Water Vapor is a Greenhouse Gas, and it counts toward global warming.

http://www.sciencemag.org/cgi/content/full/323/5917/1020

http://www.sciencemag.org/cgi/data/323/5917/1083b/DC1/1

Or as a pod cast as the final interview in this mp3 segment:

http://podcasts.aaas.org science_podcast/SciencePodcast_090220.mp3

It would be best to read the article above at a university or college library or science department with full access to Science published by the American Association for the Advancement of Science (AAAS).

Image: wikipedia, lots of water vapor in the image, you just can't see it.

In "Perspectives" published by Science 20 February 2009, an article by Andrew E. Dessler [Texas A&M University, College Station, TX 77843, USA] and Steven C. Sherwood[Climate Change Research Centre, University of New South Wales, Sydney, Australia] the authors take a look at the humidity in the atmosphere caused by the increasing amount of CO2 mankind has placed in the air, and it's contributory effects on global warming.

In lieu of reading their original perspective, I offer my own paraphrase and very loose interpretation of their article. Some of the difficulties in the absolute quantification of the effects of water vapor in the scale of the warming are discussed by them and I use their article to form the core of the following:

The difficulty of assessing the feedback of water vapor has always been it's relatively "small effect" and it's rapid decay as a component of the CO2 driven warming -- a condition of the range of temperatures we now experience on Earth. Water vapor's behavior makes weather and is the medium of life on Earth. Water vapor's residence as a vapor is NOT a consistent and easily quantifiable number. Our "weather," day to day and month to month is the story of the behavior of water vapor, and water liquid, and water solids. That water is a "special" molecule with respect to life on Earth should be clear.

Image:National Center for Atmospheric Research, UCAR education

Note that the constituents of air ignore H2O and really consider only "dry" air. One of the reasons for this is the ubiquity of water vapor in the air of Earth -- since it ranges from near 0% to 4% depending on the ambient temperature. Indeed, water vapor is a GHG, it is just so difficult to characterize globally in some constant "humidity" quantity, that it is a complication to evaluate precisely on a large or global scale. Because of this variability, locally and even regionally it is difficult to assign a nice feedback or forcing quantity to water vapor. Presently, when the variability of humidity is factored into the equation, via climate models, no true global standard quantity can be yet defended.

The molecules and elements of which Earth consists -- its lands, its ocean and its air, combine in consequence with the Earth's "place" in the solar system. This "place," 93 million miles from the Sun, at this time, is keenly coupled to the long slow evolution of the Sun's energy production. Earth's long-time constant path around this sun-star yields to the Earth, a realm of ambient temperatures that form the basic bounds and limits for the physical behavior of water in its three guises. The physics prescribes this water molecule to be gas, liquid and solid, sometimes all three at essentially the same time.

The atmospheric temperature determines the quantity of water vapor that the air can hold, but the range of equilibrium vapor pressures in what is Earth's climate regime allows supersaturation, as well as levels of humidity from the frozen near "zero humidity" of Antarctica, to the near constant, 100% humidity of the raniest parts of the Hawaiian Islands. Water vapor is, in effect, a "stronger" component of the global warming on a function basis, than is CO2.

At this stage in the Earth's existence (beneath the Sun,) water vapor is "transient" in Earth's atmosphere, and exists within the temperature realms and pressures currently existing on Earth. A billion years from now, as the Sun further evolves, these conditions will have passed, and water will only be solid high high in the air half-way to space. Ice and snow will not exist on the surface of Earth.

But now they do.

The current CO2 driven warming slightly and delicately changes the temperature realms and provides a temperature basis for more water effects. One of which is additional global warming concordant with the increase of water vapor, piled on top of the amount from the build-up of CO2.

Although everyone knows and recognizes water vapor as a greenhouse gas (GHG), observational evidence to clearly support the overall theoretical predictions is not easy to gather, and the complete energy re-radiation picture is only now beginning to be seen because of satellite data. One can quantify and model the general thermal characteristics of water vapor, and the expectation is that the atmosphere's relative humidity rises rapidly with air temperature and creates a new equilibrium vapor pressure, which then increases quickly with temperature. Satellite information supports the models.

The majority of the water vapor in Earth's atmosphere is logically present in and adjacent to the tropic, and equatorial zones of Earth and is moved by mechanical means north and south, convection transporting the vapor and then as it is cooled, the water.

http://earthobservatory.nasa.gov/Newsroom/view.php?id=22423

Maura Rabbette, et al., examined the equatorial Pacific's anomalous warm pool and found that, so far, this seems about as close to a water vapor forcing runaway greenhouse as possible now on Earth, but the relevance to the current arguments and discussions by Dessel and Sherwood bear thinking about.

