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Explaining how the water vapor greenhouse effect works

What the science says...

Select a level... Basic Intermediate
Water vapour is the most dominant greenhouse gas. Water vapour is also the dominant positive feedback in our climate system and amplifies any warming caused by changes in atmospheric CO2. This positive feedback is why climate is so sensitive to CO2 warming.

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)

Water vapour is the most dominant greenhouse gas. The greenhouse effect or radiative flux for water is around 75 W/m2 while carbon dioxide contributes 32 W/m2 (Kiehl 1997). These proportions are confirmed by measurements of infrared radiation returning to the Earth's surface (Evans 2006). Water vapour is also the dominant positive feedback in our climate system and a major reason why temperature is so sensitive to changes in CO2.

Unlike external forcings such as CO2 which can be added to the atmosphere, the level of water vapour in the atmosphere is a function of temperature. Water vapour is brought into the atmosphere via evaporation - the rate depends on the temperature of the ocean and air, being governed by the Clausius-Clapeyron relation. 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 vapour levels to 'normal levels' in short time.

Water Vapour as a positive feedback

As water vapour is directly related to temperature, it's also a positive feedback - in fact, the largest positive feedback in the climate system (Soden 2005). As temperature rises, evaporation increases and more water vapour accumulates in the atmosphere. As a greenhouse gas, the water absorbs more heat, further warming the air and causing more evaporation. When CO2 is added to the atmosphere, as a greenhouse gas it has a warming effect. This causes more water to evaporate and warm the air to a higher, stabilized level. So the warming from CO2 has an amplified effect.

How much does water vapour amplify CO2 warming? Without any feedbacks, a doubling of CO2 would warm the globe around 1°C. Taken on its own, water vapour feedback roughly doubles the amount of CO2 warming. When other feedbacks are included (eg - loss of albedo due to melting ice), the total warming from a doubling of CO2 is around 3°C (Held 2000).

Empirical observations of water vapour feedback and climate sensitivity

The amplifying effect of water vapor has been observed in the global cooling after the eruption of Mount Pinatubo (Soden 2001). The cooling led to atmospheric drying which amplified the temperature drop. A climate sensitivity of around 3°C is also confirmed by numerous empirical studies examining how climate has responded to various forcings in the past (Knutti & Hegerl 2008).

Satellites have observed an increase in atmospheric water vapour by about 0.41 kg/m² per decade since 1988. A detection and attribution study, otherwise known as "fingerprinting", was employed to identify the cause of the rising water vapour levels (Santer 2007). Fingerprinting involves rigorous statistical tests of the different possible explanations for a change in some property of the climate system. Results from 22 different climate models (virtually all of the world's major climate models) were pooled and found the recent increase in moisture content over the bulk of the world's oceans is not due to solar forcing or gradual recovery from the 1991 eruption of Mount Pinatubo. The primary driver of 'atmospheric moistening' was found to be the increase in CO2 caused by the burning of fossil fuels.

Theory, observations and climate models all show the increase in water vapor is around 6 to 7.5% per degree Celsius warming of the lower atmosphere. The observed changes in temperature, moisture, and atmospheric circulation fit together in an internally and physically consistent way. When skeptics cite water vapour as the most dominant greenhouse gas, they are actually invoking the positive feedback that makes our climate so sensitive to CO2 as well as another line of evidence for anthropogenic global warming.

Last updated on 26 June 2010 by John Cook.

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Comments 1 to 50 out of 188:

  1. A calculation of the greenhouse effect of water vapor and clouds using the GISS GCM is 85% as shown in the table at http://www.realclimate.org/index.php?p=142 . This argues against the relative values for radiative forcing of water and carbon dioxide quoted above.
  2. The watervaporfeedbacktheory isn't 'waterproof' at all.
    Humidity is not only controlled by air temperature but also by the temperature of seasurface and tropopause.
    I've been working with boundarylayer models. They showed a decrease in evaporation when increasing greenhouseeffect by CO2 at fixed SST's, especially by stratocumulus-conditions. Since greenhouse warming is strongest in troposphere and over land. Atmosphere stabilize over oceans and reduce vertical mixing. On the other hand the stratosphere surpresses deep convection. CO2-cooling of the stratospere makes the tropopause more flexible which leads to higher cumulonimbusclouds. Thereby the tops getting colder and water/ice rains out more efficiently. They observe a trend in more intensive rainfall all over the globe which IPCC destributes to higher moisture contents. It's rather a result of higher and colder tropopause, since higher watervapour contents are not measured in tropospere in recent decades. NCEP/NCAR-realyses show decrease of relative humidity on different levels and the evidence raised above are just so tiny.

    The study of Soden 2001 doesn't say anything about watervapourfeedback by greenhousewarming. First the Pinatubo caused a short cooling by reflecting solar irridiance and not a slowly warming. The area examined is too small thus doesn't yield for the total globe and the period is too short. Local interannual variations should also cause that effect.
  3. What about water vapour as a negative feedback, due to the reflecton of solar radiation away from the Earth by clouds?

    Surely if water vapour levels in the atmosphere increase then there is a greater tendency for clouds to form, reflecting more radiation away from the earth.
  4. I wish people would stop using emotively biased language instead of 'scientific' language.
    Water vapour does NOT amplify anything. It simply acts as a buffer, storing heat and releasing it according to well known physical processes.
    GG's do NOT 'force' anything : that implies they have some inherent power...which they don't. They moderate heat loss through buffering IR.
  5. A study of satellite data on clouds and water vapor indicates that prior predictions of substantial Global Warming are wrong. The study introduces a new method to diagnose the total radiative feedback parameter. Corrected analysis will result in low climate sensitivity where the GCMs predict insignificant global warming with increased atmospheric carbon dioxide. The subject is discussed at http://www.weatherquestions.com/Roy-Spencer-on-global-warming.htm .

    A completely independent analysis reveals that there is insignificant net positive feedback. This has the same effect on the climate as the finding of low climate sensitivity. Examination of the temperature data of the last and prior glaciations from NOAA as determined from Vostok ice cores reveals that temperature trends reversed direction irrespective of carbon dioxide level. This proves that there is no significant net positive feedback. Climatologists, who apparently don't know how feedback works don't realize this. Unaware of their ignorance, they impose net positive feedback in their GCMs which causes them to predict substantial warming from carbon dioxide increase. Without significant net positive feedback, the GCMs do not predict significant Global Warming.

    An assessment from a third perspective also determines that there is no significant net positive feedback. It can be seen at http://www.climate-skeptic.com/2008/01/index.html
  6. Water vapour is present in the upper troposphere; thus it radiates heat outwards and has a cooling effect, particularly in higher latitudes where insolation is lower than the tropics.
    As HS. says, increased cloud cover increases albedo, and since evaporation rate is a function of water/air temperature and turbulence, warming accelerates cloud production ...
    Increasing cloud (eventually) increases snowfall.
    All negative feedbacks.
  7. Has Global Warming Research Misinterpreted Cloud Behavior? ScienceDaily (June 12, 2008)
    ["Unfortunately, so far we have been unable to figure out a way to separate cause and effect when observing natural climate variability. That's why most climate experts don't like to think in terms of causality, and instead just examine how clouds and temperature vary together.] - Spencer
  8. #4 and #6 Mizimi - please think before posting:

    #4: Water vapour amplifies whatever source of warming caused the atmosphere to warm,since atmospheric warming results in raised atmospheric water concentrations. And water vapour is a greenhouse gas as we all know very well.

    If the earth's atmosphere cools so the water vapour drops AMPLIFYING the primary cooling effect.

    Amplification is exactly what water vapour does with respect to thermal effects on atmospheric temperature. It's exactly the right term to use. There's nothing "emotively biased" about it!

    -------------------------------
    Greenhouse gases apply a thermal FORCING. Forcing is an appropriate word to use since greenhouse gases do not instantaneously warm the atmosphere (and indirectly the surface). The apply a constant forcing that results in a shift in the atmospheric (and indirectly the surface) temperature towards a higher equilibrium value. There's nothing "emotively biased" about using appropriate terms.

    Sadly we can't "magic away" reality by semantic quibbling!

    ---------------------------------
    #6: Re feedbacks.

    Water vapour is the dominant greenhouse gas in the earth's atmnosphere. Together with CO2, water vapour supplements the earth's black body temperature (around minus 15 oC) with around 30 oC of enhanced warmth. This has been known and understood since the middle of the 19th century.

    How you can pretend that water vapour cools the atmosphere is a mystery. If only we could magic away problematic reality by asserting that things are exactly opposite to what they are in reality!

    It's easy to highlight your dull fallacy. You state -Mizimi: "Water vapour is present in the upper troposphere; thus it radiates heat outwards and has a cooling effect". Let's pretend first that the water vapour wasn't there: the infra-red radiation emitted from the earth's surface just passes freely into space. Now add back the water vapour. The water vapour absorbs the IR radiation emitted from the earth's surface AND RADIATES IT IN ALL DIRECTIONS EQUALLY (as well as passing kinetic energy to other gas molecules in its surrounds). In other words it suppresses the ability of infra-red radiation emitted from the earth to pass unhindered to space. In other, other, words it warms the atmosphere.

    Let's not make up stuff to pursue the pretence that we don't know what we do know!
  9. Amplify: lit. to increase or make bigger.
    1. To make larger or more powerful; increase.
    2. To add to, as by illustrations; make complete.
    3. To exaggerate.

    So by what means does any GG ( water vapour inc.) amplify the source of warming? It doesn't. It moderates the rate at which heat is lost which means the GMT is somewhat higher than it would otherwise be.
  10. Re #9 Yes water vapour amplifies the warming. One needs to be a bit more explicit in relation to the question of whether it amplifies the source of the warming (it does under some circumstances).

    So in general it's more explicit to state that water vapour amplifies the effect of the source of warming to which raised water vapour concentrations is a response.

    So yes, raised water vapour amplifies the warming. It "makes it larger" (it "exaggerates" or "increases" the warming). As your definitions indicate "amplification" is an appropriate word; there's nothing emotive about it! It doesn't have a cooling effect. And as you also indicate the GMT is somewhat higher than it would be otherwise be...
  11. Loss of heat from evaporation amounts to around 78 W/m or a quarter of the total insolation.
    The vapour rises into the upper troposphere and radiates heat into space. Wind driven circulation also pushes moist warm air into higher latitudes where the air is colder and drier; this warms the local atmosphere which then radiates heat into space at a greater rate as insolation is much lower.
    Yes, water vapour is a GG, but it also acts as a coolant within the hydrological cycle as a whole.

    http://eesc.columbia.edu/courses/ees/climate/lectures/o_atm.html

    http://www.env.leeds.ac.uk/envi2150/oldnotes/lecture3/lecture3.html
  12. Re #11

    What point are you trying to make Mizimi? It seems a little like that specious argument that used to be made against seat belts, that use of the latter prevented occupants from being "thrown clear" in an accident.

    Atmospheric water vapour is a greenhouse gas whose increased atmospheric concentration enhances the Earth's surface temperature. Of course the evaporation of water from the ocean surface results in transfer of the heat of evaporation into the atmosphere..it's a major mechanism by which solar thermal energy is transferred from the equatorial regions to the high latitudes....some of the thermal energy is radiated into space...

    But overall, raised atmospheric water vapour results in an increase in the Earth's surface temperature. As we've just seen (posts # 9/#10), water vapour amplifies the warming resulting from whatever forcing caused the atmosphere to warm in the first place! Water vapour is a positive feedback in the Earth's global energy budget....
  13. some errant thoughts.....
    Burn methane (CH4) and you get CO2 + 2H2O
    Ethanol (C2H5) gives 4CO2 + 5H2O
    Propane (C3H8) gives 3CO2 + 4H2O
    Benzene (C6H6) gives 6CO2 + 3H2O.
    Alkane hydrocarbons follow the formula C(n) H (2n+2) .... Heavy oils, for example, C18H34 give 18CO2 + 17H2O
    Since water vapour is around 10x more powerful a GG than CO2, it follows that the WV produced by burning gas and oils has a greater immediate warming effect than the CO2.
    Whilst generally it is held that the WV condenses out within a period of 14days, it is of course being continuously replaced so that its effect is more or less continuous. As the usage of oil and gas increase so the amount of water vapour added to the atmosphere also increases, as does the overall warming effect.
    According to WorldWatch, in 2005 we burnt some 3800M tons of oil and 2200M tons (oil equivalent) of gas, making a total of 6000M tons of FF excluding coal. Crudely speaking, we put as much WV into the air as we did CO2 ......but since it is 10x more effective a GG most of the warming actually must be coming from WV, not CO2.????
    In addition, we are pumping lots of WV into the atmosphere through other activities..

    Drax power station (UK) is a coal fired station that uses evaporative condensers..cooling towers...which take water from a local river. Of 59M tons of water taken annually, only 29M tons are returned to the river, the rest goes into the atmosphere.....the equivalent of 310M tons of CO2 or 0.01% of the 27,000M tons of CO2 emitted globally. From ONE power station.

    www.draxgroup.plc.uk/files/page/916/Drax_environmental_performance_review_2003.pdf
    Comments please???
  14. Well yes those are very errant thoughts!

    The CO2 stays in the atmosphere as a well-mixed gas. The water vapour just comes out again, since its concentrations are entirely dependent on the air temperature and pressure.

    It's easiest to see this by considering what happens in the short term. If atmospheric CO2 levels are increasing by 2 ppm per year, say, then every day (on average) an amount equivalent to around 0.005 ppm of excess CO2 is retained in the atmosphere. That's pretty small. Over a couple of weeks the atmosphere comes to equilibrium in relation to water vapour (perhaps even more quickly in relation to the near-ground level regions of the atmosphere where the water vapour is released).

    So as atmospheric CO2 levels rise relentlessly week on week, month on month, year on year, atmospheric water vapour reaches a rapid equilibration according to its vapour pressure in relation to the atmospheric presure and temperature. If this takes a week, the net "steady state" addition of water vapour is something like 7 x 0.005 x 10 x 2 of ppm CO2 "equivalents" (taking your value of the relative power of H2O compared to CO2, which actually I think is incorrect anyway, and taking account of the fact that so far only around 1/2 of the CO2 we emit stays in the atmosphere).

    So even within your scenario any excess water vapour resuting from burning fossil fuels produces a trivial excess warming - something well under that of 1 ppm of additional CO2. And of course it's a "steady state" value (both within your scenario, and likely in reality too), so you only add it once, whereas the atmospheric CO2 concentrations continually rise and rise...


    One might use your argument in reverse. As trees/plants grow they pull CO2 and water vapour out of the air:

    6CO2 + 6H2O ----> (CHOH)6 + 6O2

    where (CHOH)6 is generic carbohydrate

    During the N. hemisphere plant growth season, this is pulling more water vapour out of the air than is being released from burning fossil fuels I suspect. And in the autumn/winter months when N. hemisphere plant decay is "pumping" CO2 and water vapour back into the atmosphere, massively supplementing the water vapour released during oxidation of fossil fuels...

    ..but we don't find massive cycles of atmospheric water vapour concetrations for the reasons outlined above.

    I have a feeling that the only significant anthropogenic addition of water vapour to the atmosphere is from airlines at high altitude where the water vapour has a far higher residence time.
  15. Around 55% of the world's electrical energy is produced from coal (some 8 terawatts) and during that generation the approximate amount of water evaporated from cooling towers is about 25 million million tons/yr. or 25,000 billion tons...much the same as the CO2 from ALL fossil fuel combustion. And that figure does not include the water produced by combustion.
    The amount of WV varies according to time of day and load/location and network switching; but as coal fired stations are more difficult to modulate, they tend to be run as 'mainstay' providers, with oil or gas stations 'topping up' as necessary.
    That WV is generally enitted at around 100 to 200 meters above ground ( dependent on the type of cooling tower) and then drifts according to prevailing wind. That drift can exceed 2000 kilometers in 7 days
    ( average time before precipitation). So the next door country tends to get your WV ( rather like acid rain).
    Also, in another thread, Dan Panburn mentioned a paper which suggests the re-radiation of IR occurs within about 100 mtrs of the emitting body and thereafter conduction/convection take over. If this is the case, then adding substantial amounts of WV at relatively low altitudes would have an immediate warming effect which perhaps would be limited geographically by the WV 'shadow'.
    ????
  16. I wonder if your numbers are incorrect Mizimi.

    The total yearly worldwide emissions of CO2 are something a bit under 30,000 million tons [***].

    I suspect your 25,000 billion tons of water vapour evaporated in cooling towers should similarly be 25,000 MILLION tons and not 25,000 billion tons. Where did your numbers come from?


