Water Vapor is Almost All of the Greenhouse Effect

This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.


H2O is 95% of the Greenhouse effect, CO2 is insignificant.


According to all of the scientific literature and climate experts I am aware of regarding this question, CO2 contributes anywhere from 9 to 30% towards the overall greenhouse effect. Depending on the method you use to determine these percentages they may even add up to more then 100% due to overlap and saturation of the particular frequencies of infra-red radiation each gas absorbs. The 95% number does not appear to come from a scientific source, though it seems to get tossed around a lot.

Please see this paper, the textbook referenced here and the article at Real Climate here.

This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.

"Water Vapor is Almost All of the Greenhouse Effect" was first published here, where you can still find the original comment thread. This updated version is also posted on the Grist website, where additional comments can be found, though the author, Coby Beck, does not monitor or respond there.

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The 95% number comes from Dalton's Law; assuming an average vapor pressure of 20 Torr for the water in the atmosphere, there is about 20 times the amount of water as carbon dioxide (in terms of mole fraction). The 20 Torr value might seem high except that 2/3 of the earth's surface is water where you can assume the air is saturated.

By Mark Campbell (not verified) on 15 Feb 2009 #permalink


The 95% probably comes from the same place as the 9 to 30% comes from, in other words they are both educated guesses.

Not being any type of expert, but certainly curious, I thought I'd ask a question here to see if I can learn something.

What effect does CO2 have on water vapor content in the atmospere? According to IPCC models? According to empirical data?

How does this affect the temperature outcomes in each case?

Thanks in advance.

By Paul in MI (not verified) on 13 May 2009 #permalink

Its not the CO2 its the Water Vapor

CO2 only absorbs in 3 narrow wavebands, be generous and say that these wavebands add up to 9% of the radiated IR. Now lets be REALLY generous and say that CO2 levels will absorb ALL the IR in those wavebands. This is still not enough 'radiative forcing' to create the Gore/Hansen cataclysmic climate. (I shall omit the arguments about exponential reduction in effectiveness of CO2).

Unlike CO2, water vapor (you know the exhaust gas from those 'green' hydrogen fuel cell cars ) absorbs a far wider spectrum of IR and is by far the major green house gas. So the Anthropogenic GW models depend on water vapor increases caused they hypothesize by higher temperatures leading to higher evaporation. This hypothetical water vapor increase could then create a far larger 'greenhouse effect' as a positive feedback and could lead to the runaway 'tipping point' etc etc.

The NASA Aqua satellites (note the name) were launched with the intent of measuring how hot the model predicted water vapor concentrations in the atmosphere had become. They found that there were no water vapor hot-spots as forecast.

NOAA also launched ocean going unmanned submarines to take the deep ocean temperatures to assess how much heating there had been and they actually show slight cooling at depth. A 'slight cooling' in temperatures in the oceans is a huge amount of energy. At the same time the Sea Surface Temperatures (SST) have also been cooling over the last years amplified in the Pacific by the PDO going negative and a recent La Nina.

So it would appear that every time there has been an attempt to validate the actual real-world complex-physics of the Anthropogenic GW models, hypotheses and their forecasts - they have been falsified.

The only 'fact' that is generally agreed is that there was a general correlation between CO2 levels and atmospheric temperatures in the last 2 decades of the 20th Century. Since then CO2 has continued an inexorable rise and the global atmospheric temperatures have stabilized and in the last 2 years appear to have dropped. This drop was not forecast by the models - although some have been revisited to match the data.

Heat Transport in the Atmosphere

Much of the work on the IR and green-house effect is built on the formulae of black body radiation. But most of the heat transport in the denser layers of the atmosphere up to the tropopause is by convection, warm humid air rising and as the air cools due to the pressure drop water condenses out, giving up its latent heat to the air around it heating it and causing it to rise more. To give an idea of the amount of energy in weather systems 'an average hurricane in one day of rain and evaporation "is equivalent to 200 times the world-wide electrical generating capacity"

( see http://www.aoml.noaa.gov/hrd/tcfaq/D7.html - method 1 - method 2 looks at the level of kinetic energy)

The IR from this air that has risen to the cloud tops has less CO2 and water vapor to pass through as the atmosphere thins so the CO2 radiative forcing effect becomes less effective.
The clouds formed by the condensation also reflect incoming heat back out into space due to the albedo effects which can depend on whether the clouds are ice-crystals or water droplets. The hypotheses and arguments in this area are very heated ( ) unlike the atmosphere.
Weather systems are complex feedbacks to changes in and imbalances in the global temperatures as are the ocean currents such as El Nino. These are not understood at all well.

