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Explaining how the water vapor greenhouse effect works

What the science says...

Select a level... Basic Intermediate

Increased CO2 makes more water vapor, a greenhouse gas which amplifies warming

Climate Myth...

Water vapor is the most powerful greenhouse gas

“Water vapour is the most important greenhouse gas. This is part of the difficulty with the public and the media in understanding that 95% of greenhouse gases are water vapour. The public understand it, in that if you get a fall evening or spring evening and the sky is clear the heat will escape and the temperature will drop and you get frost. If there is a cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays quite warm. If you go to In Salah in southern Algeria, they recorded at one point a daytime or noon high of 52 degrees Celsius – by midnight that night it was -3.6 degree Celsius. […] That was caused because there is no, or very little, water vapour in the atmosphere and it is a demonstration of water vapour as the most important greenhouse gas.” (Tim Ball)

At a glance

If you hang a load of wet washing on the line on a warm, sunny day and come back later, you can expect it to be dryer. What has happened? The water has changed its form from a liquid to a gas. It has left your jeans and T-shirts for the air surrounding them. The term for this gas is water vapour.

Water vapour is a common if minor part of the atmosphere. Unlike CO2 though, the amount varies an awful lot from one part of the globe to another and through time. Let's introduce two related terms here: 'non-condensable' and 'condensable'. They set out a critical difference between the two greenhouse gases, CO2 and water vapour.

Carbon dioxide boils at -78.5o C, thankfully an uncommon temperature on Earth. That means it's always present in the air as a gas. Water is in comparison multitalented: it can exist as vapour, liquid and solid. Condensed liquid water forms the tiny droplets that make up clouds at low and mid-levels. At height, where it is colder, the place of liquid droplets is taken by tiny ice-crystals. If either droplets or crystals clump together enough, then rain, snow or hail fall back to the surface. This process is constantly going on all around the planet all of the time. That's because, unlike CO2, water vapour is condensable.

CO2 is non-condensable and that means its concentration is remarkably similar throughout the atmosphere. It has a regular seasonal wobble thanks to photosynthetic plants - and it has an upward slope caused by our emissions, but it doesn't take part in weather as such.

Although water vapour is a greenhouse gas, its influence on temperature varies all the time, because it's always coming and going. That's why deserts get very hot by day thanks to the Sun's heat with a bit of help from the greenhouse effect but can go sub-zero at night. Deserts are dry places, so the water vapour contribution to the greenhouse effect is minimal. Because clear nights are common in dry desert areas, the ground can radiate heat freely to the atmosphere and cool quickly after dark.

On the other hand, the warming oceans are a colossal source of water vapour. You may have heard the term, 'atmospheric river' on the news. Moist air blows in off the ocean like a high altitude conveyor-belt, meets the land and rises over the hills. It's colder at height so the air cools as it rises.

Now for the important bit: for every degree Celsius increase in air temperature, that air can carry another 7% of water vapour. This arrangement works both ways so if air is cooled it sheds moisture as rain. Atmospheric rivers make the news when such moisture-conveyors remain in place for long enough to dump flooding rainfalls. The floods spread down river systems, causing variable havoc on their way back into the sea.

Atmospheric rivers are a good if damaging illustration of how quickly water is cycled in and out of our atmosphere. Carbon dioxide on the other hand just stays up there, inhibiting the flow of heat energy from Earth's surface to space. The more CO2, the stronger that effect.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

When those who deny human-caused global warming use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as potent a greenhouse gas as water vapour, which there's already a lot of, adding a little more CO2 couldn't be that bad, they insist.

What this argument misses is the critical fact that water vapour in air creates what scientists call a 'positive feedback loop'. That means it amplifies temperature increases, making them significantly larger than they would be otherwise.

How does this work? The amount of water vapour in the atmosphere has a direct relation to the temperature in any given region and the availability of water for evaporation. Heard the weather-saying, "it's too cold to snow"? There's more than a grain of truth in that; very cold air has a low capacity for moisture.

But if you increase the temperature of the air, more water is able to evaporate, becoming vapour. There's a formula for this, the figure being 7% more moisture capacity for every degree Celsius of warming. All you then need is a source of water for evaporation and they are widespread - the oceans, for example.

So when something else causes a temperature increase, such as extra CO2 emissions from fossil fuel burning, more water can evaporate. Then, since water vapour is a greenhouse gas, this additional moisture causes the temperature to go up even further. That's the positive feedback loop.

How much does water vapour amplify warming? Studies show that water vapour feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C upward temperature change caused by CO2, the water vapour will cause the temperature to go up another 1°C. When other demonstrable feedback loops are included, and there are quite a few of them, the total warming from a 1°C change caused by CO2 is as much as 3°C.

The other factor to consider is that water evaporates from the land and sea and falls as rain, hail or snow all the time, with run-off or meltwater returning to the sea. Thus the amount of water vapour held in the atmosphere varies greatly in just hours and days. It's constantly cycling in and out through the prevailing weather in any given location. So even though water vapour is the dominant greenhouse gas in terms of quantity, it has what we call a short 'atmospheric residence time' due to that constant cycling in and out.

On the other hand, CO2 doesn't take an active part in the weather. It does hitch a lift on it by being slowly removed from the air as weak solutions of carbonic acid in rainwater. These solutions are key weathering agents, affecting rocks on geological time-scales. Weathering is a key part of the slow carbon cycle, with the emphasis on slow: CO2 thus stays in our atmosphere for years and even centuries. It has a long atmospheric residence time. Even a small additional amount of CO2 thus has a greater long-term effect - and in our case that additional amount is far from small.

To summarize: what deniers are ignoring when they say that water vapour is the dominant greenhouse gas, is that the water vapour feedback loop actually amplifies temperature changes caused by CO2.

When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right? What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere — making any temperature changes larger than they would be otherwise.

How does this work? The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.

How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C. When other feedback loops are included, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.

The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapour is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.

So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger.

Last updated on 23 July 2023 by John Mason. View Archives

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Further viewing

Further reading

From Ask NASA Climate - published on Feb 8, 2022:

Steamy Relationships: How Atmospheric Water Vapor Supercharges Earth's Greenhouse Effect

Denial101x video(s)

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

Additional video from the MOOC

Expert interview with Steve Sherwood

Comments

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Comments 1 to 25 out of 93:

  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

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