    [***] see table 3 at:

    http://www.eia.doe.gov/oiaf/1605/ggrpt/
  17. Yup, got my powers mixed up! That should have been 25 billion tons.
    There are a large number of water usage sites which give estimates for various activities:

    Evaporation from reservoirs: 275cubic km/yr
    (275 billion tonnes)
    World industry: 90 cubic km/yr.....90 billion tonnes
    (which includes the world's 63,590 power stations)
    Agriculture 1870 cubic km/yr ( listed as evapotranspiration as they can't tell the difference)
    So a reasonable estimate for AWV added to the atmosphere is 2200 billion tonnes a year which otherwise would not be there. Which is a lot more than the CO2 added.
  18. PS: I cannot find figures for agricultural irrigation and windage losses: some of that evapotranspiration would be offset by 'natural' plant growth, but to what extent is not clear, so that 1870 billion tonnes is probably a bit high.
  19. PPS; the figures don't include WV from combustion ( all sources of), commercial airconditioning systems, fogging systems, domestic irrigation, or simple respiration ( human component expected to increase by 50% by 2050)
  20. O.K., 25 billion tons of water vapour released in cooling towers and a bit more from other activities. Thta makes more sense. Let's use your 25 billion tons number and see whether this is significant in any way with respect to greenhouse gas warming.

    The answer is no...not really. This relates to the fact that water vapour in the atmosphere comes to a fairly fast equilibrium with respect to the atmospheric temperature and pressure (see my post #14).

    A good handle on this can be gleaned from a comparison of the water vapour released from cooling towers (and from fossil fuel burning overall) in relation to the overall amount of water vapour produced in the natural evironment by evaporation and precipitation.

    So we can use your numbers and compare the 25 billion tons of water vapour you indicate is released per year:

    25 x 10^9 tons = 2.5 x 10^13 kilograms of water

    to the total amount of evaporation/rainfall worldwide per year:

    5 x 10^17 kilograms of water.

    In other words the excess water vapour released into the atmosphere by the cooling tower/fossil fuel burning is around 0.005% of that released and precipitated yearly during the natural evaporative water cycle each year.

    in other worlds insignificant.
  21. But whilst that WV is in the atmosphere it is acting as a GG and thus delaying heat emission by the earth. The actual amount of 'natural' WV is increased by the addition of WV from man's activities and so the overall warming effect must be enhanced...after all is that not the argument for CO2? And WV is 10x more effective a GG than CO2. Unless one accepts there is a 'saturation' limit beyond which no further GG additions has an effect.
    Also if you add in the other sources of manmade WV (allowing 600B tonnes for agriculture)) we put more than 1000 billion tonnes a year into the atmosphere, which is 1 x 10^15 kg....0.2%
    My overall point here is that the amount of AWV has been increasing post 1950 ( the actual amounts I have yet to research) and the effect of that increase has to be included in any modelling.
  22. Not really Mizimi. Remember (see post #14/#20) that all we're doing is adding a small amount to the lower atmosphere which is a tiny proportion of that produced in the natural evaporation/precipitation cycle. We can blast and spray water into the atmosphere to our hearts content - it just comes out again.

    The warming effect of water vapour results from the column that exists through the entire atmosphere whose concentration responds dynamically to the atmospheric temperature and pressure. That's the greenhouse contribution: the amount of water vapour that is retained at equilibrium in relation to the atmospheric tempeature and pressure. By pumping a tiny excess amount of water vapour into the lower atmosphere, all we're doing is supplementing the already saturated amount that is there from the natural evaporation/precipitation cycle. I suppose that if one were able to measure this, there should be 0.05% more rainfall as a result!

    Water vapour isn't 10x more effective a GC than CO2. Despite the fact that the water vapour concentration of the atmosphere is 5 times that of CO2 (around 0.3% by mass for water vapour cf around 0.06% by mass for CO2), the contribution of CO2 to the greenhouse effect is at least 10% (and more like 25-30% with the water vapour feedback).

    Basically, over a period of a week or two a very tiny supplement of the natural evaporative water vapour cycle is added to the atmosphere from where it falls right out again. So if there is any additional contribution to the lower atmospheric water vapour this is a tiny steady state value that cannot increase.
  23. 1. The WV does indeed mostly stay in the lower atmosphere, but since that is the nearest to the radiating body ( the earth)it has a proportionately greater effect (exponential? the first molecule radiates in all directions and some of that is captured by the next and et seq) and whilst the amount may be small in relation to the total, the same can be said for CO2. And you don't appear to accept that when that WV is in the atmosphere ( for however long) it IS achieving some level of greenhouse effect.

    2. The WV column thro' the atmosphere is a gradient, most at the bottom of the column. This re-inforces my view that its greatest effect is at lower levels.
    The current amount of WV being 'added' by man is larger than the CO2 and logically has a greater warming effect.

    3. With regard to the absorbtion of IR I will go re-check. But by your figures there is 5x more WV than CO2 and together they account for a 33C rise in temp. This makes WV twice as powerful as CO2.

    4. I have preliminary data on world water usage which shows a rising trend from 700 to 2000 cubic kilometres from 1950 t0 2005 - - this is evaporated water from all sources. Interestingly there is a marked increase from 1980 -1990 of around 30% ( 1850 to 2360 ck)just around the time the GMT started to rise sharply. I don't see this as co-incidence.
  24. nope, you're still not getting it.

    First of all I did say that the tiny excess amount of water vapour with its short residence time might have an effect, but this will be a small steady state one, since the excess water vapour rather quickly precipitates from the lower atmosphere. In post #14 we calculated that the steady state effect might be equivalent to something under 1 ppm of additional atmospheric CO2.

    Remember that atmospheric CO2 doesn't fall out of the atmosphere. As we pump CO2 into the atmosphere it accumulates day by day, month by month, year by year.

    That can't happen with water vapor. So any tiny additional amount of water vapour we pump into the atmosphere that supplements that vastly larger amount from natural evaporative/precipitation results in a tiny steady state increase in whatever amount of warming results from the natural evaporation.

