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. This book will explore the natural climate system, and potential changes, man-made (anthropogenic) or otherwise. Its dominant theme will be the contribution of the oceans to these processes. Observable changes to the climate due to anthropogenic inputs could be expected to have already occurred; Chapters 6 and 7 will show that recent climatic shifts are, by the year 2002, increasingly likely to be largely attributable to these inputs. We will see that numerical climate models are an important means towards this recognition. The stability of the ocean's thermal and freshwater responses to change, its absorption of a significant proportion of anthropogenically derived compounds from the atmosphere, and its coupling with the atmosphere, form important pieces of the puzzle of climatic change.
To understand how the ocean affects, and is affected by, the climate we need to briefly consider the climate system as a whole. It is a complex, many-faceted system; Fig. 1.1 illustrates its major constituents and interactions. There are five components: the atmosphere, the ocean, the cryosphere (ice sheets, transient snow and sea-ice), the biosphere, and the geosphere (the solid earth). The system
Fig. 1.1. A schematic diagram of the climate system. [From Bigg, 1992d] SIJ \( T
is driven by short wavelength, principally visible and ultra-violet, solar radiation, with longer wavelength, infra-red, radiation being emitted into space to maintain thermal equilibrium. In the rest of this chapter we will briefly examine the different components of the climate system, including the timescales on which it operates and the principles behind the numerical models that increasingly are required to study it. However, we will begin with a short discussion of the basic energy source driving the climate - radiation from the Sun.
Was this article helpful?