[The Scientist (OP)]

If yes, why? So in that case can I assume that clouds too also contribute to global warming? What do you think? Thanks!

[SeanB]

Yes, but CO2 is chosen as the villain as it is easy to tax energy supplies, but hard to tax an ocean.

[peppercorn]

Yes, but CO2 is chosen as the villain as it is easy to tax energy supplies, but hard to tax an ocean.

No one dumping huge amounts more water vapour into the atmosphere though (apart from the same people releasing CO2).

[CliffordK]

When you look at the IR spectrum of the stuff in the atmosphere, you get more or less the following (from Wikipedia)

http://en.wikipedia.org/wiki/Greenhouse_gas



Note, I believe these spectra are CALCULATED, rather than Empirically Derived.

Incoming UV, Light, and IR radiation from the sun is shown in red.
Emissions from the earth (at a much cooler temperature) are shown in Blue, not counting reflected light.

Water has some very broad absorption peaks, as well as some less distinct peaks, much of which overlaps with CO₂.

Absorption of 2 materials with similar peaks would be cumulative, up to 100%.

While the spectrum of IR radiation from the planet corresponding to temperatures from about -30°C to +40°C (or so) is fairly broad, much of it gets absorbed and scattered in the atmosphere, with the main "window" being between 8μm and 14μm.

While H₂O is a relatively light molecule, water tends to condense at relatively lower atmospheric levels.  All air near the surface will have some water, whether or not clouds form.  Cold air has a lower water carrying capacity, and thus less water.  Clouds, of course, can act as a blanket keeping in night-time heat, as well as preventing the sunshine from getting through.

The question I've seen come up is whether or not the CO₂ absorption band is mostly saturated, at which point increasing CO₂ would cause little change to the planet's emissions.  However, CO2 does have a couple of perhaps not so insignificant absorption peaks around 9 & 10μm which lie in the middle of Earth's IR emission window.

Anyway, water vapor as well as clouds does play a role in both blocking sunlight coming to earth and absorbing IR emissions.  However, it is often considered a confounding variable, and considered to be independent from human activities (whether or not that is true remains to be seen), but nonetheless, there will be a maximum amount that humans will be discharging into the air.

Water may also block as much sunlight coming in as it blocks the IR emissions going out.

[CliffordK]

Here is a note from Wikipedia.

http://en.wikipedia.org/wiki/Greenhouse_gas

Greenhouse Gas Contributions:

• Water Vapor    H2O    36 – 72 % 
• Carbon Dioxide    CO2    9 – 26 %
• Methane    CH4    4 – 9 % 
• Ozone    O3    3 – 7 % 

These are, of course, absolute contributions.  Changes in concentration may NOT result in a directly proportional increase in Greenhouse Gas contribution.

I just noted on a Wikipedia Discussion page that water's contribution may actually be underestimated in that it can exist in all 3 phases in our atmosphere, solid, liquid, and gas.  And in fact, in the solid/liquid phases, refraction also becomes important due to the droplet nature of the water.

As we all know, it is a mixed bag.
On cloudy days, it is significantly cooler on the planet surface, and due to the light color of the clouds, sunlight is both absorbed in the clouds, as well as being reflected back into space.
However, cloudy nights are often significantly warmer as heat is trapped in, and reflected back to the surface.

Thermal infrared image of the Western Hemisphere from GOES.

Clouds emit thermal infrared (heat) radiation in proportion to their temperature, which is related to altitude. This image shows the Western Hemisphere in the thermal infrared. Warm ocean and land surface areas are white and light gray; cool, low-level clouds are medium gray; and cold, high-altitude clouds are dark gray and black. (NASA image courtesy GOES Project Science.)

So the greys and blacks are clouds, with the black being high altitude clouds (coldest).  And, it makes sense.  The clouds are blocking & scattering the IR from the planet.  However, the IR scan may actually be measuring the "temperature" at each level.  So, the upper atmosphere is coldest, and thus any clouds at that level would also be coldest.  But, if they capture heat, then fall to the planet in the form of rain, we loose out a bit.

People try to ignore water's contribution because at this point we have very little control over it.

It absorbs and reflects in both the sun's radiant energy spectra, as well as earth's IR spectra which make the contributions much more complex.  And, as I mentioned, most calculations seem to concentrate on only the vapor phase, rather than all 3 phases, solid, liquid, and gas.  Furthermore, it is extremely variable, dependent on temperature, the land/water below, etc.

[CliffordK]

Yes, but CO2 is chosen as the villain as it is easy to tax energy supplies, but hard to tax an ocean.

No one dumping huge amounts more water vapour into the atmosphere though (apart from the same people releasing CO2).

There are, of course, proposals to do it though, in hopes that it will cause a net decrease in temperatures. 
The ships below are supposed to blow both salt and water into the sky with the belief that it will increase the reflectivity of the clouds, tipping it away from warming.  I guess I'd rather not have excess salt in the rain, but I'm sure the Detroit Automakers are all in favor of it...  as well as other proposals to increase the sulphur and acid rain content in emissions again.

http://io9.com/5046852/a-fleet-of-1500-cloud+seeding-ships-could-stop-global-warming-say-scientists
http://physicsworld.com/cws/article/news/35693

[yor_on]

No, but with a reservation. It depends on where you look. Recently Climate scientists has found that 'stratospheric water vapor' have changed. "Since 2000, water vapor in the stratosphere decreased by about 10 percent. The reason for the recent decline in water vapor is unknown. The new study used calculations and models to show that the cooling from this change caused surface temperatures to increase about 25 percent more slowly than they would have otherwise, due only to the increases in carbon dioxide and other greenhouse gases." This vapor exist in a little researched narrow altitude region of the stratosphere that exist from about 17 km (56,000 ft) up to 51 km (32 mi; 170,000 ft). Temperature increases with height due to increased absorption of ultraviolet radiation by the ozone layer, which restricts turbulence and mixing. While the temperature may be -60° −60 °C (−76 °F; 213.2 K) at the troposphere, the top of the stratosphere is much warmer, and may be near freezing.

"An increase in stratospheric water vapor in the 1990s likely had the opposite effect of increasing the rate of warming observed during that time by about 30 percent, the authors found. Susan Solomon, Karen Rosenlof, Robert Portmann, and John Daniel, all of the NOAA Earth System Research Laboratory  (ESRL) in Boulder, Colo.; Sean Davis and Todd Sanford, NOAA/ESRL and the Cooperative Institute for Research in Environmental Sciences, University of Colorado; and Gian-Kasper Plattner, University of Bern, Switzerland. "

Right-click on it, and choose 'view image' to view a higher resolution.

Water vapor, even though being our biggest 'green house gas', at its lower levels have a natural short term effect. There will be more vapor as the global warming increase though, heating our oceans and seas. It's an important 'feedback' of heat but no 'forcing' as for example CO2, mostly due to its short cycle, around ten days. As we get more water vapor we will see more clouds, storms, and it will trap more long-wave radiation. Relative humidity might, according to some models, be expected to raise in the Tropics as this happens but decrease at mid latitudes, including the the bottom half of Australia, Chile, Argentina, South Africa. Siberia, Russia, Canada, British Isles and other countries left and right of these places.