    Of course we do know that as the troposphere warms throughout its entire height, so the saturation point of water vapour increases (warmer air holds more water vapour). And so there is a net accumulation of water vapour in response to CO2-induced atmospheric warming that does lead to a cumulative increase in atmospheric water vapour. We know this occurs, first because it's simple physics and secondly we can measure it in the real world.


    In short:

    ONE: the CO2 we pump into the atmosphere stays there (except for the amount that partitions into the oceans and is absorbed by the terrestrial environment). Therefore atmospheric CO2 concentrations rise cumulatively (and very very quickly now).

    TWO: the water vapour that we pump into the atmosphere is a tiny supplement to the natural evaporative/precipitation cycle, and since this comes straight out of the lower atmosphere within a week or two it can (a) have only a very small effect and (b) caanot be cumulative.

    THREE: AS the entire troposphere warms under the influence of cumulatively enhanced CO2 concentrations, so the atmospheric water vapour concentration rises. This element (the water vapour feedback to enhanced greenhouse warming) is cumulative, and does provide a very significant supplement (a feedback or amplification) to the primary CO2-induced warming.
  25. You seem to be arguing that AWV has virtually no effect
    but as soon as CO2 induces increase in WV by warming the atmosphere a fraction of a degree there is a WV 'positive feedback' which is admitted...even when the amount is unquantified and probably quite small itself?
    And that somehow, despite adding AWV, the atmospheric total of WV remains more or less constant because of precipitation; yet we know it doesn't. It fluctuates all the time.

    "As the temperature of the atmosphere rises, more water is evaporated from ground storage (rivers, oceans, reservoirs, soil). Because the air is warmer, the relative humidity can be higher (in essence, the air is able to 'hold' more water when its warmer), leading to more water vapor in the atmosphere. As a greenhouse gas, the higher concentration of water vapor is then able to absorb more thermal IR energy radiated from the Earth, thus further warming the atmosphere. The warmer atmosphere can then hold more water vapor and so on and so on. This is referred to as a 'positive feedback loop'.
    #However, huge scientific uncertainty exists in defining the extent and importance of this feedback loop.#
    # As water vapor increases in the atmosphere, more of it will eventually also condense into clouds, which are more able to reflect incoming solar radiation (thus allowing less energy to reach the Earth's surface and heat it up).#
    The future monitoring of atmospheric processes involving water vapor will be critical to fully understand the feedbacks in the climate system leading to global climate change.
    #As yet, though the basics of the hydrological cycle are fairly well understood, we have very little comprehension of the complexity of the feedback loops.# Also, while we have good atmospheric measurements of other key greenhouse gases such as carbon dioxide and methane, #we have poor measurements of global water vapor, so it is not certain by how much atmospheric concentrations have risen in recent decades or centuries,# though satellite measurements, combined with balloon data and some in-situ ground measurements indicate generally positive trends in global water vapor.

    http://lwf.ncdc.noaa.gov/oa/climate/gases.html
  26. Hmmm...You've made some odd misinterpretations of my post again. If you read the two sentences following "THREE" in my post just above (#24) you can see that water vapour makes a significant effect. Since this is water vapour that re-equilibrates in the atmosphere in response to anthropogenic CO2-induced warming it can be classed as anthropogenic water vapour (AWV).

    And I certainly didn't say that "the atmospheric total of WV remains more or less constant because of precipitation". If you read the two sentences following "THREE" in my post just above, it states:

    "AS the entire troposphere warms under the influence of cumulatively enhanced CO2 concentrations, so the atmospheric water vapour concentration rises."

    I'm not sure how you can interpret a statement that the WV rises as meaning that "the WV remains constant"!


    Of course the atmospheric WV fluctuates. It varies according to the local atmospheric temperature (and pressure). However on average the amount of atmospheric water vapour rises as the atmospheric temperature rises on average in a warming world. This can be measured in the real world.

    And any water vapour that doesn't partition into the atmosphere according to the local temperature and pressure just falls right out again as precipitation. If we doubled the amount of water that we pumped into the near ground atmosphere that doesn't change the fact that it is ultimately the local atmospheric temperature and pressure that governs the amount of water vapour in the atmosphere. Anything else that's pumped up there just falls right out again as precipitation.


    The NOAA site gives a pretty basic description although it's a bit out of date with respect to the determination of tropospheric water vapour in response to greenhouse warming.
  27. some sums.............contribution to greenhouse effect:

    'natural' 'man added'

    WV 94.999% 0.001%
    CO2 3.502% 0.117%
    CH4 0.294% 0.066%
    NO2 0.903% 0.047%
    rest 0.025% 0.047%

    So A.CO2 contributes around 0.117% of total greenhouse effect.....33 x 0.117% = .038 degC

    Total A GG's contribute around 0.28%...33 x 0.28% = 0.09 degC
  28. Source please Mizimi for those columns of faux-"numerology". Where did you cut and paste them from and what do the columns mean?
  29. Apologies for the format, the spaces that should have been there seem to have been ignored in uploading.
    try again:

    The figures given indicate the relative contribution of GG's to global warming as a % of the whole, ie +33C

    WV from natural sources 94.999%: manmade 0.001%
    CO2 from natural sources 3.502%: manmade 0.117%
    CH4 from natural sources 0.294%; manmade 0.066%
    NO2 from natural sources 0.903%: manmade 0.047%
    Rest ....................0.025%: manmade 0.047%

    The basic data is available from the DOE website ( other than they ignore WV as a GG) "Current Greenhouse Gas Concentrations"; " Greenhouse Gases and Global Warming Potentials"
    From the IPCC "Warming Potentials of Halocarbons and Greenhouses Gases"
  30. Mizimi those numbers are nonsense. I don't believe they're from the IPCC reports at all.

    Why not simply give us the url from the site that you cut and pasted them from?

