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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

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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 176 to 200 out of 378:

  1. bvt123, did you also read the advanced version of the article? Why less <1? Well you do the maths on the physical system (or do an experiment) and that is the number that comes out...

    When you say "average temperature of Earth was stable", what temperature range do you consider "stable"? The risk from climate change is when it happens too fast for adaptation, and having a temperature change of the order that you get from glacial to interglacial happening over say 500 years instead of 10,000 is too fast.

    Over long time scales, chemical weathering of silicates (which in turn control CO2 absorption in the ocean) act as a crude thermostat. See for example Archer 2008.

  2. @ 174, " Biotic Regulation Theory ", good read btv123, nice explanation of how changes in land use can lead to a change in weather patterns. This in turn can lead to so called 'erratic weather'. Interestingly there are claims on the web of global forest cover halving over the 20th century.

  3. I have not read all these comments. There has been so much fudging on historical climate data by GISS and other government outlets to make the past look colder than today that I don't really see any evidence the eartth is really warming at any truely measureable rate. The HADCUT data shows no warming, the ARGOS ocean data shows no warming, Antartica is growing record ice, and the Arctic is showing some signs of ice growth too. Plus the USA just experienced one of the harshest winters of all recent times. If CO2 increases water vapor in the atmosphere than why are some blaming the west coast drought on AGW? The truth is these droughts have occurred many times in the historical past in the USA. A few years ago the Southeast was in severe drought. Not any more. The evaporation process in itself is a COOLING process so more evaporation more cooling not less. When precipitation falls latent is released back into the atmosphere. There is no net warming or cooling. The earth's atmosphere is a baroclinic system which is always trying to bring equilibrium to this dynamic system. Its this natural unbalance that keeps the system in motion and always unstable. 

  4. Arthur123: If your comment survives moderation, might I recommend you read more actual science rather than disinformation. I also recommend reviewing the Skeptical Science comments policy.

  5. Arthur123 wrote "There has been so much fudging on historical climate data by GISS and other government outlets to make the past look colder than today that I don't really see any evidence the eartth is really warming at any truely measureable rate."

    You do know, don't you that the raw data is publically available, and that anybody who is skeptical of GISS can download the data (and indeed the code) and perform the analysis for themselves.  Indeed a group of (initially) skeptical scientists actually went and did this and found that the nonsense about fudging historical climate data was just that - nonsense, as they got very similar results to the existing analyses.

  6. Dikran Marsupial,


    It isn't nonsense! 

    I have seen many articles documenting the adjustments to temperature records. In almost all the cases the past is temperatures are adjusted downward to eliminate any simlar periods of past warming. Take a look at what was recently done to historical climate data at Reykjavik, Greenland. This distortion of past climate record data is happening at many, many locations around the world. The US historical record is nothing to be proud of. The group has shown so many problems with sitting of monitoring equipement hear heat sources. Most of the data used by GISS is based on many many adjustments over time. Each adustment they make always enhances there arguement the earth/USA is getting warmer. The cat is out of the bag and the American public know the truth behind the AGW SCAM!


    [PS] Arthur, please review the comment policy. Conformance is not optional. Note the bit on allegations of deception. Discussions of the temperature record are at "Temp record is unreliable" and offtopic on water vapour. I would ask arthur and anyone responding to show him how he has been taken in by disinformers to move the discussion there.

    I could also suggest Arthur that you actually read the science instead of the disinformaton for a bit so you dont make laughable comments which suggests scientists dont know about evaporative cooling. (eg look at the Trenberth Energy budget diagram)

  7. Arthur!  Dangit!  You almost made me ruin my keyboard.  I was taking a drink.

    No evidence.  No post.  bye bye.

  8. Arthur, did you go and investigate what they have done on the BEST project?  Yes or no?

    (updated to add the link again, just in case)

  9. D Marsupial,

    TThere are many documented cases already illustrating the tampering of historical climate data on the web site “WattsUpWithThat” and other similar sites. Just because you can download data off government web sites doesn’t mean it wasn’t tampered with previously. I just go over there at “WattsUpwiththat” and read them. There was a recent posting on Reykjavik, Iceland’s data tampering. The US data is constantly being adjusted by GISS. If I downloaded data 10 years ago at one meteorological site and compared it to a download today there is a better than 50 percent change some portions of the historical time line were adjusted.

    Some of you paid AGW ”schlemiels” were just attacking me because I was saying the evaporation process in itself is a COOLING process. And this is a fact. As far as people like Trenberth is concerned he is running out of explanations as to why the earth is not heating up like he wants. Now he says the heat is "hiding" in the deep ocean. Cold salt water is denser and has greater salinity than warm ocean water, therefore it will sink. If anything ocean temperatures below 2,000 meters depth is likely significantly lower than the first 2000 meters, but because there are no monitoring systems at those depths guys like Trenberth can make wild guesses where the heat has gone. Complete nonsense!

