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Climatic conditions are delicately adjusted to the composition of the Earth’s atmosphere. If there were a change in the atmosphere—for example, in the relative proportions of atmospheric gases—the climate would probably change also. A slight increase in water vapor, for instance, would increase the heat-retaining capacity of the atmosphere and would lead to a rise in global temperatures. In contrast, a large increase in water vapor would increase the thickness and extent of the cloud layer, reducing the amount of solar energy reaching the Earth’s surface.

The level of carbon dioxide, CO₂, in the atmosphere has an important effect on climatic change. Most of the Earth’s incoming energy is short-wavelength radiation, which tends to pass through atmospheric CO₂ easily. The Earth, however, reradiates much of the received energy as long-wavelength radiation, which CO₂ absorbs and then remits toward the Earth. This phenomenon, known as the greenhouse effect, can result in an increase in the surface temperature of a planet. An extreme example of the effect is shown by Venus, a planet covered by heavy clouds composed mostly of CO₂, whose surface temperatures have been measured at 430℃. If the CO₂ content of the atmosphere is reduced, the temperature falls. According to one respectable theory, if the atmospheric CO₂ concentration were halved, the Earth would become completely covered with ice. Another equally respectable theory, however, states that a halving of the CO₂ concentration would lead only to a reduction in global temperatures of 3℃.

If, because of an increase in forest fires or volcanic activity, the CO₂ content of the atmosphere increased, a warmer climate would be produced. Plant growth, which relies on both the warmth and the availability of CO₂ would probably increase. As a consequence, plants would use more and more CO₂. Eventually CO₂ levels would diminish and the climate, in turn, would become cooler. With reduced temperatures many plants would die; CO₂ would thereby be returned to the atmosphere and gradually the temperature would rise again. Thus, if this process occurred, there might be a long-term oscillation in the amount of CO₂ present in the atmosphere, with regular temperature increases and decreases of a set magnitude.

Some climatologists argue that the burning of fossil fuels has raised the level of CO₂ in the atmosphere and has caused a global temperature increase of at least 1℃. But a supposed global temperature rise of 1℃ may in reality be only several regional temperature increases, restricted to areas where there are many meteorological stations and caused simply by shifts in the pattern of atmospheric circulation. Other areas, for example the Southern Hemisphere oceanic zone, may be experiencing an equivalent temperature decrease that is unrecognized because of the shortage of meteorological recording stations.