    And how can you say one the one hand that "they ignore WV as a GG"...

    ...and on the other show data that indicates that water vapour is 94.999% of the whole?

    ...???????
  31. The numbers are not cut and pasted but taken from the sites mentioned. Check them out yourself.
    The DOE site does not include WV in their GG listing so I take that as ignoring its effect as a GG.
    The 95% WV quoted is the 'high' side figure from various sites; the 'low' side figure generally quoted is 90% from other sources and one can rerun the sums using the 90% if desired. The rest is simple maths.
    I included WV in the calcs for obvious reasons.
  32. I have checked them out. Your numbers are nonsense and don't come from the DOE or from the IPCC. The data on Greenhouse Gas concentrations from the US government CDIAC refer to anthropogenic greenhouse gas and so don't include water vapour. Is that what you're on about?

    e.g. http://cdiac.esd.ornl.gov/pns/current_ghg.html

    But we don't really know what you're talking about unless you give us a link to the data that you cut 'n pasted those weird numbers from.
  33. Try:
    http://yosemite.epa.gov/oar/globalwarming.nsf/UniqueKeyLookup/SHSU5BUM9T/$File/ghg_gwp.pdf
    which gives warming potentials; (and they acknowledge WV is a GG and address their tables to 'selected' GG's)

    and:
    http://cdiac.esd.ornl.gov/pns/current_ghg.html (which doesn't acknowledge WV...and their data is NOT just AGG's........unless you deem we are responsible for all the pre-1750 gases ???)

    If you check the math from table 2 in the EPA site ( which can be checked against the DOE site)

    CO2 accounts for ~72% of GG warming EXCLUDING WV.
    CH4 about 7% and N2O 19%, the rest is CFC's etc.
    Now factor in WV at 95% (topend) and CO2 is responsible for 72% of 5%...3.6% of the total, or 1.19C
    Take the lowend 90% WV and you get 7.2% of the total or
    2.38C
    Now run the increases due to man's contributions alone and you will get the numbers I quoted in #29
  34. Not really Mizimi:

    (i) The CDIAC is exactly about anthropogenic greenhouse gases. That's why they reference them with respect to pre 1750 levels (zero for the CFC's but not for CO2, methane, ozone, and nitrous oxide).

    (ii) both sites (EPA and CDIAC) don't include water vapour because they are considering anthropogenic greenhouse gas forcings. The EPA site has a specific section about water vapour. As they state (and we've already established this point in numerous posts above on this thread), human activities aren't believed to directly affect water vapour concentrations (it's not a forcing, it's a feedback), but the warmer atmosphere from anthropogenic greenhouse gases results in a water vapour feedback that amplifies the anthropogenic forcing from enhanced greenhouse gas concentrations.

    That's all very straightforward and easy to understand.

    (iii) Otherwise your numerology is suspect. In fact it's not possible to partition greenhouse effect contributions from the individual greenhouse gases in the manner that you have done, since the greenhouse gases don't act independently, especially when water vapour is considered.

    removing CO2 from the atmosphere results in a very large cooling, since a significant part of the water vapour contribution arises as a feedback to CO2-induced warming, and if you remove the CO2 you remove a lot of the water. Do you see why that makes a linear, discrete "partitioning" of the greenhouse effect to individual components inaccessible to simple-minded arithmetic?

    In fact this issue has been dealt with many times through the use of modelling of the effects of removing various components of the atmosphere. An early example is:

    Ramanathan V, Coakley JA (1978) Climate Modeling Through Radiative-Convective Models. Rev. Geophys. 16, 465-489.

    For example if you remove CO2 from the atmosphere the greenhouse effect is reduced by 9% and if you remove water vapour it's reduced by 36%. But if you removee CO2 and water vapour it's reduced by more than 45% (the sum of the two). Likewise if you remove everything but CO2, 26% of the longwave IR is still absorbed in the atmosphere.

    So if one wanted to put numbers to the contribution of CO2, it's somewhere between 9-26% of the greenhouse effect....


    If you find Ramanathan and Coakley heavy going, Wikipedia has a goodish account:

    http://en.wikipedia.org/wiki/Greenhouse_effect#Water_vapour_effects
  35. "a significant part of the water vapour contribution arises as a feedback to CO2-induced warming,"

    If we allow CO2 alone is responsible for a 0.17C rise/decade in GMT as per GISS data, and we allow ALL of the heat associated with that temp rise to evaporate water then an estimate of the increase in WV is around 0.07gm/kg of dry air. The atmosphere contains an estimated 5.135x10E18 kg of dry air (National Centre for Atmospheric research) which gives us a possible
    5.135x10E18 x .07x10E-3 kg of WV added by this temp rise. Which ( providing I get my powers sorted out his time!) amounts to some 36x10E13 kg, PER DECADE.
    The current estimate for mm WV additions is 2360 cubic k's which is 2.36x10E12 kg....PER ANNUM.
    So over a decadal period we would add WV equivalent to 6.5% of that caused by CO2. Not insignificant.
    Dismissing AWV on the basis it precipitates out within 7-14 days does not do away with the fact that it does have a warming effect during that time period and it is a continuous effect at that.
  36. Sorry!! got my powers screwed up again, and having re-checked the sums there appears to me to be some overestimates in the AWV basic data, especially in respect of agricultural 'evapotranspiration' and the total for evaporation from reservoirs. So let's drop these out and concentrate on industrial figures which are a lot 'harder'.
    90 billion tonnes of WV from industrial use ( excluding water from combustion) is 9 x 10E13 kg /annum or 3 times that produced by CO2 warming. AND it is increasing.
    There is still the 30% rise in usage recorded from 1980 to 1990 which just happens to coincide with the upturn in GMT. Just a coincidence?
    Also your figures comparing the 'turnover' of WV in the atmosphere (5 x 10E17kg/annum) only use the WV reckoned to be evaporated from cooling towers, not ALL AWV.
  37. Some figures from the World Water Council on evaporation from human sources (not abstraction which is much much higher)

    1940 1950 1960 1970 1980 1990 2000
    50...80..155..245.. 285..320..515

    figures in cubic k's and exclude agriculture and domestic/municipal usage. Including these quadruples the figures.
    It is clear from the data that AWV has been steadily increasing since 1940 and thus the average global atmospheric water content will have risen since the atmosphere is not saturated. This addition occurs at low altitudes...precisely where one expects the warming effect of GG's to be the greatest.
  38. The article concerns itself with radiative forcing and positive feedback and does not address basics....

