Climate can be deﬁned as “the synthesis of weather” considered over a time interval long enough to determine its essential statistical properties. More broadly, it is the time-averaged state of the physical system that involves the atmosphere, hydrosphere, cryosphere, lithosphere and biosphere (Bolle, 1985) and their interactions on many different time and space scales (Figure 1.1). The main domains that comprise the climate system have a wide range of equilibration times, as shown schematically in Figure 1.2, illustrating the many time scales involved. Internally, the climate system as a whole is a closed system for exchanges of matter but is subject to external forcing by solar radiation, gravitational forces, geological processes, and human activity. The whole complex constitutes a “cascading system,” or chain of subsystems, interconnected by ﬂows of energy, matter, and momentum.