• The radiative energy budget of the Earth is a balance between the absorption of incoming SW (mainly visible) solar radiation and outgoing LW (IR) thermal radiation. • The huge complexity of the physical processes that control these energy fluxes means that we cannot presently model or predict many aspects of climate and how it will change with adequate accuracy. • Many problems, e.g. :    +Greenhouse warming vs. natural variability?    +How well balanced?    +Role of clouds in equilibrium and in feedback?….