The Global Climate System

Undoubtedly the most important outcome of work in the second half of the twentieth century was the recognition of the existence of the global climate system (see Box 1.1). The climate system involves not just the atmosphere elements, but the five major


The idea of studying global climate through co-ordinated intensive programmes of observation emerged through the World Meteorological Organization (WMO: and the International Council on Science (ICSU: in the 1970s. Three 'streams' of activity were planned: a physical basis for long-range weather forecasting; interannual climate variability; and long-term climatic trends and climate sensitivity. Global meteorological observation became a major concern and this led to a series of observational programmes. The earliest was the Global Atmospheric Research Programme (GARP). This had a number of related but semi-independent components. One of the earliest was the GARP Atlantic Tropical Experiment (GATE) in the eastern North Atlantic, off West Africa, in 1974 to 1975. The objectives were to examine the structure of the trade wind inversion and to identify the conditions associated with the development of tropical disturbances. There was a series of monsoon experiments in West Africa and the Indian Ocean in the late 1970s to early 1980s and also an Alpine Experiment. The First GARP Global Experiment (FGGE), between November 1978 and March 1979, assembled global weather observations. Coupled with these observational programmes, there was also a co-ordinated effort to improve numerical modelling of global climate processes.

The World Climate Research Programme (WCRP:, established in 1980, is sponsored by the WMO, ICSU and the International Ocean Commission (IOC). The first major global effort was the World Ocean Circulation Experiment (WOCE) which provided detailed understanding of ocean currents and the global thermohaline circulation. This was followed in the 1980s by the Tropical Ocean Global Atmosphere (TOGA).

Current major WCRP projects are Climate Variability and Predictability (CLIVAR:, the Global Energy and Water Cycle Experiment (GEWEX), and Stratospheric Processes and their Role in Climate (SPARC). Under GEWEX are the International Satellite Cloud Climatology Project (ISCCP) and the International Land Surface Climatology Project (ISLSCP) which provide valuable datasets for analysis and model validation. A regional project on the Arctic Climate System (ACSYS) is nearing completion and a new related project on the Cryosphere and Climate (CliC: has been established.


Houghton, J. D. and Morel, P. (1984) The World Climate Research Programme. In J. D. Houghton (ed.) The Global Climate, Cambridge University Press, Cambridge, pp. 1-11.

subsystems: the atmosphere (the most unstable and rapidly changing); the ocean (very sluggish in terms of its thermal inertia and therefore important in regulating atmospheric variations); the snow and ice cover (the cryosphere); and the land surface with its vegetation cover (the lithosphere and biosphere). Physical, chemical and biological processes take place in and among these complex subsystems. The most important interaction takes place between the highly dynamic atmosphere, through which solar energy is input into the system, and the oceans which store and transport large amounts of energy (especially thermal), thereby acting as a regulator to more rapid atmospheric changes. A further complication is provided by the living matter of the biosphere. The terrestrial biosphere influences the incoming radiation and outgoing re-radiation and, through human transformation of the land cover, especially deforestation and agriculture, affects the atmospheric composition via greenhouse gases. In the oceans, marine biota play a major role in the dissolution and storage of CO2. All subsystems are linked by fluxes of mass, heat and momentum into a very complex whole.

The driving mechanisms of climate change referred to as 'climate forcing' can be divided conveniently into external (astronomical effects on incoming short-wave solar radiation) and internal (e.g. alterations in the composition of the atmosphere which affect outgoing long-wave radiation). Direct solar radiation measurements have been made via satellites since about 1980, but the correlation between small changes in solar radiation and in the thermal economy of the global climate system is still unclear. However, observed increases in the greenhouse gas content of the atmosphere (0.1 per cent of which is composed of the trace gases carbon dioxide, methane, nitrous oxide and ozone), due to the recent intensification of a wide range of human activities, appear to have been very significant in increasing the proportion of terrestrial long-wave radiation trapped by the atmosphere, thereby raising its temperature. These changes, although small, appear to have had a significant thermal effect on the global climate system in the twentieth century. The imbalance between incoming solar radiation and outgoing terrestrial radiation is termed 'forcing'. Positive forcing implies a heating up of the system, and adjustments to such imbalance take place in a matter of months in the surface and tropospheric subsystems but are slower (centuries or longer) in the oceans. The major greenhouse gas is water vapour and the effect of changes in this, together with that of cloudiness, are as yet poorly understood.

The natural variability of the global climate system depends not only on the variations in external solar forcing but also on two features of the system itself -feedback and non-linear behaviour. Major feedbacks involve the role of snow and ice reflecting incoming solar radiation and atmospheric water vapour absorbing terrestrial re-radiation, and are positive in character. For example: the earth warms; atmospheric water vapour increases; this, in turn, increases the greenhouse effect; the result being that the earth warms further. Similar warming occurs as higher temperatures reduce snow and ice cover allowing the land or ocean to absorb more radiation. Clouds play a more complex role by reflecting solar (short-wave radiation) but also by trapping terrestrial outgoing radiation. Negative feedback, when the effect of change is damped down, is a much less important feature of the operation of the climate system, which partly explains the tendency to recent global warming. A further source of variability within the climate system stems from changes in atmospheric composition resulting from human action. These have to do with increases in the greenhouse gases, which lead to an increase in global temperatures, and increases in particulate matter (carbon and mineral dust, aerosols). Particulates, including volcanic aerosols, which enter the stratosphere, have a more complex influence on global climate. Some are responsible for heating the atmosphere and others for cooling it.

Recent attempts to understand the global climate system have been aided greatly by the development of numerical models of the atmosphere and of climate systems since the 1960s. These are essential to deal with non-linear processes (i.e. those which do not exhibit simple proportional relationships between cause and effect) and operate on many different timescales.

The first edition of this book appeared some thirty-five years ago, before many of the advances described in the latest editions were even conceived. However, our continuous aim in writing it is to provide a nontechnical account of how the atmosphere works, thereby helping the understanding of both weather phenomena and global climates. As always, greater explanation inevitably results in an increase in the range of phenomena requiring explanation. That is our only excuse for the increased size of this eighth edition.

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