Tropospheric Water Vapor and Climate Sensitivity

Estimates are made of the effect of changes in tropospheric water vapor on the climate sensitivity to doubled carbon dioxide (CO2), using a coarse resolution atmospheric general circulation model coupled to a slab mixed layer ocean. The sensitivity of the model to doubled CO2 is found as the difference between the equilibrium responses for control and doubled CO2 cases. Clouds are specified to isolate the water vapor feedback. Experiments in which the water vapor distribution is specified rather than internally calculated are used to find the contribution of water vapor in various layers and latitude belts to the sensitivity. The contribution of water vapor in layers of equal mass to the climate sensitivity varies by about a factor of 2 with height, with the largest contribution coming from layers between 450 and 750 mb, and the smallest from layers above 230 mb. The positive feedback on the global mean surface temperature response to doubled CO 2 from water vapor above 750 mb is about 2.6 times as large as that from water vapor below 750 mb. The feedback on global mean surface temperature due to water vapor in the extratropical free troposphere (above 750 mb) is about 50% larger than the feedback due to the lower-latitude free troposphere water vapor. Several important sources of nonlinearity of the radiative heating rates were identified in the process of constructing the specified cloud and water vapor fields. These are (i) the interaction of clouds and solar radiation, which produces much more reflection of solar radiation for time mean clouds than for the instantaneous clouds; (ii) the correlation of clouds and water vapor, which produces less downward longwave radiation at the ground for correlated clouds and water vapor than when these fields are independent; and (iii) the interaction of water vapor with longwave radiation, which produces less downward longwave radiation at the ground for the average over instantaneous water vapor distributions than for the time mean water vapor distribution.