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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Explaining how the water vapor greenhouse effect works

What the science says...

Select a level... Basic Intermediate

Increased CO2 makes more water vapor, a greenhouse gas which amplifies warming

Climate Myth...

Water vapor is the most powerful greenhouse gas

“Water vapour is the most important greenhouse gas. This is part of the difficulty with the public and the media in understanding that 95% of greenhouse gases are water vapour. The public understand it, in that if you get a fall evening or spring evening and the sky is clear the heat will escape and the temperature will drop and you get frost. If there is a cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays quite warm. If you go to In Salah in southern Algeria, they recorded at one point a daytime or noon high of 52 degrees Celsius – by midnight that night it was -3.6 degree Celsius. […] That was caused because there is no, or very little, water vapour in the atmosphere and it is a demonstration of water vapour as the most important greenhouse gas.” (Tim Ball)

At a glance

If you hang a load of wet washing on the line on a warm, sunny day and come back later, you can expect it to be dryer. What has happened? The water has changed its form from a liquid to a gas. It has left your jeans and T-shirts for the air surrounding them. The term for this gas is water vapour.

Water vapour is a common if minor part of the atmosphere. Unlike CO2 though, the amount varies an awful lot from one part of the globe to another and through time. Let's introduce two related terms here: 'non-condensable' and 'condensable'. They set out a critical difference between the two greenhouse gases, CO2 and water vapour.

Carbon dioxide boils at -78.5o C, thankfully an uncommon temperature on Earth. That means it's always present in the air as a gas. Water is in comparison multitalented: it can exist as vapour, liquid and solid. Condensed liquid water forms the tiny droplets that make up clouds at low and mid-levels. At height, where it is colder, the place of liquid droplets is taken by tiny ice-crystals. If either droplets or crystals clump together enough, then rain, snow or hail fall back to the surface. This process is constantly going on all around the planet all of the time. That's because, unlike CO2, water vapour is condensable.

CO2 is non-condensable and that means its concentration is remarkably similar throughout the atmosphere. It has a regular seasonal wobble thanks to photosynthetic plants - and it has an upward slope caused by our emissions, but it doesn't take part in weather as such.

Although water vapour is a greenhouse gas, its influence on temperature varies all the time, because it's always coming and going. That's why deserts get very hot by day thanks to the Sun's heat with a bit of help from the greenhouse effect but can go sub-zero at night. Deserts are dry places, so the water vapour contribution to the greenhouse effect is minimal. Because clear nights are common in dry desert areas, the ground can radiate heat freely to the atmosphere and cool quickly after dark.

On the other hand, the warming oceans are a colossal source of water vapour. You may have heard the term, 'atmospheric river' on the news. Moist air blows in off the ocean like a high altitude conveyor-belt, meets the land and rises over the hills. It's colder at height so the air cools as it rises.

Now for the important bit: for every degree Celsius increase in air temperature, that air can carry another 7% of water vapour. This arrangement works both ways so if air is cooled it sheds moisture as rain. Atmospheric rivers make the news when such moisture-conveyors remain in place for long enough to dump flooding rainfalls. The floods spread down river systems, causing variable havoc on their way back into the sea.

Atmospheric rivers are a good if damaging illustration of how quickly water is cycled in and out of our atmosphere. Carbon dioxide on the other hand just stays up there, inhibiting the flow of heat energy from Earth's surface to space. The more CO2, the stronger that effect.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

When those who deny human-caused global warming use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as potent a greenhouse gas as water vapour, which there's already a lot of, adding a little more CO2 couldn't be that bad, they insist.

What this argument misses is the critical fact that water vapour in air creates what scientists call a 'positive feedback loop'. That means it amplifies temperature increases, making them significantly larger than they would be otherwise.

How does this work? The amount of water vapour in the atmosphere has a direct relation to the temperature in any given region and the availability of water for evaporation. Heard the weather-saying, "it's too cold to snow"? There's more than a grain of truth in that; very cold air has a low capacity for moisture.

