[Lonesome in Massachussets]

Clouds are not water vapor, they are condensate. Clouds tend to contribute strongly to global cooling. The tops of clouds reflect far more sunlight into space than the bottoms reflect IR back to the ground. Water vapor is colorless and orderless. In the atmosphere it tends to absorb infrared radiation, causing the atmosphere to warm.

AGWT depends on positive feedback from water vapor to amplify the minor effects of CO2.

The atmosphere is very bad at absorbing heat from sunlight. Sunlight does not effectively heat the atmosphere. Rather, sunlight heats the surface and things on the surface. The surface warms the atmosphere two ways. One by convection and the other by radiation. The sun warmed surface radiates infrared radiation. Water vapor, CO2 and methane, inter alia, absorb the infrared radiation and are thereby heated. They collide with and heat the other molecules in the atmosphere. (A real greenhouse works by limiting convection to the air mass trapped inside. Misnamed “greenhouse” gases make the atmosphere more effective at absorbing infrared.)

More water vapor implies more clouds. Whether the net marginal effect of water vapor in heating the atmosphere by absorbing infrared radiation or cooling it by increasing the number of clouds is really the question on which the whole of AGW catastrophism hinges. The available observations seem to support water vapor as contributing to net cooling. If true, AGW catastrophism collapses.

[Mmogamer]

That volcano eruption in Iceland last year put more “greenhouse” gasses into the atmosphere than man has since
he started using fire. Billions of times more.

[SuziQ]

For an interesting read on the effect of clouds on global climate, and why those clouds form, read The Chilling Stars: A Cosmic View of Climate Change, by Henrik Svensmark. Last I heard, he was trying to get some cloud experiments set up at CERN, but they were delayed by the damage the Large Hadron Collider sustained in its first run.

[exDemMom]

In the atmosphere it tends to absorb infrared radiation, causing the atmosphere to warm.

The only kind of absorbance I can think of leads almost immediately to emission at a slightly longer wavelength, regardless of where in the spectrum the original absorbed photon existed. Since the slight difference in energy would be retained by the absorbing molecule as internal energy (thus causing warming through increasing the kinetic energy), I don't see how how such a phenomenon could be limited to only absorbance/emission occurring in the IR range of the spectrum. It seems to me that such a mechanism of heat gain by the molecule would occur at any wavelength where the molecule absorbs (which would be why highly absorptive colored objects on the ground get so much warmer than the air).

Or is there another kind of absorbance other than that which is part of fluorescence of which I am not aware?