A relatively recent study of the atmospheric impact of water vapor in the tropics has come to a decidedly unique conclusion: once temperatures hit a certain point, the infamously powerful greenhouse gas called 'water vapor' has a cooling effect. Read more:

During the composite oscillation’s rainy, tropospheric warming phase, the longwave flux anomalies unexpectedly transitioned from warming to cooling, behavior which was traced to a decrease in ice cloud coverage. This decrease in ice cloud coverage is nominally supportive of Lindzen’s ‘‘infrared iris’’ hypothesis. While the time scales addressed here are short and not necessarily indicative of climate time scales, it must be remembered that all moist convective adjustment occurs on short time scales. Since these intraseasonal oscillations represent a dominant mode of convective variability in the tropical troposphere, their behavior should be considered when testing the convective and cloud parameterizations in climate models that are used to predict global warming.

The full study, in PDF format, behind this link.

To rehash: