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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 101 to 125 out of 378:

  1. What "wins" in the long run, an oscillation or a secular trend?
    Response: For discussion relating climate change to the PDO, please use the It's Pacific Decadal Oscillation thread.
  2. @cruzn246: " The PDO and NAO basically put a stop on a warming trend from about the late 40s through the late 70s." No, it didn't. The cooling mid-century was due mostly to aerosols. Why did climate cool in the mid-20th Century? Of course, now that you've been shown wrong on WV, you're going to try to change the subject yet again. That's a textbook denier tactic, and I'm not falling for it. "You can quit insulting me any time you like." Stop making false statements and I'll stop calling you on them. Start listening and learning and I'll be there to help you. "I have an IQ in about the 135 range and have been around for a few more years than you, I'm sure." Not that IQ really measures anything else but the ability to answer IQ tests, but my own scores vary between 146 and 154 ("ordinary" IQ tests aren't too precise in that range). I'm also 40 years old, but that's completely irrelevant to the fact that you haven't been able to present a compelling argument to make your case. I'll also note that English is not my first language, and yet I seem to make much less typos and grammatical errors. Perhaps you should be less emotional about this and take it as an opportunity to learn. "I know what i am talking about but you see things in only your frame of refernece. I can't help you there." I disagree. I think it's clear from your various errors and misuse of graphs that you have *no* idea what you're talking about, and we won't be able to help you with that until you first admit it.
    Response: Comments by anyone, about PDO, will be deleted from this thread.
  3. The water vapor in the atmosphere plays the role of GHG but also has the job of conductor of heat as is a locomotive pulling a train heating. To solve the problem of global warming we need a mechanism to pierce the layer of GHG taking the heat for the top. This work is carried by water that evaporates from the soil. Unfortunately our soil is impermeable. “What should we do about climate change?” We need to fix the environment's thermostat. The negative feedback for an incoming heat is provided by water vapor. See for yourself on this animation:

  4. A few thoughts on the above post... A comparison is made stating that CO2 is an external forcing and water vapour is a result of temperature. This is is misleading, since CO2 levels in the atmosphere can be affected by temperature (for example oceans) while water vapour is added to the atmosphere by power plants and other industry. Therefore are they actually similar in the source for both gases - both affected by temparature and man made sources. The overall point of the article to state that water vapour warming will amplify CO2 warming is misleading, since there is nothing special about the CO2 warming. A natural sun-inducing warming would have similar effects on water vapour. So it is dangerous to state without qualification that 'Water vapour is also the dominant positive feedback in our climate system and amplifies any warming caused by changes in atmospheric CO2. This positive feedback is why climate is so sensitive to CO2 warming.' It could just as easily read 'This postive feedback is why climate is so sensitive to sun warming'. With this in mind, it leads me to ask why such postive feedbacks inherent in the atmosphere have not lead to runway global warming in the past from any naturally induced changes. The reference to Kiehl 1997 shows the clouds also reflect some sunlight back into space (diagram on page 10). Why is this negative feedback not discussed?
    Response: I fixed your extraneous line breaks. Please use the Preview button before using the Submit button. Clouds are not water vapor; they are condensed water. This post is about water vapor. See the post Positive feedback means runaway warming.
  5. I thought the residence time for atmospheric CO2 was verified as about 5.4 years as determined in this article: Carbon cycle modelling and the residence time of natural and anthropogenic atmospheric CO2. Also, in another article: What is the Major Culprit for Global Warming: CFC's or CO2? It's stated that CO2 played hardly any role at all in the global warming since 1950- 2000 and blames it on CFCs. It's also stated that we are facing several decades of global cooling beginning in 2002.
    Response: See the post "CO2 has a short residence time," and comment over there. For the CFCs topic, comment on that post, not this one. Off topic comments usually are deleted after one warning.
  6. #105: See comment re CFCs here.
  7. I am not a scientist but lived with a lot of observation in my life. Can someone tell me if many are actually missing the point and getting to technical? From my observation and reading many papers on line, it would seem that CO2 is far from the cause of global warming, Co2 is a result of global warming from what I can glean. So if water vapour is causing the the increase in temperature, then the increase in temperature is the cause of CO2 rises. To explain: It would appear that after a period of warming and even when the mini ice age came through, CO2 continued to rise to a peak before declining. It would appear that greater degeneration of plant life is caused through warming emitting faster quantities of CO2 into the atmosphere, this in turn creates greater growth of natural flora and fauna to again be degenerated and emitting greater flows of CO2. These growths are also as a result of increasing rains, and higher dew points. Research also shows without being specific, that in Roman times CO2 was higher then than now, that the global temperature was 3 degrees Celsius higher then than now and greater vapour was evident in some history books. If this is the case why are we being led to believe otherwise and that the planet is in danger of burning out if we do not get CO2 under control? I would just like to add as an aside , that Australia despite having serious rain problems and flooding this year, our harvest in South Australia has exceeded all expectations and 300,000 tonnes greater output than the highest ever record reached. As CO2 and water are a necessary plant food, could the small rise be sufficient to warrant awarding the increase to these two elements? Maybe we should be applauding the fact that we have this rise to create greater stock piles before it recedes to almost famine growth as it was in the 1950's with most of our water vapour again locked into glaciers and pole caps. Sometimes to much knowledge in one area can be detrimental to a whole picture which has not been formulated yet. Apologies if this has been addressed as I have not had time to read the many posts, yet!
    Response: Welcome to Skeptical Science! At the left of every page is a list of Most Used Skeptic Arguments. To see all of them, click the link "View All Arguments" at the bottom of that short list. You can also get there by clicking the "Arguments" link in the blue horizontal bar at the top of every page. You'll find the answers to all your questions there. For example, see "Water vapor is the most powerful greenhouse gas," "CO2 lags temperature," "Warming causes CO2 rise," "CO2 is not a pollutant," and "It’s not bad." There are more, but I'm tired of typing. Your best first stop, though, is the Home page, where you should click on the two boxes at the upper right labeled "Newcomers, Start Here" and "The Big Picture." By the way, one of the rules is that you must post comments on appropriate threads. This first comment of yours is too broad for this thread, but that's okay because this is your first time. But in future please find the best thread ("Argument") to post each comment on. And split up your comments into multiple narrow ones, one comment per topic and therefore one comment per thread. Also take advantage of the Search field at the top left of every page.
