The structure and circulation of the atmosphere

The atmosphere exists as a thin, almost spherical, shell around the earth. Within the atmosphere differences in the thermal structure characterise a sequence of layers from the troposphere, adjacent to the earth's surface, through to the thermosphere and beyond. Figure 2.1 shows an average profile of the temperature of the atmosphere, indicating the layers and the boundaries between them.

In the study of greenhouse gases, the most important distinction is that between the troposphere and the stratosphere. The troposphere comprises about the lowest 85% of the atmosphere (in terms of mass) and is characterised by a negative lapse rate, i.e. temperature generally decreases with height. In contrast, in the stratosphere, temperature generally increases with height. While Figure 2.1 shows a mean distribution, the height of the tropopause varies significantly with latitude, being higher (typically 16 km) in the tropics than it is at high latitudes (typically 8 km).

Table 2.1 lists some of the components of the dry atmosphere. The main constituents, those in the upper group in the table, are distributed virtually uniformly in space and time, although it has recently become possible to measure the very small changes in oxygen content and use them to interpret aspects of the carbon cycle.