A process which alters the energy balance of the Earth-atmosphere system (see Figure 1.3) is known as a radiative forcing mechanism (Shine et al., 1990). These may include variations in the Earth’s orbit around the Sun, solar radiation, volcanic activity and atmospheric composition. Associating a particular cause with a particular change, however, is extremely difficult. The interlinked nature of the climate system ensures that there are feedbacks; a change in one component leads to a change in most, if not all, other components. The concept of feedback is discussed more thoroughly in section 2.7.
Before investigating some of the more important forcing mechanisms, both internal and external, there is one factor that needs elaborating: time scale.