But if you increase the temperature of the air, more water is able to evaporate, becoming vapour. There's a formula for this, the figure being 7% more moisture capacity for every degree Celsius of warming. All you then need is a source of water for evaporation and they are widespread - the oceans, for example.

So when something else causes a temperature increase, such as extra CO2 emissions from fossil fuel burning, more water can evaporate. Then, since water vapour is a greenhouse gas, this additional moisture causes the temperature to go up even further. That's the positive feedback loop.

How much does water vapour amplify warming? Studies show that water vapour feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C upward temperature change caused by CO2, the water vapour will cause the temperature to go up another 1°C. When other demonstrable feedback loops are included, and there are quite a few of them, the total warming from a 1°C change caused by CO2 is as much as 3°C.

The other factor to consider is that water evaporates from the land and sea and falls as rain, hail or snow all the time, with run-off or meltwater returning to the sea. Thus the amount of water vapour held in the atmosphere varies greatly in just hours and days. It's constantly cycling in and out through the prevailing weather in any given location. So even though water vapour is the dominant greenhouse gas in terms of quantity, it has what we call a short 'atmospheric residence time' due to that constant cycling in and out.

On the other hand, CO2 doesn't take an active part in the weather. It does hitch a lift on it by being slowly removed from the air as weak solutions of carbonic acid in rainwater. These solutions are key weathering agents, affecting rocks on geological time-scales. Weathering is a key part of the slow carbon cycle, with the emphasis on slow: CO2 thus stays in our atmosphere for years and even centuries. It has a long atmospheric residence time. Even a small additional amount of CO2 thus has a greater long-term effect - and in our case that additional amount is far from small.

To summarize: what deniers are ignoring when they say that water vapour is the dominant greenhouse gas, is that the water vapour feedback loop actually amplifies temperature changes caused by CO2.

When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. 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.

How does this work? The 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 further—a positive feedback.

How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So 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, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.

The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapour is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.

So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger.

Last updated on 23 July 2023 by John Mason. View Archives

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Further viewing

Further reading

Denial101x video(s)

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Additional video from the MOOC

Expert interview with Steve Sherwood


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Comments 376 to 378 out of 378:

  1. So if CO2 increases temperature that increases H2O and H20 increases temperature, why doesn't that increase temperature forever or is there a mechanism that cools the climate before that happens?

  2. Likeitwarm:

    You like asking questions that you think are "gotcha's", don't you?

    Tell you what: what you are describing is an infinite series. Put the infinite series into a mathematical form, including a variable that tells you how much H2O increases by for a given temperature increase, and then how much that temperature increase will increase H2O, and see how it behaves?

    You will probably find that not all infinite series lead to infinite increases. Some of them have finite limits and asymptotically approach a limiting value (as long as the appropriate mutiplliers fall within certain limits).

    When you have your mathematical expression of the problem you are asking about, get back to us.

  3. Oh, darn. The moderator has pointed Likeitwarm to a place where the answers to my homework assignment can be found.

    The infinite series I had in mind is:

    1 + x + x*x + x*x*x....


    1+ x + x2 + x3 ...

    where x is the additional single-step feedback temperature rise added to the initial 1 degree rise. At each subsequent step, x acts on the extra rise from the previous step, hence the x2, x3 terms.

    Brilliant mathematicians have managed to find a closed form (finite)  solution to that infinite sum, for x less than 1.

    sum = 1/(1-x)

    As long as x < 1, there is an eventual stable sum. If x = 1, the denominator becomes zero and the sum becomes infinite. If x>1 the sum at each step  represents an exponential increase.

  4. ScienceTruther @379 ,

    we have been over all this, with other threads and other monikers, so many times before.

    You do not even try to understand the very basic physics.  You do not get to be published in Nature journal, and you do not get a Prize in Stockholm.  At a long shot, you might have a chance at an IgNobel.


