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Climate Hustle

Climate change: Water vapor makes for a wet argument

Posted on 2 September 2010 by James Frank

When skeptics use the argument 'Water vapor is the most powerful greenhouse gas', 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.

This post is the Basic version (written by James Frank with a contribution from John Russell) of the skeptic argument "Water vapor is the most powerful greenhouse gas".

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Comments 1 to 50 out of 144:

  1. So where is all this heat?
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    Moderator Response: Go to the home page. At the top right, click on the big image that says "Global Warming Is Still Happening." In the resulting page, click the "Intermediate" tab.
  2. One consideration neglected by skeptic and non-skeptic alike is: water vapor is a GHG at some altitudes, but not all.

    Or so I learned years ago. The example given was that water vapor emissions from high altitude airliners was a problem, but water vapor at lower altitudes is not. Or has this result been overturned since then?
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  3. Here's the thing, though. Water vapor accounts for just over 50% of the total Greenhouse effect-on average (it obviously varies according to temporal & geographic factors). Wheras CO2 accounts for just under 20% on-average. Yet water vapor accounts for between 1% to 3% of the atmosphere, whilst CO2 accounts for less than 0.1% of the atmosphere. This suggests that, on a ppmv basis, CO2 is clearly a more potent greenhouse gas than water vapor.
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  4. Re: MattJ
    Vapour trails or contrails, by affecting the Earth's radiation balance, act as a radiative forcing. Studies have found that vapour trails or contrails trap outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) at a greater rate than they reflect incoming solar radiation (negative radiative forcing). Therefore, the overall net effect of contrails is positive, i.e. a warming.

    However, the effect varies daily and annually, and overall the magnitude of the forcing is not well known: globally (for 1992 air traffic conditions), values range from 3.5 mW/m² to 17 mW/m². Other studies have determined that night flights are mostly responsible for the warming effect: while accounting for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative forcing. Similarly, winter flights account for only 22% of annual air traffic, but contribute half of the annual mean radiative forcing.
    Easy Teasy Lemon Squeezy Source

    Higher humidity at lower elevations means that the water vapor in the exhaust is released below the saturation point of the air, thus no condensation/crystalization of the vapor & no contrail.

    The Yooper
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  5. Marcus, i believe the figures you are using are derived by subtracting the persistent gases effect in isolation from the total 33k effect... but then water vapor should still be around the 65% mark... but in reality is more inline with about 80% i believe on "average".

    But if you stripped all the water vapor out of the atmosphere the temp doesn't drop by 26.4k, but more by around the 65%-70% mark (21.45k-23.1k) There is a good thread on it at real climate.

    And yes mattj, i too have been learning about the non linear response o water vapor over at science o doom. A not so simple Q with water vapor... even taken in isolation(ignoring possible albedo effects)
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  6. Yes, Virginia, water vapor IS a GHG, regardless of altitude.

    Except for the white lumpy kind that collects & causes ice ages. :)

    The Yooper
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  7. This is a straw argument.

    Both water vapor and CO2 absorb IR radiation. Water vapor is a broad spectrum absorber with a large gap in absorption at IR wavelengths that effect the temperature regulation of the planet's atmosphere. CO2 is a notch absorber that overlaps and somewhat extends into the lower end (higher wavelength end) of the gap in water vapor's absorption. As the temperature of the atmosphere increases so the wavelength of the IR carrying heat by radiation shortens and moves away from the wavelength that CO2 absorbs and into the center of the gap in the water vapor absorption band allowing more IR energy to escape. The higher the temperature of the atmosphere, the less the effect of CO2. Of course it will be pretty toasty when that day comes.

    Anybody that has looked at the absorption of CO2 and methane overlaid on that of H2O would know this.

    Secondly H2O has another nice property. When it absorbs IR it causes the air in which it exists to rise with it. When the lapse rate causes the water vapor to condense the heat is released to continue traveling up while the air below is cooled. CO2 doesn't have this nice phase change under normal atmospheric conditions. But it goes along for the ride anyway.