  8. In different water vapor threads I have seen comments that continually added water vapor(i.e. fossil fuel combustion/irrigation) might act as an anthropogenic forcing of climate change. This makes sense to me (continual addition of vapor overcoming the short residence time) so I looked around and found a couple of places this was addressed: one is a grad student at UCBoulder's website and I'm not sure how good his numbers are but the explanation and detail is very easy to understand, and he concludes that water vapor from combustion is a trivial addition. The other is an article (Boucher 2004), again I'm not qualified to say how "good" the article is, but they estimate that irrigation *does* add to global radiative forcing by up 0.03 to 0.1 W/m2, again a trivial amount compared to C02 or total water vapor forcing. If the mods/author thinks these links look "up to snuff", they might be a useful addition to add to the article for people looking at this aspect of the argument and wondering about "directly anthropogenic" water vapor.
  9. Neglected to mention that in the Boucher article, they note that there is some significant counteracting cooling as well that occurs due to the irrigation/evaporation cooling the area irrigated (not to mention land albedo changes from vegetation etc...)
  10. On another thread Ken wrote: "When Water Vapor is included as a GHG, it represents 95% of the GHGs." Water vapor represents about 0.4% (or 4000 ppm) of the total atmosphere. CO2 is at about 395 ppm currently. The other greenhouse gases are all measured in parts per BILLION or less and thus disappear in the rounding. Based on these numbers water vapor molecules would account for about 91% of all greenhouse gas molecules. The warming impact of water vapor obviously varies by location (i.e. not much in deserts, rather alot in humid areas), but on average it represents 36% (if we exclude water vapor impacts which would also be caused by other GHGs) or 72% (if we exclude impacts by other GHGs which would also be caused by water vapor) of the total greenhouse effect. Thus, I'm not clear where your figure of "95% of the GHGs" comes from. The closest is the 91% of total molecules figure... but that value has no direct relevance to the warming impact you were referring to in your post.
  11. @ CBDunkerson - At times it certainly could be 95%. - Water vapor varies by 3 orders of magnitude... ... and a major defect in the CO2 models is that they assume it is constant. - It is also different in concentration [and behavior with light] at different altitudes in addition to varying between humid and desert areas near the surface. - Plus it's been going up right along with CO2. The CO2 models I've dug into all assume cloud configurations that are 10 to 40 years old. - With a ~10 day Residence Time it not only varies greatly it varies quickly -and- it also moves a great deal of heat around with it when it moves. - Assuming a huge and rapidly changing factor [one that varies by 3 orders of magnitude] as a constant is a lapse of reasonable thought. - There isn't gong to be a good climate model until water vapor is properly accounted for. .
  12. 'and a major defect in the CO2 models is that they assume it is constant" Sorry, perhaps you would like to explain this further (with references). "It is also different in concentration [and behavior with light] at different altitudes in addition to varying between humid and desert areas near the surface." Not exactly news to climate modellers. What is your point here?
  13. PCB, yes I noted that water vapor concentrations vary significantly... but the fact that it can get up to 95% of total greenhouse warming, total greenhouse gas molecules, relative humidity, or other factors does not explain what 95% factor Ken was referring to as a constant. As to your claims about water vapor variations not being considered in climate models... they just aren't true. At that, the very term 'CO2 models' suggests a fictional belief system... climate models account for a large number of forcings and feedbacks, not just CO2.
  14. I am trying to understand this. Water vapor causes warming which causes more water vapor which causes more warming ... What is keeping the earth from vaporizing because of this? What is the GWP of water vapor?
    Response: The feedback is a gain less than 1. That means the increase gets progressively smaller. See Positive feedback means runaway warming.
  15. William, (linking from here) it doesnt make much sense to talk about GWP of water vapour as it is a feedback not a forcing. Water vapour concentration depends on temperature. If you tried to put water data into the GWP equation you run into a problem with the time part of the equation - residence time will depend on temperature. However, if you kind of ignored that, you would end up with a GWP of about 0.23.
  16. Thank you for your reply and for giving me a value for the GWP of water vapor. Water vapor alone is a green house gas and contributes to the insulating effects of the atmosphere as do the other gasses. Water vapor causes warming which causes more water vapor ... I doubt that the world would be at 0 degrees K if there were no CO2. How did you arrive at a value of .23?