    By the way I am a meteorologist, graduated from CCNY back in the late 1970's and have been watching this whole AGW madness/scam unfold over the years and now I see its end nearing. Look recently what happened to Professor Lennart Bengtsson, who was a member of Global Warming Policy Foundation. He had to resign because he expressed his personal views that he was no longer sure he could support the anthropogenic global warming theory, because of the 17 year global temperature pause and the inability of global GCM models to reliability predict the pause. What happened to scientific freedom and inquiry???
    Scientist by their very nature should always take the opposing view on any theory such as “AGW” especially since the real metrological data does not support run away temperatures in the first place.

    The last Ice Age peaked approximately 18,000 years ago. Let’s see if any reasonable AWG proponent can explain to me how 2-3 mile thick (10,000 to 15,000 feet) solid glacial ice melted completely away in such a short geological time span. CO2 levels in those days were lower than today. What melted 15,000 feet of solid ice that was covering most of the Northern Hemisphere including the NYC area??? The Atlantic Ocean was so much lower that the east coast of the USA was 60 miles further out. For sure, climate change does happen but is a natural process that happens on a scale many more times higher in magnitude than anything caused by small changes in CO2. Imagine ice that was 10 times the height of the Empire State Building all gone today. 18,000 years ago is a blink of the eye in the earth’s life. No GCM model can model that for sure. What melted all that ice???


    [TD] Your comment violates the comment policy at the very least by being off topic.  Post your comments on the relevant threads, or your comments will be deleted without warning in the future.  Here are some suggestions; many of these have Basic, Intermediate, and Advanced tabbed panes, so be sure to read all of those, in that order:

    Your first paragraph:  Temperature Record Is Unreliable

    "The evaporation process in itself is a cooling process":  (Do not use all caps; use bold or italic instead.)  Evaporation cannot remove energy from the total Earth system (i.e., to space), so evaporation cannot cool the total Earth system, which is the system that global warming is all about.  There are lots of places to learn about this, but you might start with "A Rough Guide to the Components of the Earth's Climate System."

    Regarding the slower surface heating in the past few years than in the previous few:  That is only slightly mysterious and most certainly not surprising.  There are many posts about that, such as "Global Cooling - Is Global Warming Still Happening?," and "Climate Models Show Remarkable Agreement With Recent Surface Warming."

    You are incorrect that that cold ocean water must immediately and permanently sink below warm water; there are long-understood mechanisms by which cold deeper water is pushed/pulled above warmer water.  Also read "Correction to Curry's Erroneous Comments on Ocean Heating."

    AGW does not mean runaway warming; you misunderstand what "positive feedback" means.

    Regarding the last ice age (i.e., glacial period), the root-causal orbital variations are called Milankovitch Cycles.  The other contributors, which are triggered by those orbital variations, actually are very well modeled

  10. arthur, I'll take that as a "no" then.  Sorry, I have better things to do than answer questions from those that are not interested in any answer other than the one they started out with (especially if they are going to be needlessly insulting).

  11. Arthur, if you vomit unsubstantiated claims, your vomit will be deleted. If you really believe what you say, and you have evidence, then take your arguments to the appropriate threads and present the evidence.  That's all anyone here is asking--well, that and that you argue in good faith (i.e., you recognize evidence given by others).  

    It's easy to "win" arguments if you get to define what is "truth."

  12. Arthur123 @184

    With respect to your comment on Bengtsson, you might like to read through this which includes this comment by one of Bengtsson's peers on the reasons for the rejection of his paper

    “What counts are the reasons the editor gave for rejection. They were because the paper contained important errors and didn’t add enough that was new to warrant publication. Indeed, looking at all the comments by the reviewer they suggested how the paper might be rewritten in the future to make it a solid contribution to science. That’s not suppressing a dissenting view, it’s what scientists call peer review.*

  13. There's also the statement released by the publisher of the journal Bengtsson submitted his work to.

  14. I have a question. Since warming causes more atmospheric water vapor and more atmospheric water vapor causes warming, why doesn't the water vapor feedback become self sustaining as a result of any natural change in global temperature, such as an El Nino? The effect must be self limiting or the planet would just keep heating up. What breaks, interrupts or stalls the feedback loop?


    [TD]  Good question!  See the answer in the post on positive feedback.

  15. I would be grateful if somebody could give a link to a source of data on long term absolute and relative humidity levels for the atmosphere as a whole.  I am struggling to find such a source (if there actually is one).  Has anybody done any empirical work on the relationship between past temperature changes and changes in humidity?  Thanks.

  16. Matthew L @190.

    This NOAA-ESRL site might be the sort of thing you're looking for.

  17. I am working to understand the complex interactions involved in the most complicated thing there is, climate. Water vapor and clouds seem to be where the most uncertainty lies, according to my understanding of IPCC reports.

    Have there been any research papers done on the diurnal temperature changes in desert areas ?

    It would seem to me in the near absence of water vapor (low humidity) the difference between day time highs and night time lows over time should correspond to the increase in co2 during the same time period. That is since the Industrial revolution the difference between highs and lows should be decreasing proportional to the increase in co2. They would be inversely proportional.