    Water vapor is the most powerful greenhouse gas. That statement doesn't imply anything about the importance of CO2. CO2 is also a greenhouse gas. Speaking about CO2 out of the context of it's relation to the water vapor can be and is misleading.
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  8. No Joe Blog, the estimates I gave are based on the reports of J. T. Kiehl & Kevin E. Trenberth in "Earth’s Annual Global Mean Energy Budget"-from the "Bulletin of the American Meteorological Society 78 (2): 197–208." The point remains the same though, which is that on a ppmv basis CO2 is a significantly more powerful GHG than Water vapor, by at least 3:1.

    @ Daniel Bailey #6. Seriously, this kind of childish post might be acceptable for your fellow Denialists, but it really does nothing at all to advance anyone's understanding when you put up posts that are devoid of any intelligent analysis (like the fact that Ice & Snow don't *cause* Ice Ages, but are a symptom of Ice Ages caused by significantly reduced insolation.)
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  9. One of the Easy Teasy Lemon Squeezy sources says:

    It is currently not clear how much of the correlation between air traffic and cirrus cloudiness is actually due to a causal relationship. Hence the determination of the radiative forcing of contrail cirrus is fraught with large uncertainties; studies to resolve the differences and to constrain the error margins are certainly needed. All studies suggest that air traffic actually induces additional cirrus clouds, which seems plausible. However it is extremely difficult to demonstrate and prove such a correlation because the variation of cirrus cloudiness due to natural influences is much larger than the possible aviation effect. Hence, to look for the latter is like looking for a signal hidden in strong noise.

    This particular article is primarily concerned with parameters for models.

    Another one attributes a 1.2C increase in Tmax and a .3C increase in Tmin in areas with high contrail concentrations. This study used the paucity of civilian air traffic during 9/11 - 9/14/2001 to obtain temperature sensitivity to contrails. One of the factors that had to be corrected for was that the CONUS was essentially cloud free during that period.
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  10. Re: Marcus

    Denialist? Moi??? Been accused of many things in my life (never convicted) but never a denialist.

    Dude, you must've missed the smiley emoticon...see Poe's Law.

    The Yooper
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  11. Yes I did miss the emoticon. I read some of your other posts, in other threads, & realized that (a) denialist you are not & (b) you usually provide some excellent analysis. Consider my comment retracted :)!
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  12. TOP - the point about water vapour is that it is a feedback not a forcing. You dont have a physical process that increase or decrease water vapour content independent of temperature. ANY forcing that increases or decrease surface temperature will be amplified by the feedback of water vapour. With solar and aerosols stable or decreasing, its GHG that is the dominant post-1975 forcing. This isnt hand-waving - its backed by solid measurement of the extra radiation heating the surface matching the theoretical calculation of GHG effect resulting from the observed increases.
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  13. Water vapor - albedo - temperature ...

    Present here we see that it changes inversely as the temperature.

    Clouds - questioned the measurements on ISCCP high clouds, but low clouds affect the albedo (high thin clouds increase the FF - is a positive feedback to CO2) - are changing just like albedos. Overall, the circa to 2000 yr . decreasing cloud cover - now slightly increases - the opposite as the temperature.

    Professor Ole Humulum writes: “Within the still short period of satellite cloud cover observations, the total global cloud cover reached a maximum of about 69 percent in 1987 and a minimum of about 64 percent in 2000 (see diagram above), a decrease of about 5 percent. This decrease roughly corresponds to a radiative net change of about 0.9 W/m2 [!!!] within a period of only 13 years, which may be compared with the total net change from 1750 to 2006 of 1.6 W/m2 of all climatic drivers as estimated in the IPCC 2007 report, including release of greenhouse gasses from the burning of fossil fuels. These observations leave little doubt that cloud cover variations may have a profound effect on global climate and meteorology on almost any time scale considered.

    Where is the most water vapor - trapped energy ?

