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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)

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.

Basic rebuttal written by James Frank


Update July 2015:

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

 

Last updated on 5 July 2015 by pattimer. View Archives

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Comments 51 to 75 out of 136:

  1. All that grade school "numerology" isn't going to get you very far Mizimi. re #48 and "water vapour emitted by air breathers". Remember that every molecule of water vapour returned to the atmosphere by metabolic respiration was a water molecule pulled out of the atmosphere by photosynthesis: nCO2 + nH2O ------> (CHOH)n + nO2 photosynthesis where (CHOH)n is generic carbohydrate (CHOH)n + nO2 ------> nCO2 + nH2O respiration re #49 and atmospheric relative humidity. Remember that the bulk of the atmosphere is much colder than 15 oC and so the water content at 50% relative humidity is far lower rather quickly as one ascends re #47/#49 Which paper are you talking about (mizimi: "just re-read that paper....")? You should really give your sources when you start expounding numbers. You make so many mistakes that it would be helpful if you could simply state where you obtained your numbers/ideas from so that we could assess them ourselves. Otherwise it's hardly worth addressing many of your points since they are based on series of unattributed numbers which very often turn out to be incorrect...
  2. Chris: I have difficulty some points you have mentioned. "Water Vapour as a positive feedback" If this were true, then what prevents the water from providing it's own feedback into what is essentially a thermal runaway? As well, it's a well documented fact that carbon dioxide dissolves in rain water, resulting in a typical pH of 6.5. So how can the lifetime of CO2 be so large if it's readily washed out by rain fall?
  3. Chris; the paper is Santer 2007 noted above. And you didn't answer the question....how do climate models incorporate the heat emitted from life which is independant of IR ? Also, as you have previously averred, the WV content of the atmosphere is dependent on P~T, (subject to availability)so when it is removed during photsynthesis it is readily replaced..returning to "equilibrium"; so WV emitted by air breathers adds to the atmospheric content.
  4. Help please! I have not be able to find the paper disclosing the physics of the carbon dioxide and water interactions. Here I see only references to climate models and empirical studies, I would prefer to see the description of the actual phyical chemistry.
  5. According to http://wattsupwiththat.com/2008/06/21/a-window-on-water-vapor-and-planetary-temperature-part-2/ and http://landshape.org/enm/greenhouse-thermodynamics-and-water-vapor/ atmospheric WV appears to be decreasing at the height where GCM's predict it should increase.
  6. "We have tried to outline some of the unresolved issues concerning water in the atmosphere. But there are others. For example, it is well known that at low temperature pairs of water molecules will stick together to form a weakly bound molecule known as a dimer. The absorption properties of the water dimer at visible wavelengths will be different from those of a single water molecule, but these remain to be characterized. Furthermore, it has so far proved impossible to determine the proportion of atmospheric water molecules that are present as dimers in either laboratory or atmospheric measurements. And we have not even dared to discuss the many problems in understanding clouds. Clouds are highly variable in their make-up, distribution and size. They contain aerosols and mini droplets of water vapour, which have spectroscopic properties that are even more uncertain than those of normal water vapour. " http://physicsworld.com/cws/article/print/17402 A very interesting read.
  7. Don't know that anyone will come back to this discussion, but just in case... Let's pretend that about 2/3 of the earth is covered in water. If that were the case, shouldn't the atmosphere, in general, already be effectively saturated with respect to WV? What happens when the air becomes over saturated; the water precipitates out. True, if you irrigate a field in a dry region or exhaust WV from a hydrocarbon engine, you add to the WV downwind of that, but that doesn't change the thermodynamic properties of CO2. Nor does it change the fact that water evaporates more when it is warmer. With regard to #2, why would the sea surface temperature remain fixed when it is receiving more IR radiation?
