Is or isn't water vapor "the most important greenhouse gas"?

One of the most significant unresolved issues in the global warming/climate change debate is the role of water vapor as a greenhouse gas. The most recent IPCC reports neither raise nor address this issue.

I just ran across the name of a Stanford "climatologist", Stephen H. Schneider. I searched for his name and the phrase "water vapor" and ran across a web page entitled "Testimony of Stephen H. Schneider - Professor, Department of Biological Sciences - Stanford University - July 10, 1997 - CLIMATE CHANGE: CAUSES, IMPACTS AND UNCERTAINTIES" which appears to be his testimony as a witness before the U.S. Senate Committee on Environment and Public Works. In it I find this reference to water vapor (my emphasis):

The atmosphere is more opaque to terrestrial infrared radiation than it is to incoming solar radiation, simply because the physical properties of atmospheric molecules, cloud and dust particles tend on average to be more transparent to solar radiation wavelengths than to terrestrial radiation. These properties create the large surface heating that characterizes the greenhouse effect, by means of which the atmosphere allows a considerable fraction of solar radiation to penetrate to the earth's surface and then traps (more precisely, intercepts and re-radiates) much of the upward terrestrial infrared radiation from the surface and lower atmosphere. The downward re- radiation further enhances surface warming and is the prime process causing the greenhouse effect.

This is not a speculative theory, but a well understood and validated phenomenon of nature. The most important greenhouse gas is water vapor, since it absorbs terrestrial radiation over most of the infrared spectrum. Even though humans are not altering the average amount of water vapor in the atmosphere very much by direct injections of this gas, increases in other greenhouse gases which warm the surface cause an increase in evaporation which increases atmospheric water vapor concentrations, leading to an amplifying or "positive" feedback process known as the "water vapor-surface temperature-greenhouse feedback." The latter is believed responsible for the bulk of the climate sensitivity (IPCC, 1996a). Carbon dioxide is another major greenhouse gas. Although it absorbs and re-emits considerably less infrared radiation than water vapor, CO2 is of intense interest because its concentration is increasing due to human activities. Ozone, nitrogen oxides, some hydrocarbons, and even some artificial compounds like chlorofluorocarbons are also greenhouse gases. The extent to which they are important to climate depends upon their atmospheric concentrations, the rates of change of those concentrations and their effects on depletion of stratospheric ozone --which in turn, can indirectly modify the radiative forcing of the lower atmosphere thus changing climate -- currently offsetting a considerable fraction of the otherwise expected greenhouse warming signal.

Note those words: "The most important greenhouse gas is water vapor..."

If this is the case, why no mention of water vapor as a greenhouse gas and its positive feedback effect in the latest IPCC reports or in the media coverage?

One statement by Professor Schneider which has no scientific citation, but which certainly merits one is his claim that "humans are not altering the average amount of water vapor in the atmosphere very much by direct injections of this gas", especially since the chemical process of "burning" fossil fuels produced water vapor as well as carbon dioxide.

We read here the standard claim that "CO2 is of intense interest because its concentration is increasing due to human activities", without any citation, and no discussion of the rationale or evidence for believing that carbon dioxide concentrations are increasing while it is believed that water vapor concentrations are not increasing due to human activities. It would be nice to see some real data, some real science on these important scientific issues.

-- Jack Krupansky