    In the tropics
    - which coincides with decreasing cloud cover in the tropics.
    Of course, I do not say that it is responsible for the cosmic radiation - according to me is of little importance, but I add the most important: the three explosions, "sulfur" volcanoes that destroyed the ozone and limited the tropical algae - cloud condensation nuclei, decrease of water vapor in the stratosphere and ...
    ... summed up this (depending on - now in superposition - solar cycles) factors together ...
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  14. FF - RF should be, very sorry ...
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  15. TOP #7, the fact that the IR absorption bands of water vapor largely overlap those of CO2 and methane does NOT prevent those gases from causing warming in those IR bands. That argument was first made by Angstrom in 1900 (who also thought water vapor overlapped entirely because instruments of the time lacked sufficient resolution). However, it has since been proven false;

    Water vapor is generally not found in the high atmosphere (see the discussion about jet contrails above for an exception). However, carbon dioxide and methane ARE. Which means that IR photons which manage to get through the 'water vapor layer' then get absorbed and re-emitted by CO2 and methane higher up. Without the CO2 and methane they'd all have escaped to space immediately once past the water vapor... with these other GHGs they get bounced around in the atmosphere more and this delay in radiating to space means more warming.

    So no, rising temperatures do NOT lead to decreased IR absorption by CO2. The 'band overlap' argument is simply false... though a reasonable mistake a hundred years ago.
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  16. Andrea, the FACT that CO2 and other greenhouse gases slow the rate at which infrared radiation leaves the atmosphere was first PROVEN by John Tyndall in the 1850s. You will not find a single remotely respected scientist, even amongst the global warming skeptics, who disputes this.

    No one claims that CO2 'holds heat' within itself... that simply isn't how the greenhouse effect works. The fundamental flaw in your argument is that it assumes this is the ONLY way heat can be retained within the climate system. Greenhouse gases absorb and then re-emit infrared radiation. Thus, radiation which WAS traveling from the planet's surface out towards space instead gets 'bounced back' down towards the surface... which has lots of rocks and trees and water and all sorts of other things which can 'hold' that heat just like water vapor does. The more greenhouse gas in the atmosphere the more this heat has to bounce around before it eventually makes its way out to space and thus the hotter the planet's surface.
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  17. I've always tried to explain this one to people this way (maybe someone can correct me if it's wrong)...

    Water vapor in the atmosphere is dependent on temperature. Warmer air holds more water vapor. Cooler air holds less.

    Absent all greenhouse gasses the earth would cool by ~30C.

    If you were to remove all the OTHER greenhouse gasses (CO2, CH4, etc.) other than water vapor the moisture would quickly freeze out. It would have no mechanism to remain in the air. Solar irradiance does not provide enough warming to hold water vapor in the air on it's own.

    Slowly add the other "trace" GHG's back into the atmosphere and you get incremental warming, which allows water vapor to stay in the air. A little more CO2 then feeds back more water vapor, etc.

    This is Richard Alley's "Biggest Control Knob." Dial up CO2, more warming. Dial down CO2, less warming.
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  18. robhon, even at ~30 C lower global temperatures there ought to be regions along the equator which were still warm enough to have some water vapor in the atmosphere. Possibly billions of years ago when solar output was significantly lower it could have resulted in 0% absolute humidity worldwide (c.f. 'Snowball Earth' theory). Otherwise your explanation matches my understanding.
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  19. Rob,

    I'd say you have it pretty well covered. Water becomes vapour because the heat excites the vibrations between different molecules. These are low energy: 20 times smaller than vibrations of the atoms within a molecule. Excite them enough and individual molecules will start "breaking free", Dah Dah: water vapour.

    Some people get confused about the fact that the feedback from water vapour doesn't explode, giving infinite heating, especially if they are told that the heating contribution from H2O is greater than CO2. A good way to explain that is to use the factor 2/3 as an example. So a 1 degree rise due to CO2 gives an extra 0.66667 due to H2O, which in turn induces 2/3 of that (i.e. 4/9 of the original). So already by the second iteration the total contribution from water vapour is greater than CO2 (1.1111 or 10/9) but, because the numbers at each iteration are smaller than the previous one the feedback is not explosive.
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  20. What's your take on the Spencer/Braswell paper in Journal of Geophysical Research questioning the cause and effect assumptions of the water vapor feedback theory?
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  21. Doug... There is a good piece on the Spencer/Braswell paper here at Real Climate.
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  22. Just some general thoughts:

    Without more water vapor, there will not be more clouds. Else, what are the more clouds made of?

    With higher temps, water vapor will stay in gas phase longer than with cooler temps.

