ABSTRACT
Climate is defined in terms of statistical averages such as air temperature, hours of bright sunshine, and precipitation. There are periodic changes in climate over diurnal and annual time scales, which are an integral part of the operation of the Earth-atmosphere system and which do not involve longterm structural changes. However, it is possible to identify longer-term changes in climate occurring over periods of decades and centuries. Some of these changes represent considerable modifications to the mode of operation of the Earth’s surface and the atmospheric system. The Earth’s surface reveals evidence of periods of extreme cold, when large areas were under thick sheets of ice. Areas that are deserts today contain landforms that are clearly fluvial in origin, suggesting periods of wetter climate conditions. Measurements of climatic parameters (which have been available only since the early 18th century) reveal, for example, general increase in global mean air temperature over the period from the late 19th century. More recently, the greater frequency of failure of monsoon rains to reach northwest India and the extension of desert conditions along the south side of Sahara in the Sahel in Africa indicate that major changes in the Earth-atmosphere system are currently taking place. These changes can be viewed alternatively as a result of the changing operation of factors external to the Earth-atmosphere system or as modifications of the disposition of internal energy and matter. For example, changes in temperature of the lower troposphere may be due to changing radiant energy exchange between the Earth-atmosphere system and its surroundings (space). Alternatively, it may be that structural modifications within the system have resulted in more or less energy being stored as sensible heat within the atmosphere. If we view the Earth-atmosphere system as a black box, then our first alternative concerns a discussion of the relationships between input and output and we may make general statements regarding throughput. Changes within the system concern its response to energy inputs, in which case we must consider the pathways along which energy and matter are transferred, the nature of stores, and the rate at which energy and matter are transferred between them. In this respect, the regulators, which are control points within the system, are critical.