    [BL] Note that the comment being responded to no longer exists.

  5. The commenter @379 presently calling themselves ScienceTruther is entirely incorrect to cite either Pierrehumbert (2011) or Zhong & Haigh (2013) to support the assertion that "it was predicted from advanced spectroscopy calculations that a four-fold increase in CO2 would be needed for even a detectable, much less dangerous, change in temperature due to CO2." Indeed, Zhong & Haigh demonstrate the exact opposite.


    [BL] Note that the comment being responded to no longer exists.

  6. There hasn't been a single experiment comparing temperatures of infrared transparent airtight capsules with different CO2 concentrations, including on with current-atmosphere concentration... also, comparing with different infrared filters in the "crystal" and heights, so where's the real falsifiable science? It's all based on assumptions without being able to discriminate confounding variables!

  7. Prof Nazar:

    Are you trying to claim that there is no experimental evidence that CO2 will absorb IR radiation at certain wavelengths?

    Are you trying to claim that the amount of absorption does not vary with CO2 concentration?

    If so, then why is it that I can buy commercial off-the-shelf technology that measures CO2 concentrations in air using those principles?

    Are you arguing that the absorption of IR radiation by CO2 does not increase the energy level of that CO2 molecule? And thus the average temperature of the air?

    If so, you are taking a position that does not respect the conservation of energy.

    If you are just saying "I haven't seen anything that convinces me", then you are probably making an Argument from Incredulity.

    You are long on assertions, and really, really short on presenting any actual evidence for your assertion.

  8. Isn't i so that greenhouse gases emits radiation into space at a more or less fixed temperature? Say, CO2 emits radiation to space at 220 kelvin, while water vaport emits radiation to space at 260 kelvin.

    As far as my understanding goes, the only increased greenhouse effect is that higher concentration of these gases makes the absorption spectra broader so that more radiation to space happens at those lower temperatures. Since water vapor is the dominant greenhouse gas, accounting for approx 50% of the greenhouse effect, and its content in the atmosphere has increased by 'only' 7% due to the 1°C increase in atmospheric temperatures, how much additional greenhouse effect can this actually have? I would believe that 1°C additional warming from 7% more water vapor is an overstatement. What do you think?

  9. Vidar2032:

    I think you have some fundamental misunderstandings of the emission and absorption of radiation.

    First, gases do not emit radiation "at a fixed temperature". Temperature is not a characteristic of radiation. Temperature is a measure of thermal energy, and that thermal energy is available to be converted to radiation (i.e., emitted). Gases will emit radiation at selected individual wavelengths, related to the structure of the gas. But they will emit radiation at those wavelengths at any temperature.

    The way that temperature links to emission is by the quantity of energy available. Higher temperature? More frequent emissions of photons, which carry more energy. (But each individual photon at a specific wavelength will contain the same amount of energy.) For a blackbody (not a gas) the higher temperature also tends to increase the amount of radiation more at shorter wavelengths, so you see a shift in the wavelength with the peak emissions:

    Planck curves

    Increased concentration of gases means more molecules to absorb radiation, which means that individual photons will travel shorter distances before being absorbed. This is simply due to the number of extra CO2 molecules, not their concentration in ppm relative to the remaining gases. You can read about the proper way to use measurements of gas concentrations for radiation absorption calculations in this blog post:

    The comments in that blog post are also useful in explaining some of the related effects.

    Once the proper calculations of the effects of adding CO2 and water vapour are done - yes, the seemingly small increase in water vapour will have that 1C warming effect.

  10. Vidar2032 @ post#383 ,

    No.  What you propose about infrared emissions is bizarrely wrong.

    To educate yourself, please go back to Physics 101.  From what you have said, you have a great deal of reading to do, to get up to understanding the very basics about radiation and molecules.   You have a lot of work ahead of you !