    Water vapor is always a GHG; there just isn't very much of it above the tropopause.

    Water vapor has a tendency to precipitate out of the atmosphere; CO2 does not precipitate at conditions commonly found on earth.

    Regarding the overlap of water vapor and CO2, I've found these links to be useful. Basically, there is enough non-overlap for CO2 to have effects.

    We are mostly interested in what CO2 does around the 667 wavenumber. I like to switch the graphs to absorbance for clarity.

    An alternate view of the same information
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  23. scaddenp at 17:36 PM, water vapour feedback or forcing is almost a chicken and the egg question.
    Evaporation of surface moisture, and the resultant water vapour, depends on a number of factors, however the two that drive it most are solar radiation and wind.
    Ambient air temperature whilst a factor is not so nearly important as soil temperature which in turn responds directly to solar radiation.
    The depth of heat penetration of the soil is dependent not only on soil composition, but soil moisture levels with moist soil allowing the heat to penetrate as much as 3 times the depth as with dry soil.
    This reservoir of both moisture and stored heat is then most responsive to wind which without on a day of high solar radiation causes soil temperatures to rise, but with wind, even on a day with lower solar radiation will rapidly remove soil moisture. Wind is perhaps the most important single factor in evaporation.

    Ambient air temperature, or surface temperature is generally understood as not being that on the actual surface, but that at a point above the surface, in the case of instrument readings, 1.4 metres IIRC is the standard height.
    It as this standard height that a person can quite comfortably stand fully clothed whilst frying an egg on a steel plate on the ground heated solely by solar radiation, as long as the surface is protected from the wind.

    When the question of correlating water vapour content with atmospheric temperatures, it needs to be looked at from at least two different perspectives. Firstly what drives the change of state process that results in water vapour entering the atmosphere in the first place, and secondly what drives the process that causes it to change state again in the atmosphere and complete the hydrological cycle.
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  24. Chris G @21

    Assuming GHGs are the prime cause of atmospheric temp increase, then the greater their presence, water vapor increases. But the more water vapor present means an increase in clouds, reducing the level of solar radiation reaching the surface of the earth.

    Is it not true to argue that increased atmospheric water vapor acts not only to amplify temperature increases caused by GHGs but also acts to limit that increase by reducing solar energy reaching the earths surface?
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    Response: Tricky question - clouds not only limit the amount of incoming sunlight but also trap more infrared radiation coming up from the Earth's surface. The amount of heat trapped depends on the altitude of the clouds. The evidence currently indicates a slightly positive feedback from clouds (eg - a net warming effect) but this is an area with much uncertainty. Certainly a worthy topic for a more detailed future blog post!
  25. johnd - I am sorry but I can make no sense of that. Firstly, the water vapour in the atmosphere is mostly from evaporation out at sea and surface sea temperature is what is important.

    If the feedback from temperature didnt have an equilibrium point (k<1), then we would have runaway. To change climate and create trends, then you have to perturb the system. You cant do that by suddenly finding a way to add water, but you can if you increase the radiation received by the surface. Can you honestly believe that extra W/m2 of radiation is somehow NOT going to warm the surface?
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  26. Agnostic,
    It is an interesting question, although, I'll make the assumption that you mean that the water vapor which condenses to form clouds is what reduces solar energy reaching the surface. In addition to the moderator response, you also have to keep in mind that during the day, clouds reduce inbound flow of energy, but they always reduce the outbound flow, night and day. A cloudy day is cooler than a clear day, but a cloudy night is warmer than a clear night. I don't know how they balance out offhand.

    If the negative, cooling feedback of clouds were enough to offset the initial forcing plus the positive feedback of water vapor, then I don't think it would be possible to have periods as warm as those which exist in the geologic record.
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  27. Clouds cause an approximate net effect o 18wm2 cooling... they prevent on average 48wm2 o solar radiation, and reflect 30wm2 LW. Clouds are an area of uncertainty.
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  28. Ooops, forgot the link ;-)
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  29. scaddenp at 11:33 AM, obviously the majority of evaporation occurs off the oceans, but the process is the same as what occurs on land, all the same factors are involved.
    The main reason I used the situation on land is because it is not only easier to measure, but all the relevant factors are well measured and understood because of it's relevance to agriculture.
    However the other reason was that I assumed most people could easily relate how the various forces come into the equation from what they have observed in their everyday lives, whether it is observing how quickly the ground dries out on a seemingly mild, but windy day as opposed to a hot still day, or perhaps drying the clothes on a line.
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  30. Johnd - but all your stuff about 1.4m for measurement etc is irrelevant. The process that increases water evaporation is increased SST. The process (on the whole) that increases SST is more radiation. The cause of more radiation is more GHG.

