Claim: Water Vapor is vastly more important than CO2 in the greenhouse effect. CO2 is a tiny component of the atmosphere and contributes only a tiny amount of greenhouse warming. And besides, the absorption lines of CO2 are saturated and so adding more CO2 to the atmosphere won't add more greenhouse warming.

Why these claims are wrong or deliberately misleading:

1. Water vapor IS a bigger source of greenhouse warming than CO2 (it's about 60% of the total GH effect). Water vapor forces greenhouse heating at 75 W/sq-meter compared to 32 W/sq-meter for CO2 alone. But water vapor is not the initiating cause for the rapid warming seen in the past ~60 years. The reasons are many... First, CO2 absorption lines are saturated at the Earth's surface and only in the central core wavelength, adding more CO2 adds opacity in the wings (wavelengths slightly different than the theoretical wavelength of maximum absorption) of the absorption lines and traps additional heat. Also, because of the saturated line core, detailed calculations show that heat trapped only rises as the logarithm of CO2 rise (not linearly), in agreement with observations. - More important, there is always a height in the atmosphere above which the absolute density of CO2 is low enough that the absorption lines are not saturated. As CO2 levels continue to rise, the mean altitude where the bulk of the re-absorption of

outgoing IR radiation happens will slowly rise higher (but still far below the stratosphere) - it certainly does not mean that heat trapped is constant with CO2 concentration. The saturated CO2 lines argument is just naive and false. If you'd like a little more detail, this link describes the situation well, even for the spectroscopically challenged. SkepticalScience.org has a well-linked explanation of how bogus this argument is. The absolute humidity at water vapor saturation rises fully 7% for every degree Celsius of air temperature rise. In other words, air that is 1 degree Celsius warmer can hold 7% more greenhouse-causing water vapor (Held and Soden 2006). Data over the past 110 years shows an even stronger link: 10% per degree Celsius (That's a strong positive feedback. The increased warming due to CO2 is amplified by the resulting higher water vapor in the atmosphere and its greenhouse effect, by roughly an equal amount, so the net effect of CO2 greenhouse warming is fully doubled by adding in the increased water vapor. In absolute terms, existing water vapor contributes 2.3 times more heating than does existing CO2 alone (Keihl 1997). Rising total atmospheric water vapor has been confirmed by observations (Santer 2007, see figure at left), and confirmed by careful attribution analysis to be due to human generated greenhouse gases, not "natural variation" with high confidence. Even if the CO2 absorption lines were all completely saturated, rising CO2 would still cause rising surface temperatures for another reason - the altitude above which the absorption lines become unsaturated will rise to higher colder levels as CO2 levels rise, so there will be reduced IR emission to outer space. This, in fact, has been observed for many years by satellites looking down at outgoing IR radiation. Absolute humidity is observed indeed to be rising, and it is confirmed to be due not to "natural variation", but to the feedback described above, in response to human-produced CO2 (Chung et al. 2014)

2. The water cycle is rapid. Water is evaporated from the surface, and precipitates back down to the ground with a time scale very short , while CO2 cannot liquify out of the atmosphere and cycles through the Earth system on time scales of millenia to millions of years. This means that water vapor in the atmosphere is very close to equilibrium. We cannot FORCE more water into the atmosphere because, other things being equal, it will just condense and rain right back out of the atmosphere very quickly. However, when we burn coal and oil which has been buried underground and taken millions of years to accumulate, and we do it at a rate which will exhaust it in just a few hundred years, the Earth and oceans cannot absorb the resulting CO2 fast enough to take it back out of the atmosphere. This is why CO2 levels are rising so rapidly while water vapor levels are not - water vapor remains in approximate equilibrium because it can liquify and fall immediately out of the atmosphere as rain - CO2 cannot. Now however, if you warm the air, you raise the ability of air to hold water vapor, and not by just a little - but by a lot. Warm the air by only 1 degree C and you raise the saturation point by fully 7% by mass. This illustrates a large positive feedback - raise temperatures by adding CO2 which can't be diffused into the ocean or ground fast enough, thereby evaporating more water and raising the average absolute humidity, which raises further greenhouse warming due to water vapor. As of 2012, the average global troposheric humidity has gone up 4% in the past 35 years. And human-generated CO2 dumping into the atmosphere is the ultimate culprit, even when some of the additional greenhouse heating of the Earth is due to water vapor itself.

