Estimates are made of the role of changes in upper tropospheric water vapor (UTWV) in the climate sensitivity to doubled carbon dioxide (CO2). A coarse resolution atmospheric general circulation model to double CO2 is found as the difference between the equilibrium responses for control and doubled CO2 cases. Experiments in which UTWV is specified rather than internally calculated areused to isolate the UTWV contribution to the sensitivity. In the specified UTWV experiments, the water vapor mixing ratio for pressures less that 740 mb is taken to be that of the control simulation climatology. It is found that the feedback factor describing the influence of globally specified UTWV is close to 0.5, indicating that, in teh absence of other feedbacks, the UTWV feedback approximately doubles the sensitivity of the model to increase CO2. An estimate for the feedback factor for UTWV between latitudes 30°S and 30°S is 0.23. Feedback estimates due to UTWV outside fo this belt range from 0.19 to 0.27. The feedback factor due to lower tropospheric water vapor is estimated to be 0.07.
The treatment of clouds in the specified water vapor experiments complicates the design of the experiments and the interpretation of results. Two techniques were used, both of which gave similar results. The analysis of the cloud effects shows that 1) that local positive correlation between clouds and water vapor in the upper troposphere cools the model's global mean surface temperature by about 1°C, and 2) these cloud interactions do not appear to have a major effect on the sensitivity of the global mean surface temperature to doubled CO2 with fixed UTWV.
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Center for Ocean-Land-Atmosphere Studies
last update: 13 August 1997
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