    You seem to postulating that in steady state (no FORCING in albedo, solar, aerosol or albedo), that is somehow a mechanism that could cause more water vapour to enter the atmosphere and that this process is not damped, and could create a temperature trend over a 30 year cycle? Have I got that right?
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  31. Clouds also act as a buffer. High clouds such as the type cirrus - they are thin and have small albedo retain heat (long wave), in the tropics where high clouds are thicker, the albedo is higher - energy balance may be negative.
    At night, clouds not only retain but also to remove the energy - on its top surface - through the so-called: "thermal chimney". In high latitudes, "thermal chimney" is weaker than the effect of stopping energy through the clouds of radiation the Earth's surface - at night.
    In the tropics, such removal of energy - for example, are often very complex cumulonimbus (upward - until the tropopause and the stratosphere, even) can be even (so) larger than the energy of radiation stopped from the earth's surface.

    There should not be also consider the global energy balance cloud cover - although already a bit "already old" criticism on RealClimate, the tropics, can - through internal forcing of the Earth's climate system - significantly affect the size of the current warming. Not only as Mr Spencer says, but many researchers, in this and the IPCC (eg Solomon).
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  32. scaddenp at 13:44 PM, Phil, what I was trying to do is examine the link between two things.
    The known relationship between water vapour and atmospheric temperature, and the contentious issue of the apparent current lack of increased solar output whilst temperatures are supposedly still rising.
    Whether the output of shortwave radiation by the sun varies or not, it is not the only factor that determines the energy that is absorbed by the land and sea surfaces, which must occur before it can then be radiated off as longwave radiation.
    Variation in clouds is a significant controlling factor which is readily observed over short time frames with indications of longer term trends.
    With water vapour, although there is a known relationship between the saturation point and temperature, the evaporation process is driven by other factors that are not necessarily directly tied to temperature, those being the direct input of solar shortwave radiation and wind.
    Even though the oceans cover a larger area than land, I used the example of what occurs on land as it is well measured and understood, and something that I assume has been observed by most people, especially those with more than a passing interest in the processes that drive both the weather and the climate.
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  33. With regard to the JohnD/scaddenp conversation; be careful not to confuse rate with equilibrium. The speed at which any process occurs bears no relationship to how far it can go, and changes to the rate will not affect the "equilibrium" value. This sounds somewhat counter-intuitive, but that is a consequence of the fact that, in the real world, we rarely get a view of a complete system at equilibrium.

    Factors affecting the rate of water evaporation such as "global dimming", atmospheric "churn" (wind) and local heating of wet surfaces will not affect the total amount of water vapour the atmosphere can hold, but will affect (locally at least) the rate at which water evaporates. A high rate of evaporation is what is required for drying clothes.
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  34. @Johnd

    You are not answering the fundamental question.
    The earth has been a system more or less in equilibrium for thousands of years . For thousands of years the amount of water evaporating has been roughly the same a the amount of water vapor condensing. The mechanisms of evaporation you describe have existed as long as the earth exists.

    The question is: What has caused the earth’s climate to deviate from this situation ? You cannot say “an increase in water vapor concentration in the atmosphere”, because then you have to explain why this concentration has increased in the first place. You cannot pull yourself up by your own hairs. The consequence can not be its own cause.

    The generally accepted answer to the question is: CO2. We have put an enormous amount of CO2 in the atmosphere, causing the levels of CO2 to increase by 35% compared to pre-industrial times. This caused a temperature rise, the temperature rise caused more water to evaporate, and more water vapour in the atmosphere in turn leads to even higher temperatures.

