SENSITIVITY OF THE EARTHS CLIMATE TO HEIGHT-DEPENDENT CHANGES IN THE WATER-VAPOR MIXING-RATIO

THE atmospheric water vapour feedback is thought to amplify the global climate response to increased concentrations of greenhouse gases 1. As the oceans and atmosphere warm, there is increased evaporation, and it is generally believed that the additional moisture then adds to the greenhouse effect by trapping more infrared radiation. Lindzen 2-4 has suggested that climate models overestimate this response; he argues that increased convective activity in a warmer climate will lead to a drying rather than a moistening of the upper troposphere (but see refs 5-7 for other views). An important part of Lindzen's argument is that it is only changes in upper tropospheric water vapour that can alter the radiative budget of the atmosphere significantly, and hence contribute to the water vapour feedback. Here we use radiative transfer calculations to show that this seems to be true if the water vapour concentration is perturbed by a constant absolute amount at all heights. But observations of the seasonal change in water vapour concentrations 6,8 suggest that the climate response to increased concentrations of greenhouse gases may be closer to a constant relative change at all heights, corresponding to much larger absolute changes in the lower troposphere. We find that the Earth's radiation budget is most sensitive to changes in lower tropospheric water vapour concentrations, when such relative perturbations are considered.