[dnautics]

Not only that, all greenhouse models require a 'water feedback loop'.

Of course there are plenty of anthropogenic water emissions to go about. When you irrigate, you are effectively throwing water into the air - by increasing the surface area of water evaporation through plant transpiration networks.

I have never seen anyone talk about this alternative hypothesis.

A disclaimer - I am all for reducing CO2 emissions strictly on principle (this is not dependent on the effects of CO2) and have put my money behind my beliefs - I have driven hybrid vehicles to support technology towards that end since 2003. But in my judgement as a scientist in general (and to some degree as a chemist who has done advanced spectroscopy including IR spectroscopy) I am losing confidence in climatology.

[chris_va]

It's a reasonable question. While I am not a climatologist...

Anthropogenic evaporation is probably minuscule next to deviations in natural evaporation due to small changes in temperature. A single hurricane, for example, dumps more power (as heat) than all human power generation.

Also, unlike CO2, I would also guess that atmospheric water probably reaches equilibrium much faster (half life ~1 year instead of ~30 years).

[dnautics]

There is basically no equilibrium condition here. If you mean that there is a more 'inelastic' equilibrium constant, then keep in mind that the concentration level governed by such a constant is dependent on rate in versus rate out, and there are certainly ongoing inputs of ag water. We've completely drained several seas and underground aquifers as a result of human activity in the last 50 years.

[chris_va]

The amount of water used globally (approx 10^13 m^3) is only about 2.5% the total yearly evaporation off of the ocean (approx 4*10^14 m^3).

I suspect small changes in climate have a much greater effect on net evaporation.

[hackuser]

I'm pretty sure your concerns have been addressed. Is it reasonable to 'lose confidence' because you haven't read the research?

[dnautics]

pray tell, enumerate my concerns and show me how they've been addressed. Is it reasonable to assume they've been addressed just 'because scientists'?

[ska]

does 2.5.6 from http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-cha... address this adequately?

(I was curious from your comment, but only spent a minute looking)

[dnautics]

Read the section yourself and ask if it does. They say right there that they don't do the calculations because it's too uncertain.

[ska]

Ah, but that's not at all the same as not addressing it, or as you state "I have never seen anyone talk about this alternative hypothesis." (of course I read it, and asked that question)

They are talking about it, and saying that it's a potential source of error that nobody knows how to account for yet, which is pretty common in science. If someone can demonstrate that the potential effect is dominant, it's a real problem. If someone can demonstrate that the potential effect is trivial it can safely be left alone. Otherwise you do the best modeling you can with the best data you can and talk about the possible confounding issues as so we proceed.

So I don't see any particularly damning issue here, have I missed something? It would be nice to resolve the differences between estimates. Is i the lack of analysis of radiative forcing that is bothering you? Do we have any reason to believe it can be a significant term? These sources of vapor are certainly not large (relative to total sources of water vapor in the atmosphere) so I'm not seeing the mechanism.

This really isn't my area, and as always I'm more than happy to be wrong (I learn more that way).