Traditionally climate was defined as the average atmospheric state over at least a score of years, modulated by the seasonal cycle.

Such a definition concealed the temporal variability which produced the mean state, and the complexity of the underlying physical, chemical, biological, geographical and astronomical processes contributing to the climate system. It also implicitly assumed that the climate of a locality does not change over decades, while accepting long-term changes such as glacial periods. In recent decades this perspective of climate has changed. The strong coupling of different constituents of the climate system is now widely recognized, as is the fragility of ‘stable’ climate.

The traditional definition of climate contains two elements which lead us towards the concept of climatic change. First, the seasonal cycle demonstrates one scale of change through the direct impact of the annual change in solar radiation on the atmosphere and the biosphere (the collective term for plant and animal life). Contrastingly, the implication of stability over a number of years implies that basic balances exist within the system. It also implies that any decadal changes in the surface forcing of the atmosphere, from the land or ocean, are small; Chapters 5 and 6 will show that this is not always the case but in general this is a reasonable claim.

In recent decades concern over potential climatic changes due to humanity's activities has arisen, although acknowledgement of the possibility was first made early in the nineteenth century.