单项选择题

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Climatic conditions are delicately adjusted to the composition of the Earth’s atmos phere. If there were a change in the atmosphere—for example, in the relative proportions of atmosphere gases—the climate would probably change also. A slight increase in water vapor, for instance, would increase the hem-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 mount of solar en ergy reaching the Earth’surface.
The level of carbon dioxide, CO₂ in the atmosphere has 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 re ceived energy as a 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 the Ve- nus, a planet covered by heavy clouds composed mostly of CO₂, whose surface tempera tures have been measured at 43℃. If the CO₂ content of the atmosphere is reduced, the temperature falls. According to one respectable theory, if the atmospheric CO₂ concentra tion 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 cli mate, in turn, would become cooler. With reduced temperatures many plants would die; CO2 would thereby be returned to the atmosphere and gradually the temperatures 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 increase and decrease 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 rise of at least 1℃. But a supposed global temperature rise of 1℃ may in reality be only several regional temperature increase, 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 Hemi sphere oceanic zone, may be experiencing an equivalent temperature decrease that is unrec ognized because of the shortage of meteorological recording stations.
The passage suggests that a large decrease in the amount of CO₂ in the atmosphere would result in______.