The global climate of the Earth has significantly varied over the last millennia. On a regional scale, the climate has varied and does presently vary on many different time scales, leading to a continuously changing pattern of temperatures, humidity, precipitation, with important effects on the whole terrestrial biosphere. Physicist are interested in understanding the mechanism at work by gathering data and properly analysing them, by building theoretical models and, if possible, making predictions on the future evolution of the system. Along these lines, an important question is to understand.