Is Water Vapor a Greenhouse Gas? If not what are the implications?

This is a spin off global warming thread from several GW threads that populate the poli board on a regular basis.

So I was thinking, in none of these GW threads the term Greenhouse effect has never been defined. I mean the entire bases of GW/AGW theory is based on the definition of the Greenhouse effect. Therefore at a minimum I should have a good understanding of what the Greenhouse effect is...so as any 5th grader should I looked up the definition. From the IPCC website:

quote:

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Greenhouse effect
Greenhouse gases effectively absorb thermal infrared radiation, emitted by the Earth’s surface, by the atmosphere itself due to the same gases,and by clouds. Atmospheric radiation is emitted to all sides, including downward to the Earth’s surface. Thus, greenhouse gases trap heat within the surface-troposphere system. This is called the greenhouse effect. Thermal infrared radiation in the troposphere is strongly coupled
to the temperature of the atmosphere at the altitude at which it is emitted. In the troposphere, the temperature generally decreases with height. Effectively, infrared radiation emitted to space originates from an altitude with a temperature of, on average, -19°C, in balance with the net incoming solar radiation, whereas the Earth’s surface is kept at a much higher temperature of, on average, 14°C. An increase in the concentration of greenhouse gases leads to an increased infrared opacity of the atmosphere and therefore to an effective radiation into space from a higher altitude at a lower temperature. This causes a radiative forcing that leads to an enhancement of the greenhouse effect, the so-called enhanced greenhouse effect.

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Note the bolded part. That caught my attention. The way I interpret the bolded part is simple; with an increase in a greenhouse gas there is a corresponding increase in global temperature. This is called a positive feedback effect. That is, as the concentration of greenhouse gases increase the temperature increases within a system until the system changes (or the boundary conditions of the system change).

So I read further in the IPCC glossary and the next defined term was "Greenhouse gas":

quote:

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Greenhouse gas
Greenhouse gases are those gaseous constituents of the atmosphere,both natural and anthropogenic, which absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and ozone (O3) are the primary greenhouse gases in the Earth’s atmosphere. Moreover, there are a number of entirely human-made greenhouse gases in the atmosphere, such as the halocarbons and other chlorine- and bromine-containing substances, dealt with under the Montreal Protocol. Besides CO2, N2O, and CH4, the Kyoto Protocol deals with the greenhouse gases sulfur hexafluoride (SF6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs).

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I bolded Water Vapor here because as I read through the list of greenhouse gases based the definition of greenhouse effect, I thought to myself,"wait, water vapor does not behave like this...water vapor does not impart positive feedback mechanism in the greenhouse effect...it's negative. And since water vapor is negative it falls outside the definition of a greenhouse gas according to the IPCC."

I only know water vapor has a negative feedback effect on temperature simply by observing it. If you are metro New Orleans resident you can easily observe the negative feedback effect that water vapor has on temperature. For example, New Orleans consistently has a dew point (measure of water vapor) higher than Mandeville which is only 25 miles north of the city separated by Lake Pontchartrain. Because Mandeville generally has lower dew points, due to the lake effect on New Orleans, Mandeville averages lower temperatures. The less water vapor in the Mandeville air allows daytime temperatures to increase at a higher rate than New Orleans temperatures and vise versa at night. This is a negative feedback. That is, the higher the concentration of water vapor the lower the rate of change of temperature.

So I googled my observation and of course I was not the only one to observe the negative feedback effect of water vapor. I found an awesome youtube video demonstrating what I described between New Orleans and Mandeville with other cities around the world. I can't link a youtube video on this computer so just google "youtube negative feedback effect of water vapor" and you'll see the youtube video I'm referring to.

Along with the youtube video proving the negative feedback effect of water vapor I also came across this scholarly paper which has huge implications: LINK

quote:

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We have analyzed a wide variety of albedo and IR differences which are associated with rainfall variations on many different space and time scales. Our goal is to determine the extent to which we are able to accept or reject thereality of the Global Climate Model (GCM) simulations. The following analysis indicates that the GCM simulation of the influence of a doubling of CO2 give far too much global warming. We anticipate that a doubling of CO2 will act in a way to cause the global hydrologic cycle to increase in strength by approximately 3-4 percent. Our analysis indicates that there will be very little global temperature increase (~0.3oC) for a doubling of CO2, certainly not the 2-5oC projected by the GCMs.

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Conclusion 1: instead of seeing higher temperatures due to increased CO2 levels we will see greater precipitation.

Conclusion 2: water vapor is not a greenhouse gas according to IPPC because it lacks a positive feedback effect.

Thoughts?

I know, tl;dr.