Though the concept associated with the term `climate' is various among scientists as well as among people in general, most of people will admit to choose averages of meteorological variables (i.e. physical quantities related to the atmosphere, such as temperature and precipitation) in a time interval of the order of a month as a representative variable of climate. Despite these are atmospheric variables, we can hardly think that variation of climate with a time scale larger than several decades are caused by a mechanism intrinsic to the atmosphere. The time scale of the atmosphere itself is shorter. For example, suppose that the atmospheric temperature suddenly becomes lower by some reason. then the amount of terrestrial radiation emitted to space decreases, which act to restore the temperature. The time scale of this restoring action is estimated to be a month. On the other hand, the atmosphere closely exchanges energy with the ocean and the land, and the heat capacity of the ocean is much larger than that of the atmosphere. If the temperature of the ocean changes remarkably, it is much larger change of energy than change of the atmosphere with similar magnitude in terms of temperature, and the time scale to restore the perturbation is also longer. Thus, in order to consider climatic changes, we need to think of the atmosphere, the ocean, the cryosphere and the land (here `the land' generically refers to terrestrial water, vegetation, soil etc.) together, and their interactions. The term `climate system' means the whole of these interacting components as one entity.
Causes of variations of the climate system may be sought outside the system as well as within the system. By `internal cause' I mean the possibility that the climate system can spontaneously generate variations even without changes of external condition. The two types of causes are not independent, however. As we see in such a simple example as a swing, resonance can happen if a system can spontaneously generate an oscillation of a certain period, and it is forced at a period similar to that. I think that both external and internal causes are involved in the actual variation of climate.
Feedback
feedback is an important concept concerning systems. The change of a certain quantity can affect, through changes of other quantities, the quantity itself. If the roundabout effect acts to strengthen the first change, here is positive feedback. If it acts to weaken it, here is negative feedback. The role of outgoing terrestrial radiation to temperature as discussed above is an example of negative feedback.