    If you have an alternative theory, I’d like to hear it.
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  35. -I'm no scientist, but I want one who is to explain how the phase change of water affects water vapor's role as a green house gas relative to CO2. My understanding is that when water vapor condenses as water droplets in clouds, it releases its latent heat energy at that point. So, water constantly is gaining and loosing heat energy, and CO2 just holds on to the heat energy?
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  36. davidwwalters, no CO2 doesn't 'hold onto' heat energy at all. Think of each CO2 molecule as a tiny spinning mirror which is transparent to visible light, but reflective to infrared. A photon of sunlight hits the CO2 molecule and goes right through... down to the surface where it heats up a rock or something. The rock then emits a photon of infrared radiation upwards towards space. With no greenhouse gases in the atmosphere that heat would just go up and escape. WITH CO2 and other GHGs it goes up, hits a CO2 molecule, and bounces off in a random direction. Maybe it continues going upwards (to escape or bounce off another GHG molecule) or maybe it goes back down and 're-heats' the surface.

    The greenhouse effect does not work by 'trapping heat' within molecules of greenhouse gases. It works by slowing down the rate at which heat escapes the atmosphere into space. That heat winds up in molecules of other gases, liquids, and solids rather than just traveling up and out to space.
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  37. "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"

    Not so fast. Water vapor distribution in the atmosphere is always uneven (it is not a so called well mixed gas). GHG effect also depends on its distribution (which is pretty fractal-like), not only on its quantity. If upon heating this distribution does not simply scale up, but gets a bit more uneven, WV "feedback" can easily get neutral or even negative.

    Any empirical data on higher moments of atmospheric humidity distribution?

    One thing is sure. The more uneven water vapor distribution gets, the higher the radiative entropy production of the Earth system becomes. In an open thermodynamic system with many degrees of freedom the maximum entropy production principle tends to hold. It is like self organized criticality (e.g. in sandpile dynamics).

    In even more simple terms as soon as some radiative flux gets "trapped" by GHSs, it punches a hole in the water vapor canopy where the excess radiation can freely escape to space. The exact mechanism by which it happens is not important, it should happen somehow due to pretty basic principles.

    Anyway, hot and humid air usually ascends, expands, cools down adiabatically, water condenses, latent heat released, precipitation falls down to surface, relatively hot dry air is released at cloud top, moves sideways, cools down radiatively to space, descends. It is a hole like the one I've mentioned above.
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  38. CBDunkerson@ 35,
    Thanx for replying.....I understand now the mechanism of how greenhouse gasses work. I guess my question now is why CO2 trumps water vapor as the prime greenhouse gas even though there is so much more water vapor in the atmosphere relative to CO2 and other greenhouse gases.
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  39. It seems to me I was reading just last week that an increase in co2 causes a corresponding decrease in air saturation by co2 and therefore removes the warming effect of co2.average humidity has decreased worldwide since the 1940's ,so there might be some truth in that theory.whats up with that?
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  40. Johnd - still not making sense. I think the drivers for evaporation for warm water is perfectly well understood. I cannot understand what model for warming you are proposing - the some supposed interaction between clouds and wind can change the overall energy balance?? Seriously?
    On the other hand you seem perfectly content the logical, obvious way of increasing evaporation - more radiation warming the surface.
    Do accept the more radiation globally warming the ocean will increase water vaour?
    If not, what is postulated way of saving First law if the increased radiation is not warming the waters.

    If you are trying to make any sense at all, then please give us physically reasonable model so we can examine, make predictions and compare with reality.
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  41. Re: davidwwalters (37)
    I guess my question now is why CO2 trumps water vapor as the prime greenhouse gas even though there is so much more water vapor in the atmosphere relative to CO2 and other greenhouse gases.
    Short answer? The concentrations of water vapor in the atmosphere are pretty stable. Variations from background concentration levels normalize within a 9-day period (the residence time of water vapor in the atmosphere). Thus, while there is vastly more water vapor in the atmosphere than CO2, the greenhouse gas effects of that water vapor change little. Also, those effects are played out in the lower troposphere (because temps decline with increasing altitude, extra water vapor precipitates out...and when the freezing point of water is passed, the air gets pretty dry indeed).