3. 99% of the Atmosphere is Made up of ~non-Greenhouse Gases. Denialists who loudly remind you that CO2 is only 0.04% of the atmosphere hope you will not notice that more than 99% of the atmosphere is made up of diatomic nitrogen (78%), diatomic oxygen (21%), and argon (1%), none of which are greenhouse gases in the long wave IR because they are symmetric (or single atoms in the case of Argon) and therefore have very small electric dipole moments and absorb very little far infrared radiation. Asymmetric molecules have many modes of internal quantum excitation and absorb infrared radiation very well (CO2, water vapor, methane, nitrogen oxides, other man-made chemicals...). It's not at all surprising that raising CO2 levels by 43%, as we have done, therefore has a profound effect on climate, both directly, and indirectly as its warming allows more water vapor into the atmosphere. As warming continues, we are also seeing methane trapped in the permafrost liberated as well, and it is ~100 times more powerful a greenhouse gas as CO2, pound for pound on a few year time scale. Methane levels have more than doubled since the dawn of the Industrial Age. All very well known physics established decades or centuries ago.

4. This reason is the most important: the basic physics of radiation absorption and transport by molecules described above is well understood. Though the calculations are non-trivial - we do need quantum mechanics and good Monte Carlo codes to calculate absorption coefficients vs wavelength for all the relevant molecules - they are nevertheless straightforward and confirmed to be correct in experiments. We know the density, composition, and temperature profile of the atmosphere and this allows us to take those calculations as confirmed by lab results and extend them to the atmosphere as a whole. There are no significant uncertainties here.

I can't emphasize this enough - Human-caused climate change denialists not only need to come up with another culprit for observed global warming, they also need to come up with some sort of amazing physics which will somehow nullify over a hundred years of solid molecular physics and observations both in and out of the laboratory demonstrating the reality of CO2-induced greenhouse warming, dating back to the discovery of the greenhouse effect by Tyndall in 1861. They have done none of this.

Water Vapor Trends
Climate skeptics also point out that absolute humidities should have risen as temperatures have risen during the past 30 years, and claim they have not. The graph shown on this subject at "WattsUpWithThat" blog, is a dead link. The graph below is a consolidation of many sources for water vapor in the stratosphere. It shows an increase until after the strong El Nino of 1998, and then some decrease, as the ENSO oscillation has been predominantly in the cooler phase. The data suggest that most of the water vapor at this height comes from convection from increased El Nino activity, and so it is perhaps not surprising that water vapor levels have been lower here. It is also true that temperatures of the stratosphere are expected to cool as CO2 levels rise, and this is confirmed by observations. But there is a major caveat, and that is that stratospheric cooling is ALSO due to man-made chloroflourocarbons, which were not outlawed until the early 1980's, and which persist over many decades and continue to depress the ozone heating of the stratosphere (a very slow rise in stratosphere ozone has been seen in data since the Montreal Accords banned CFC's). The bottom line is that the failure of water vapor levels at such high altitudes to rise with global warming does not invalidate the fact that the total water vapor at all altitudes is indeed rising, nor the long term expected effect on climate.

If you're adventurous, try digesting this excellent for-the-advanced-layman long description of the Greenhouse Effect and water vapor included, on RealClimate.org's site maintained by climate scientists to help the public understand and ask them about climate issues.

If you want a more entertaining lead in to the science, try this 8 minute video "Water Vapor and Climate" from Climate Crock of the Week

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