Image:NASA, NOAA-CIRES, Note the "warm pool" area north of Australia, in the darkest shade presented. This area corresponds to a near supersaturation of water vapor at moderate tropospheric altitudes. The scale is in temperature C. Now, max temperature of the ocean is creeping up, not as much as the cold ice melts, but in the past we had hot-tub ocean's near 107 F for some time, millions of years. 87 F is the current rough max sea surface temperature. (That will change.)

NASA Text: Rabbette analyzed clear-sky data above the tropical Pacific from March 2000 to July 2001. She determined that water vapor above 5 kilometers (3 miles) altitude in the atmosphere contributes significantly to the runaway greenhouse signature. She found that at 9 kilometers (5.6 miles) above the Pacific warm pool, the relative humidity in the atmosphere can be greater than 70 percent - more than three times the normal range. In nearby regions of the Pacific where the sea surface temperature is just a few degrees cooler, the atmospheric relative humidity is only 20 percent. These drier regions of the neighboring atmosphere may contribute to stabilizing the local runaway greenhouse effect, Rabbette said.

IMAGE: Wikipedia, dewpoint -- satutration

Dessel and Sherwood write "The water vapor feedback mainly results from changes in humidity in the tropical upper troposphere, where temperatures are far below that of the surface and the vapor is above most of the cloud cover."

Now, water vapor is distributed from the tropics or any warm area producing the vapor state, controlled by planetary scale winds patterns which drive these masses of vapor, and thus spread the vapor great distances so that the relative humidity does not settle to a single level over a broad area of surface, but it is close to a "middling" value over a large planetary surface area. Granted, that the dew-point can remain relatively constant over time scales of a few days to a week or so, there is enough variability to make it difficult to derive the GHG forcing it produces.

Solar driven diurnal temperature variations, are coupled to wind transport of "humidity" over large areas with the energy of the vapor contained in large volumes of atmosphere. As well as mixing and exchanging this energy with ocean realms and land surface realms and yielding vapor energy to vast volumes of air, these energy transportation mechanisms change and spread this energy beneath the vapor, although phase changes in the water consume and release this energy and tend to alter the overall humidity, pushing it globally higher with global temperature rise.

I can only see one element of the climate system capable of generating these fast, global changes, that is, changes in the tropical atmosphere leading to changes in the inventory of the earth's most powerful greenhouse gas-- water vapor. -- Wallace Broecker, in lecture presented in 1996 AGU meeting.

We have all learned since 1996.

Image: Wikipedia

Sherwood and Dessel state that, over all, there is local variation and local uncertainty in the global humidity increase and its precise feedback --what is clear is that the global humidity is increasing as temperature is driven upward with the carbon dioxide, and other GHG's. Although the exact magnitude of the global water vapor heating component, "feedback" is still being settled, it is virtually certain to be strongly positive and most evidence supports 1.5 to 2 Watts/m2/K -- sufficient to very roughly double the warming from CO2 that occurs. Every degree increment in temperature produced by the rest of the greenhouse gases, doubles the resultant effective temperature rise.

As humanity continues this unwise experiment data acquisition continues, and the time series will eventually allow a very precise and almost certainly positive quantity to emerge.

Dessel and Sherwood conclude: "There remain many uncertainties in our simulations of the climate, but evidence for the water vapor feedback--and the large future climate warming it implies--is now strong."
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A note: My political "science" approach. . .

The scheme to cool the Earth by heating the ocean's surface waters and vaporizing sea water to vapor into the air likely results in a heating of the Earth's air, exacerbating the warming instead of cooling and mitigating the AGW ongoing. Removal of anthropogenic CO2 from the air and reducing such things as CH4 emissions quickly, using natural gas for example, until neutral biofuel or better batteries can help us, are likely going to be major job-producing techniques in the near future. As I have said before, elsewhere, this is the key to scheduling an ice age if we want to have one ahead in time.

We do not have to just reduce fossil carbon based emissions, we have to halt them, and actively reverse the process -- removing the CO2 from the air.

This is why Obama's call for a Carbon Cap and Trade mechanism should be denied. Cap and Trade exacerbates the problem and extends it, and the warming worsens as a result.

Instead, and to use the market and entrepreneurship for a solution, the Carbon Tax and 100% dividend return to those who will drive the market is needed. Sorry, Al Gore. Kyoto should be dumped and solved with green tech and Carbon Tax with 100% dividend.

Sorry too, for the Middle East's oil. You fossil carbon purveyors must switch to sunlight and wind, and you might best link your grid to the rest of the world. Playing with old-fission nuclear is not a good idea, though it will produce lots of clean energy, nuclear weapons add difficulty -- but you could enlist the French or Israeli approach and work with strong international oversight -- or wait until the US finishes the 4th-generation fast neutron nuclear plants and sells you the proven technology.

The coal and oil ways are a dead-end for the planet and for economies based on them.