  8. Chris G: Think cycles. The earth rotates so it goes through a heating/cooling cycle. WV taken up during the day will precipitate out at night if it contacts a surface cool enough....dew. Obviously if the temp drops sufficiently the general air mass will reach saturation point ( dewpoint). At sunrise, this saturated air is warmed and the dewpoint rises so the air is no longer saturated and can take up more WV. Whether the mass of air ever reaches saturation point during the day depends on how much water is available to evaporate and also that there is sufficient energy to evaporate it. The Saharan atmosphere is dry because although it has enough heat it doesn't have the water. Tropical forests can almost saturate the local atmosphere as both heat and water are present in sufficient quantities to allow this.
  9. Anybody read this http://www.landshape.org/dokuwiki/doku.php?id=introduction which is an outline of a new climate 'theory' by Ferenc Miskolczi (ex NASA mathemetician) and if so...any comments?
  10. Hello guys. Has anyone considered the basis of a biological understanding of global warming? For instance your assumptions about the behaviour of water vapour are explained in terms of temperature, evaporation and condesation. I have read an interesting article from CSIRO SUSTAINABILITY NETWORK UPDATE – No. 64E is title "The biology of global warming and its profitable mitigation" and written by Dr Walter Jehne. I have a MSWord copy of the article though do not know how to added it to the Articles list.
  11. AldousH: I have read the article summary and in general terms agree with the conclusions. However it does seem to concentrate on deforestation as the prime mover and disregards other matters such as WV emissions from industrial processes, increased evaporation from man-made dams, lakes etc. Whilst these may be (relatively) small, they are not inconsequential; neither is the increase in heat and WV from animal life.
  12. There is considerable evidence that man's activities are changing the distribution of WV with consequential effect on climate. Deforestation has decreased evaporation by around 3000cubic/k/a which is mostly balanced by increases from agricultural evaporation of # 2600c/k/a. To that must be added the estimated loss of 400 c/k/a from industry, commercial, municipal use and reservoirs. The system seems to be in balance. However, the flow pattern of WV has changed, with subsequent effects on climate....this is explored from an agricultural view in "Human modification of global water vapor flows from the land surface." published online at PNAS ( the address is just too long to post) and the authors conclude that until modellers include redistribution of WV, GCM's cannot be considered to adequately describe real world conditions.
  13. Something to consider. The global dew point has been rising faster than global temperature. At the same time the pan evaporation rate has been dropping with an increase in global temperature.
  14. I would like to see all energy inputs and outputs and also the real estimates of the amounts of water in oceans water in atmosphere carbon sources ,co2 production from nature and man methane from nature and man i would lke to see evidence of temperatures in the distant past with co2 levels and if possible water vapour levels say over millenia. my expectation is that atmospheric carbon and water vapour levels are tails of very big dogs in the oceans and carbon sources. that we are dealing with small numbers in a system of much bigger numbers so conclusions we reach about global warming based on atmosheric gases are very problematic. what does co2 energy absorption in its narrow absorption band really have on energy balance.could it be it is reradiated at another frequency to space ? please explain how the green house effect of a gas actually works in detail I can appreciate the effect of WV in that clouds capture heat energy from the sun and nett radiate convect and reflect more heat than if the clouds were not there.water vapour that is not in a cloud presumably is causing a radiation block from the earth. i am not sure of the deatil
    Response: I've recently posted about an analysis of all energy inputs and outputs. and posted on man's co2 emissions. For more on the carbon cycle, see this page on human versus natural co2 emissions.