    The mass of CO2 in the atmosphere is around 3x 10E15 kg.
    The mass of WV is around 12.7 x 10E15 kg.
    The specific heat of CO2 @ 275K is 0.819kj/kg
    The specific heat of water @ 275K is around 4.2kj/kg
    Simplistically ( I can see the objections coming!) the heat content increase for a 1C temp increase is thus:
    CO2: 3x10E15 x 0.819 x 1 = 2.45 x 10E15 kj
    Water: 12.7 x 10E15 x 4.2 x 1 = 53.3 x 10E15 kj
    IE: the heat content available from wv is 22 x greater than CO2 disregarding any IR effect.
  39. A fine debate! Thanks to both of you. The questions are probably as important as the answers. If cooling towers are significant contributors to global warming, the alternatives to coal-fired power stations need to be reassessed.

    Surely there is a complication, though. The effluent from chimneys and (probably) cooling towers forms plumes, which are not only blown down-wind but also rise up through the atmosphere, cooling as they do so, remaining warmer and lighter than the surrounding atmosphere until completely mixed. I have a vague (and unsubstantiated) idea that these plumes rise to considerable altitudes – so is it possible that a large part of the AWV (and CO2) mixes into the atmosphere at the middle or top of the troposphere? Here, the influence of the AWV might be more significant…

    I have limited knowledge of climate modelling, so find it difficult to progress this idea. Any comments would be gratefully received.
  40. Wavelength: The answer is : It depends!
    I live 26km from a coal fired power station/cement works
    and when the prevailing wind is northerly I can see, out at sea a horizontal band of brown haze..the plume. On other days with higher winds it disperses more quickly and does not appear here.
    On still winter days smoke from local bonfires rises straight up and flattens out at around 100m and is dispersed by around 200m
    How high and far the plume goes depends entirely on 'local' weather conditions. Back a few years the scandinavian countries suffered 'acid rain' from british power stations.
  41. Wavelength: Air temperature decreases with altitude..this is called the 'lapse rate' and is approx -6.5C/km, so at around 5km the air temp is -13C and the pressure is about 0.5bar (half surface pressure).
    As WV content of air is temperature/pressure related then the amount of WV decreases as you ascend.
    Both these factors influence how high the plume can ascend without 'external' help from air turbulence or other factors.

    Therefore I would expect a greater warming effect from WV at surface level ( say up to 500metres)and then a steady decline towards zero at around 3000m
  42. Somewhere around 50% of the world’s CO2 and 75% of the AWV is produced by large point sources, in contrast to natural evaporation from oceans or lakes - so to me it seems reasonable to ask if this is likely to affect climate models. I have not seen it mentioned in the reading I have done.

    Since your previous response, I have looked on the web and found some information, such as: http://ams.confex.com/ams/88Annual/techprogram/paper_136670.htm
    This shows an average height of plumes above wildfires to be 2.3 km worldwide with 3 km for North America. This presumably only refers to the visible plume from particles/cloud formation, but is quite a bit below the 10-15 km that I had in mind. Also, the updraft from wildfires is probably hotter than from cooling towers, although with a lower relative humidity.

    The point-source question remains to niggle me. I suspect it is not significant, but still have no definitive argument to dismiss it.
  43. There's a new satellite-based study of the relationship of water vapor to CO2 by Dessler and colleagues on NASA.gov (the Earth section). It nails down the specific feedback effect's size.
  44. Had a quick look at the summary page which talks about measuring total WV content of the atmosphere....but no mention of assigning values to WV from global warming as distinct from WV evaporating from human activities. It;s all lumped together as 'WV'.
  45. "a network of 124 U.S. military weather stations with continuous human observations provides useful information of total cloud cover averaged over the contiguous United States, and suggests an increasing trend (~1.4% of the sky covered per decade) in U.S. total cloud cover from 1976 to 2004, with increases over most of the country except the Northwest."

    http://www.tiimes.ucar.edu/highlights/fy06/dai.html

    Clear indication of increasing WV over the time period global warming has 'accelerated'. The question is, how much is due to man's additions and how much to CO2 induced warming?
  46. re #45

    It's all due to CO2-induced warming Mizimi.....at least none of it is due to "addition" by man (or ladies for that matter). Man can't "add" water vapour to the atmosphere. The atmospheric water vapour levels are essentially "defined" by the atmospheric temperature and pressure.

    We've already determined (see post #20) that the amount of water vapour released into the atmosphere by burning stuff, cooling towers and so on is a tiny proportion of the water vapour released into the atmosphere by the natural evaporative cycle (we calculated mankinds contribution is around 0.005% of that released by natural evaporation).

    What happens to all of that water (e.g the vast amount from natural evaporation)?. It all comes straight out as precipitation. What stays in the atmosphere is what the atmosphere can support in relation to the atmospheric temperature and pressure. In fact the research indicates that the atmosphere tends to maintain a relatively constant relative humidity (i.e. raised absolute humidity with increasing temperature and vice versa).

