Under present conditions a temperature of 18℃ can be observed at an altitude of 5 to 6 kilometers above the Earth. Below this altitude , the tem- perature increases by about 6~C per kilometer approaching the Earth s surface, where the average temperature is about 15~C. An increase in the amount of carbon dioxide means that there are more molecules of carbon dioxide to absorb infrared radiation. As the capaci- ty of the atmosphere to absorb infrared radiation increase, the radiating level and the. tem- perature of the surface must rise.

　　One mathematical model predicts that doubling the atmospheric carbon dioxide would raise the global mean surface temperature by 2. 5℃. This model assumes that the atmos- phere s relative humidity remains constant and the temperature decreases with altitude at a rate of 6.5~C per kilometer.

　　The assumption of constant relative humidity is important, because water vapor in the atmosphere is another efficient absorber of radiation at infrared wavelengths. Because warm air can hold more moisture than cool air, the relative humidity will be constant only if the amount of water vapor in the atmosphere increases as the temperature rises. There- fore, more infrared radiation would be absorbed and reradiated back to the Earths sur- face. The resultant warming at the surface could be expected to melt snow and ice, reduc- ing the Earths reflectivity. More solar radiation would then be absorbed, leading to a fur-ther increase in temperature.

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