    Examination of past temperature change alongside CO2 change is examined on the CO2 lags temperature page as well as empirical determinations of climate sensitivity.

    The conclusion that atmospheric gases are causing global warming is based on empirical observations. Satellite observations of radiation escaping to space find that less radiation is escaping at wavelengths that CO2 absorb. This is confirmed by surface measurements of downward longwave radiation that also find increasing downward radiation at CO2 wavelengths.
  15. The article states: If extra water is added to the atmosphere, it condenses and falls as rain or snow within a week or two. Similarly, if somehow moisture was sucked out of the atmosphere, evaporation would restore water vapor levels to 'normal levels' in short time. Implicitly that says that humans can not add water vapor to the atmosphere. But from the pure logical point of view this contradicts the very first statement (from the green box): Water vapor is the most dominant greenhouse gas. If it is a greenhouse gas then it will act as such i. e. warm the atmosphere in effect. As a result the atmosphere will hold more water vapor including a fraction of what was added by men. If it's not, then all the talk about feedback is just gibberish and all conclusions from it, including (I assume) all the models, can surely be entrusted to the trash can. Please don't even consider the argument about the tiny amount by comparison. That has clearly been ruled out in the global warming debate. What I'd like to see are some serious estimations about the anthropogenic part when it comes to water vapor. Including besides the obvious ones e. g. the amount of water contained in a swamp vs. a palm oil farm per square kilometer or tropical rain forest vs. Cattle pasture or corn field. So if you start thinking about it there is hardly an end to find even restricted to the respect of land use that might result in releasing water to the atmosphere. Which then leads me to my last point. That is about direct heating the atmosphere by our energy production (of cause from burning fossil fuels and using nuclear - because all other sources are more or less conversions from sunlight). I think this belongs here because in most cases water vapor acts as a transport medium in the process. It gets vaporized by the produced energy and releases it due to condensation in the atmosphere. Due to the overall efficiency of our industry we speak about 50+ % of all energy generated. So far I have been unfortunate in finding anything about that matter so I thought it might be a good idea posting this question here.
  16. h-j-m, there is no actual contradiction between "extra" water falling out of the atmosphere, and the increased temperature due to the presence of extra water vapor allowing more water vapor to stay in the atmosphere. Additional water vapor increases the atmosphere's temperature by enough to allow an increase of the atmosphere's water-vapor-holding-capacity by only a fraction--a proportion less than 1. That resulting increase in water vapor then repeats the cycle, but now only that same fraction of the previous fraction. It is a converging series. The increases are the same percentage each round, but since the percentage of increase is less than 100%, the increase gets progressively smaller until it reaches zero.
  17. h-j-m, the amount of energy added directly to the atmosphere by humans is a forcing of less than 1% of the forcing from greenhouse gases added by humans.
  18. h-j-m, evidence supporting my contention of the triviality of energy humans add directly to the atmosphere is in my two comments on another thread here and here.
  19. h-j-m, I'm sorry, my previous reply regarding extra water falling out of the atmosphere did not answer your question. Please let me try again: There are vast pools of liquid water available to go into the atmosphere, and vast seeds for condensation to help water vapor drop out of the atmosphere. Indeed, both those activities happen constantly. So neither of those is a limitation on the amount of water vapor in the atmosphere at any one time. Humans' provision of more water is only a drop in the bucket. What does primarily limit the amount of water vapor in the (Earth's) atmosphere is the atmosphere's temperature. At a given temperature, adding more water vapor "nearly instantly" forces water vapor to drop out of the atmosphere. "Nearly instantly" in this context means "so fast that there is no time for significant atmospheric heating from the extra water vapor." The opposite happens as well: Water vapor removed from the atmosphere merely leaves room for the other water vapor that is constantly being added. The net effect all those processes is no change in temperature nor in the amount of water vapor. Water vapor is not a "forcing" of temperature. All the above is not just theory; it is observed fact. It was true before humans had even evolved. If it were not true--if water vapor was not limited by temperature--then there would no longer be liquid water on the Earth's surface. It would all have evaporated and none would have condensed. Water vapor could be a forcing if there weren't any liquid water lying around. On some other planet that doesn't have enough water to fill its atmosphere's capacity for water vapor, adding water vapor to the atmosphere certainly would cause that vapor to stay in the atmosphere. But here on Earth, we've got an abundance of water. What's needed to increase water vapor for more than 10 days is an increase in atmospheric temperature. That initial increase can't come from added water vapor (as I just explained), but it can come from anything else--anything that is a temperature forcing. For example, it can come from an increase in the Sun's output, or an increase in greenhouse gases. Once the temperature has increased, less water vapor drops out of the atmosphere. That does indeed then increase the temperature, which is why water vapor is a "feedback" from other causes of temperature increase. But the amount of temperature increase is strictly limited by the converging series I described in my previous comment.