The world warmed significantly in the last decades of the twentieth century.
CO2 concentrations have continued to rise at a rapid rate to the present day
The world has stopped warming in the last few years,
CO2 radiative forcing is real but is not enough alone to have caused the rise in global atmospheric temperatures.
The hypothesis that CO2 radiative forcing has caused a rise in water vapor in the atmosphere as a positive feedback has been falsified
The major transport of heat from the surface to the tropopause is by convection of warm humid air

This then leads to the questions

What _does_ cause the global warming and now cooling that are indubitable?
Is there any other variable or set of variables with correlations with the temperature change that could have a causal relation to global temperature changes?
Is this causal factor anything to do with humankind's activities
Will 'carbon caps', 'carbon trading' and taxes, have any effect on these changes in atmospheric temperatures?

Now of course i am but a simple denier (as opposed to a skeptic apparently) so maybe you should not listen to me

There is a theory about the greenhouse effect that I have not been able to dismiss. It is essentially about the contribution of the entire atmosphere to the greenhouse effect, defined as the difference between the surface temperature and the blackbody temperature of the earth as seen from outer space. The earth as a whole will always try to equally distribute energy between its constituents, and since the gravitational potential energy is lower at the surface, the kinetic energy (temperature) of the particles should be greater there. I guess the tricky question is whether the Stefan-Bolzmann radiation law is applicable to solids which are not in vacuum. We can consider the following thought experiment. Suppose we place a hot object in outer space, then it will radiate at a magnitude proportional to T^4. Now let it be surrounded by a cloud of only nitrogen and oxygen which is at a lower temperature than the body. Will the body now loose energy at a faster or slower pace? Supposing that blackbody radiation and heat loss through convection are additive the answer is faster. However, I had the following thought, since the atmosphere has a lower temperature than the body, a temperature gradient is formed which makes the body surface to be at a temperature somewhere in between. Since radiation scales as T^4 this results in substatially reduced radiation, which makes the energy loss slower. Could you help me clear my mind?

Its about that: H2O absorption bands are saturated, CO2 are to a great extend but not fully.

Its Lambert-Beers Law... If you double the amount of water vapor, transmission will decrease only half, if you double again, transmission will only increase 1/4 and so on.. so if theres much water vapor, more water vapor does no real additional effect.. Thats the situation, water vapor bands are fully absorbed. And of course thats 95% or something, but the point is these 95% do not change.

With CO2, most bands are saturated as well, but there are some bands where absorption still increases...

Btw, methane has few more unsaturated bands then CO2.. Guess what, methane is 50 times more powerful in global warming then CO2.. And water vapor has no effect in ANTHROPOGENIC, that is man-made greenhouse effect AT ALL.

another thing: latent heat transport: again, it is RIGHT that latent heat transport into higher athmosphere by water vapor is a major process, that by the way isnt anticipated in the models fully yet.

BUT: guess what... Absorption bands in the higher atmosphere are saturated TO A LESSER DEGREE, as when radiation comes from the earth's surface. Normally, radiation can escape through so called radiation windows, that are bands that are not fully saturated...

Now you got to understand... a man made greenhouse effect can only occur when there are open windows that can be closed. Only then a temperature rise can occur due to more greenhouse gases. That's the cause why water vapor has no effect at all, because there are no open windows in the water absorption bands BECAUSE there is so much water.

You understand, only open windows can be closed, and only due to that process a temperature rise occurs.. Now that is the case in the upper atmosphere, where the latent heat is transported and converted to radiation. Due to the lower thickness of the absorbing layer that is... So latent heat transport does in fact strenghten the power of greenhouse gases.. But stopping latent heat transport would make things worse on the other hand as well.. Because from earth's surface radiation can escape even less...

Hope you understand (especially crakar14)

@1 The 95% can't be from Dalton's law as that law does not address the absorption of radiation.

@2 No, the 95% is not an informed figure. I've pursued sources for it, and it appears that the original source is a nonscience web page, maintained by a nonscientist, who made up the figure himself (or took a particular other unsourced mention, and thence became the source; hard to tell given his citation practices).

CO2 is good for 12% of the absorption in the present atmosphere -- that is, if you removed it all, absorption would drop by 12%. On the other hand, if it were the only gas present, it would absorb 36% of what the earth emits. Water vapor is good for 36% of the absorption, if the only thing done is to remove it from the atmosphere (same experiment as gives CO2 12%).

@3: What sorts of effects are you thinking about? CO2 will dissolve a little in to liquid water. But the two gases don't care much about each other. The main thing CO2 does is support a baseline warm temperature. Without that, once you had some rain, the atmosphere would be drier, thence colder, and then you condense more out and speedily run out of greenhouse effect. With CO2 (and other non-condensing greenhouse gases), it rains and snows without such drastic consequence.