  11. Actually, I think that Vidar2032 @383 is correct. When he/she says GHGs emit at a fixed temperature, I believe he/she means at the temperature of the atmosphere as fixed by the atmospheric temperature profile. The 1976 U.S. Std Atmosphere for the tropopause, where CO2 emits to space, is close to 220K, while the emitting layer of H2O vapor in the troposphere is about 240-270 K. When he/she says that the effect of increased concentration is to broaden the band, that also is correct when considering that increasing concentration strengthens weak absorption lines. Look at the Figure in Bob Loblaw @7 in his linked thread to Beer’s Law above, which Bob kindly produced for me at that time. The weak absorption lines on the wings get stronger as concentration increases. There is sufficient path length in the tropopause to bring most of the absorption lines for the CO2 band between 14-16 microns close to 1.0, which means that the emittance is close to 1.0. Stacking the strong absorption lines in the middle of the band, which means increasing the path length and bringing an emittance of close to 1.0 even closer to 1.0, is not how increasing CO2 increases the emittance. Note that increasing emittance means more energy is emitted from a colder temperature which has less intensity than the energy emitted from a lower altitude at a warmer temperature. This is in accordance with the Planck black body distribution curves that Bob presents. The difference between a black body and a gas is that a black body absorbs/emits at all wavelengths while gases absorb/emit only at wavelengths specific to their molecular structure. What would be interesting, if only I could post my own Figure, would be the HITRAN absorption lines for CO2 at conditions of the tropopause and H2O for the troposphere.

    Meanwhile, Vidar’s question is an excellent opportunity to use the Univ of Chicago link to MODTRAN Infrared Light in the Atmosphere. Choose the 1976 U.S. Std Atmosphere. All one has to do is increase the water vapor scalar to 1.07 to show a 7% increase, then adjust the temperature offset until the original value is matched. It turns out to be about 0.25 C. Better, to see if 7 % is about right, set CO2 to 280, CH4 to 0.7, and Freon to 0 to get pre-industrial conditions. Save the run to background. Then change CO2 to 415, CH4 to 1.8, and Freon to 1.0 to get current conditions, adjust the temperature offset to match the starting value, and choose holding fixed relative humidity. The raw model output shows that it changes the water vapor by about 6%, and the temperature offset is about 1.0 C. It's a very good approximation, but be careful not to place too high of an expectation on the accuracy and precision of this model. Realize that it is designed to be an educational tool with high computational speed and limited flexibility that provides good results, but better models exist for professional use.

  12. Charlie_Brown  @386 ,

    No.  Please remember the old adage about "not seeing the forest for the trees".     ;-)

  13. I also disagree with part of what Charlie Brown has said in comment 386. Although it is reasonable to say that the IR radiation emitted to space looks like it is being emitted from a single layer at temperature X, the losses to space are an integration of IR radiation emitted at many layers of differing temperatures.

    A lot of IR loss to space comes from the stratosphere. In the Beer's Law thread I linked to, in comment #15, I give the modelling results from Manabe and Wetherald, 1967, which shows a cooling of the stratosphere with increased CO2. That is because adding CO2 also increases the ability to emit radiation, as well as to absorb it. In the stratosphere, that means that the temperature change is dominated by the fact that the same IR radiation can be emitted a a lower temperature. That would not make any sense if IR loss to space only came from a single height. Here is the figure I included in that oher comment:

    Manabe and Wetherald 1967 figure 16

    IR radiation transfer in the atmosphere cannot really be dealt with as a single-layer item , except as a useful approximation to illustrate certain characteristics. It is a continuous system of many layers, with absorption/emission sequences that depend on all of the following: temperatures, atmospheric composition, and the wavelength of radiation (since greenhouse gasses absorb and emit at specific wavelengths).

