation take place. We will detail the analogy, and investigate the nature of thetransitions, the number of degrees of freedom that characterizes the latter and discuss the effect of reducing the number ofdegrees of freedom in such systems. We will also discuss the role of fluctuations and their origin, and stress the importance ofdescribing very small scales to capture fluctuations of correct intensity and scale.