@5 I can't figure out what you really have in mind. Consider a globe without oceans, and then put an Oxygen-Nitrogen atmosphere on top of it. (While you're at it, ensure that the nearby star doesn't emit enough UV to produce an ozone layer -- ozone is itself a greenhouse gas.) The surface absorbs radiation from the star and gets hotter. Some of that energy is radiated into space, and some is absorbed by conduction in to the O2-N2 atmosphere. As far as this consideration goes, then, adding the atmosphere cools the surface, since it acts as an additional energy sink. But don't run off to conclusion, because we've only considered what happens during daytime. At night, the atmosphere will resupply energy to the surface, again by condition from the (now) warmer atmosphere to the cooling surface.

What you really need is to start setting up the equations for conservation of energy and see what they say. Just how conductive is the atmosphere? The surface material? What are their heat capacities? How bright (and in what spectral ranges) is the star? ...

Well that cleared that up!

This is all what you believe v what I believe. There is nothing here with which to convince a skeptic or explain things to a layman.

By Stuart Duff (not verified) on 13 Dec 2009 #permalink

Stuart, see the links at the top of the page. Simply put:

- CO2 is responsible for between 9 -30% of the Earths Greenhouse Effect. Because of the overlapping absorption bands with other greenhouse gases that's the best estimate.

- If not for the Greenhouse Effect the Earth's global temperatures would be 33 C degrees colder.

- CO2 in the atmosphere has increased over 35% due to human emissions.

- Why would this not enhance the Greenhouse Effect?.

By Dappledwater (not verified) on 14 Dec 2009 #permalink

"The fact that water vapor absorbs to some extent in the same spectral interval as carbon dioxide is the basis for the usual objection to the carbon dioxide theory. According to this argument the water vapor absorption is so large that there would be virtually no change in the outgoing radiation if the carbon dioxide concentration should change. However, this conclusion was based on early, very approximate treatments of the very complex problem of the calculation of the infrared flux in the atmosphere. Recent and more accurate calculations that take into account the detailed structure of the spectra of these two gases show that they are relatively independent of one another in their influence on the infrared absorption. There are two main reasons for this result: (1) there is no correlation between the frequencies of the spectral lines for carbon dioxide and water vapor and so the lines do not often overlap because of nearly coincident positions for the spectral lines; (2) the fractional concentration of water vapor falls off very rapidly with height whereas carbon dioxide is nearly uniformly distributed. Because of this last fact, even if the water vapor absorption were larger than that of carbon dioxide in a certain spectral interval at the surface of the Earth, at only a short distance above the ground the carbon dioxide absorption would be considerably larger than that of the water vapor. Careful estimates show that the temperature changes given above for carbon dioxide would not be reduced by more than 20 per cent because of water vapor absorption."


By pyrexolotyl (not verified) on 30 May 2010 #permalink

Water vapor almost completely masks CO2 absorption because water is a broad spectrum absorber (bond angle effects, stretch and rotation).

In the lower Troposphere where all the "OMG, it is warming" temperature measurements are made, the VERY SHORT mean free path means radiation is a tiny fraction of terrestrial heat rejection. The majority heat rejection is through convection, as much as half of the total solar energy flux is dissipated by condensation in clouds. This heat rejection cannot be measured by simple thermometers as it is latent heat.

Now, when you get to the Stratosphere which by its name, is stratified, radiation becomes the major mode of terrestrial heat rejection. Well, until you get to the Mesosphere where again, a lapse rate forms and voila, you get CLOUDS again...due to the low pressure, these are very sparse and limited to the higher latitudes, during the summer months only...noctilucent clouds, FYI.

It would help if you modeled the Earth as a control volume instead of the myopic "climate" models. Meaning a boundary is needed, pick the exosphere as this is most logical. Is the exosphere heating or cooling?

Spam alert on #14.

re 13: "Water vapor almost completely masks CO2 absorption because water is a broad spectrum absorber (bond angle effects, stretch and rotation)."

Nope, they're still fairly discrete. Go use a high resolution model to show you.

And there's still a hell of a lot of opacity at IR levels above the tropopause. Since the heat of that water vapour cannot get out because of all that CO2 in the way, you still have CO2 as the main cause of IR retardation at CO2 levels.

If you don't believe models, there are satellites that are sensitive to discrete wavelengths that show that the earth is radiating less at CO2 absorption levels.

Watch your myopia, keith.

Well Crank14 here i would like to add that if CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right? What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere making any temperature changes larger than they would be otherwise and amount of water vapor in the atmosphere exists in direct relation to the temperature.

If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even, studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. I am going to cover this on Technogie.

one thing more that if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C. When other feedback loops are included.

Given that 25% of current GHG warming is from CO2 and 65% from H2O, that would mean that for every 1C of warming from CO2, H2O has added 2.6C, making a current sensitivity aggregate of 3.6x the CO2 effect.

If Crank14 thinks that this current historical and absolutely measured (not modelled) sensitivity figure is not going to hold in the future, I would ask how he knows this.