    [DB] "Water vapor alone is a green house gas"

    Vastly incorrect.  Read more, post less.

    "Thank you for your reply and for giving me a value for the GWP of water vapor"

    You ignore the caveats given.  Read my response to you here.

    "How did you arrive at a value of .23?"

    The GWP equation can be found here.

  17. William keeps posting, in probable innocence, asking the question:
    "What is the GWP of water vapor?"
    Without realizing that the question itself is a non sequiter. As scaddenp points out above, water vapor concentrations are a function of air temperature, and thus serve as a feedback to the other GHGs. As a condensable GHG, water vapor does not even have a GWP even calculated for it. Only the non-condensable GHG's like CO2 and CH4 have GWPs calculated, as shown here: William, I strongly recommend watching this video on why CO2 is the biggest temperature control knob.
  18. William, the amount of water vapor that the atmosphere can hold depends on the atmosphere's temperature. So if the CO2 were gradually removed from the atmosphere, the temperature would fall, which would cause a fall in the amount of water vapor in the atmosphere, which would cause more cooling of the atmosphere. CO2 also falls as ocean temperature falls, because cooler water absorbs more CO2.
  19. Thank you for your reply. So if the ocean temperature falls the amount of CO2 in the atmosphere will fall and vis versa. This seems to appear in the historical record. Yes the warmer the atmosphere the more water vapor it can hold but that does not assure that more water vapor will actually appear in the atmosphere. As ocean surface temperatures warm I would expect that more water will evaporate into the atmosphere but it also depends on humidity next to the water, winds,and sea state. I believe that much of the temperatures that I experience where I live are a result of convective heat transfer between the ocean and the atmosphere. Local ocean temperatures depend a lot on current and current changes. The ocean where I live has apparently had various cycles of warming and cooling that do not seem to be related to at least short term variations in CO2.
  20. Your hypothesis that water is the dominant greenhouse gas is correct. No one in climate science is disputing that. The question is so what? Because it condenses, you cant change the global water vapour content without first changing the global temperature. Water vapour acts an amplifier to any temperature change caused by say sun, aerosols, GHG changes. CO2 is also an amplifier since its concentration is also temperature-dependent but over very long time-scales. How did I calculate it? Crudely. Including overlap, water vapour = 60% CO2 = 26% (From Keihl and Trenberth) 60/26 = 2.3 10 molecules of water for 1 of CO2 so per molecule cf CO2 = 0.23 If I used Schmidt et al 2010 I guess I would calculate 0.25
  21. William - AGW is about global warming not your local weather. On global basis, the various interactions do indeed result in water vapour being closely tied to global average temperature. There are indeed decadal cycles associated with internal variability of the system (distributing heat around the globe) which is why climate is about 30 year averages not tomorrows weather. However, if you want to discuss this further put it on an appropriate thread, not here.
  22. 119. William Haas; Air temperatures are linked to sea surface temperatures pretty strongly, and higher temperatures mean a higher water vapour content in the atmosphere. Satellites measure a pretty constant relative humidity, so it suggests this is true. You asked in another thread about the GWP of water vapour. Global Warming Potential is defined as the integrated heating caused by a molecule over some time: typically 100 years after you've added it to the atmosphere. Water vapour's mean life is about 10 days in the atmosphere. Iirc, on a molecule-per-molecule basis, largely due to saturation, water vapour is weaker than CO2 by an order of magnitude or two. So the GWP of a water vapour molecule could be estimated as between 0.00003 and 0.000003 relative to CO2. However, this is an overestimate because water vapour evaporates and condenses. It will condense in the upper atmosphere, releasing heat which 'short circuits' a chunk of the greenhouse effect. The water vapour feedback is reduced by about half by this, so you're more likely looking at water vapour molecule = 0.000001 CO2 molecule. That's only water added by combustion and volcanoes etc: water vapour from the ocean is constrained by temperatures and can't act as a long term forcing.
  23. If the temperature increases, the amount of water vapor should also increase (Clausius-Clapeyron law). Some graphs, however, seem to show that the humidity is decreasing Could someone explain. Thanks in advance.
  24. Emilio - I would assume this is Paltridge's long-debunked inference from the NCEP reanalysis. In the paper itself it was noted: "It is of course possible that the observed humidity trends from the NCEP data are simply the result of problems with the instrumentation and operation of the global radiosonde network from which the data are derived." though I doubt climate4you would be pointing that out. Since the NCEP reanalysis had known flaws and was in disagreement with practically every other measurement, this was a drawing a long bow, but then in denial-land, nothing like that gets replaced. It is contradicted by later reanalysis products. I think you can find better graph in AR4.
  25. I'm not a scientist by formal training, but I question the H2O effect. Precipitation has an effect of lowering temperatures, but with 67% of precipitation re-entering the atmosphere through evapotranspiration how is this taken account for? With less ground water, doesn't the earth heat with less energy since the water mass isn't there to help absorb the energy? So, when precipitation contacts with the warmer surface, evaporation is accelerated? Owing to Conservation of Energy- wouldn't this mean more energy is transferred to the atmosphere rather than the Earth, where I suppose this hypothesis has its roots. Wouldn't it stand to reason that the less water in the ground, the more in the atmosphere?

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