    In the paper[***] that Tom Dayton refers to (Tom links to an info summary), the water vapour levels increase right throughout the troposphere to the high altitude/low pressure regions. The authors note that as the atmospheric temperature fluctuates (e.g. the strongish La Nina cooling), so the atmospheric water vapour levels follows (such that a constant relative humidity is roughly maintained).

    Each of these observations is consistent with our understanding of the response of the atmospheric water vapour concentration to temperature, and its rather short term response to temperature changes (after Pinatubo the atmospheric water vapour levels dropped even ‘though we were still releasing water vapour into the atmosphere from cooling towers!), and is incompatible with the notion that mankind's emissions (a miniscule proportion of the natural evaporative contribution), can somehow affect the atmospheric water vapour levels.

    [***] A. E. Dessler et al (2008) Water-vapor climate feedback inferred from climate fluctuations, 2003–2008. Geophys. Res. Lett. 35, L20704,

    abstract: Between 2003 and 2008, the global-average surface temperature of the Earth varied by 0.6°C. We analyze here the response of tropospheric water vapor to these variations. Height-resolved measurements of specific humidity (q) and relative humidity (RH) are obtained from NASA's satellite-borne Atmospheric Infrared Sounder (AIRS). Over most of the troposphere, q increased with increasing global-average surface temperature, although some regions showed the opposite response. RH increased in some regions and decreased in others, with the global average remaining nearly constant at most altitudes. The water-vapor feedback implied by these observations is strongly positive, with an average magnitude of λq = 2.04 W/m2/K, similar to that simulated by climate models. The magnitude is similar to that obtained if the atmosphere maintained constant RH everywhere.



    I'm not totally convinced by the clouds snippet you linked to. Again reading the full paper[*****] a couple of things stand out:

    (i) the large proportion of those 124 stations are in S. Calif, along the Gulf coast and along the Eastern seaboard. So vast regions of the US aren't covered, and it's possible that a small rise in cloudiness in these built up regions may relate to their location in built up areas of the US (??). The authors acknowledge this:

    "Quantitative estimates of the uncertainties of the
    U.S. cloud trend during 1976–2004 shown in Fig. 8b
    are difficult to derive because of the poor coverage
    by the 124 military stations and the subjective nature
    of the human observations."

    and:

    "Although the 124 U.S. military stations provide useful
    data for total cloud amount up to the present, they
    have limited spatial coverage and are inadequate
    for monitoring regional trends in the western and
    other parts of the country."


    (ii) satellite data show a decreasing cloud trend during this period worldwide. So either the military station trend is a localized one..or isn't correct...or the satellite trend is incorrect...or the two series are measuring different things (e.g. low level clouds from ground observations vs high level clouds from satellites).

    I think one can quite reasonably be skeptical about drawing too many inferences from empirical cloud data so far. The main point of Dai and Trenberth's paper is that replacement of human cloud monitoring (as done in the military bases) by automatic monitoring systems, is a backwards step..


    Note that although mankinds water vapour emissions don't have a significnat effect on greenhouse levels of atmospheric water vapour, we can probably influence the hydrological cycle by land use effects. and of course we are indirectly influencing the atmospheric water vapour levels by our CO2, methane, nitrous oxide emissions!


    [*****]Dai, A., T. R. Karl, B. Sun, and K. E. Trenberth, (2006) Recent trends in cloudiness over the United States: A tale of monitoring inadequacies. Bull. Amer. Meteor. Soc., 87, 597-606
  47. So we don't warm the atmosphere directly? The 14 terawatts of heat released by us annually has no effect on the global temp? It doesn't raise the temp at all and thus increase Tmin?
    The 6.7x 10E15 watts emitted annualy by the human population just by being alive doesn't affect Tmin either? Or the 3x 10E15 watts emitted by cows? All that heat is dissipated into the atmosphere and provides energy for WV to increase.
    Bear in mind too, that industrial WV emissions INCLUDE the energy needed to vaporise the water...no heat is required from the atmosphere, unlike 'natural' evaporation.
    One of the papers you referred to in another post indicated an increase in WV of 0.4gm/kg dry air over the oceans since 1988...far too much for just CO2 induced warming.
  48. PS:
    Have you considered the net contribution to the earth's energy budget made by Life? All metabolic processes are exothermic, from whales to bacteria. Even decomposition produces heat. And the source of that heat is the sun. Not IR, but mostly visible light in the green/yellow band, converted by photosynthesis into complex organic compounds which are then metabolised....giving out heat.
    In addition water vapour is emitted by air breathers ( roughly 60% of metabolic heat is emitted in the form of WV)adding to the atmospheric WV total.
    And whilst you will no doubt argue that compared to IR it is tiny - it is also iterative.
  49. #47....just re-read that paper and confirmed that often we see what we think we should see rather than what is there. The paper actually states 0.4kg/m2 increase in WV through the lower troposphere. Which if you assume is 8km deep allows an increase of 400/8000 gm/m3 or .05gm/m3.
    Air at 15C/~50%RH contains about 5.5gms/m3, so this increase is pretty insignificant and probably less than background 'noise', especially when you consider this is over a 10yr period.
  50. A few sums ....various sources give our annual energy usage from FF as around 14 terawatts.
    Looking around the Australian Bureau of Statistics gives the following population figures....

    People 21 million
    Horses 400,000
    Kangaroos 23 million
    Camels 400,000
    Cattle (dairy and beef) 26 million
    Sheep 20 million
    Rabbits 250 million

    Simple maths - multiplying numbers by the basal metabolic rate at rest of each species gives a daily heat emission of 315 x 10E9 watts or 114 x 10E12 watts per annum. This figure increases with physical activity.

    In other words, the small selction of life forms listed from ONE country put 9 times more heat into the atmosphere than man does through FF consumption....and they represent a tiny fraction of the worlds animal species.
    How does the GW model accomodate this?

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