  20. Dear Tom Dayton, deliberately I war referring to applied logic in my argument because I did not want to delve into the complexities we are dealing with here if only for the reason I don't know enough to argue in any specialized field. But I fear your reply forces me to elaborate on my point. As to my knowledge anything happening in the atmosphere is occurring locally depending on a lot of causes that are satisfied just at the spot. Any method of generalizing and narrowing down to only a few or even one cause (atmosphere temperature in this case) is not appropriate in my view due to the fact that we are dealing with nature here and for quite a long time now it is known that nature no way can be understood in a linear fashion as your argumentation suggests. I might mention that only a few decades ago it was the complexity of the earth's atmospheric system that triggered a whole new branch in mathematical science known as chaos theory. The atmosphere being a chaotic system clearly rules out any notion of insignificance due to amount unless any significance is disproved. Anyway to state my point in a somewhat broader consent: The matter of water and how we handle that should be given a much higher priority not only but also with respect to global warming. The reason should be obvious: The fact that it got harder in the last decades to get access to fresh water due to dwindling surface reservoirs and by now we already started using fossil reservoirs of it poses an immediate threat to mankind's existence as devastating as global warming if not even worse.
  21. h-j-m wrote "for quite a long time now it is known that nature no way can be understood in a linear fashion as your argumentation suggests." The role of water vapor in global warming is not at all "linear." It is rather complicated, and so is scientific knowledge of it. Introductory explanations of it are less complicated to suit readers who "did not want to delve into the complexities we are dealing with here if only for the reason I don't know enough to argue in any specialized field," as you wrote. If you suspect an explanation is unrealistically simple, you should pursue a more complete explanation. That is easy by clicking on the links to scientific papers provided in John Cook's original post, and indeed by clicking on the link I provided in my earlier response to you. One particularly relevant and short article is by Dessler and Sherwood (2009, Science, available for free), which specifically mentions local versus global effects, and which I think you in particular would find very informative and comprehensible.
  22. For one: The reason "I don't know enough to argue in any specialized field" is quite simple due to the fact that next to the majority of what is to be read is not worth it. A lot of it is in my view often stating trivialities cloaked on science language. On the other hand by far too often statements are made that can't stand any initial logical evaluation. Let's take as an example for the latter the wikipedia article on water vapor you referred to. Now let's just take a look at the paragraph on Condensation. I will include my comments in lines starting by the usual comment sign known from programming # continued citation will be marked with an >. It states: Water vapor will only condense onto another surface when that surface is cooler than the dew point temperature, or when the water vapor equilibrium in air has been exceeded. When water vapor condenses onto a surface, a net warming occurs on that surface. # not necessary because that would depend on the ability of that surface to hold additional heat (energy). > The water molecule brings a parcel of heat with it. # why referring here at molecules when this amount is given in the properties section as 2.27 MJ/kg. I'm too lazy to look up what that would make in watts but I'm pretty sure we are talking about real energy here and not some negligible portion as indicated by the term parcel. >In turn, the temperature of the atmosphere drops slightly. # That is absolute nonsense it implies that the surface it condenses to absorbs more energy (heat) than is set free in the process. > In the atmosphere, condensation produces clouds, fog and precipitation (usually only when facilitated by cloud condensation nuclei). # Does that mean when taking place in the atmosphere no energy (heat) is released at all or maybe absorbed by the condensation nuclei? That would be a rather absurd notion and to my knowledge contradict the first law of thermodynamics. ># I canceled the rest of the paragraph. So far so well (or not so well). I hope this made a bit more clear what I mean when speaking of "not knowing enough". Now, this is what happens in my view: Given the right conditions water evaporates mainly near the earth surface. Due to it's density it then starts a path to higher altitudes. If then again the conditions are right it condenses and continues it's route until at last it comes down again as rain, snow or hail. In the process of evaporating the amount of 2.27 MJ/kg of energy is used. Logic dictates that the same amount has to be released at condensation. I hardly dare saying that it should be obvious that at the point where condensation occurs it's getting warmer. Now given the fact that on average water stays only 10 days in the atmosphere this is a real fast process. Now I'd like to know if you can find something wrong in this statement. Anyway it would prove a better starting point for a real discussion because it is more suitable to uncover differences in the understanding of the basic principles.
  23. Greenman3610 just posted a Climate Denial Crock of the Week on water vapor.
    Response: Thanks for the heads up, I've embedded the video above.
  24. John, the embedded video is just a blank box for me. But I'm using Firefox on a Mac; maybe it's platform-specific.
    Response: Hmm, I see a blank box in Firefox on the PC but it works fine in Internet Explorer. Any YouTube boffins have any solution to this conundrum?
  25. I really don'y buy the positive feedback model of water vapor. All positive feedback loops are unstable. The planet should have gone wildly hot millions of years ago in a positive feedback model. We have to conclude that water is in a negative feedback loop, either alone, as from the reflection of clouds alone, or in concert with other elements.

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