  18. In response to this question, per Lacis et al 2010:

    "Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth's atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can, and does.

    Non-condensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect.

    Without the radiative forcing supplied by CO2 and the other non-condensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state."

    Per Lacis et al 2013:

    "The climate system of the Earth is endowed with a moderately strong greenhouse effect that is characterized by non-condensing greenhouse gases (GHGs) that provide the core radiative forcing. Of these, the most important is atmospheric CO2. There is a strong feedback contribution to the greenhouse effect by water vapor and clouds that is unique in the solar system, exceeding the core radiative forcing due to the non-condensing GHGs by a factor of three. The significance of the non-condensing GHGs is that once they have been injected into the atmosphere, they remain there virtually indefinitely because they do not condense and precipitate from the atmosphere, their chemical removal time ranging from decades to millenia. Water vapor and clouds have only a short lifespan, with their distribution determined by the locally prevailing meteorological conditions, subject to Clausius-Clapeyron constraint.

    Although solar irradiance is the ultimate energy source that powers the terrestrial greenhouse effect, there has been no discernable long-term trend in solar irradiance since precide monitoring began in the late 1970s. This leaves atmospheric CO2 as the effective control knob driving the current global warming trend.

    Over geologic time scales, volcanoes are the principal source of atmospheric CO2, and the weathering of rocks is the principal sink, with the biosphere particpating as both a source and a sink. The problem at hand is that human indistrial activity is causing atmospheric CO2 to increase by 2 ppm/yr, whereas the interglacial rate has been 0.005 ppm/yr. This is a geologically unprecedented rate to turn the CO2 climate control knob. This is causing the global warming that threatens the global environment."

  19. A response to new commentator POJO:

    The output graphic you linked to specifically excludes relative humidity outside a narrow band of the tropics (5S to 10N).  Looking at global RH, we see that the atmospheric composition of water vapor has increased by about 5% since 1970 (Trenberth & Fasullo, 2009; pp 317). As a result, the atmosphere now holds the approximate equivalent of an extra volume of Lake Erie in it, spread throughout.

    Also see my comment above about research done by Lacis on water vapor vs CO2 as drivers of temperature change (hint:  water vapor is not a driver and CO2 is).

  20. Thanks Daniel,

    I am getting annihilated at work.  The guys have rightly shown me for which i cannot dispute that the tropics is best where the amplification is to take place.  Hence why I have shown the narrow band.  Is this incorrect? 

  21. "have rightly shown me for which i cannot dispute that the tropics is best where the amplification is to take place"

    How did they rightly show you?  Do these people maintain that water vapor somehow "knows" that it can only amplify temperatures in a narrow band of the tropics and not anywhere else?  Sounds like magic thinking, to me.

    Ask them for links to the primary literature establishing such.  And please keep the same user name here.

  22. Thanks for the response Daniel,

    Well the IPCC stated that warming is expected to be amplified in the tropics in the upper troposphere.  I harp on about radio sonde not being acceptable but they then site references where the ipcc accept it in the ipcc ar5.  I am left with very little to argue with here.   Anyway here is the link for world Water Vapour.  Help me out will you. 

  23. Daniel Bailey @196.

    The choice of a narrow tropical latitude band used to produce the graphic originally presented by POJO doesn't make a great deal of difference to the graph plotted for all latitudes.

    What is being plotted in these graphs is the RH at 300mb in the upper troposphere. (The mention on the graph axes legends of "(up to 300mb only)" is confusingly reproducing an instruction that the data available for plotting does not extend above 300mb.) The web-utility that generates them is here.

    The problem with the data at high altitude is that it is very unreliable being much troubled by calibration issues. A plot of this global RH data for 1000mb, 600mb & 300mb & suitably smoothed (usually 2 clicks to 'download your attachment') shows that at lower altitudes where calibration is more certain RH is increasing.

  24. Correcte link to global RH at 300mb.

    Also the final graphic linked is SH not RH.

  25. MA Rodger,

    Is there a problem with the NOAA data.  Just saying the data is no good does not wash with these guys.  They have cited references from IPCC which make it very dificult to argue against.  Particulary BOX 2.3 of section 2 of the AR5.  Page 185 of the document clearly states that the NOAA reanalysis datasets are aceptable.

    They have also shown me another part of section 2 where it states that BOX 2.3 is the latest and greatest.  re page 164

    "In recent decades, advances in the global climate observing system have contributed to improved monitoring capabilities. In particular, satellites provide additional observations of climate change, which have been assessed in this and subsequent chapters together with more traditional ground-based and radiosonde observations. Since AR4, substantial developments have occurred including the production of revised data sets, more digital data records, and new data set efforts. New dynamical reanalysis data sets of the global atmosphere have been published (Box 2.3). These various innovations have improved understanding of data issues and uncertainties (Box 2.1)."

    So if that is the case i am at aloss to argue with these guys.

    The NOAA data used is referenced in this box 2.3

     Help me out will you.

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