    CO2 concentrations, on the other hand, are rising (about 40% above the interglacial range). And much of their effects occur in the upper troposphere, above the point where water vapor plays much of its King Lear role. Must-see video: Richard Alley AGU talk: CO2 Biggest Control Knob.

    For more in-depth background on greenhouse gases, global warming & climate science in general, go here.

    A little bit of CO2 goes a long ways.

    Hope this helps,

    the Yooper
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  42. Whoops should read:

    On the other hand you seem perfectly content to dismiss the logical, obvious way of increasing evaporation - more radiation warming the surface.
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  43. Phil at 19:44 PM, rate and equilibrium are closely related. With two opposing forces, in this case incoming energy and outgoing energy, equilibrium will only be found at the point where the energy gained balances the energy being lost.
    But is equilibrium ever found?
    In the case of a single location where the length of days and nights vary, and so too the rate at which energy is gained or lost, at what point of any day, or at what day of any annual cycle can the system be said to be at equilibrium?
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  44. Ann at 19:56 PM , what do you mean "more or less" in equilibrium?
    Seeking equilibrium is totally different to being at equilibrium.
    An active person seeking to match food intake with energy output is more or less seeking some form of equilibrium.
    A dead person may be at equilibrium in terms of the same energy input and output, but if not, then certainly at rest.
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  45. scaddenp at 12:19 PM, read more carefully, you have not grasped what has been written.
    Solar shortwave radiation provides the energy required for evaporation to occur.
    IR radiation, heat, can only be emitted after the solar energy has been absorbed by any matter on the surface, transforming the energy from light to heat, the air is then warmed from below by contact with such matter.
    Without solar radiation first transferring it's energy to any form of matter, such matter cannot emit IR radiation, heat, nor would water have the energy it requires to evaporate.
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  46. AdrianSmits - can you provide a source for your statement that humidity is reduced (Please realize that "Watts up with that" is not a source)? Can you possibly restate your first point - I don't understand it as written. Thanks.
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  47. johnd - heating the water to cause evaporation can be done by short-wave AND long wave. That is why radiation from GHGs matter. I am perfectly ready to accept that without solar radiation coming in then you dont get any IR going out. The point is that the GHG re-radiate outgoing IR back to the surface causing more warming of the water.

    A careful statement of your alternative theory would be appreciated.
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  48. scaddenp at 08:56 AM, forgetting about clouds for a moment that also trap IR radiation over about 2/3rds of the earths surface, water also covers about 2/3rds of the earths surface.
    Thus one would expect that if longwave radiation is a factor of any significance in increasing evaporation than perhaps 2/3rds of that radiation is a result of the initial absorption of solar shortwave radiation by the water suggesting that for at least 2/3rd of any IR driven evaporation it is a case of the water heating itself by the heat already dissipated.
    Perhaps if could provide a critical path chart or flow chart that would assist in conveying how this circular flow of energy all fits into place.
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  49. Johnd, firstly, forget clouds etc - you MEASURE the radiation coming from the atmosphere (not the sun), that is warming the sea. However you slice the cake, you have more radiation warming the sea with GHG than without them. And for equal windspeed, waves etc. warmer water will result in more evaporation than cooler water.

    The chart which shows all the energy interactions from
    from Trenberth & Kiehl. Latent heat etc. that you keep asking about.

    I would recommend the Science of Doom series Earth Energy Budget for the gory detail.
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  50. scaddenp at 14:39 PM, Phil, it is not really a critical path chart which would show the sequence of events, such a chart probably doesn't exist such is the complexity of the processes.

    However the question this chart immediately raises is that as evaporation requires an input of energy, how can IR radiation provide this.
    The chart shows a value of 390 for surface radiation and 324 for back radiation. Clearly there is a nett deficit of about 66 where an excess would required if energy was to be put into the evaporation process.
    From the chart it appears the process relies on incoming solar energy to provide the energy that not only drives the evaporation and other consumers of energy but to make up the nett loss in IR radiation.
    IR radiation can only enter the system after solar radiation has already transferred it's energy to the surface including the water that covers 2/3 of the surface.
    Immediately the solar radiation ceases, the losses of IR increase rapidly leaving even less energy to contribute to evaporation, even if there was excess available in the first place.
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