    It is correct that water vapour is concentrated in lower layers of the troposphere,  where the temperatures are warmer - whereas CO2 is relatively uniformly mixed through the troposphere and stratosphere. But both exist in a continuum. The symbols in the figure I give above represent the different layers that were used in the Manabe and Wetherald model. Still a set of discrete altitudes (heck it was 1967, so the computer they used was far less complex that your current cell phone) - but a lot more layers than just "this one for water vapour, that one for CO2".

  14. Manabe's Nobel Prize was very well deserved, that's for sure.

  15. Bob @388
    We agree more than you think. I did not intend to imply that only two atmospheric layers need to be considered for emittance. I was intending to illustrate the many strong and weak absorption lines in the layers that were most important for emittance from CO2 and H2O. A useful concept is to look down from the top of the atmosphere. Integrate absorptance/emittance for energy loss to space from the top and descend until a value of 1.0 is reached. MODTRAN Infrared Light in the Atmosphere (MILIA) model, which is a multilayer model, does the calculations.

    Maybe we disagree on descriptions for the magnitude of contribution to the upward heat flux from the tropopause and the stratosphere, because I conclude that the major emitting layer for CO2 is the tropopause (11-20 km in the 1976 U.S. Std atmosphere) and the stratosphere contributes only a small amount. This can be demonstrated by changing the altitude in MILIA from 20 km (217 K) to 50 km (271 K). At 20 km, the bottom of the spectrum in the CO2 band of 14-16 microns reaches 217 K, which matches the Planck distribution. Raising the altitude to 50 km brings the bottom of the band up a little bit to 222 K, but not to the higher temperature of the stratosphere. Also, and very interesting, is the appearance of a sharp peak at 14.9 microns that matches a Planck temperature of 240 K. This is caused by the contribution from a few very strong CO2 absorption lines.

    We agree that Manabe’s work is awesome, but I don’t think that it necessarily supports a description “A lot of IR loss to space comes from the stratosphere.” It demonstrates a significant effect of CO2 on the temperature of the stratosphere. However, the stratosphere has so few molecules that small differences in the total IR energy flux can have large differences in temperature.

  16. OK, Charlie Brown. I agree that I overstated it when I said that "a lot of IR loss to space comes from the stratosphere". But the important point is that what is seen at any height, looking up or down, is not from a single level above or below the observation height. I think we agree on that.

    The IR emitted in the stratosphere will easily be lost to space, due to the small number of molecules at that height, as you mention. We've discussed on other threads how the view of  upward-directed IR at high altitude has its origins at different levels, in a kind of inverse view of Beer's Law. Beer's Law tells us the probability that a photon will be transmitted through  X path lengths, and the flip side is that it tells us the probability that a photon seen at altitude Z came from a source X path lengths away. A high probability that it came from somewhere close; a low probability that it came from somewhere far away. Exact values of "close" and "far" are dependent on the absorption coefficient for that particular wavelength.

    When we "see" an individual photon, we have no idea how far it has travelled, as all photons of that wavelength look the same and carry no memory of the temperature they were emitted at. With a large number of photons, we can start to talk of the probability distribution that it came from altitude Z. It could have been emitted by a layer just below, or well below, or just emitted locally.

    In a model, you will have access to internal calculations that tell you how much of the  upward flux was transmitted from lower layers, versus how much was emitted within the layer. Field measurements will just give you the sum of the two, though. (In part, it's because field measurements don't have "layers" in the way a model does.)

    The MODTRAN page you link to has a button "Show raw model output". That has a lot of detail. I don't know if there is a way to parse that to get the split between "transmitted" "emitted" in the flux. It is a very useful site, none-the-less.

    All of which is to say that the words Vidar2032 posted in #383 have serious shortcomings. I don't know if we will see Vidar return with any clarification on his/her thoughts.

  17. Please note: the basic version of this rebuttal has been updated on July 23, 2023 and now includes an "at a glance“ section at the top. To learn more about these updates and how you can help with evaluating their effectiveness, please check out the accompanying blog post @

    The intermediate version was updated as well to update some links.

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